The viscoelastic properties of microvilli are dependent upon the cell-surface molecule

Johanne L. Python, Kristal O. Wilson, Jeremy H. Snook, Bin Guo and William H. Guilford

We studied at nanometer resolution the viscoelastic properties of microvilli and tethers pulled from myelogenous cells via P-selectin glycoprotein ligand 1(PSGL-1) and found that in contrast to pure membrane tethers, the viscoelastic properties of microvillus deformations are dependent upon the cell-surfacemolecule through which load is applied. A laser trap and polymer bead coated with anti-PSGL-1 (KPL-1) were used to apply step loads to microvilli. The lengthening of the microvillus in response to the induced step loads was fitted with a viscoelastic model. The quasi-steady state force on the microvillus at any given length was approximately fourfold lower in cells treated with cytochalasin D or when pulled with concanavalin A-coated rather than KPL-1-coated beads. These data suggest that associations between PSGL-1 and the underlying actin cytoskeleton significantly affect the early stages of leukocytedeformation under flow.

DOI

Optical path clearing and enhanced transmission through colloidal suspensions

J. Baumgartl, T. Čižmár, M. Mazilu, V. C. Chan, A. E. Carruthers,B. A. Capron, W. McNeely, E. M. Wright, and K. Dholakia

We utilize advanced laser fields to clear a path through a dynamic turbid medium, a concept termed “Optical path clearing (OPC).” Particles are evacuated from a volume of the medium using the gradient and/or scattering forces due to an applied laser field with a suitably tailored spatial profile. Our studies encompass both an analytical model and proof-of-principle experiments where paths are cleared in dense bulk colloidal suspensions. Based on our results we suggest that high-performance and high efficiency OPC will be achieved by multiple-step clearing using dynamic laser fields based on Airy or inverted axicon beams.

DOI

Geometrical optics calculation of forces and torques produced by a ringed beam on a prolate spheroid

Alberto Hinojosa-Alvarado and Julio C. Gutiérrez-Vega

The axial and transverse optical forces and torques exerted by circularly ringed beams on an arbitrarily oriented and homogeneous spheroid are calculated and studied within the framework of the geometrical optics regime. The results are applied to study the behavior of the forces in a counter-propagating optical trap. We calculate the trapping efficiencies and torques for several values of physical parameters, including the beam waist separation distance, the equivalent spheroid radius, the spheroid eccentricity, and the refractive index ratio between the particle and the surrounding medium.

DOI

Observation of optical guiding using thermal light

Carlos López-Mariscal and Julio C Gutiérrez-Vega

We report the use of light from a thermal source for optical guiding of microscopic particles. We demonstrate successful guiding of dielectric spheres using light from a high-pressure gas lamp shaped into an ad hocBessel wavefield with an enhanced transverse optical gradient and long focal depth. The broadband light source is spectrally characterized and potential applications of our observations are outlined.

DOI

Calculation of optical trapping forces on dielectric spheres at an oil-water interface with ray-optics model

Mincheng Zhong, Jinhua Zhou, and Yinmei Li

The transverse trapping forces on a dielectric sphere located at an oil-water interface are theoretically investigated with the ray-optics model. The transverse trapping forces rely on the internal property of the particle-interface system, and increase with either the decrease of three phase contact angles at the oil-water interface or the use of oil phase with low refractive index. The numerical results also show that the transverse trapping forces can be improved by either decreasing the numerical aperture of the microscope objective or shrinking the diameter of the trapping laser beam.

DOI

Dynamic ray tracing for modeling optical cell manipulation

Ihab Sraj, Alex C. Szatmary, David W. M. Marr, and Charles D. Eggleton

Current methods for predicting stress distribution on a cell surface due to optical trapping forces are based on a traditional ray optics scheme for fixed geometries. Cells are typically modeled as solid spheres as this facilitates optical force calculation. Under such applied forces however, real and non-rigid cells can deform, so assumptions inherent in traditional ray optics methods begin to break down. In this work, we implement a dynamic ray tracing technique to calculate the stress distribution on a deformable cell induced by optical trapping. Here, cells are modeled as three-dimensional elastic capsules with a discretized surface with associated hydrodynamic forces calculated using the Immersed Boundary Method. We use this approach to simulate the transient deformation of spherical, ellipsoidal and biconcave capsules due to external optical forces induced by a single diode bar optical trap for a range of optical powers.

DOI

Reduction of optical forces exerted on nanoparticles covered by scattering cancellation based plasmonic cloaks

Simone Tricarico, Filiberto Bilotti, and Lucio Vegni 

Scattering cancellation approach has been recently proposed as a promising route to design invisibility cloaks. However, reduced observability of an object is just one of the potential applications of this technique. In this paper, we investigate the possibility to reduce optical forces exerted on a given nanoparticle by covering it with a properly designed plasmonic cloak. We show, in fact, that conditions similar to those used to make spherical and cylindrical nanoparticles invisible to the electromagnetic field by using the scattering cancellation approach, may be straightforwardly applied also to minimize both gradient and scattering optical forces exerted by the illuminating radiation on the same covered nanoparticles. These results are then validated through full-wave simulations, properly considering both dispersion and losses of the plasmonic materials used to design the cloaks. We also extend our speculations to the case of optical torques exerted on spheroidal and cylindrical Rayleigh particles, deriving the conditions to obtain stable equilibrium positions. This investigation leads to the anomalous result that the usual unstable equilibrium positions of uncovered particles may result stable ones when properly designing the particle cover. Finally, in order to apply the proposed theoretical speculations to more complex cases, we derive the conditions for minimizing optical forces exerted on a cloaked Rayleigh particle placed above a dielectric half space. These results may find interesting applications in different fields of nanotechnology.

DOI

Measurement of Macrophage Adhesion at Various pH Values by Optical Tweezers with Backward-Scattered Detection

Yi-Jr Su and Long Hsu

Optical tweezers have emerged as a powerful tool with broad applications in biology and physics. In force-measuring applications, the trapped bead position is usually accurately determined by forward-scattered detection. The current study discusses both backward-scattered detection and forward-scattered detection related to the linear detection range for a 3 µm bead and the distance between the two laser system focuses, confirming the optimum positions of the two focuses. The result indicates that the linear detection range of backward-scattered detection is longer than the forward-scattered one. Finally, this work investigates real-time adhesion force measurements between human macrophages and 3 µm trapped beads coated with lipopolysaccharides at various pH values by optical tweezers with backward-scattered detection.

DOI

Secondary convergence in femtosecond laser trapping

Jiang, YQ, Ma, CG, Oh, I, Hosokawa, Y, Masuhara, H

When femtosecond laser pulses pass through a trapped polystyrene bead, water breakdown is induced even though the energy of laser pulse is much lower compared to the threshold value of breakdown when the femtosecond laser directly irradiates in water. This mechanism is assigned to the secondary convergence of the laser by the trapped bead.

DOI

Manipulating CD4+ T cells by optical tweezers for the initiation of cell-cell transfer of HIV-1

McNerney, G.P., Hüner, W., Chen, B.K., Huser, T.

Cell-cell interactions through direct contact are very important for cellular communication and coordination-especially for immune cells. The human immunodeficiency virus type I (HIV-1) induces immune cell interactions between CD4+ cells to shuttle between T cells via a virological synapse. A goal to understand the process of cell-cell transmission through virological synapses is to determine the cellular states that allow a chance encounter between cells to become a stable cell-cell adhesion. We demonstrate the use of optical tweezers to manipulate uninfected primary CD4+ T cells near HIV Gag-iGFP transfected Jurkat T cells to probe the determinants that induce stable adhesion. When combined with fast 4D confocal fluorescence microscopy, optical tweezers can be utilized not only to facilitate cell-cell contact, but also to simultaneously track the formation of a virological synapse, and ultimately to probe the events that precede virus transfer. HIV-1 infected T cell (green) decorated with uninfected primary T cells (red) by manipulating the primary cells with an optical tweezers system.

DOI

Optical micromanipulations in the non-diffractive regime

Smitha S. Varghese, Min Gu

The ever-evolving topic of optical micromanipulation has established itself as a discipline over the last three decades, and is of much interest to a wide research community due to constantly emerging new applications across the various key disciplines. Performing optical manipulation using evanescent waves is termed near-field optical manipulation, which is essentially the manipulation of particles in the non-diffractive regime. The concept of the breaking of diffraction limit is the spur driving near-field optics studies, as opposed to all far field optical applications where light cannot be focused to a spot smaller than the diffraction limited value, which is about half the wavelength of light in the medium. The authors present a review of the various near-field optical manipulation techniques and then report on the observation of erythrocyte pearl chains by a near-field optical tweezer.

DOI

Real time characterization of hydrodynamics in optically trapped networks of micro-particles

Arran Curran, Alison Yao, Graham Gibson, Richard Bowman, Jon Cooper, Miles Padgett

The hydrodynamic interactions of micro-silica spheres trapped in a variety of networks using holographic optical tweezers are measured and characterized in terms of their predicted eigenmodes. The characteristic eigenmodes of the networks are distinguishable within 20-40 seconds of acquisition time. Three different multi-particle networks are considered; an eight-particle linear chain, a nine-particle square grid and, finally, an eight-particle ring. The eigenmodes and their decay rates are shown to behave as predicted by the Oseen tensor and the Langevin equation, respectively. Finally, we demonstrate the potential of using our micro-ring as a non-invasive sensor to the local environmental viscosity, by showing the distortion of the eigenmode spectrum due to the proximity of a planar boundary.

DOI

Optical Trapping of Quantum Dots Based on Gap-Mode-Excitation of Localized Surface Plasmon

Yasuyuki Tsuboi, Tatsuya Shoji, Noboru Kitamura, Mai Takase, Kei Murakoshi, Yoshihiko Mizumoto and Hajime Ishihara

One of the recent hot topics in the fields of plasmonics and related nanophotonics is optical trapping of nano/microparticles based on surface plasmon. Experimental demonstration of such trapping by gap-mode plasmon has hitherto been limited so far to a few reports in which submicrometer polymer beads were trapped with intense irradiation at MW/cm2, satisfying an energetic condition of U > kT. (U is the potential energy of the trap and kT is an averaged thermal background energy.) We demonstrate plasmon-based optical trapping of a luminescent quantum dot (Q dot, diameter ≥10 nm) with a very weak irradiation (0.5−10 kW/cm2). The most important discovery is that the Q dot trapping was clearly observed through luminescence detection even under an energetic condition of U < kT, on the basis of which we propose a novel concept that is peculiar to plasmon-based trapping at a nanogap.

DOI

Optical trapping with π-phase cylindrical vector beams

Brian J Roxworthy and Kimani C Toussaint Jr

The use of π-phase radially and azimuthally polarized vector beams in optical trapping is investigated. We find that by tuning the relative phase between the eigenmodes comprising the beams, the optical forces applied to a trapped particle are modified. In particular, axial trapping efficiency is enhanced with increasing zpolarization and the lateral trapping efficiency of the vector beams is reduced compared to a Gaussian input beam. In addition, this is the first experimental demonstration of low-power optical trapping in an aqueous environment using vector beams, which may have important applications in biological systems.

DOI

Focusing of concentric piecewise vector Bessel–Gaussian beam

Jinsong Li, Ying Fang, Shenghua Zhou and Youxiang Ye

The focusing properties of a concentric piecewise vector Bessel–Gaussian beam are investigated in this paper. The beam consists of three portions: the center circular portion and outer annular portion are radially polarized, while the inner annular portion is generalized polarized with tunable polarized angle. Numerical simulations show that the evolution of focal pattern is altered considerably with different Bessel parameters in the Bessel term of the vector Bessel–Gaussian beam. The polarized angle also affects the focal pattern remarkably. Some interesting focal patterns may appear, such as two-peak, dark hollow focus; ring focus; spherical shell focus; cylindrical shell focus; and multi-ring-peak focus, and transverse focal switch occurs with increasing polarized angle of the inner annular portion, which may be used in optical manipulation.

DOI

Quantitative measurements of absolute dielectrophoretic forces using optical tweezers

Yoochan Hong, Jin-Woo Pyo, Sang Hyun Baek, Sang Woo Lee, Dae Sung Yoon,Kwangsoo No, and Beop-Min Kim

Optical tweezers were used for quantitative measurement of the absolute dielectrophoresis (DEP) forces acting on polystyrene microparticles. The electrodes and tweezers were configured to create one-dimensional DEP forces acting perpendicular to the tweezers’ beam. The influences of various external factors, such as applied voltage frequency, conductivity of the medium, and particle size on the measurement were estimated. By accounting for these factors, actual measurements were in close agreement with theoretical predictions. Our results show that the optical tweezers may serve as a unique tool for the measurement of DEP forces in various applications.

DOI

Nanostructure-enhanced laser tweezers for efficient trapping and alignment of particles

Benjamin K. Wilson, Tim Mentele, Stephanie Bachar, Emily Knouf, Ausra Bendoraite, Muneesh Tewari, Suzie H. Pun, and Lih Y. Lin

We propose and demonstrate a purely optical approach to trap and align particles using the interaction of polarized light with periodic nanostructures to generate enhanced trapping force. With a weakly focused laser beam, we observed efficient trapping and transportation of polystyrene beads with sizes ranging from 10 μm down to 190 nm as well as cancer cell nuclei. In addition, alignment of non-spherical dielectric particles to a 1-D periodic nanostructure was achieved with low laser intensity without attachment to birefringent crystals. Bacterial cells were trapped and aligned with incident optical intensity as low as 17 μW/μm2.

DOI

Mechanical forces exerted by a dipole emitter on an interface

D. Petrov

Mechanical forces exerted by an emitting dipole on the interface between two media with different dielectric susceptibilities are found for different distances between the dipole and the interface. Estimations of the force are given based on known values of molecular polarizabilities for inelestic processes such as Raman scattering and fluorescence including those that occur near metal structures.

DOI

Different Modulation Mechanisms of Attractive Colloidal Interactions by Lipid and Protein Functionalization

Yupeng Kong and Raghuveer Parthasarathy

The nature of attractive interactions observed between like-charged microparticles near a confining wall remains an outstanding puzzle in colloidal science. The shortage of experimental systems that provide tunable attractions contributes to the lack of progress in solving this mystery. We have recently shown that the functionalization of microspheres with lipid membranes allows simple control of interparticle interactions as a function of membrane composition (Kong, Y.; Parthasarathy, R. Soft Matter 2009, 5, 2027−2032). Here we introduce a new approach to biomembrane-mediated control in which varying amounts of a peripheral membrane protein, cholera toxin subunit B, are bound to the surface of lipid-functionalized silica particles. Protein functionalization again provides a family of tunable attractive pair interactions, measured using an optical line trap. Surprisingly, however, the form of interactions is strikingly different for particles with protein-plus-lipid membranes than for particles with lipid-only membranes, displaying opposite correlations between the depth of the attractive potential well and the spatial range of the interaction as well as between the well depth and the distance to the confining wall. Our findings and their distinctiveness from previous membrane-functionalized systems not only demonstrate an orthogonal route to the practical control of colloidal assembly but also, more fundamentally, show that multiple physical mechanisms or mechanisms that are especially sensitive to particle surface chemistries may be responsible for governing like-charge attraction in colloidal systems.

DOI

Effect of internal flow on the photophoresis of a micron-sized liquid droplet

Takafumi Iwaki

Light irradiation can induce the vectorial motion of an aerosol particle. This phenomenon is often explained in terms of inelastic collision between gas molecules and the aerosol particle under a temperature gradient. We considered the photophoresis of a micron-sized liquid droplet in a rarefied gas atmosphere based on the Boltzmann equation for the atmosphere coupled with the Navier-Stokes equation for the droplet. Two features attributable to induced internal flow in the droplet are analyzed: the contribution of homogeneous energy inflow to the motion of the droplet and the nonlinear scaling of the photophoretic velocity depending on the irradiated light intensity.

DOI

Light-driven nanoscale plasmonic motors

Ming Liu, Thomas Zentgraf, Yongmin Liu, Guy Bartal & Xiang Zhang

When Sir William Crookes developed a four-vaned radiometer, also known as the light-mill, in 1873, it was believed that this device confirmed the existence of linear momentum carried by photons, as predicted by Maxwell's equations. Although Reynolds later proved that the torque on the radiometer was caused by thermal transpiration, researchers continued to search for ways to take advantage of the momentum of photons and to use it for generating rotational forces. The ability to provide rotational force at the nanoscale could open up a range of applications in physics, biology and chemistry, including DNA unfolding and sequencing and nanoelectromechanical systems. Here, we demonstrate a nanoscale plasmonic structure that can, when illuminated with linearly polarized light, generate a rotational force that is capable of rotating a silica microdisk that is 4,000 times larger in volume. Furthermore, we can control the rotation velocity and direction by varying the wavelength of the incident light to excite different plasmonic modes.

DOI

Axon repair: surgical application at a subcellular scale

Wesley C. Chang, Elizabeth Hawkes, Christopher G. Keller, David W. Sretavan


Injury to the nervous system is a common occurrence after trauma. Severe cases of injury exact a tremendous personal cost and place a significant healthcare burden on society. Unlike some tissues in the body that exhibit self healing, nerve cells that are injured, particularly those in the brain and spinal cord, are incapable of regenerating circuits by themselves to restore neurological function. In recent years, researchers have begun to explore whether micro/nanoscale tools and materials can be used to address this major challenge in neuromedicine. Efforts in this area have proceeded along two lines. One is the development of new nanoscale tissue scaffold materials to act as conduits and stimulate axon regeneration. The other is the use of novel cellular-scale surgical micro/nanodevices designed to perform surgical microsplicing and the functional repair of severed axons. We discuss results generated by these two approaches and hurdles confronting both strategies.


DOI

Comparison of silicon photonic crystal resonator designs for optical trapping of nanomaterials

X Serey, S Mandal and D Erickson

The use of silicon photonic devices for optical manipulation has recently enabled the direct handling of objects like nucleic acids and nanoparticles that are much smaller than could previously be trapped using traditional laser tweezers. The ability to manipulate even smaller matter however requires the development of photonic structures with even stronger trapping potentials. In this work we investigate theoretically several photonic crystal resonator designs and characterize the achievable trapping stiffness and trapping potential depth (sometimes referred to as the trapping stability). Two effects are shown to increase these trapping parameters: field enhancement in the resonator and strong field containment. We find trapping stiffness as high as 22.3 pN nm − 1 for 100 nm polystyrene beads as well as potential depth of 51 000 kBT at T = 300 K, for one Watt of power input to the bus waveguide. Under the same conditions for 70 nm polystyrene beads, we find a stiffness of 69 pN nm − 1 and a potential depth of 177 000 kBT. Our calculations suggest that with input power of 10 mW we could trap particles as small as 7.7 nm diameter with a trapping depth of 500 kBT. We expect these traps to eventually enable the manipulation of small matter such as single proteins, carbon nanotubes and metallic nanoparticles.

DOI

Effect of long- and short-term exposure to laser light at 1070 nm on growth of Saccharomyces cerevisiae

Thomas Aabo, Ivan R. Perch-Nielsen, Jeppe Seidelin Dam, Darwin Z. Palima, Henrik Siegumfeldt, Jesper Glückstad, Nils Arneborg

The effect of a 1070-nm continuous and pulsed wave ytterbium fiber laser on the growth ofSaccharomyces cerevisiae single cells is investigated over a time span of 4 to 5 h. The cells are subjected to optical traps consisting of two counterpropagating plane wavebeams with a uniform flux along the x, y axis. Even at the lowest continuous power investigated—i.e., 0.7 mW—the growth of S. cerevisiae cell clusters is markedly inhibited. The minimum power required to successfully trap single S. cerevisiae cells in three dimensions is estimated to be 3.5 mW. No threshold power for the photodamage, but instead a continuous response to the increased accumulated dose is found in the regime investigated from 0.7 to 2.6 mW. Furthermore, by keeping the delivered dose constant and varying the exposure time and power—i.e. pulsing—we find that the growth of S. cerevisiae cells is increasingly inhibited with increasing power. These results indicate that growth of S. cerevisiae is dependent on both the power as well as the accumulated dose at 1070 nm.

DOI

Shape anisotropy induces rotations in optically trapped red blood cells

Kapil Bambardekar, Jayashree A. Dharmadhikari, Aditya K. Dharmadhikari, Toshihoro Yamada, Tsuyoshi Kato, Hirohiko Kono, Yuichi Fujimura, Shobhona Sharma, Deepak Mathur

A combined experimental and theoretical study is carried out to probe the rotational behavior of red blood cells (RBCs) in a single beam optical trap. We induce shape changes in RBCs by altering the properties of the suspension medium in which live cells float. We find that certain shape anisotropies result in the rotation of optically trapped cells. Indeed, even normal (healthy) RBCs can be made to rotate using linearly polarizedtrapping light by altering the osmotic stress the cells are subjected to. Hyperosmotic stress is found to induce shape anisotropies. We also probe the effect of the medium's viscosity on cell rotation. The observed rotations are modeled using a Langevin-typeequation of motion that takes into account frictional forces that are generated as RBCs rotate in the medium. We observe good correlation between our measured data and calculated results.

DOI

Power spectrum analysis with least-squares fitting: Amplitude bias and its elimination, with application to optical tweezers and atomic force microscope cantilevers

Simon F. Nørrelykke and Henrik Flyvbjerg

Optical tweezers and atomic force microscope (AFM) cantilevers are often calibrated by fitting their experimental power spectra of Brownian motion. We demonstrate here that if this is done with typical weighted least-squares methods, the result is a bias of relative size between −2/n and +1/n on the value of the fitted diffusion coefficient. Here, n is the number of power spectra averaged over, so typical calibrations contain 10%–20% bias. Both the sign and the size of the bias depend on the weighting scheme applied. Hence, so do length-scale calibrations based on the diffusion coefficient. The fitted value for the characteristic frequency is not affected by this bias. For the AFM then, force measurements are not affected provided an independent length-scale calibration is available. For optical tweezers there is no such luck, since the spring constant is found as the ratio of the characteristic frequency and the diffusion coefficient. We give analytical results for the weight-dependent bias for the wide class of systems whose dynamics is described by a linear (integro)differential equation with additive noise, white or colored. Examples are optical tweezers with hydrodynamic self-interaction and aliasing, calibration of Ornstein–Uhlenbeck models in finance, models for cell migration in biology, etc. Because the bias takes the form of a simple multiplicative factor on the fitted amplitude (e.g. the diffusion coefficient), it is straightforward to remove and the user will need minimal modifications to his or her favorite least-squares fitting programs. Results are demonstrated and illustrated using synthetic data, so we can compare fits with known true values. We also fit some commonly occurring power spectra once-and-for-allin the sense that we give their parameter values and associated error bars as explicit functions of experimental power-spectral values.

DOI

Controlled rotation and orientation of rubrene particles and Escherichia coli using optical tweezers

X. C. Li and X. D. Sun

Optical trapping and rotating of suspended micro-sized rubrene particles were performed using optical tweezers with circularly polarized light. The experimental results show that the rotation speed of the rubrene particles is proportional to the laser power, and the orientation of the rubrene particles can be controlled by the optical tweezers with linearly polarized light. Interestingly, by combining with the rubrene particle, the Escherichia coli (E. coli) can be rotated and oriented by optical tweezers. However, the rotating and orientating are mainly determined by the characteristics of rubrene particles. Our experiment provides a simple and convenient way to orient biological particles even if they are not sensitive to the polarization of the laser beam. Moreover, the rubrene can emit strong fluorescence when excited by the laser at the wavelength of 532 nm, and which can be potential applied to manipulate other particles with the fluorescence characteristics.

DOI

Analytical particle measurements in an optical microflume

Joseph D. Taylor, Alex Terray and Sean J. Hart

In this work, microscopic particles in a fluid flow are manipulated using forces generated by a high power laser beam. The resulting manipulations on the particles are imaged using a microscope lens connected to a CCD camera. Differential forces on particles of varying physical and chemical composition have been measured. The goal is to measure the optical forces on a diverse range of particles and catalog the associated chemical and physical differences to understand which properties and mechanisms result in the largest force differentials. Using these measurements our aim is to better understand differences between similar microspheres in terms of size, morphology, or chemical composition. Particles of the same size, but different composition show large variations in optical pressure forces and are easily discernable in the present analytical system. In addition, we have demonstrated the ability to differentiate a 70 nm size difference between two NIST precision size standard polystyrene microspheres, corresponding to a 2.0 pN difference in optical force. Lastly, the instrument was used to measure differences between biological samples of similar size, demonstrating the ability to make precise analytical measurements on microorganism samples.

DOI

Published Paper Statistics (first half of 2010)

Here is the results for the first half of 2010 for published papers on optical tweezers, micromanipulation and trapping.

The top Journals (more than 2% hits) are:

1. Optics Express  13.5%
2. Proceedings of the National Academy of Sciences 4.9%
3. Optics Letters 4.3%
4. Physical Review E 3.8%
5. Physical Review Letters 3.8%
6. Biophysical Journal 3.2%
7. Lab on a Chip 2.7%
8. Nano Letters 2.7%
9. Nanotechnology 2.7%
10. Physical Review A 2.7%
11. Review of Scientific Instruments 2.7 %

Below is a tag cloud for the corresponding words found in the title and abstracts for the period: