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Wednesday, July 23, 2014

Microcapsule mechanics: From stability to function

Martin P. Neubauer, Melanie Poehlmann, Andreas Fery

Microcapsules are reviewed with special emphasis on the relevance of controlled mechanical properties for functional aspects. At first, assembly strategies are presented that allow control over the decisive geometrical parameters, diameter and wall thickness, which both influence the capsule's mechanical performance. As one of the most powerful approaches the layer-by-layer technique is identified. Subsequently, ensemble and, in particular, single-capsule deformation techniques are discussed. The latter generally provide more in-depth information and cover the complete range of applicable forces from smaller than pN to N. In a theory chapter, we illustrate the physics of capsule deformation. The main focus is on thin shell theory, which provides a useful approximation for many deformation scenarios. Finally, we give an overview of applications and future perspectives where the specific design of mechanical properties turns microcapsules into (multi-)functional devices, enriching especially life sciences and material sciences.

DOI

Tuesday, July 22, 2014

Optical trapping of individual human immunodeficiency viruses in culture fluid reveals heterogeneity with single-molecule resolution

Yuanjie Pang, Hanna Song, Jin H. Kim, Ximiao Hou & Wei Cheng

Optical tweezers use the momentum of photons to trap and manipulate microscopic objects, contact-free, in three dimensions. Although this technique has been widely used in biology and nanotechnology to study molecular motors, biopolymers and nanostructures, its application to study viruses has been very limited, largely due to their small size. Here, using optical tweezers that can simultaneously resolve two-photon fluorescence at the single-molecule level, we show that individual HIV-1 viruses can be optically trapped and manipulated, allowing multi-parameter analysis of single virions in culture fluid under native conditions. We show that individual HIV-1 differs in the numbers of envelope glycoproteins by more than one order of magnitude, which implies substantial heterogeneity of these virions in transmission and infection at the single-particle level. Analogous to flow cytometry for cells, this fluid-based technique may allow ultrasensitive detection, multi-parameter analysis and sorting of viruses and other nanoparticles in biological fluid with single-molecule resolution.

DOI

Sunday, July 20, 2014

Equilibrium orientations of oblate spheroidal particles in single tightly focused Gaussian beams

Yongyin Cao, Wenhe Song, Weiqiang Ding, Fangkui Sun, and TongTong Zhu

Based on a hybrid discrete dipole approximation (DDA) and T-matrix method, a powerful dynamic simulation model is used to find plausible equilibrium orientation landscapes of micro- and nano-spheroids of varying size and aspect ratio. Orientation landscapes of spheroids are described in both linearly and circularly polarized Gaussian beams. It’s demonstrated that the equilibrium orientations of the prolate and oblate spheroids have different performances. Effect of beam polarization on orientation landscapes is revealed as well as new orientation of oblate spheroids. The torque efficiencies of spheroids at equilibrium are also studied as functions of tilt angle, from which the orientations of the spheroids can be affirmed. This investigation elucidates a solid background in both the function and properties of micro-and nano-spheroidal particles trapped in optical tweezers.

DOI

Speckle optical tweezers: micromanipulation with random light fields

Giorgio Volpe, Lisa Kurz, Agnese Callegari, Giovanni Volpe, and Sylvain Gigan

Current optical manipulation techniques rely on carefully engineered setups and samples. Although similar conditions are routinely met in research laboratories, it is still a challenge to manipulate microparticles when the environment is not well controlled and known a priori, since optical imperfections and scattering limit the applicability of this technique to real-life situations, such as in biomedical or microfluidic applications. Nonetheless, scattering of coherent light by disordered structures gives rise to speckles, random diffraction patterns with well-defined statistical properties. Here, we experimentally demonstrate how speckle fields can become a versatile tool to efficiently perform fundamental optical manipulation tasks such as trapping, guiding and sorting. We anticipate that the simplicity of these “speckle optical tweezers” will greatly broaden the perspectives of optical manipulation for real-life applications.

DOI

Thursday, July 17, 2014

Volatility and Oxidative Ageing of Aqueous Maleic Acid Aerosol Droplets and the Dependence on Relative Humidity

Benjamin J. Dennis-Smither , Frances H. Marshall , Rachael E.H. Miles , Thomas C Preston, and Jonathan Philip Reid

The microphysical structure and heterogeneous oxidation by ozone of single aerosol particles containing maleic acid (MA) has been studied using aerosol optical tweezers and cavity enhanced Raman spectroscopy. The evaporation rate of MA from aqueous droplets has been measured over a range of relative humidities and the pure component vapour pressure determined to be (1.7± 0.2)×10^-3 Pa. Variation in the refractive index (RI) of an aqueous MA droplet with relative humidity (RH) allowed the sub-cooled liquid RI of MA to be estimated as 1.481 ± 0.001. Measurements of the hygroscopic growth are shown to be consistent with equilibrium model predictions from previous studies. Simultaneous measurements of the droplet composition, size and refractive index have been made during ozonolysis at RHs in the range 50 to 80 %, providing insight into the volatility of organic products, changes in the droplet hygroscopicity and optical properties. Exposure of the aqueous droplets to ozone leads to the formation of products with a wide range of volatilities spanning from involatile to volatile. Reactive uptake coefficients show a weak dependence on ozone concentration, but no dependence on RH or salt concentration. The time evolving RI depends significantly on the RH at which the oxidation proceeds and can even show opposing trends; while the RI increases with ozone exposure at low relative humidity, the RI decreases when the oxidation proceeds at high relative humidity. The variations in RI are broadly consistent with a framework for predicting RIs for organic components published by Cappa et al.1 Once oxidised, particles are shown to form amorphous phases on drying rather than crystallisation, with slow evaporation kinetics of residual water.

DOI

Unravelling the effects of radiation forces in water

Nelson G. C. Astrath, Luis C. Malacarne, Mauro L. Baesso, Gustavo V. B. Lukasievicz & Stephen E. Bialkowski

The effect of radiation forces at the interface between dielectric materials has been a long-standing debate for over a century. Yet there has been so far only limited experimental verification in complete accordance with the theory. Here we measure the surface deformation at the air–water interface induced by continuous and pulsed laser excitation and match this to rigorous theory of radiation forces. We demonstrate that the experimental results are quantitatively described by the numerical calculations of radiation forces. The Helmholtz force is used for the surface radiation pressure. The resulting surface pressure obtained is consistent with the momentum conservation using the Minkowski momentum density expression assuming that the averaged momentum per photon is given by the Minkowski momentum. Considering the total momentum as a sum of that propagating with the electromagnetic wave and that deposited locally in the material, the Abraham momentum interpretation also appears to be appropriate.

DOI

Plasmofluidic single-molecule surface-enhanced Raman scattering from dynamic assembly of plasmonic nanoparticles

Partha Pratim Patra, Rohit Chikkaraddy, Ravi P. N. Tripathi, Arindam Dasgupta & G. V. Pavan Kumar

Single-molecule surface-enhanced Raman scattering (SM-SERS) is one of the vital applications of plasmonic nanoparticles. The SM-SERS sensitivity critically depends on plasmonic hot-spots created at the vicinity of such nanoparticles. In conventional fluid-phase SM-SERS experiments, plasmonic hot-spots are facilitated by chemical aggregation of nanoparticles. Such aggregation is usually irreversible, and hence, nanoparticles cannot be re-dispersed in the fluid for further use. Here, we show how to combine SM-SERS with plasmon polariton-assisted, reversible assembly of plasmonic nanoparticles at an unstructured metal–fluid interface. One of the unique features of our method is that we use a single evanescent-wave optical excitation for nanoparticle assembly, manipulation and SM-SERS measurements. Furthermore, by utilizing dual excitation of plasmons at metal–fluid interface, we create interacting assemblies of metal nanoparticles, which may be further harnessed in dynamic lithography of dispersed nanostructures. Our work will have implications in realizing optically addressable, plasmofluidic, single-molecule detection platforms.

DOI

Left-handed optical radiation torque

Davit Hakobyan & Etienne Brasselet

Optical forces and torques are two mechanical degrees of freedom available to manipulate matter, and form the basis of optical tweezing strategies. In contrast to the Keplerian intuition that objects should be pushed downstream an incident photon flux, the concept of ‘negative’ optical forces has recently been described and has triggered many developments. Here, we report on the counterintuitive angular analogue of negative optical forces by demonstrating that circularly polarized Gaussian light beams give rise to torque with opposite sign to that of the incident optical angular momentum. Such a ‘left-handed’ mechanical effect is demonstrated by the use of an inhomogeneous and anisotropic transparent macroscopic medium. Practical difficulties associated with the direct observation of optically induced spinning of a macroscopic object are circumvented via the rotational Doppler effect. These results shed light on spin–orbit optomechanics and equip the left-handed optomechanical toolbox with angular features.

DOI