Friday, May 22, 2015

Chiral discrimination in optical binding

Kayn A. Forbes and David L. Andrews

The laser-induced intermolecular force that exists between two or more particles in the presence of an electromagnetic field is commonly termed “optical binding.” Distinct from the single-particle forces that are at play in optical trapping at the molecular level, the phenomenon of optical binding is a manifestation of the coupling between optically induced dipole moments in neutral particles. In other, more widely known areas of optics, there are many examples of chiral discrimination—signifying the different response a chiral material has to the handedness of an optical input. In the present analysis, extending previous work on chiral discrimination in optical binding, a mechanism is identified using a quantum electrodynamical approach. It is shown that the optical binding force between a pair of chiral molecules can be significantly discriminatory in nature, depending upon both the handedness of the interacting particles and the polarization of the incident light, and it is typically several orders of magnitude larger than previously reported.


Joule heating monitoring in a microfluidic channel by observing the Brownian motion of an optically trapped microsphere

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

Electric fields offer a variety of functionalities to Lab-on-a-Chip devices. The use of these fields often results in significant Joule heating, affecting the overall performance of the system. Precise knowledge of the temperature profile inside a microfluidic device is necessary to evaluate the implications of heat dissipation. This article demonstrates how an optically trapped microsphere can be used as a temperature probe to monitor Joule heating in these devices. The Brownian motion of the bead at room temperature is compared with the motion when power is dissipated in the system. This gives an estimate of the temperature increase at a specific location in a microfluidic channel. We demonstrate this method with solutions of different ionic strengths, and establish a precision of 0.9 K and an accuracy of 15%. Furthermore it is demonstrated that transient heating processes can be monitored with this technique, albeit with a limited time resolution.


Long Working-Distance Optical Trap for in Situ Analysis of Contact-Induced Phase Transformations

Ryan D. Davis, Sara Lance, Joshua A. Gordon, and Margaret A. Tolbert

A novel optical trapping technique is described that combines an upward propagating Gaussian beam and a downward propagating Bessel beam. Using this optical arrangement and an on-demand droplet generator makes it possible to rapidly and reliably trap particles with a wide range of particle diameters (∼1.5–25 μm), in addition to crystalline particles, without the need to adjust the optical configuration. Additionally, a new image analysis technique is described to detect particle phase transitions using a template-based autocorrelation of imaged far-field elastically scattered laser light. The image analysis allows subtle changes in particle characteristics to be quantified. The instrumental capabilities are validated with observations of deliquescence and homogeneous efflorescence of well-studied inorganic salts. Then, a novel collision-based approach to seeded crystal growth is described in which seed crystals are delivered to levitated aqueous droplets via a nitrogen gas flow. To our knowledge, this is the first account of contact-induced phase changes being studied in an optical trap. This instrument offers a novel and simple analytical technique for in situ measurements and observations of phase changes and crystal growth processes relevant to atmospheric science, industrial crystallization, pharmaceuticals, and many other fields.


Microfluidics for Research and Applications in Oncology

Parthiv Chaudhuri, Majid Ebrahimi Warkiani, Tengyang Jing, Kenry Kenry and Chwee Teck Lim

Cancer is currently one of the top non-communicable human diseases and continual research and developmental efforts are being made to better understand and manage this disease. More recently, with improved understanding in cancer biology as well as advancement made in microtechnology and rapid prototyping, microfluidics is increasingly being explored and even validated for use in the detection, diagnosis and treatment of cancer. With inherent advantages such as small sample volume, high sensitivity and fast processing time, microfluidics is well-positioned to serve as a promising platform for applications in oncology. In this review, we look at recent advances in the use of microfluidics - from basic research such as understanding cancer cell phenotypes as well as metastatic behaviors to applications such as detection, diagnosis, prognosis and drug screening. We then conclude with a future outlook on this promising technology.


Stick-slip motion of surface point defects prompted by magnetically controlled colloidal-particle dynamics in nematic liquid crystals

Michael C. M. Varney, Qiaoxuan Zhang, and Ivan I. Smalyukh

We explore the dynamics of topological point defects on surfaces of magnetically responsive colloidal microspheres in a uniformly aligned nematic liquid crystal host. We show that pinning of the liquid crystal director to a particle surface with random nanostructured morphology results in unexpected translational dynamics of both particles and topological point defects on their surfaces when subjected to rotating magnetic fields. We characterize and quantify the “stick-slip” motion of defects as a function of field rotation rates as well as temperature, demonstrating the roles played by the competition of elastic forces, surface anchoring, and magnetic torques on the sphere as well as random-surface-mediated pinning of the easy axis of the nematic director on colloidal microspheres. We analyze our findings through their comparison to similar dynamic processes in other branches of science.


Thursday, May 14, 2015

Untethered photonic sensor for wall pressure measurement

Maurizio Manzo and Tindaro Ioppolo

In this Letter, we study a novel untethered photonic wall pressure sensor that uses as sensing element a dome-shaped micro-scale laser. Since the sensor does not require any optical or electrical cabling, it allows measurements where cabling tends to be problematic. The micro-laser is made by a mixture of Trimethylolpropane Tri(3-mercaptopropionate), commercial name THIOCURE and Polyethylene (glycol) Diacrylate (PEGDA) mixed with a solution of rhodamine 6G. Two different volume ratios between the THIOCURE and the PEGDA are studied, since different ratios lead to different mechanical properties. In addition, two different sensor configurations are presented: (i) sensor coupled to a membrane, that allows differential wall pressure measurement and (ii) sensor without membrane that allows absolute wall pressure measurement. The sensitivity plots are presented in the paper for both sensor configurations and polymer ratios.


Chirality in Optical Trapping and Optical Binding

David S. Bradshaw , Kayn A. Forbes , Jamie M. Leeder and David L. Andrews
Optical trapping is a well-established technique that is increasingly used on biological substances and nanostructures. Chirality, the property of objects that differ from their mirror image, is also of significance in such fields, and a subject of much current interest. This review offers insight into the intertwining of these topics with a focus on the latest theory. Optical trapping of nanoscale objects involves forward Rayleigh scattering of light involving transition dipole moments; usually these dipoles are assumed to be electric although, in chiral studies, magnetic dipoles must also be considered. It is shown that a system combining optical trapping and chirality could be used to separate enantiomers. Attention is also given to optical binding, which involves light induced interactions between trapped particles. Interesting effects also arise when binding is combined with chirality.


Wednesday, May 13, 2015

A Molecular Tuning Fork in Single-Molecule Mechanochemical Sensing

Shankar Mandal, Deepak Koirala, Sangeetha Selvam, Chiran Ghimire and Prof. Hanbin Mao

The separate arrangement of target recognition and signal transduction in conventional biosensors often compromises the real-time response and can introduce additional noise. To address these issues, we combined analyte recognition and signal reporting by mechanochemical coupling in a single-molecule DNA template. We incorporated a DNA hairpin as a mechanophore in the template, which, under a specific force, undergoes stochastic transitions between folded and unfolded hairpin structures (mechanoescence). Reminiscent of a tuning fork that vibrates at a fixed frequency, the device was classified as a molecular tuning fork (MTF). By monitoring the lifetime of the folded and unfolded hairpins with equal populations, we were able to differentiate between the mono- and bivalent binding modes during individual antibody-antigen binding events. We anticipate these mechanospectroscopic concepts and methods will be instrumental for the development of novel bioanalyses.

Femtosecond Optical Trap-assisted Nanopatterning Through Microspheres by a Single Ti:Sapphire Oscillator

Aleksander Shakhov , Artyom Astafiev , Dmytro Plutenko , Oleg M. Sarkisov , Anatoly I. Shushin , and Victor Andreevich Nadtochenko

A new approach to fabricating the various range of patterns using femtosecond optical trap assisted nanopatterning is presented. Here we report how a single Gaussian laser beam from a 55 fs 80 MHz 780 nm Ti:Sapphire oscillator trapping dielectric microspheres near surfaces can be used to enable near-field direct-write subwavelength ~λ/6 (~130 nm) 2D nanopartening of polymer surface. We discuss the stability conditions for effective manipulation of the particle by the pulsed beam. A Klein-Kramers and Brownian motion models were used to analyze the positional accuracy of femtosecond tweezers. We studied effects of the microsphere size, pulsed laser energy and light polarization, spacing between objective focal plane and polymer surface on the pattern size experimentally and theoretically. Microspheres with a diameter of about 1 μm provide the smallest patterns. The experimental results are reasonably matched by Generalized Lorentz Mie theory.


Photon momentum transfer in inhomogeneous dielectric mixtures and induced tractor beams

Cheng-Wei Qiu, Weiqiang Ding, M.R.C. Mahdy, Dongliang Gao, Tianhang Zhang, Fook Chiong Cheong, Aristide Dogariu, Zheng Wang and Chwee Teck Lim

The determination of optical force as a consequence of momentum transfer is inevitably subject to the use of the proper momentum density and stress tensor. It is imperative and valuable to consider the intrinsic scheme of photon momentum transfer, particularly when a particle is embedded in a complex dielectric environment. Typically, we consider a particle submerged in an inhomogeneous background composed of different dielectric materials, excluding coherent illumination or hydrodynamic effects. A ray-tracing method is adopted to capture the direct process of momentum transfer from the complex background medium, and this approach is validated using the modified Einstein–Laub method, which uses only the interior fields of the particle in the calculation. In this way, debates regarding the calculation of the force with different stress tensors using exterior fields can be avoided. Our suggested interpretation supports only the Minkowski approach for the optical momentum transfer to the embedded scatterer while rejecting Peierls's and Abraham's approaches, though the momentum of a stably moving photon in a continuous background medium should be considered to be of the Abraham type. Our interpretation also provides a novel method of realizing a tractor beam for the exertion of negative force that offers an alternative to the use of negative-index materials, optical gain, or highly non-paraxial or multiple-light interference.