Tuesday, June 30, 2015

Optical regulation of cell chain

Xiaoshuai Liu, Jianbin Huang, Yao Zhang & Baojun Li

Formation of cell chains is a straightforward and efficient method to study the cell interaction. By regulating the contact sequence and interaction distance, the influence of different extracellular cues on the cell interaction can be investigated. However, it faces great challenges in stable retaining and precise regulation of cell chain, especially in cell culture with relatively low cell concentration. Here we demonstrated an optical method to realize the precise regulation of cell chain, including removing or adding a single cell, adjusting interaction distance, and changing cell contact sequence. After injecting a 980-nm wavelength laser beam into a tapered optical fiber probe (FP), a cell chain of Escherichia colis (E. colis) is formed under the optical gradient force. By manipulating another FP close to the cell chain, a targeted E. coli cell can be trapped by the FP and removed from the chain. Further, the targeted cell can be added back to the chain at different positions to change the cell contact sequence. The experiments were interpreted by numerical simulations and the impact of cell sizes and shapes on this method was analyzed.


Deformation of phospholipid vesicles in an optical stretcher

Ulysse DELABRE, Kasper Feld, Eleonore Crespo, Graeme Whyte, Cecile Sykes, Udo Seifert and Jochen Guck

Phospholipid vesicles are common model systems for cell membranes. Important aspects of membrane function relate to its mechanical properties. Here we have investigated the deformation behaviour of phospholipid vesicles in a dual-beam laser trap, also called an optical stretcher. This study explicitly makes use of the inherent heating present in such traps to investigate the dependence of vesicle deformation on temperature. By using lasers with different wavelengths, optically induced mechanical stresses and temperature increase can be tuned fairly independently with a single setup. The phase transition temperature of vesicle can be clearly identified by an increase in deformation. In the case of no heating effects, a minimal model for drop deformation in an optical stretcher and a more specific model for vesicle deformation that take explicitly into account the angular dependence of the optical stress are presented to account for the experimental results. Elastic constants are extracted from the fitting procedures, which agree with literature data. This study demonstrates the utility of optical stretching, which is easily combined with microfluidic delivery, for future serial, high-throughput study of the mechanical and thermodynamic properties of phospholipid vesicles.


A combined electrochemical and optical trapping platform for measuring single cell respiration rates at electrode interfaces

Benjamin J. Gross and Mohamed Y. El-Naggar

Metal-reducing bacteria gain energy by extracellular electron transfer to external solids, such as naturally abundant minerals, which substitute for oxygen or the other common soluble electron acceptors of respiration. This process is one of the earliest forms of respiration on earth and has significant environmental and technological implications. By performing electron transfer to electrodes instead of minerals, these microbes can be used as biocatalysts for conversion of diverse chemical fuels to electricity. Understanding such a complex biotic-abiotic interaction necessitates the development of tools capable of probing extracellular electron transfer down to the level of single cells. Here, we describe an experimental platform for single cell respiration measurements. The design integrates an infrared optical trap, perfusion chamber, and lithographically fabricated electrochemical chips containing potentiostatically controlled transparent indium tin oxide microelectrodes. Individual bacteria are manipulated using the optical trap and placed on the microelectrodes, which are biased at a suitable oxidizing potential in the absence of any chemical electron acceptor. The potentiostat is used to detect the respiration current correlated with cell-electrode contact. We demonstrate the system with single cell measurements of the dissimilatory-metal reducing bacterium Shewanella oneidensis MR-1, which resulted in respiration currents ranging from 15 fA to 100 fA per cell under our measurement conditions. Mutants lacking the outer-membrane cytochromes necessary for extracellular respiration did not result in any measurable current output upon contact. In addition to the application for extracellular electron transfer studies, the ability to electronically measure cell-specific respiration rates may provide answers for a variety of fundamental microbial physiology questions.


Optimized three-dimensional trapping of aerosols: the effect of immersion medium

S. Mohammad-Reza Taheri, Ebrahim Madadi, Mohammad Sadeghi, and S. Nader S. Reihani

Optical tweezers have proven to be indispensable micromanipulation tools especially in aqueous solutions. Because of the significantly larger spherical aberration induced by the refractive index mismatch, trapping aerosols has always been cumbersome if not impossible. We introduce a simple but very efficient method for optimized aerosol trapping at a desired depth. We show that a wise selection of the immersion medium and the mechanical tube length not only enables trapping of objects that are known to be untrappable but also provides a way to tune the trappable depth range.


Thursday, June 25, 2015

Synthesis and photo-postmodification of zeolite L based polymer brushes

Tim Buscher, Álvaro Barroso, Cornelia Denz and Armido Studer

A novel type of zeolite L/polymer hybrid material is presented. Polymer brush particles are generated using surface-functionalized zeolite L crystals as macroinitiators in controlled radical polymerization processes. Copolymerization of a photo-cleavable monomer and subsequent spin trapping of functionalized nitroxides under UV irradiation lead to a variety of highly functionalized zeolite L based core–shell particles.


Controlling dispersion forces between small particles with artificially created random light fields

Georges Brügger, Luis S. Froufe-Pérez, Frank Scheffold & Juan José Sáenz

Appropriate combinations of laser beams can be used to trap and manipulate small particles with optical tweezers as well as to induce significant optical binding forces between particles. These interaction forces are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic, translationally invariant, van der Waals and, in general, Casimir–Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. Here we show, both theoretically and experimentally, that dispersion forces between small colloidal particles can also be induced and controlled using artificially created fluctuating light fields. Using optical tweezers as a gauge, we present experimental evidence for the predicted isotropic attractive interactions between dielectric microspheres induced by laser-generated, random light fields. These light-induced interactions open a path towards the control of translationally invariant interactions with tuneable strength and range in colloidal systems.


Optical pulling forces in hyperbolic metamaterials

Alexander S. Shalin, Sergey V. Sukhov, Andrey A. Bogdanov, Pavel A. Belov, and Pavel Ginzburg

Control over mechanical motion of nanoscale particles is a valuable functionality desired in a variety of multidisciplinary applications, e.g., biophysics, and it is usually achieved by employing optical forces. Hyperbolic metamaterials enable tailoring and enhancing electromagnetic scattering and, as the result, provide a platform for a new type of optical manipulation. Here optical pulling forces acting on a small particle placed inside a hyperbolic metamaterial slab were predicted and analyzed. In order to attract particles to a light source, highly confined extraordinary modes of hyperbolic metamaterial were excited via scattering from an imperfection situated at the slab's interface. This type of structured illumination together with remarkable scattering properties, inspired by the hyperbolic dispersion in the metamaterial, creates optical attraction. Forces acting on high-, low-index dielectric, and gold particles were investigated and it was shown that the pulling effect emerges in all of the cases. The ability to control mechanical motion at nanoscale using auxiliary photonic structures paves the way for investigation of various phenomena, e.g., biochemical reactions, molecular dynamics, and more.


Tuesday, June 23, 2015

Low-cost optical manipulation using hanging droplets of PDMS

Craig McDonald and David McGloin

We propose and demonstrate a low-cost optical micromanipulation system that makes use of simple microfabricated components coupled to a smartphone camera for imaging. Layering hanging droplets of polydimethylsiloxane (PDMS) on microscope coverslips, and curing with a 100 W bulb, creates lenses capable of optical trapping. Optically trapped 3.93 μm silica beads were imaged with a second hanging droplet lens, doped with 1400 μg mL−1 Sudan II dye. Through doping, a lens with an integrated long-pass filter that effectively blocks the 532 nm trapping light was produced. Illumination was provided by shining a lamp into polystyrene foam packaging, perpendicular to the imaging path, producing a diffuse light source. We observed two dimensional trapping and report a Q value of [similar]8.9 × 10−3. The techniques here are an addition to the growing body of work on low cost and adaptable microfluidics.


Numerical study on rotational force in a bi-layered gammadion chiral metamaterial

Yohan Lee, Eui-Young Song, Seung-Yeol Lee, Joonsoo Kim, Jaebum Cho and Byoungho Lee

Recently it has been demonstrated that chiral metamaterials show giant optical activity and circular dichroism. However, it has been rarely known about rotational force generated by incident wave in chiral metamaterials. We calculate the rotational force in the bilayered gammadion chiral metamaterial. In addition, we show that a bi-layered structure, which has greater optical activity than a single-layered structure, changes the rotational force tendency greatly. This result can be used as a possible optical manipulation in biological sciences.


The tethering of chromatin to the nuclear envelope supports nuclear mechanics

Sarah M. Schreiner, Peter K. Koo, Yao Zhao, Simon G. J. Mochrie & Megan C. King

The nuclear lamina is thought to be the primary mechanical defence of the nucleus. However, the lamina is integrated within a network of lipids, proteins and chromatin; the interdependence of this network poses a challenge to defining the individual mechanical contributions of these components. Here, we isolate the role of chromatin in nuclear mechanics by using a system lacking lamins. Using novel imaging analyses, we observe that untethering chromatin from the inner nuclear membrane results in highly deformable nuclei in vivo, particularly in response to cytoskeletal forces. Using optical tweezers, we find that isolated nuclei lacking inner nuclear membrane tethers are less stiff than wild-type nuclei and exhibit increased chromatin flow, particularly in frequency ranges that recapitulate the kinetics of cytoskeletal dynamics. We suggest that modulating chromatin flow can define both transient and long-lived changes in nuclear shape that are biologically important and may be altered in disease.


All-silica microfluidic optical stretcher with acoustophoretic prefocusing

Giovanni Nava, Francesca Bragheri, Tie Yang, Paolo Minzioni, Roberto Osellame, Ilaria Cristiani, Kirstine Berg-Sørensen

Acoustophoresis is a widely reported and used technique for microparticle manipulation and separation. In the study described here, acustophoresis is employed to prefocus the flow (i.e., focusing occurring upstream of the analysis region) in a microfluidic chip intended for optical trapping and stretching. The whole microchip is made of silica with optical waveguides integrated by femtosecond laser writing. The acoustic force is produced by driving an external piezoelectric ceramic attached underneath the microchip at the chip resonance frequency. Thanks to an efficient excitation of acoustic waves in both water and glass, acoustophoretic focusing is observed along the channel length (>40 mm) and it is successfully demonstrated both with polystyrene beads, swollen red blood cell, and cells from mouse fibroblast cellular lines (L929). Moreover, by comparing results of cell stretching measurements, we demonstrate that acoustic waves do not alter the optical deformability of the cells and that the acoustic prefocusing results in a considerable enhancement of throughput in optical stretching experiments.


Monday, June 22, 2015

Elegant Gaussian beams for enhanced optical manipulation

Christina Alpmann, Christoph Schöler and Cornelia Denz

Generation of micro- and nanostructured complex light beams attains increasing impact in photonics and laser applications. In this contribution, we demonstrate the implementation and experimental realization of the relatively unknown, but highly versatile class of complex-valued Elegant Hermite- and Laguerre-Gaussian beams. These beams create higher trapping forces compared to standard Gaussian light fields due to their propagation changing properties. We demonstrate optical trapping and alignment of complex functional particles as nanocontainers with standard and Elegant Gaussian light beams. Elegant Gaussian beams will inspire manifold applications in optical manipulation, direct laser writing, or microscopy, where the design of the point-spread function is relevant.


The potential of optical tweezer (ot) for viscoelastivity measurement of nanocellulose solution

Wan Nor Suhaila Wan Aziz, Shahrul Kadri Ayop, Sugeng Riyanto

In this paper, we review the recent applications of optical tweezer (OT) in studying the microrheology of variety of polymeric solution. Our aim is to expose optical tweezer research to the public and newcomer. This paper highlights and summarizes the advantages of optical tweezer as compared with the conventional method, introduces the benefit of nanocellulose and also presents an overview of the potential in the measurement of nanocellulose solution’s viscoelasticity by using optical trapping method.


A Temperature-Jump Optical Trap for Single-Molecule Manipulation

Sara de Lorenzo, Marco Ribezzi-Crivellari, J. Ricardo Arias-Gonzalez, Steven B. Smith, Felix Ritort

To our knowledge, we have developed a novel temperature-jump optical tweezers setup that changes the temperature locally and rapidly. It uses a heating laser with a wavelength that is highly absorbed by water so it can cover a broad range of temperatures. This instrument can record several force-distance curves for one individual molecule at various temperatures with good thermal and mechanical stability. Our design has features to reduce convection and baseline shifts, which have troubled previous heating-laser instruments. As proof of accuracy, we used the instrument to carry out DNA unzipping experiments in which we derived the average basepair free energy, entropy, and enthalpy of formation of the DNA duplex in a range of temperatures between 5°C and 50°C. We also used the instrument to characterize the temperature-dependent elasticity of single-stranded DNA (ssDNA), where we find a significant condensation plateau at low force and low temperature. Oddly, the persistence length of ssDNA measured at high force seems to increase with temperature, contrary to simple entropic models.


Ultrastable measurement platform: sub-nm drift over hours in 3D at room temperature

Robert Walder, D. Hern Paik, Matthew S. Bull, Carl Sauer, and Thomas T. Perkins

Advanced optical traps can probe single molecules with Ångstrom-scale precision, but drift limits the utility of these instruments. To achieve Å-scale stability, a differential measurement scheme between a pair of laser foci was introduced that substantially exceeds the inherent mechanical stability of various types of microscopes at room temperature. By using lock-in detection to measure both lasers with a single quadrant photodiode, we enhanced the differential stability of this optical reference frame and thereby stabilized an optical-trapping microscope to 0.2 Å laterally over 100 s based on the Allan deviation. In three dimensions, we achieved stabilities of 1 Å over 1,000 s and 1 nm over 15 h. This stability was complemented by high measurement bandwidth (100 kHz). Overall, our compact back-scattered detection enables an ultrastable measurement platform compatible with optical traps, atomic force microscopy, and optical microscopy, including super-resolution techniques.


Friday, June 19, 2015

The Cryptococcus neoformans capsule: lessons from the use of optical tweezers and other biophysical tools

Bruno Pontes and Susana Frases

The fungal pathogen Cryptococcus neoformans causes life-threatening infections in immunocompromised individuals, representing one of the leading causes of morbidity and mortality in AIDS patients. The main virulence factor of C. neoformans is the polysaccharide capsule; however, many fundamental aspects of capsule structure and function remain poorly understood. Recently, important capsule properties were uncovered using optical tweezers and other biophysical techniques, including dynamic and static light scattering, zeta potential and viscosity analyses. This review provides an overview of the latest findings in this emerging field, explaining the impact of these findings on our understanding of C. neoformans biology and resistance to host immune defenses.


Optical Fiber Tweezers Fabricated by Guided Wave Photo-Polymerization

Rita S. Rodrigues Ribeiro, Raquel Queirós, Olivier Soppera, Ariel Guerreiro and Pedro A. S. Jorge

In this work the use of guided wave photo-polymerization for the fabrication of novel polymeric micro tips for optical trapping is demonstrated. It is shown that the selective excitation of linear polarized modes, during the fabrication process, has a direct impact on the shape of the resulting micro structures. Tips are fabricated with modes LP02 and LP21 and their shapes and output intensity distribution are compared. The application of the micro structures as optical tweezers is demonstrated with the manipulation of yeast cells.


Optical trapping of gold nanoparticles in air

Liselotte Jauffred , S. Mohammad-Reza Taheri , Regina Schmitt , Heiner Linke , and Lene B Oddershede

Most progress on optical nanoparticle control has been in liquids while optical control in air has proven more challenging. By utilizing an air chamber designed to have a minimum of turbulence and a single laser beam with a minimum of aberration, we trapped individual 200 to 80 nm gold nanoparticles in air and quantified the corresponding trapping strengths. These results pave the way for construction of metallic nanostructures in air away from surfaces.


Direct measurements of magnetic interaction-induced cross-correlations of two microparticles in Brownian motion

Maria N. Romodina, Maria D. Khokhlova, Evgeny V. Lyubin & Andrey A. Fedyanin
The effect of magnetic interactions on the Brownian motion of two magnetic microparticles is investigated. The cross-correlations of the thermal fluctuations of the two magnetic microbeads are directly measured using double-trap optical tweezers. It is experimentally demonstrated that the cross-correlation function is governed by the gradient of the magnetic force between the microparticles. The magnetic forces are measured with femtonewton precision, and the magnetic dipole moments of individual microparticles are determined within an accuracy on the order of fA-m2.


Thursday, June 18, 2015

Axial Optical Traps: A New Direction for Optical Tweezers

Samuel Yehoshua, Russell Pollari, Joshua N. Milstein

Optical tweezers have revolutionized our understanding of the microscopic world. Axial optical tweezers, which apply force to a surface-tethered molecule by directly moving either the trap or the stage along the laser beam axis, offer several potential benefits when studying a range of novel biophysical phenomena. This geometry, although it is conceptually straightforward, suffers from aberrations that result in variation of the trap stiffness when the distance between the microscope coverslip and the trap focus is being changed. Many standard techniques, such as back-focal-plane interferometry, are difficult to employ in this geometry due to back-scattered light between the bead and the coverslip, whereas the noise inherent in a surface-tethered assay can severely limit the resolution of an experiment. Because of these complications, precision force spectroscopy measurements have adapted alternative geometries such as the highly successful dumbbell traps. In recent years, however, most of the difficulties inherent in constructing a precision axial optical tweezers have been solved. This review article aims to inform the reader about recent progress in axial optical trapping, as well as the potential for these devices to perform innovative biophysical measurements.


Optical chirality exhibited by two axially propagating electromagnetic waves in counter-rotations

Jinsik Mok and Hyoung-In Lee

The optical chirality of two axially propagating electromagnetic waves is investigated. These two waves of different nature are in counter-rotations, thereby being non-plane waves. From the resulting spatial distributions of energy and chirality, we find not only enhancements but also cancellations due to the interferences in these hybrid waves. In addition, the roles of the axial wave number and interference phase are illustrated by introducing a proper figure of merit for relevant device performances.


Macroscopic Theory of Optical Momentum

Brandon A. Kemp

Light possesses energy and momentum within the propagating electromagnetic fields. When electromagnetic waves enter a material, the description of energy and momentum becomes ambiguous. In spite of more than a century of development, significant confusion still exists regarding the appropriate macroscopic theory of electrodynamics required to predict experimental outcomes and develop new applications. This confusion stems from the myriad of electromagnetic force equations and expressions for the momentum density and flux. In this review, the leading formulations of electrodynamics are compared with respect to how media are modeled. This view is applied to illustrate how the combination of electromagnetic fields and material responses contribute to the continuity of energy and momentum. A number of basic conclusions are deduced with the specific aim of modeling experiments where dielectric and magnetic media are submerged in media with a differing electromagnetic response. These conclusions are applied to demonstrate applicability to optical manipulation experiments.


Radiation pressure of active dispersive chiral slabs

Maoyan Wang, Hailong Li, Dongliang Gao, Lei Gao, Jun Xu, and Cheng-Wei Qiu

We report a mechanism to obtain optical pulling or pushing forces exerted on the active dispersive chiral media. Electromagnetic wave equations for the pure chiral media using constitutive relations containing dispersive Drude models are numerically solved by means of Auxiliary Differential Equation Finite Difference Time Domain (ADE-FDTD) method. This method allows us to access the time averaged Lorentz force densities exerted on the magnetoelectric coupling chiral slabs via the derivation of bound electric and magnetic charge densities, as well as bound electric and magnetic current densities. Due to the continuously coupled cross-polarized electromagnetic waves, we find that the pressure gradient force is engendered on the active chiral slabs under a plane wave incidence. By changing the material parameters of the slabs, the total radiation pressure exerted on a single slab can be directed either along the propagation direction or in the opposite direction. This finding provides a promising avenue for detecting the chirality of materials by optical forces.


Optical torque on small bi-isotropic particles

Manuel Nieto-Vesperinas

We establish the equations for the time-averaged optical torque on dipolar bi-isotropic particles. Due to the interference of the scattered fields, it has a term additional to the one that is commonly employed in theory and experiments. Its consequences for conservation of energy, angular momentum, and effects like negative torques are discussed.


Wednesday, June 17, 2015

Polar features in the flagellar propulsion of E. coli bacteria

S. Bianchi, F. Saglimbeni, A. Lepore, and R. Di Leonardo

E. coli bacteria swim following a run and tumble pattern. In the run state all flagella join in a single helical bundle that propels the cell body along approximately straight paths. When one or more flagellar motors reverse direction the bundle unwinds and the cell randomizes its orientation. This basic picture represents an idealization of a much more complex dynamical problem. Although it has been shown that bundle formation can occur at either pole of the cell, it is still unclear whether these two run states correspond to asymmetric propulsion features. Using holographic microscopy we record the 3D motions of individual bacteria swimming in optical traps. We find that most cells possess two run states characterized by different propulsion forces, total torque, and bundle conformations. We analyze the statistical properties of bundle reversal and compare the hydrodynamic features of forward and backward running states. Our method is naturally multi-particle and opens up the way towards controlled hydrodynamic studies of interacting swimming cells.


Rotational Efficiency of Photo-Driven Archimedes Screws for Micropumps

Chih-Lang Lin, Yu-Sheng Lin and Patrice L. Baldeck

In this study, we characterized the rotational efficiency of the photo-driven Archimedes screw. The micron-sized Archimedes screws were fabricated using the two-photon polymerization technique. Free-floating screws trapped by optical tweezers align in the laser irradiation direction and rotate spontaneously. The influences of the screw pitch and the number of screw blades have been investigated in our previous studies. In this paper, the blade thickness and the central rod of the screw were further investigated. The experimental results indicate that the blade thickness contributes to rotational stability, but not to rotational speed, and that the central rod stabilizes the rotating screw but is not conducive to rotational speed. Finally, the effect of the numerical aperture (NA) of the optical tweezers was investigated through a demonstration. The NA is inversely proportional to the rotational speed.


Optical trap for both transparent and absorbing particles in air using a single shaped laser beam

Brandon Redding and Yong-Le Pan

Optical trapping of airborne particles is emerging as an essential tool in applications ranging from online characterization of living cells and aerosols to particle transport and delivery. However, existing optical trapping techniques using a single laser beam can trap only transparent particles (via the radiative pressure force) or absorbing particles (via the photophoretic force), but not particles of either type—limiting the utility of trapping-enabled aerosol characterization techniques. Here, we present the first optical trapping technique capable of trapping both transparent and absorbing particles with arbitrary morphology using a single shaped laser beam. Such a general-purpose optical trapping mechanism could enable new applications such as trapping-enabled aerosol characterization with high specificity.


Flying particle sensors in hollow-core photonic crystal fibre

D. S. Bykov, O. A. Schmidt, T. G. Euser & P. St. J. Russell

Optical fibre sensors make use of diverse physical effects to measure parameters such as strain, temperature and electric field. Here we introduce a new class of reconfigurable fibre sensor, based on a ‘flying-particle’ optically trapped inside a hollow-core photonic crystal fibre and illustrate its use in electric field and temperature sensing with high spatial resolution. The electric field distribution near the surface of a multi-element electrode is measured with a resolution of ∼100 μm by monitoring changes in the transmitted light signal due to the transverse displacement of a charged silica microparticle trapped within the hollow core. Doppler-based velocity measurements are used to map the gas viscosity, and thus the temperature, along a hollow-core photonic crystal fibre. The flying-particle approach represents a new paradigm in fibre sensors, potentially allowing multiple physical quantities to be mapped with high positional accuracy over kilometre-scale distances.


Tuesday, June 16, 2015

Mechanochemical tuning of myosin-I by the N-terminal region

Michael J. Greenberg, Tianming Lin, Henry Shuman, and E. Michael Ostap

Myosins are molecular motors that generate force to power a wide array of motile cellular functions. Myosins have the inherent ability to change their ATPase kinetics and force-generating properties when they encounter mechanical loads; however, little is known about the structural elements in myosin responsible for force sensing. Recent structural and biophysical studies have shown that myosin-I isoforms, Myosin-Ib (Myo1b) and Myosin-Ic (Myo1c), have similar unloaded kinetics and sequences but substantially different responses to forces that resist their working strokes. Myo1b has the properties of a tension-sensing anchor, slowing its actin-detachment kinetics by two orders of magnitude with just 1 pN of resisting force, whereas Myo1c has the properties of a slow transporter, generating power without slowing under 1-pN loads that would stall Myo1b. To examine the structural elements that lead to differences in force sensing, we used single-molecule and ensemble kinetic techniques to show that the myosin-I N-terminal region (NTR) plays a critical role in tuning myosin-I mechanochemistry. We found that replacing the Myo1c NTR with the Myo1b NTR changes the identity of the primary force-sensitive transition of Myo1c, resulting in sensitivity to forces of <2 pN. Additionally, we found that the NTR plays an important role in stabilizing the post–power-stroke conformation. These results identify the NTR as an important structural element in myosin force sensing and suggest a mechanism for generating diversity of function among myosin isoforms.


Origin and Future of Plasmonic Optical Tweezers

Jer-Shing Huang and Ya-Tang Yang

Plasmonic optical tweezers can overcome the diffraction limits of conventional optical tweezers and enable the trapping of nanoscale objects. Extension of the trapping and manipulation of nanoscale objects with nanometer position precision opens up unprecedented opportunities for applications in the fields of biology, chemistry and statistical and atomic physics. Potential applications include direct molecular manipulation, lab-on-a-chip applications for viruses and vesicles and the study of nanoscale transport. This paper reviews the recent research progress and development bottlenecks and provides an overview of possible future directions in this field.


Trapping two types of particles with a focused generalized Multi-Gaussian Schell model beam

Xiayin Liu, Daomu Zhao

We numerically investigate the trapping effect of the focused generalized Multi-Gaussian Schell model (GMGSM) beam of the first kind which produces dark hollow beam profile at the focal plane. By calculating the radiation forces on the Rayleigh dielectric sphere in the focused GMGSM beam, we show that such beam can trap low-refractive-index particles at the focus, and simultaneously capture high-index particles at different positions of the focal plane. The trapping range and stability depend on the values of the beam index N and the coherence width. Under the same conditions, the low limits of the radius of low-index and high-index particles for stable trapping are indicated to be different.


Microfluidic Single-Cell Analysis with Affinity Beads

Michael Werner, Raghavendra Palankar, Loïc Arm, Ruud Hovius and Horst Vogel

To investigate functional heterogeneity between individuals in a cell population, affinity beads are transferred into living mammalian cells by H. Vogel and co-workers. On page 2607, beads coated with specific capture agents efficiently bind and up-concentrate cytosolic target proteins on their surface. For single-cell analysis, an optically trapped bead with bound target proteins is extracted from its host cell in a microfluidic device. Compared to classical chemical cytometry, this method has the potential to reach high analytical sensitivity and throughput for single-cell analysis in many biomedical fields.


Monday, June 15, 2015

Water diffusion in atmospherically relevant α-pinene secondary organic material

Hannah C. Price, Johan Mattsson, Yue Zhang, Allan K. Bertram, James F. Davies, James W. Grayson, Scot T. Martin, Daniel O'Sullivan, Jonathan P. Reid, Andrew M. J. Rickards and Benjamin J. Murray
Secondary organic material (SOM) constitutes a large mass fraction of atmospheric aerosol particles. Understanding its impact on climate and air quality relies on accurate models of interactions with water vapour. Recent research shows that SOM can be highly viscous and can even behave mechanically like a solid, leading to suggestions that particles exist out of equilibrium with water vapour in the atmosphere. In order to quantify any kinetic limitation we need to know water diffusion coefficients for SOM, but this quantity has, until now, only been estimated and has not yet been measured. We have directly measured water diffusion coefficients in the water soluble fraction of α-pinene SOM between 240 and 280 K. Here we show that, although this material can behave mechanically like a solid, at 280 K water diffusion is not kinetically limited on timescales of 1 s for atmospheric-sized particles. However, diffusion slows as temperature decreases. We use our measured data to constrain a Vignes-type parameterisation, which we extend to lower temperatures to show that SOM can take hours to equilibrate with water vapour under very cold conditions. Our modelling for 100 nm particles predicts that under mid- to upper-tropospheric conditions radial inhomogeneities in water content produce a low viscosity surface region and more solid interior, with implications for heterogeneous chemistry and ice nucleation.


Linear microrheology with optical tweezers of living cells 'is not an option'!

Manlio Tassieri

Optical tweezers have been successfully adopted as exceptionally sensitive transducers for microrheology studies of complex fluids. Despite the general trend, in this article I explain why a similar approach should not be adopted for microrheology studies of living cells. This conclusion is reached on the basis of statistical mechanics principles that indicate the unsuitability of optical tweezers for such purpose.


Optical tweezing electrophoresis of single biotinylated colloidal particles for avidin concentration measurement

Toon Brans, Filip Strubbe, Caspar Schreuer, Kristiaan Neyts and Filip Beunis

We present a novel approach for label-free concentration measurement of a specific protein in a solution. The technique combines optical tweezers and microelectrophoresis to establish the electrophoretic mobility of a single microparticle suspended in the solution. From this mobility measurement, the amount of adsorbed protein on the particle is derived. Using this method, we determine the concentration of avidin in a buffer solution. After calibration of the setup, which accounts for electro-osmotic flow in the measurement device, the mobilities of both bare and biotinylated microspheres are measured as a function of the avidin concentration in the mixture. Two types of surface adsorption are identified: the biotinylated particles show specific adsorption, resulting from the binding of avidin molecules with biotin, at low avidin concentrations (below 0.04 μg/ml) while at concentrations of several μg/ml non-specific on both types of particles is observed. These two adsorption mechanisms are incorporated in a theoretical model describing the relation between the measured mobility and the avidin concentration in the mixture. This model describes the electrophoretic mobility of these particles accurately over four orders of magnitude of the avidin concentration.


Non-contact intracellular binding of chloroplasts in vivo

Yuchao Li, Hongbao Xin, Xiaoshuai Liu & Baojun Li

Non-contact intracellular binding and controllable manipulation of chloroplasts in vivo was demonstrated using an optical fiber probe. Launching a 980-nm laser beam into a fiber, which was placed about 3 μm above the surface of a living plant (Hydrilla verticillata) leaf, enabled stable binding of different numbers of chloroplasts, as well as their arrangement into one-dimensional chains and two-dimensional arrays inside the leaf without damaging the chloroplasts. Additionally, the formed chloroplast chains were controllably transported inside the living cells. The optical force exerted on the chloroplasts was calculated to explain the experimental results. This method provides a flexible method for studying intracellular organelle interaction with highly organized organelle-organelle contact in vivo in a non-contact manner.


Dynamic regulation of transcription factors by nucleosome remodeling

Ming Li, Arjan Hada, Payel Sen, Lola Olufemi, Michael A Hall, Benjamin Y Smith, Scott Forth, Jeffrey N McKnight, Ashok Patel, Gregory D Bowman, Blaine Bartholomew, Michelle D Wang

The chromatin landscape and promoter architecture are dominated by the interplay of nucleosome and transcription factor (TF) binding to crucial DNA sequence elements. However, it remains unclear whether nucleosomes mobilized by chromatin remodelers can influence TFs that are already present on the DNA template. In this study, we investigated the interplay between nucleosome remodeling, by either yeast ISW1a or SWI/SNF, and a bound TF. We found that a TF serves as a major barrier to ISW1a remodeling, and acts as a boundary for nucleosome repositioning. In contrast, SWI/SNF was able to slide a nucleosome past a TF, with concurrent eviction of the TF from the DNA, and the TF did not significantly impact the nucleosome positioning. Our results provide direct evidence for a novel mechanism for both nucleosome positioning regulation by bound TFs and TF regulation via dynamic repositioning of nucleosomes.


Friday, June 12, 2015

The lifetime of the actomyosin complex in vitro under load corresponding to stretch of contracting muscle

Salavat R. Nabiev, Denis A. Ovsyannikov, Andrey K. Tsaturyan, Sergey Y. Bershitsky

During eccentric contraction, muscle is lengthening so that the actin-myosin cross-bridges bear a load that exceeds the force they generate during isometric contraction. Using the optical trap technique, we simulated eccentric contraction at the single molecule level and investigated the effect of load on the skeletal actomyosin lifetime at different ATP concentrations. The range of the loads was up to 17 pN above the isometric level. We found that the frequency distribution of the lifetime of the actin-bound state of the myosin molecule was biphasic: it quickly rose and then decreased slowly. The rate of the slow phase of this distribution increased with both the load and the ATP concentration. The fast phase accelerated sharply with the load, but it was independent of ATP concentration. The presence of the fast phase demonstrates that some transition(s) in the actomyosin complex occur before the myosin head becomes able to bind ATP and detach from actin. Its high sensitivity to the load indicates that the transition is load-dependent.


A Model for the Force Exerted on a Primary Cilium by an Optical Trap and the Resulting Deformation

Ian Lofgren and Andrew Resnick

Cilia are slender flexible structures extending from the cell body; genetically similar to flagella. Although their existence has been long known, the mechanical and functional properties of non-motile (“primary”) cilia are largely unknown. Optical traps are a non-contact method of applying a localized force to microscopic objects and an ideal tool for the study of ciliary mechanics. We present a method to measure the mechanical properties of a cilium using an analytic model of a flexible, anchored cylinder held within an optical trap. The force density is found using the discrete-dipole approximation. Utilizing Euler-Bernoulli beam theory, we then integrate this force density and numerically obtain the equilibrium deformation of the cilium in response to an optical trap. The presented results demonstrate that optical trapping can provide a great deal of information and insight about the properties and functions of the primary cilium.


Reflection Microspectroscopic Study of Laser Trapping Assembling of Polystyrene Nanoparticles at Air/Solution Interface

Shun-Fa Wang , Ken-ichi Yuyama , Teruki Sugiyama , and Hiroshi Masuhara
We present the formation of a single nanoparticle assembly with periodic array structure induced by laser trapping of 200-nm polystyrene nanoparticles at air/solution interface of the colloidal heavy water solution. Their trapping and assembling behavior is observed by monitoring transmission and backscattering images and measuring reflection spectra under a microscope. Upon the laser irradiation into the solution surface layer, nanoparticles are gathered at and around the focal spot, and eventually a nanoparticle assembly with the size much larger than the focal volume is formed. The assembly gives structural color in visible range under halogen lamp illumination, indicating that constituent nanoparticles are periodically arrayed. Reflection spectra of the assembly show a reflection band, and its peak position is gradually shifted to short wavelength and the band width becomes narrow with time, depending on the distance from the focal spot. After the laser is switched off, red-shift is observed in the reflection band. These results indicate that nanoparticles are rearranged into a densely packed periodic array during laser irradiation and diffused out to the surrounding solution after turning off the laser. This dynamics is discussed from the viewpoints of the attractive optical trapping force and the electrostatic repulsive force among nanoparticles.


Fiber based optical tweezers for simultaneous in situ force exertion and measurements in a 3D polyacrylamide gel compartment

Chaoyang Ti, Gawain M Thomas, Yundong Ren, Rui Zhang, Qi Wen, and Yuxiang Liu

Optical tweezers play an important role in biological applications. However, it is difficult for traditional optical tweezers based on objective lenses to work in a three-dimensional (3D) solid far away from the substrate. In this work, we develop a fiber based optical trapping system, namely inclined dual fiber optical tweezers, that can simultaneously apply and measure forces both in water and in a 3D polyacrylamide gel matrix. In addition, we demonstrate in situ, non-invasive characterization of local mechanical properties of polyacrylamide gel by measurements on an embedded bead. The fiber optical tweezers measurements agree well with those of atomic force microscopy (AFM). The inclined dual fiber optical tweezers provide a promising and versatile tool for cell mechanics study in 3D environments.


Pushing the limit: investigation of hydrodynamic forces on a trapped particle kicked by a laser pulse

Naja Villadsen, Daniel Ø. Andreasen, Jesper Hagelskjær, Jan Thøgersen, Alberto Imparato, and Søren Rud Keiding

We introduce a new optical technique where a train of short optical pulses is utilized to disturb a trapped microscopic particle. Using fast (250 kHz) and accurate (nm) detection of the position of the particle, accurately synchronized to the repetition rate of the laser pulses, we can coherently superimpose the displacement caused by each individual laser pulse. Thereby we are able to both bypass the influence from the Brownian motion of the trapped particle and to simultaneously increase the ability to localize its average trajectory by n⎯⎯√, where n is the number of repetitive pulses. In the results presented here we utilize a train of 1200 pulses to kick a 5 μm polystyrene sphere and obtain a spatial resolution corresponding to 0.09 nm and a time resolution of 4 μs. The magnitude of the optical force pushing the particle corresponds to ∼ 104g and enables an investigation of both the hydrodynamical drag and the inertial effects caused by the particle and the surrounding liquid. Our results enables a more accurate testing of the existing extended models for the hydrodynamic drag and we discuss the observed agreement between experiments and theory.


Calculating the torque of the optical vortex tweezer to the ellipsoidal micro-particles

Lie Zhu, Zhongyi Guo, Qiang Xu, Jingran Zhang, Anjun Zhang, Wei Wang, Yi Liu, Yan li, Xinshun Wang, Shiliang Qu

In this paper, we have accurately computed the torque of the optical vortex tweezers to the ellipsoidal micro-particles with the method of finite-difference time-domain (FDTD). The transferred orbital angular momentum (OAM) from the vortex beam to the micro-particles can be obtained based on the scattering phase function (SPF) of the micro-particles. We have verified that the calculated SPF of a spherical particle by FDTD agrees well with that by Mie theory, which indicates that the SPF of micro-particles with any shapes can be calculated by FDTD accurately. In addition, with the method of FDTD, we have obtained the SPFs of the different-shape ellipsoidal micro-particles with same volume, including prolate ellipsoids and oblate ellipsoids. Meanwhile, the transferred OAM between the light and the ellipsoidal micro-particles have been deduced analytically by the relative formulas. And the rotating angular velocities of the trapped ellipsoidal micro-particles have been investigated and discussed in detail based on the obtained corresponding SPFs.


Tuesday, June 9, 2015

Active cell mechanics: Measurement and theory

Wylie W. Ahmed, Étienne Fodor, Timo Betz

Living cells are active mechanical systems that are able to generate forces. Their structure and shape are primarily determined by biopolymer filaments and molecular motors that form the cytoskeleton. Active force generation requires constant consumption of energy to maintain the nonequilibrium activity to drive organization and transport processes necessary for their function. To understand this activity it is necessary to develop new approaches to probe the underlying physical processes. Active cell mechanics incorporates active molecular-scale force generation into the traditional framework of mechanics of materials. This review highlights recent experimental and theoretical developments towards understanding active cell mechanics. We focus primarily on intracellular mechanical measurements and theoretical advances utilizing the Langevin framework. These developing approaches allow a quantitative understanding of nonequilibrium mechanical activity in living cells. This article is part of a Special Issue entitled: Mechanobiology.


Raman Spectra Analysis for Single Mitochondrias after Apoptosis Process of Yeast Cells Stressed by Acetic Acid

Bing LI, Ming-Qian LU, Qiao-Zhen WANG, Gui-Yu SHI, Wei LIAO, Shu-Shi HUANG

Laser tweezers Raman spectroscopy (LTRS) as a non-invasive tool for mitochondria analysis, combined with oxygen electrode and ultraviolet spectrophotometric method, was used to investigate the Raman spectra of mitochondria of yeast cells in vitro and in vivo after induced with acetic acid. The results showed that when the mitochondria of yeast cells in vivo was induced by acetic acid, the spectral peaks of the nucleic acid (1081 and 1301 cm−1), proteins (872, 1604, 1445 and 1657 cm−1), lipids (1125, 1301, 1445 and 1657 cm−1), cytochrome c (750 and 1125 cm−1) and mitochondria respiration (1604 cm−1) were significantly decreased as a function of the duration of the acetic acid stress, and the obtained physiological and biochemical indexes of respiration rate, phosphorous/oxygen (P/O) ration and cytochrome c content were similar to those by conventional method. Furthermore, when the mitochondria of yeast cells in vitro was induced by acetic acid, the spectral peaks of the nucleic acid (1081 and 1301 cm−1), proteins (872, 1604, 1445 and 1657 cm−1), lipids (1125, 1301, 1445 and 1657 cm−1), cytochrome c (750 and 1125 cm−1) and mitochondria respiration (1604 cm−1) were significantly decreased as the function of the duration of the acetic acid stress, and the obtained physiological and biochemical indexes of respiration rate, P/O ration and cytochrome c content were similar to those by conventional method. The results indicated that acetic acid could penetrate into the cell interior and directly impacted the mitochondria possibly, resulting in the release of inclusions from the mitochondria, subsequently, causing the apoptosis of yeast cells via mitochondrial pathway-induced apoptosis.


Reconstructing folding energy landscapes from splitting probability analysis of single-molecule trajectories

Ajay P. Manuel, John Lambert, and Michael T. Woodside
Structural self-assembly in biopolymers, such as proteins and nucleic acids, involves a diffusive search for the minimum-energy state in a conformational free-energy landscape. The likelihood of folding proceeding to completion, as a function of the reaction coordinate used to monitor the transition, can be described by the splitting probability, pfold(x). Pfold encodes information about the underlying energy landscape, and it is often used to judge the quality of the reaction coordinate. Here, we show how pfold can be used to reconstruct energy landscapes from single-molecule folding trajectories, using force spectroscopy measurements of single DNA hairpins. Calculating pfold(x) directly from trajectories of the molecular extension measured for hairpins fluctuating in equilibrium between folded and unfolded states, we inverted the result expected from diffusion over a 1D energy landscape to obtain the implied landscape profile. The results agreed well with the landscapes reconstructed by established methods, but, remarkably, without the need to deconvolve instrumental effects on the landscape, such as tether compliance. The same approach was also applied to hairpins with multistate folding pathways. The relative insensitivity of the method to the instrumental compliance was confirmed by simulations of folding measured with different tether stiffnesses. This work confirms that the molecular extension is a good reaction coordinate for these measurements, and validates a powerful yet simple method for reconstructing landscapes from single-molecule trajectories.


Vesicle Fusion Triggered by Optically Heated Gold Nanoparticles

Andreas Rørvig-Lund, Azra Bahadori, Szabolcs Semsey, Poul Martin Bendix, and Lene B. Oddershede

Membrane fusion can be accelerated by heating that causes membrane melting and expansion. We locally heated the membranes of two adjacent vesicles by laser irradiating gold nanoparticles, thus causing vesicle fusion with associated membrane and cargo mixing. The mixing time scales were consistent with diffusive mixing of the membrane dyes and the aqueous content. This method is useful for nanoscale reactions as demonstrated here by I-BAR protein-mediated membrane tubulation triggered by fusion.


Experimental evidence for Abraham pressure of light

Li Zhang, Weilong She, Nan Peng and Ulf Leonhardt

The question of how much momentum light carries in media has been debated for over a century. Two rivalling theories, one from 1908 by Hermann Minkowski and the other from 1909 by Max Abraham, predict the exact opposite when light enters an optical material: a pulling force in Minkowski's case and a pushing force in Abraham's. Most experimental tests have agreed with Minkowski's theory, but here we report the first quantitative experimental evidence for Abraham's pushing pressure of light. Our results matter in optofluidics and optomechanics, and wherever light exerts mechanical pressure.


Tuesday, June 2, 2015

Inducing dynamical bistability by reversible compression of an optical piston

Gabriel Schnoering and Cyriaque Genet

We study the reversible crossover between stable and bistable phases of an overdamped Brownian bead inside an optical piston. The interaction potentials are solved developing a method based on Kramers's theory that exploits the statistical properties of the stochastic motion of the bead. We evaluate precisely the energy balance of the crossover. We show that the deformation of the optical potentials induced by the compression of the piston is related to a production of heat balanced between potential energy changes and the total amount of work performed by the piston. This reveals how specific thermodynamic processes can be designed and controlled with a high level of precision by tailoring the optical landscapes of the piston.


Entangled F-actin displays a unique crossover to microscale nonlinearity dominated by entanglement segment dynamics

Tobias T. Falzone, Savanna Blair and Rae M. Robertson-Anderson

We drive optically trapped microspheres through entangled F-actin at constant speeds and distances well beyond the linear regime, and measure the microscale force response of the entangled filaments during and following strain. Our results reveal a unique crossover to appreciable nonlinearity at a strain rate of [small gamma, Greek, dot above]c ≈ 3 s−1 which corresponds remarkably well with the theoretical rate of relaxation of entanglement length deformations 1/τent. Above [small gamma, Greek, dot above]c, we observe stress stiffening which occurs over very short time scales comparable to the predicted timescale over which mesh size deformations relax. Stress softening then takes over, yielding to an effectively viscous regime over a timescale comparable to the entanglement length relaxation time, τent. The viscous regime displays shear thinning but with a less pronounced viscosity scaling with strain rate compared to flexible polymers. The relaxation of induced force on filaments following strain shows that the relative relaxation proceeds more quickly for increasing strain rates; and for rates greater than [small gamma, Greek, dot above]c, the relaxation displays a complex power-law dependence on time. Our collective results reveal that molecular-level nonlinear viscoelasticity is driven by non-classical dynamics of individual entanglement segments that are unique to semiflexible polymers.


Self-assembled photonic-plasmonic nanotweezers for directed self-assembly of hybrid nanostructures

Dakota O'Dell, Xavier Serey and David Erickson

We demonstrate a technique for assembling photonic-plasmonic nanotweezers by optically driving the adsorption of multi-walled carbon nanotubes onto a silicon waveguide. The nanotweezers are then used to trap and release individual polystyrene beads. Additionally, we demonstrate the ability to localize the deposition of metallic nanoparticles to the intersection points between multiple carbon nanotubes with the goal of forming more complex hybrid nanostructures.


Molecular Machines Like Myosin Use Randomness to Behave Predictably

Peter Karagiannis, Yoshiharu Ishii, and Toshio Yanagida

We use machines to move, to lift, and to build. In a similar way, cells use machines to grow, to reproduce, and to live. These machines play essential roles in cellular functions such as cell signaling, energy transduction, and motion, to name just a few. Because they operate inside a cell, they are tiny and operate on a physical scale that makes them very different from the manmade, macroscopic objects we normally imagine when we hear the word “machine”. Further, their size and soft structure allows them to be much more dynamic and robust than artificial machines. They work needing very little input, as energy levels not far from average thermal energy (k B T) are sufficient for a given task. 1 This property too contrasts with artificial machines, which work much more rapidly, accurately, and deterministically, but with higher energy demands and less adaptability. To elucidate how molecular machines operate, single molecule techniques have been developed to measure the dynamic behavior of individual biomolecules with an accuracy that can correlate thermal effects on machine function. Understanding the uniqueness of how molecular machines operate and exploiting this mechanism will reveal the strategies used by nature to build its machines and, therefore, new paradigms for how we can build ours.


Dynamics of aerosol size during inhalation: Hygroscopic growth of commercial nebulizer formulations

Allen E. Haddrell, James F. Davies, Rachael E.H. Miles, Jonathan P. Reid, Lea Ann Dailey, Darragh Murnane

The size of aerosol particles prior to, and during, inhalation influences the site of deposition within the lung. As such, a detailed understanding of the hygroscopic growth of an aerosol during inhalation is necessary to accurately model the deposited dose. In the first part of this study, it is demonstrated that the aerosol produced by a nebulizer, depending on the airflows rates, may experience a (predictable) wide range of relative humidity prior to inhalation and undergo dramatic changes in both size and solute concentration. A series of sensitive single aerosol analysis techniques are then used to make measurements of the relative humidity dependent thermodynamic equilibrium properties of aerosol generated from four common nebulizer formulations. Measurements are also reported of the kinetics of mass transport during the evaporation or condensation of water from the aerosol. Combined, these measurements allow accurate prediction of the temporal response of the aerosol size prior to and during inhalation. Specifically, we compare aerosol composed of pure saline (150 mM sodium chloride solution in ultrapure water) with two commercially available nebulizer products containing relatively low compound doses: Breath®, consisting of a simple salbutamol sulfate solution (5 mg/2.5 mL; 1.7 mM) in saline, and Flixotide® Nebules, consisting of a more complex stabilized fluticasone propionate suspension (0.25 mg/mL; 0.5 mM in saline. A mimic of the commercial product Tobi© (60 mg/mL tobramycin and 2.25 mg/mL NaCl, pH 5.5–6.5) is also studied, which was prepared in house. In all cases, the presence of the pharmaceutical was shown to have a profound effect on the magnitude, and in some cases the rate, of the mass flux of water to and from the aerosol as compared to saline. These findings provide physical chemical evidence supporting observations from human inhalation studies, and suggest that using the growth dynamics of a pure saline aerosol in a lung inhalation model to represent nebulizer formulations may not be representative of the actual behavior of the aerosolized drug solutions.


Monday, June 1, 2015

Quantification of High-Efficiency Trapping of Nanoparticles in a Double Nanohole Optical Tweezer

Abhay Kotnala and Reuven Gordon

We measure the dynamics of 20 nm polystyrene particles in a double nanohole trap to determine the trap stiffness for various laser powers. Both the autocorrelation analysis of Brownian fluctuations and the trapping transient analysis provide a consistent value of ∼0.2 fN/nm stiffness for 2 mW of laser power, which is similar to theoretical calculations for aperture trapping. As expected, the stiffness increases linearly with laser power. This is comparable to the stiffness obtained for conventional optical traps for trapping, but for ten times smaller dielectric particles and less power. This approach will allow us to quantitatively evaluate future aperture-based optical traps, with the goal of studying the folding dynamics of smaller proteins (∼10 kDa) and small-molecule interactions.


Selective Trapping or Rotation of Isotropic Dielectric Microparticles by Optical Near Field in a Plasmonic Archimedes Spiral

Wei-Yi Tsai, Jer-Shing Huang, and Chen-Bin Huang

We demonstrate selective trapping or rotation of optically isotropic dielectric microparticles by plasmonic near field in a single gold plasmonic Archimedes spiral. Depending on the handedness of circularly polarized excitation, plasmonic near fields can be selectively engineered into either a focusing spot for particle trapping or a plasmonic vortex for particle rotation. Our design provides a simple solution for subwavelength optical manipulation and may find applications in micromechanical and microfluidic systems.


Optical extinction efficiency measurements on fine and accumulation mode aerosol using single particle cavity ring-down spectroscopy

Michael I. Cotterell, Bernard J. Mason, Thomas C. Preston, Andrew J. Orr-Ewing and Jonathan P. Reid

A new experiment is presented for the measurement of single aerosol particle extinction efficiencies, Qext, combining cavity ring-down spectroscopy (CRDS, λ = 405 nm) with a Bessel beam trap (λ = 532 nm) in tandem with phase function (PF) measurements. This approach allows direct measurements of the changing optical cross sections of individual aerosol particles over indefinite time-frames facilitating some of the most comprehensive measurements of the optical properties of aerosol particles so far made. Using volatile 1,2,6-hexanetriol droplets, Qext is measured over a continuous radius range with the measured Qext envelope well described by fitted cavity standing wave (CSW) Mie simulations. These fits allow the refractive index at 405 nm to be determined. Measurements are also presented of Qext variation with RH for two hygroscopic aqueous inorganic systems ((NH4)2SO4 and NaNO3). For the PF and the CSW Mie simulations, the refractive index, nλ, is parameterised in terms of the particle radius. The radius and refractive index at 532 nm are determined from PFs, while the refractive index at 405 nm is determined by comparison of the measured Qext to CSW Mie simulations. The refractive indices determined at the shorter wavelength are larger than at the longer wavelength consistent with the expected dispersion behaviour. The measured values at 405 nm are compared to estimates from volume mixing and molar refraction mixing rules, with the latter giving superior agreement. In addition, the first single-particle Qext measurements for accumulation mode aerosol are presented for droplets with radii as small as [similar]300 nm.


Microscopic dynamics of synchronization in driven colloids

Michael P.N. Juniper, Arthur V. Straube, Rut Besseling, Dirk G.A.L. Aarts & Roel P.A. Dullens

Synchronization of coupled oscillators has been scrutinized for over three centuries, from Huygens’ pendulum clocks to physiological rhythms. One such synchronization phenomenon, dynamic mode locking, occurs when naturally oscillating processes are driven by an externally imposed modulation. Typically only averaged or integrated properties are accessible, leaving underlying mechanisms unseen. Here, we visualize the microscopic dynamics underlying mode locking in a colloidal model system, by using particle trajectories to produce phase portraits. Furthermore, we use this approach to examine the enhancement of mode locking in a flexible chain of magnetically coupled particles, which we ascribe to breathing modes caused by mode-locked density waves. Finally, we demonstrate that an emergent density wave in a static colloidal chain mode locks as a quasi-particle, with microscopic dynamics analogous to those seen for a single particle. Our results indicate that understanding the intricate link between emergent behaviour and microscopic dynamics is key to controlling synchronization.


Optomagnetically Controlled Microparticles Manufactured with Glancing Angle Deposition

Joseph L. Lawson, Nathan J. Jenness and Robert L. Clark

Optical trapping and magnetic trapping are common micromanipulation techniques for controlling colloids including micro- and nanoparticles. Combining these two manipulation strategies allows a larger range of applied forces and decoupled control of rotation and translation; each of which are beneficial properties for many applications including force spectroscopy and advanced manufacturing. However, optical trapping and magnetic trapping have conflicting material requirements inhibiting the combination of these methodologies. In this paper, anisotropic microscaled particles capable of being simultaneously controlled by optical and magnetic trapping are synthesized using a glancing angle deposition (GLAD) technique. The anisotropic alignment of dielectric and ferromagnetic materials limits the optical scattering from the metallic components which typically prevents stable optical trapping in three dimensions. Compared to the current state of the art, the benefits of this approach are twofold. First, the composite structure allows larger volumes of ferromagnetic material so that larger magnetic moments may be applied without inhibiting the stability of optical trapping. Second, the robustness of the synthesis process is greatly improved. The dual optical and magnetic functionality of the synthesized colloids is demonstrated by simultaneously optically translating and magnetically rotating a magnetic GLAD particle using a custom designed optomagnetic trapping system.