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Photophoretic forces confine particles by a combination of levitation and active trapping. To generate the latter, the component of photophoretic force transverse to gravity generates a torque, which leads to complex rotation of the particle, thereby generating a restoring force. This force has not been understood quantitatively. In this paper, we study this component of photophoretic force by trapping a single absorbing particle using a loosely focused Gaussian laser beam and modulating the trap-center spatially using a superposition of multiple sinusoidal frequencies applied to the trapping beam. We vary the modulation amplitude in proportion to the excitation frequency so as to compensate for the reduced particle response at higher frequencies and thus obtain an almost constant response of the particle over the entire excitation range. We use the amplitude and phase response to determine the resonance frequency and the trap stiffness—which we study as a function of laser power and the intensity experienced by trapped particles of different sizes. We observe that, while the trapping stiffness is proportional to laser power, the optical intensity experienced by a particle at different powers is the same. This is reasonable in order to facilitate optical levitation but implies that the trap stiffness actually depends on an "effective intensity," which may be a combination of the particle accommodation coefficient and the laser power. [ABSTRACT FROM AUTHOR]

The dynamics of fine particles of different sizes in the vicinity of DC conductors have been studied. It was observed that the particles aggregated with each other and were distributed in parallel chains on the surface of the conductors. Moreover, an interesting "layering phenomenon" was discovered in the spaces where particles concentrated. Variations in the concentration of particles of different sizes were measured in the sump. A higher voltage and a larger particle size were found to be associated with a faster decrease in the particle concentration. Based on a coupled model of the electric field and the force field, the different forces acting on particles of different sizes were analyzed. Particles of different sizes were observed to reunite into a spindle shape in the calculation space, with large particles playing a leading role. According to the calculation results, the particles around the conductor will settle and reunite, and the gathering of particles accelerates the settlement. [ABSTRACT FROM AUTHOR]

Ultrasound directed self-assembly (DSA) enables noninvasively aligning high aspect ratio particles in three-dimensional (3D) user-specified orientations, which finds application in a myriad of engineering applications, including manufacturing engineered materials. However, the number of ultrasound transducers and their spatial arrangement limit the accuracy of the particle alignment with any 3D user-specified orientation. We define a set of 3D user-specified orientations and use numerical simulations to quantitatively evaluate the effect of the number of ultrasound transducers, their spatial arrangement including a sphere, cube, and two parallel plates, and the size of the spatial arrangement on the orientation error of a high aspect ratio particle in a standing ultrasound wave field. We demonstrate that a spatial arrangement of ultrasound transducers with more than two unique wave propagating directions is required to orient a high aspect ratio particle in 3D, and we determine that the orientation error decreases with the increasing number of unique wave propagation directions. Furthermore, we show that in a spherical arrangement of ultrasound transducers, the orientation error is independent of the size of the arrangement of transducers. This knowledge facilitates using ultrasound DSA as a fabrication method for engineered composite materials that derive their function from the location and orientation of particle inclusions. [ABSTRACT FROM AUTHOR]

The transport of soft particles through narrow channels or pores is ubiquitous in biological systems and industrial processes. On many occasions, the particles deform and temporarily block the channel, inducing a built-up pressure. This pressure buildup often has a profound effect on the behavior of the respective system; yet, it is difficult to be characterized. In this work, we establish a quantitative correlation between the built-up pressure and the material and geometry properties through experiments and mechanics analysis. We fabricate microgels with a controlled diameter and elastic modulus by microfluidics. We then force them to individually pass through a constrictive or straight confining channel and monitor the pressure variation across the channel. To interpret the pressure measurement, we develop an analytical model based on the Neo-Hookean material law to quantify the dependence of the maximum built-up pressure on the radius ratio of the elastic sphere to the channel, the elastic modulus of the sphere, and two constant parameters in the friction constitutive law between the sphere and the channel wall. This model not only agrees very well with the experimental measurement conducted at large microgel deformation but also recovers the classical theory of contact at small deformation. Featuring a balance between accuracy and simplicity, our result could shed light on understanding various biological and engineering processes involving the passage of elastic particles through narrow channels or pores. [ABSTRACT FROM AUTHOR]

Controlled assembly of microscale objects can be achieved by exploiting interactions that dominate at that length scale. Capillary interactions are an excellent candidate for this purpose; microparticles trapped at fluid interfaces disturb the interface shape, migrate, and assemble to minimize the interfacial area. These interactions are independent of microparticle material properties and so can be used to assemble objects of arbitrary materials. By using a magnetic robot as a mobile distortion source, additional control over assembly can be achieved. For example, millimeter-scale magnetic robots that are heavy enough to distort the interface have been used to generate long range capillary attractions and collect passive particles that are hundreds of micrometers in diameter. However, for smaller robots and particles, gravity is less important, and capillary interactions rely on interface distortions from undulated contact lines. We use a magnetic microrobot to manipulate passive microparticles at the water/hexadecane interface via an interplay of hydrodynamic and capillary interactions. Furthermore, we demonstrate preferred docking at corners of a square microrobot without the need for high resolution position control. We modulate the strength of docking interactions, allowing structure assembly and release. Finally, we design undulated docking stations with multiple stable sites for cargo delivery. The ability to dynamically manipulate microparticles and their structures at fluid interfaces creates new possibilities for manufacturing of complex microstructures. [ABSTRACT FROM AUTHOR]

Particle focusing in viscoelastic fluid flow is a promising approach for inducing particle separations in microfluidic devices. The results from theoretical studies indicated that multiple stream particle focusing can be realized with a large magnitude of the elastic second normal stress difference (N2). For dilute polymer solutions, theoretical and experimental studies show that the magnitude of N2 is never large, no matter how large the polymer molecular weight nor how high the shear rate. However, for concentrated entangled polymer solutions, the magnitude of N2 becomes large at high shear rates. Therefore, in order to test the hypothesis that N2 can be used to induce multiple particle stream focusing behavior, we perform the systematic study of the effects of increasing carrier fluid polymer concentrations in a microchannel containing fluorescent particles. In a dilute polymer solution, multiple particle stream focusing is not observed, even at high shear rates and large dimensionless Weissenberg number values (Wi ≈ 30) at which the elastic first normal stress difference (N1) and the viscosity shear-thinning should be very large, while in a concentrated entangled polymer solution, we observe that particle streams focused upon the channel centerline bifurcate to form two symmetric off-channel particle streams at higher shear rates. This particle focusing behavior is different from previous multiple-stream focusing phenomena, and that we attribute to the influence of the second normal stress difference N2. This N2 induced multiple stream focusing phenomenon provides a different approach for manipulating the particle trajectory and separation in a microchannel. [ABSTRACT FROM AUTHOR]

A special arrangement of electrically small particles for infrared imaging application is presented. The proposed configuration is composed of split ring resonators surrounding eight tapered microstrip lines and sits on a dielectric substrate with a backed metallic layer. Simulation and numerical calculations are carried out to investigate and prove the proposed structure's power absorption ability. The structure is designed to absorb incident infrared radiation in the 8–12 μm band. Approximately 52% absorption efficiency is achieved during this study. An absorption efficiency of more than 90% is realized by utilizing an infinite array of the proposed structure. [ABSTRACT FROM AUTHOR]

Displays have revolutionized the way we work and learn, and thus, the development of display technologies is of paramount importance. The possibility of a free-space display in which 3D graphics can be viewed from 360° without obstructions is an active area of research—holograms or lightfield displays can realize such a display, but they suffer from clipping and a limited field of view. Here, we use a phased array of ultrasonic emitters to realize a volumetric acoustophoretic display in which a millimetric particle is held in midair using acoustic radiation forces and moved rapidly along a 3D path. Synchronously, a light source illuminates the particle with the target color at each 3D position. We show that it is possible to render simple figures in real time (10 frames per second) as well as raster images at a lower frame rate. Additionally, we explore the dynamics of a fast-moving particle inside a phased-array levitator and identify potential sources of degradation in image quality. The dynamics are nonlinear and lead to distortion in the displayed images, and this distortion increases with drawing speed. The created acoustophoretic display shows promise as a future form of display technology. [ABSTRACT FROM AUTHOR]

We report the velocity measurement of microscopic particles flowing at ultrahigh speed with optofluidic time-stretch microscopy at high throughput. This is a study of using optical time-stretch microcopy as a tool for particle velocimetry, where we developed a custom algorithm to process the images acquired from the optofluidic platform for the velocity calculation of individual particles. We experimentally determined the actual flow velocities for polystyrene microspheres with different sizes and traveling through the microchannel at a throughput of ∼10 000 particles/s. We also examined microfluidic channels with different aspect ratios (depth-to-width) for particle velocimetry. The result indicates a measurable flow velocity up to 2.51 m/s. Our method provides a promising tool for label-free and high-throughput particle velocimetry at high velocity magnitudes. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a stable three-dimensional trap and manipulation of a micron-sized strongly absorbing particle in pure liquid glycerol by using a single tight focused TEM00 Gaussian beam. We employ a bottom-side bidirectional view observation system to observe the trapped particle. We use the light at 980 nm to trap the absorbing particle and the light at 532 nm to indicate the distribution of the temperature field around the trapped particle. The trapping position of the absorbing particle is related to the incident laser power; the lower the incident laser power, the longer the particle shift distance. Our approach provides full control over trapped absorbing particles and expands optical manipulation of strong absorbing particles into a liquid media. [ABSTRACT FROM AUTHOR]

An innovative, single-shot, non-intercepting monitor of the transverse profile of plasma-accelerated electron beams is presented, based on the simultaneous measurement of the electron energy and the betatron radiation spectra. The spatial resolution is shown to be down to few tens of nanometers, important for high-precision applications requiring fine shaping of beams and detailed characterizations of the electron transverse phase space at the exit of plasma accelerating structures. [ABSTRACT FROM AUTHOR]

In this letter, SiC nanowires were adopted to reinforce the nanoparticle-based shear thickening fluid (STF) to improve its rheological properties. The reinforced STF showed a significant increase in viscosity. A Split-Hopkinson pressure bar was implemented to evaluate the dynamic response of STF at strain rates in the range of 3×103–1.2×104/s. For the pure STF, the flow stress reaches a saturation value with increasing strain rates and shows almost no strain rate sensitivity, whereas the flow stress of the reinforced STF increases with strain rates, and the strain rate sensitivity to flow stress is obvious owing to the resistance of nanowires. The essence of this study is to reveal that there is a limiting value of the flow stress of traditional nanoparticle-based STF at high strain rates due to the lubrication force among particles. SiC nanowires can be used to break this limitation of the nanoparticle-based STF. [ABSTRACT FROM AUTHOR]

Acoustic manipulation of particles, as a non-contact and non-damage method, has attracted much interest in recent years. Here, we present a platform for sound-driven particle delivery realized on an artificially engineered metal plate with manipulated, graded acoustic field distribution. By fabricating gratings with graded height on one surface of the structured plate, we obtain graded acoustic pressure distribution near the smooth surface of the plate. The acoustic field can be tuned at different positions by regulating the operating frequency, which originates from the gratings of different heights corresponding to different resonant frequencies. Therefore, from the effect of the acoustic radiation force exerted by this gradient field, a particle will transfer on the plate just by the frequency being tuned, without moving the acoustic source. Our theoretical analysis agrees well with the experimental demonstration. This work will lead to potential applications in drug delivery and microfluidics. [ABSTRACT FROM AUTHOR]

Acoustic holography has extensive possibilities in acoustic sensing, acoustic illusion, contactless particle manipulation, and medical imaging. Based on coating unit cells and perforated panels, an acoustic composite metasurface is constructed with a decoupled modulation of phase and amplitude, which has been used to design acoustic holography. This proposal not only has lower complexity than conventional acoustic holography of active arrays due to the avoidance of complex structures and circuits but also provides more flexibility than acoustic holography based on the acoustic metasurface with phase-only modulation benefitting from the efficient decoupled modulation of phase and amplitude. We have further demonstrated three acoustic holographic applications, such as multi-directional transmission, multi-focal focusing, and holographic imaging. Due to the low complexity and the great flexibility, this proposal has potential to achieve the high-quality holograms with high information content, fine resolution, and large scale. [ABSTRACT FROM AUTHOR]

The separation of target objects conjugated with magnetic particles is a significant application in biomedicine and clinical diagnosis. Conventional magnetophoresis-based devices use a sheath flow to pre-focus the particles into a single stream and typically operate at a low flow rate. We demonstrate in this work a high-throughput, sheathless, magnetophoretic separation of magnetic and nonmagnetic beads in a groove-based channel, and also report on an interesting phenomenon where the same magnetic beads in the same microchannel, but with different setups, has a different particle tracing; a binary mixture of magnetic and non-magnetic beads in a diluted ferrofluid, is then fed into the channel. These magnetic beads are focused near the centreline of the channel by exploiting positive magnetophoresis and microvortices generated by grooves, whereas the non-magnetic beads are focused along the sidewalls of the channel by negative magnetophoresis and hydrophoresis. These magnetic and non-magnetic beads are separated in a wide range of flow rates (up to 80 µl min-1). [ABSTRACT FROM AUTHOR]

Weighing individual micro- or nanoscale particles in solution using dynamic micromechanical sensors is quite challenging: viscous losses dramatically degrade the sensor's performance by both broadening the resonance peak and increasing the effective total mass of the resonator by the dragged liquid. While the virtual mass of the resonator was discussed frequently, little attention has been paid to the virtual mass of particles attached to the resonator's surface and its impact on the accuracy of mass sensing. By means of the in situ detection of a polystyrene microbead in water using a bridge-based microresonator, we demonstrate that the virtual mass of the bead significantly affects the observed frequency shift. In fact, 55% of the frequency shift was caused by the virtual mass of the adsorbed bead, predicted by Stoke's theory. Based on the observed shift in the resonator's quality factor during particle adsorption, we confirm this significant effect of the virtual mass. Thus, a quantitative analysis of the mass of a single adsorbed particle is strongly diminished if dynamic micromechanical sensors are operated in a liquid environment. [ABSTRACT FROM AUTHOR]

At the micrometer-scale and below, particle adhesion becomes particularly relevant as van der Waals force often dominates volume and surface proportional forces. The rolling resistance of microparticles and their critical rolling angles prior to the initiation of free-rolling and/or complete detachment are critical in numerous industrial processes and natural phenomenon involving particle adhesion and granular dynamics. The current work describes a non-contact measurement approach for determining the critical rolling angle of a single microparticle under the influence of a contact-point base-excitation generated by a transient displacement field of a prescribed surface acoustic wave pulse and reports the critical rolling angle data for a set of polystyrene latex microparticles. [ABSTRACT FROM AUTHOR]

When subjected to a temperature gradient, the liquid film formed on a particle surface in nearcritical vapor may undergo osmotic flow. Such a flow can induce a normal vapor flux onto (away from) the particle surface to compensate the loss (release the excess) of liquid as a result of condensation (evaporation) and produce significant thermophoretic mobility due to the extreme fluid compressibility during the phase change. Based on this principle, we have formulated this unique process and solved for the phoretic mobility by applying the boundary layer method. Unlike the classic mechanisms, this term of phase transition-induced phoretic mobility scales reversely with particle size, which becomes dominant for nanoparticles. [ABSTRACT FROM AUTHOR]

Spontaneous dewetting of a silver layer on a templated silica substrate is proposed as a promising low-cost process to produce self-organized metallic nanostructures. Periodic gratings with inverted pyramid pattern and periods ranging from 200 to 1000 nm are fabricated by nanoimprint on a solgel silica layer. A silver layer is then deposited on the templated substrate by magnetron sputtering and annealed to form an array of well-organized islands by solid-state dewetting. The resulting islands are shown to have reduced diameter and size dispersion compared to arrays obtained in the same conditions on flat substrates. The density of defects in the periodic array is determined as a function of silver layer thickness and is lower than 10% in optimal conditions. Optical transmission spectra of periodic arrays are measured, showing extinction peaks that can be related to plasmon resonance. This resonance can be tuned by adjusting the period and particle diameter. [ABSTRACT FROM AUTHOR]

A series of reverse-impact experiments were performed on vanadium at shock pressure ranging from 32GPa to 88GPa. Particle velocity profiles measured at sample/LiF window interface were used to estimate the sound velocities, shear modulus, and yield stress in shocked vanadium. A phase transition at ~60.5 GPa that may be the body-centered cubic (BCC) to rhombohedral structure was identified by the discontinuity of the sound velocity against shock pressure. This transition pressure is consistent with the results from diamond anvil cell (DAC) experiments and first-principle calculations. However, present results show that the rhombohedral phase has higher strength and shear modulus than the BCC phase, which is contrast to the findings from DAC experiments and theoretical work. [ABSTRACT FROM AUTHOR]

Optical trapping of core-shell magnetic microparticles is experimentally demonstrated by using cylindrical vector beams. Second, we investigate the optical trapping efficiencies. The results show that radially and azimuthally polarized beams exhibit higher axial trapping efficiencies than the Gaussian beam. Finally, a trapped particle is manipulated to kill a cancer cell. The results make possible utilizing magnetic particles for optical manipulation, which is an important advantage for magnetic particles as labeling agent in targeted medicine and biological analysis. [ABSTRACT FROM AUTHOR]

We introduce a surface acoustic wave (SAW) based method for acoustically controlled concentration, capture, release, and sorting of particles in a microfluidic system. This method is power efficient by the nature of its design: the vertical direction of a traveling acoustic wave, in which the majority of the energy at the SAW-water interface is directed, is used to concentrate particles behind a microfabricated polydimethylsiloxane membrane extending partially into a channel. Sorting is also demonstrated with this concentration shown to be size-dependent. Low-power, miniature SAW devices, using methods such as the one demonstrated here, are well placed for future integration into point-of-care diagnostic systems. [ABSTRACT FROM AUTHOR]

We present experimental results of laser-to-proton conversion efficiency as a function of lateral confinement of the refluxing electrons. Experiments were carried out using the T-Cubed laser at the Center for Ultrafast Optical Science, University of Michigan. We demonstrate that the laser-to-proton conversion efficiency increases by 50% with increased confinement of the target from surroundings with respect to a flat target of the same thickness. Three-dimensional hybrid particle-in-cell simulations using LSP code agree with the experimental data. The adopted target design is suitable for high repetition rate operation as well as for Inertial Confinement Fusion applications. [ABSTRACT FROM AUTHOR]

Nanoscale Nd-Fe-B particles were fabricated with room temperature coercivity of 4.9 kOe by attrition (stirred media) milling. For longer milling times, more than 80% of the particles were less than 500 nm diameter, and 50-200 nm particles can be selected by a decanting process. Anisotropy was demonstrated by field alignment, as shown by creation of strong texture in x-ray diffraction (XRD) patterns. Limited peak broadening in XRD patterns indicates that attrition milling produces less deformation and amorphous material than high-energy milling. Compared to other magnetic nanoparticles, the higher coercivity is attributed to limited plastic deformation and larger sized particles. [ABSTRACT FROM AUTHOR]

A method to control the permittivity and tunability in composite materials formed by conductive nanoparticles embedded in a flexible nonlinear dielectric matrix is proposed. The local field distributions in composite structures were determined and the field-induced permittivity was estimated for different volume fraction of conductive particles. The predicted tunability behavior agrees well with the observed trends obtained for gold nanoparticles embedded in chitosan matrix. The paper demonstrates the concept of engineered local fields in nanocomposites by using metallic nanoparticles as fillers in polymer matrix for tailoring the permittivity and tunability values. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a rapid and simple method to evaluate the trapping performance of high frequency focused ultrasonic transducers for acoustic tweezer applications. The method takes into consideration the friction between the particle to be trapped and the surface that it resides on. As a result it should be more reliable and accurate than the methods proposed previously. The trapping force produced by a 70-MHz press-focused transducer was measured to evaluate the performance of this approach. This method demonstrates its potential in optimizing the excitation conditions for acoustic tweezer applications and the design of acoustic tweezers. [ABSTRACT FROM AUTHOR]

The forced transverse vibration of a single-walled carbon nanotube (SWNT) embedded with atomic-size particles was investigated using molecular dynamic simulations. The particles inside the cylindrical cantilever can be trapped near the antinodes or at the vicinity of the SWNT tip. The trapping phenomenon is highly sensitive to the external driving frequencies such that even very small changes in driving frequency can have a strong influence on the probability of the location of the particle inside the SWNT. The trapping effect could potentially be employed to realize the atomic scale control of particle position inside an SWNT via the finite adjustment of the external driving frequency. It may also be suggested that the trapping phenomenon could be utilized to develop high-sensitive mass detectors based on a SWNT resonator. [ABSTRACT FROM AUTHOR]

The remote manipulation of micro and nano-sized magnetic particles carrying molecules or biological entities over a chip surface is of paramount importance for future on-chip applications in biology and medicine. In this paper, we present a method for the on-chip demultiplexing of individual magnetic particles using bifurcated magnetic nano-conduits for the propagation of constrained domain walls (DWs). We demonstrate that the controlled injection and propagation of a domain wall in a bifurcation allow capturing, transporting, and sorting a single magnetic particle between two predefined paths. The cascade of n levels of such building blocks allows for the implementation of a variety of complex sorting devices as, e.g., a demultiplexer for the controlled sorting among 2n paths. [ABSTRACT FROM AUTHOR]

Frequency dependent absorption of a given material at distinct frequencies in the terahertz (THz) range is commonly used as a spectral fingerprint for material identification and classification. However, in the presence of strong scattering, these features can often become distorted or altered. Thus, there is an important need to understand how scattering from a sample alters the THz signal. In this letter, we propose an iterative algorithm that builds on the effective field theory proposed by P. C. Waterman and R. Truell [J. Math. Phys. 2, 512-537 (1961)] and offers a rather simple and computationally efficient method for accurately explaining the multiple scattering response of a medium. [ABSTRACT FROM AUTHOR]

We have experimentally monitored the growth rate of dust particles in a low pressure Ar/C2H2 radiofrequency discharge as a function of the gas temperature Tg and independent of the C2H radical density and the gas density. Used diagnostics are laser light scattering and measurements of the phase angle between the RF voltage and current. In contrast to most literature, we demonstrate that the growth rate is not a monotonically decreasing function of Tg but shows a maximum around Tg = 65 °C. In addition, we demonstrate that the phase angle is an accurate measure to monitor the particle growth rate. [ABSTRACT FROM AUTHOR]

We demonstrate a versatile concept for the quasi simultaneous and quantitative measurement of light extinction and scattering cross section spectra of nanoparticles in a darkfield microscope. We validate this method by the analysis of an aqueous suspension of gold nanorods and comparison with both numerical simulations and standard spectrophotometry measurements. Our approach holds the promise to allow one to map the principal optical properties of nanoparticles in a biological sample with μm spatial resolution, which is an issue of particular relevance for applications in biomedical optics such as photothermolysis and laser hyperthermia. [ABSTRACT FROM AUTHOR]

Scanning capacitance microscopy is used to characterize the electronic properties of ErAs nanoparticles embedded in GaAs pn junctions grown by molecular beam epitaxy. Voltage-dependent capacitance images reveal localized variations in subsurface electronic structure near buried ErAs nanoparticles at lateral length scales of 20-30 nm. Numerical modeling indicates that these variations arise from inhomogeneities in charge modulation due to Fermi level pinning behavior associated with the embedded ErAs nanoparticles. Statistical analysis of image data yields an average particle radius of 6-8 nm-well below the direct resolution limit in scanning capacitance microscopy but discernible via analysis of patterns in nanoscale capacitance images. [ABSTRACT FROM AUTHOR]

We report on room temperature low frequency noise due to magnetic inhomogeneities/domain walls (MI/DWs) in elliptic submicron FeCoB/MgO/FeCoB magnetic tunnel junctions with an area between 0.0245 and 0.0675 μm2. In the smaller area junctions we found an unexpected random telegraph noise (RTN1), deeply in the parallel state, possibly due to stray field induced MI/DWs in the hard layer. The second noise source (RTN2) is observed in the antiparallel state for the largest junctions. Strong asymmetry of RTN2 and of related resistance steps with current indicate spin torque acting on the MI/DWs in the soft layer at current densities below 5 × 105 A/cm2. [ABSTRACT FROM AUTHOR]

We investigate the settling of spherical particles through a pinching gap created by a cylindrical obstacle and a vertical wall. These macroscopic experiments capture the essence of pinched-flow-fractionation in microfluidics and highlight its deterministic nature. In the absence of pinching, we observe asymmetric trajectories consistent with a hard-core model of particle-obstacle repulsion that leads to separative lateral displacement. Then, we show that pinching promotes the onset of these short-range repulsion forces, amplifying the relative separation in the outgoing trajectory of different-size particles. Inertia effects, however, tend to reduce such relative separation and lead to a more complex behavior. [ABSTRACT FROM AUTHOR]

Ince-Gaussian (IG) beams are a third complete family of solutions of the paraxial Helmholtz equation. While many applications of Hermite-Gaussian and Laguerre-Gaussian beams have been demonstrated for manipulation of microparticles, the potential of the more general class of IG beams has not yet been exploited at all. We describe the unique properties of IG beams with respect to optical trapping applications, demonstrate a flexible experimental realization of arbitrary IG beams and prove the concept by creating two- and three-dimensional, highly ordered assemblies of typical microparticles. The concept is universal and can easily be integrated into existing holographic optical tweezers setups. [ABSTRACT FROM AUTHOR]

We develop a stochastic model for the charge fluctuations on a microscopic dust particle resting on a surface exposed to plasma. We find in steady state that the fluctuations are normally distributed with a standard deviation that is proportional to (CTe)1/2, where C is the particle-surface capacitance and Te is the plasma electron temperature. The time for an initially uncharged ensemble of particles to reach the steady state distribution is directly proportional to CTe. [ABSTRACT FROM AUTHOR]

We report an experimental result that the effective ion transport pressure in nanopores of a zeolite Y is strongly affected by the graphite particles outside. This phenomenon may be related to the electrification of zeolite-graphite interface, which is negligible for large pores but can be greatly amplified by the small nanopore size. [ABSTRACT FROM AUTHOR]

We demonstrated a method to accurately measure the zeta potentials and surface charges of individual particles by surface plasmon resonance microscopy (SPRM). The principle is based on the sensitive dependence of surface plasmons in a metal surface on the distance between a particle and the surface. By applying a periodic (ac) electric field to the metal surface, the charged particle oscillates, which is measured with SPRM, from which the zeta potential and the surface charge of the particle can be determined. The ac method reduces the electro-osmotic effect and noises induced by Brownian motions and allows for the rapid determination of the zeta potentials of individual particles. [ABSTRACT FROM AUTHOR]

Acoustophoretic separation in microchannels offers a promising avenue for high-throughput, label-free, cell and particle separation for many applications. However, previous acoustophoretic separation approaches have been limited to a single size separation threshold, analogous to a binary filter, (i.e., high-pass or low-pass). Here, we describe a tunable acoustophoretic separation architecture capable of sorting cells and particles based on a range of sizes, analogous to a band-pass filter. The device is capable of sorting an arbitrary range of particle sizes between 3 and 10 μm in diameter with high efficiency (transfer fraction=0.98±0.02) at a throughput of ∼108 particles/h/microchannel. [ABSTRACT FROM AUTHOR]

We describe an inertial measurement device based on an electrodynamically trapped proof mass. Mechanical constraints are replaced by guiding fields, permitting the trap stiffness to be tuned dynamically. Optical readout of the proof mass motion provides a measurement of acceleration and rotation, resulting in an integrated six degree of freedom inertial measurement device. We demonstrate such a device—constructed without microfabrication—with sensitivity comparable to that of commercial microelectromechanical systems technology and show how trapping parameters may be adjusted to increase dynamic range. [ABSTRACT FROM AUTHOR]

Janus polystyrene particles with gold and dielectric multilayer coatings on one hemisphere are shown to drastically alter the dielectrophoresis (DEP) behavior of homogeneous precursor particles under ac-electric fields. Alkanethiol coatings on the gold-coated hemisphere can effectively modify the DEP crossover frequency (ωc) with a negative (n) to positive (p) (p→n) DEP transition as increasing ac-frequency, in contrast to p→n DEP transition observed with precursor particles. All measured n→p DEP crossover frequencies with dependence on particle size, alkanethiol thickness, and medium conductivity are collapsed to yield a scaling with the resistance-capacitance (RC) time of the alkanethiol layer capacitance and the conductive medium resistance. [ABSTRACT FROM AUTHOR]

This study reveals that the Ce4+ to Ce3+ ratio in CeO2 nanoparticles (<20 nm) increases with increasing temperature under a hydrogen atmosphere. Such anomalous behavior is due to the physical effect of nanoparticle sizes on increasing the oxidation state of Ce ions in CeO2, which is stronger than the chemical effect of hydrogen reduction. This discovery offers an effective way to independently control the specific surface area and Ce3+ concentration of CeO2 nanoparticles. [ABSTRACT FROM AUTHOR]

Particle collection at low frequencies has been erstwhile demonstrated with dry particles and particles floating on the liquid surface. Nevertheless, the ability to collect suspended particles in a fluid offers arguably wider usage in the context of microfluidic or “lab-on-a-chip” systems. This is demonstrated here via an approach of vibrating a droplet to form resonant shapes on the liquid-gas interface. This results in particles ranging in size from 40 to 120 μm being collected predominantly at the solid-liquid interface due to a hydrodynamic focusing mechanism that develops through multiple cycles. [ABSTRACT FROM AUTHOR]

The effects of varying the ratio of the optical force to the viscous drag force, termed S, on cross-type optical particle separation were investigated experimentally to test previous theoretical predictions. The experiments were performed for various flow velocities, powers of the laser beam, and radii of the laser beam waist and the particles. The behaviors of the particles during optical separation were examined by measuring the retention distances and analyzing the particle trajectories. For small values of S, the particles move with constant velocity in the flow direction and the retention distance increases linearly with S. However, the particles accelerate and decelerate within the laser beam and the retention distance increases nonlinearly with S when S increases further. [ABSTRACT FROM AUTHOR]

Fluid in porous media flows through tortuous paths. If the size of the solid particles suspended in the fluid and the fluid velocities are large enough, Brownian motion effects may not be dominant. In this parameter regime, the average velocity of the particles is different than that of the fluid. We obtain the relation between the average velocity of the particles and the average velocity of the fluid in the context of a simple mathematical model and we apply our results to flows near wells. [ABSTRACT FROM AUTHOR]

We investigate the evolution of the laser-induced breakdown of a suspended micrometer-sized carbon particle at 460 torr N2 gas environment. An elliptical hot gas resulting from the ionizations of the particle breakdown and the gas breakdown is observed. We show that the deformation of the elliptical gas starts from the vertical direction and the counterpropagating flows leads to a protruding tail at later stage. The rarefaction wave is suspected to be responsible for the deformation of the gas core and the formation of the vortex ring structure, which is different from the previous laser-induced gas breakdown investigations. [ABSTRACT FROM AUTHOR]

We report the viscosity (η) and thermal conductivity (k) enhancement in a stable magnetic nanofluid containing particles size <10 nm as a function of volume fraction (vol %), shear rate, magnetic field, and time. Without field, the η enhancement of nanoparticle is much larger than that of k. In a nanofluid with 0.078 vol %, the k/η ratio is tunable from 0.725 to 2.35 by controlling the applied field. Our results show that magnetic nanofluids with tunable k/η ratio can be used as multifunctional “smart materials” for miniature cooling cum damping applications. [ABSTRACT FROM AUTHOR]

An in situ formed intermetallic phase/bulk metallic glass composite with high strength and good plasticity was fabricated by casting Zr55.0Cu29.0Ni8.0Al8.0 melts. In situ formed tetragonal structured (Zr,Ni,Al)2(Cu,Ni,Al) intermetallic particles with a hardness of 9.6±0.3 GPa improve the fracture strength of the composite. Micrographs of the fractured samples reveal that the shear band spacing is smaller than the intermetallic particles, indicating that they can effectively block shear band propagation before catastrophic fracture. [ABSTRACT FROM AUTHOR]

Nanometer scale metallic islands were manipulated on a graphite surface by placing the tip of an atomic force microscope on top of the particles. Above a certain lateral force threshold particle sliding is observed, which allows us to quantify the transition from static to kinetic friction. This transition shows hysteretic character in the force domain and is characterized by a constant ratio of kinetic versus static friction of one half. [ABSTRACT FROM AUTHOR]

Accurate estimation of the three-dimensional (3D) position of particles is critical in applications like biological imaging, atom/particle-trapping, and nanomanufacturing. While it is well-known that localization accuracy better than the Rayleigh resolution limit is possible, it was recently shown that, for photon-limited cases, 3D point spread functions (PSFs) can be shaped to increase accuracies over a 3D volume [Pavani and Piestun, Opt. Express 16, 22048 (2008)]. Here, we show that in the detector-limited regime, the gain in accuracy occurs in all three dimensions throughout the axial range of interest. The PSF is shaped as a double helix, resulting in a system with fundamentally better 3D localization accuracies than standard PSF systems, capable of achieving single-image subnanometer accuracies. [ABSTRACT FROM AUTHOR]