CiteULike: dchen's cool

Direct-Current Nanogenerator Driven by Ultrasonic Waves

Xudong Wang — 2007-07-25T03:43:40-00:00

Science, Vol. 316, No. 5821. (6 April 2007), pp. 102-105.

We have developed a nanowire nanogenerator that is driven by an ultrasonic wave to produce continuous direct-current output. The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric-semiconducting coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential solution for powering nanodevices and nanosystems. 10.1126/science.1139366

Progess in superhydrophobic surface development

Paul Roach — 2008-06-20T20:19:01-00:00

Soft Matter, 2008, 4, 224 - 240, DOI: 10.1039/b712575p

Research into extreme water-repellent surfaces began many decades ago, although it was only relatively recently that the term superhydrophobicity appeared in literature. Here we review the work on the preparation of superhydrophobic surfaces, with focus on the different techniques used and how they have developed over the years, with particular focus on the last two years. We discuss the origins of water-repellent surfaces, examining how size and shape of surface features are used to control surface characteristics, in particular how techniques have progressed to form multi-scaled roughness to mimic the lotus leaf effect. There are notable differences in the terminology used to describe the varying properties of water-repellent surfaces, so we suggest some key definitions.

Soft and hard nanomaterials for restoration and conservation of cultural heritage

Piero Baglioni — 2008-06-20T19:46:59-00:00

In this review we report the most recent applications of nanotechnology to the conservation and restoration of the worlds cultural heritage. Nanoparticles of humble calcium and magnesium hydroxide and carbonate can be used to restore and protect wall paints and to de-acidify paper and wood. We highlight the synthetic pathways that can be used to produce nanoparticles, and some applications for the conservation of Maya wall paints in Mexico and to the de-acidification of paper documents and wood.

Shape and erosion of pebbles

DJ Durian — 2007-03-19T13:52:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 2. (2007)

The shapes of flat pebbles may be characterized in terms of the statistical distribution of curvatures measured along their contours. We illustrate this method for clay pebbles eroded in a controlled laboratory apparatus, and also for naturally occurring rip-up clasts formed and eroded in the Mont St.-Michel bay. We find that the curvature distribution allows finer discrimination than traditional measures of aspect ratios. Furthermore, it connects to the microscopic action of erosion processes that are typically faster at protruding regions of high curvature. We discuss in detail how the curvature may be reliably deduced from digital photographs.

Aging after shear rejuvenation in a soft glassy colloidal suspension: Evidence for two different regimes

F Ianni — 2008-06-11T19:44:12-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

The aging dynamics after shear rejuvenation in a glassy clay suspension have been investigated through dynamic light scattering (DLS). Two different aging regimes are observed: one is attained if the sample is rejuvenated before its gelation and one after the rejuvenation of the gelled sample. In the first regime, the application of shear fully rejuvenates the sample, as the system dynamics soon after shear cessation follow the same aging evolution characteristic of standard aging. In the second regime, aging proceeds very fast after shear rejuvenation, and classical DLS cannot be used. An original protocol to measure an ensemble-averaged intensity-correlation function is proposed and its consistency with classical DLS is verified. The fast aging dynamics of rejuvenated gelled samples exhibit a power-law dependence of the slow relaxation time on the waiting time.

Critical packing in granular shear bands

S Fazekas — 2008-06-11T19:37:09-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

In a realistic three-dimensional setup, we simulate the slow deformation of idealized granular media composed of spheres undergoing an axisymmetric triaxial shear test. We follow the self-organization of the spontaneous strain localization process leading to a shear band and demonstrate the existence of a critical packing density inside this failure zone. The asymptotic criticality arising from the dynamic equilibrium of dilation and compaction is found to be restricted to the shear band, while the density outside of it keeps the memory of the initial packing. The critical density of the shear band depends on friction (and grain geometry) and in the limit of infinite friction it defines a specific packing state, namely the dynamic random loose packing.

Vertical ordering of rods under vertical vibration

M Ramaioli — 2008-06-10T23:33:20-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 2. (2007)

Granular media composed of elongated particles rearrange and order vertically upon vertical vibration. We perform pseudo-two-dimensional discrete element model simulations and show that this phenomenon also takes place with no help from vertical walls. We quantitatively analyze the sizes of voids forming during vibrations and consider a void-filling mechanism to explain the observed vertical ordering. Void filling can explain why short rods are less prone to align vertically than long ones. We cannot, however, explain, invoking just void filling, the existence of an optimum acceleration to promote vertical ordering and its dependence on particle length. We finally introduce an interpretation of the phenomenon, by considering the energetic barriers that particles have to overcome to exit a horizontal or a vertical lattice. By comparing these energetic thresholds with the peak mean particle fluctuant kinetic energy, we identify three different regimes. In the intermediate regime a vertical lattice is stable, while a horizontal is not. This interpretation succeeds in reconciling both dependencies on vibration acceleration and on particle length.

Lubrication effects on the flow of wet granular materials

Qing Xu — 2008-01-23T19:06:56-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We investigate the dynamics of a partially saturated grain-liquid mixture with a rotating drum apparatus. The drum is partially filled with the mixture and then rotated about its horizontal axis. We focus on the continuous avalanching regime and measure the impact of the volume fraction and viscosity of the liquid on the dynamic surface angle. The inclination angle of the surface is observed to increase sharply to a peak and then decrease as a function of liquid volume fraction. The height of the peak is observed to increase with rotation rate. For higher liquid volume fractions, the inclination angle of the surface can decrease with viscosity before increasing. The viscosity where the minimum occurs decreases with the rotation rate of the drum. Limited measurements of the flow depth were made, and these were observed to show only fractional changes with volume fraction and rotation speeds. We show that the qualitative features of our observations can be understood by analyzing the effect of lubrication forces on the time scale over which particles come in contact.

Forces between single pairs of charged colloids in aqueous salt solutions

C Gutsche — 2007-10-02T11:29:36-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

Forces between single pairs of negatively charged micrometer-sized colloids in aqueous solutions of monovalent, divalent, or trivalent counter-ions at varying concentrations have been measured by employing optical tweezers. The experimental data have been analyzed by using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and a numerical solution of the Poisson-Boltzmann (PB) equation. With monovalent counterions, the data are well described by the DLVO and PB theories, suggesting that the DLVO theory is adequate to describe the colloidal forces at these conditions. At higher counter-ion valence, the approximations within the two theories become evident.

Dune formation on the present Mars

Eric Parteli — 2007-11-14T22:22:22-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

We apply a model for sand dunes to calculate formation of dunes on Mars under the present Martian atmospheric conditions. We find that different dune shapes as those imaged by Mars Global Surveyor could have been formed by the action of sand-moving winds occuring on today's Mars. Our calculations show, however, that Martian dunes could be only formed due to the higher efficiency of Martian winds in carrying grains into saltation. The model equations are solved to study saltation transport under different atmospheric conditions valid for Mars. We obtain an estimate for the wind speed and migration velocity of barchan dunes at different places on Mars. From comparison with the shape of bimodal sand dunes, we find an estimate for the time scale of the changes in Martian wind regimes.

Capillarylike Fluctuations at the Interface of Falling Granular Jets

Yacine Amarouchene — 2008-06-05T20:44:07-00:00

Physical Review Letters, Vol. 100, No. 21. (2008)

We study the interface fluctuations of a granular jet falling under gravity and show that for small scales they are the analog of the thermally induced capillary waves. Experimental results from radial height and velocity fluctuations, static correlation functions and capillary ripple velocities allow us to estimate a granular surface tension. The ultralow interfacial tensions measured (of the order of 100 µN/m) can be rationalized using a simple model.

Ferromagnetic Microswimmers

Feodor Ogrin — 2008-06-05T20:32:23-00:00

Physical Review Letters, Vol. 100, No. 21. (2008)

We propose a model for a novel artificial low Reynolds number swimmer, based on the magnetic interactions of a pair of ferromagnetic particles: one with hard and the other with soft magnetic properties, connected by a linear spring. Using a computational model, we analyze the behavior of the system and demonstrate that for realistic values of the parameters involved, the swimmer is capable of self-propelling with average speeds of the order of hundreds of micrometers per second.

Emulsification of Partially Miscible Liquids Using Colloidal Particles: Nonspherical and Extended Domain Structures

PS Clegg — 2008-05-15T00:03:54-00:00

Langmuir, Vol. 23, No. 11. (22 May 2007), pp. 5984-5994.

Abstract: We present microscopy studies of particle-stabilized emulsions with unconventional morphologies. The emulsions comprise pairs of partially miscible fluids and are stabilized by colloids. Alcohol-oil mixtures are employed; silica colloids are chemically modified so that they have partial wettability. We create our morphologies by two distinct routes: starting with a conventional colloid-stabilized emulsion or starting in the single-fluid phase with the colloids dispersed. In the first case temperature cycling leads to the creation of extended fluid domains built around some of the initial fluid droplets. In the second case quenching into the demixed region leads to the formation of domains which reflect the demixing kinetics. The structures are stable due to a jammed, semisolid, multilayer of colloids on the liquid-liquid interface. The differing morphologies reflect the roles in formation of the arrested state of heterogeneous and homogeneous nucleation and spinodal decomposition. The latter results in metastable, bicontinuous emulsions with frozen interfaces, at least for the thin-slab samples, investigated here.

Trapping an Intensely Bright, Stable Sonoluminescing Bubble

Raúl Urteaga — 2008-05-07T23:29:17-00:00

Physical Review Letters, Vol. 100, No. 7. (2008)

Previous works on single bubble sonoluminescence in sulfuric acid solutions have stressed the fact that the sonoluminescence (SL) emissions are the highest ever found, but at the same time the bubble moves in orbits. We have fixed the SL bubble spatially and at the same time we have reached higher SL emissions using another harmonic acoustic signal to produce the acoustic excitation. Multiple harmonic excitation produces up to a fourfold increase in SL emissions, reaching the peak value of about 40 µW for a moving bubble and 15 µW for a nonmoving bubble. The ability to have a bright stationary bubble also opens new research opportunities. In particular, we develop a new method to measure the absolute radius evolution of the bubble that exploits this stability.

In Situ Observation of Fringing-Field-Induced Phase Separation in a Liquid-Crystal--Monomer Mixture

Hongwen Ren — 2008-03-24T17:17:09-00:00

Physical Review Letters, Vol. 100, No. 11. (2008)

Fringing-field-induced phase separation dynamics in liquid-crystal–(LC-)monomer mixtures is investigated via a microscope. At a low LC concentration, the fringing field converts the randomly dispersed LC droplets to an ordered droplet array, while at a high LC concentration the fringing field converts the amorphous LC-monomer system to a composite film. Because the LC and monomer are immiscible, the converted morphologies are stable even after the voltage is removed. Using the fringing field-induced phase separation, it is possible to prepare different polymer-dispersed LC morphologies.

Thermal Convection and Emergence of Isolated Vortices in Soap Bubbles

F Seychelles — 2008-05-06T21:07:23-00:00

Physical Review Letters, Vol. 100, No. 14. (2008)

A novel thermal convection cell consisting of half a soap bubble heated at the equator is introduced to study thermal convection and the movement of isolated vortices. The soap bubble, subject to stratification, develops thermal convection at its equator. A particular feature of this cell is the emergence of isolated vortices. These vortices resemble hurricanes or cyclones and similarities between our observed structures and these natural objects are found. This is brought forth through a study of the mean square displacement of these objects showing signs of superdiffusion.

Human Skin as Arrays of Helical Antennas in the Millimeter and Submillimeter Wave Range

Yuri Feldman — 2008-05-07T01:00:17-00:00

Physical Review Letters, Vol. 100, No. 12. (2008)

Recent studies of the minute morphology of the skin by optical coherence tomography showed that the sweat ducts in human skin are helically shaped tubes, filled with a conductive aqueous solution. A computer simulation study of these structures in millimeter and submillimeter wave bands show that the human skin functions as an array of low-Q helical antennas. Experimental evidence is presented that the spectral response in the sub-Terahertz region is governed by the level of activity of the perspiration system. It is also correlated to physiological stress as manifested by the pulse rate and the systolic blood pressure.

Formation of a New Dynamical Mode in alpha-Uranium Observed by Inelastic X-Ray and Neutron Scattering

ME Manley — 2008-05-05T16:32:03-00:00

Physical Review Letters, Vol. 96, No. 12. (2006)

Phonon dispersion curves were obtained from inelastic x-ray and neutron scattering measurements on -uranium single crystals at temperatures from 298 to 573 K. Both measurements showed a softening and an abrupt loss of intensity in the longitudinal optic branch along [00] above 450 K. Above the same temperature a new dynamical mode of comparable intensity emerges along the [01] zone boundary with energy near the top of the phonon spectrum. The new mode forms without a structural transition but coincides with an anomaly in the mechanical deformation behavior. We argue that the mode is an intrinsically localized vibration and formed as a result of a strong electron-phonon interaction.

Delaying Transition to Turbulence by a Passive Mechanism

Jens Fransson — 2008-05-05T16:29:56-00:00

Physical Review Letters, Vol. 96, No. 6. (2006)

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.

Optical Spin-to-Orbital Angular Momentum Conversion in Inhomogeneous Anisotropic Media

L Marrucci — 2008-05-05T16:01:46-00:00

Physical Review Letters, Vol. 96, No. 16. (2006)

We demonstrate experimentally an optical process in which the spin angular momentum carried by a circularly polarized light beam is converted into orbital angular momentum, leading to the generation of helical modes with a wave-front helicity controlled by the input polarization. This phenomenon requires the interaction of light with matter that is both optically inhomogeneous and anisotropic. The underlying physics is also associated with the so-called Pancharatnam-Berry geometrical phases involved in any inhomogeneous transformation of the optical polarization.

Soft Nanopolyhedra as a Route to Multivalent Nanoparticles

Jiunn Roan — 2008-05-05T15:43:40-00:00

Physical Review Letters, Vol. 96, No. 24. (2006)

Computer simulations show that end-grafted immiscible homopolymers can confer multivalence to nanoparticles, resulting in soft nanopolyhedra with structures identical to those found in small clusters of colloidal microspheres. Unprecedented structure tunability is demonstrated by several structure transition sequences, including a reentrant transition, induced by varying composition, polymer lengths, or grafting patterns. These results suggest a new method for fabricating nanoparticles with precisely controlled numbers and locations of functional sites (i.e., multivalent nanoparticles).

Microevaporators for Kinetic Exploration of Phase Diagrams

Jacques Leng — 2008-05-05T00:02:09-00:00

Physical Review Letters, Vol. 96, No. 8. (2006)

We use pervaporation-based microfluidic devices to concentrate species in aqueous solutions with spatial and temporal control of the process. Using experiments and modeling, we quantitatively describe the advection-diffusion behavior of the concentration field of various solutions (electrolytes, colloids, etc.) and demonstrate the potential of these devices as universal tools for the kinetic exploration of the phases and textures that form upon concentration.

Strong far-field coherent scattering of ultraviolet radiation by holococcolithophores

Quintero Torres — 2008-05-04T23:04:15-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 74, No. 3. (2006)

By considering the structure of holococcoliths (calcite plates that cover holococcolithophores, a haploid phase of the coccolithophore life cycle) as a photonic structure, we apply a discrete dipolar approximation to study the light backscattering properties of these algae. We show that some holococcolith structures have the ability to scatter the ultraviolet radiation. This property may represent an advantage for holococcolithophores possessing it, by allowing them to live higher in the water column than other coccolithophores.

Controllable Snail-Paced Light in Biological Bacteriorhodopsin Thin Film

Pengfei Wu — 2008-05-04T19:36:43-00:00

Physical Review Letters, Vol. 95, No. 25. (2005)

We observe that the group velocity of light is reduced to an extremely low value of 0.091 mm/s in a biological thin film of bacteriorhodopsin at room temperature. By exploiting unique features of a flexible photoisomerization process for coherent population oscillation, the velocity is all-optically controlled over an enormous span, from snail-paced to normal light speed, with no need of modifying the characteristics of the incident pulse. Because of the large quantum yield for the photoreaction in this biochemical system, the ultraslow light is observed even at low light levels of microwatts, indicating high energy efficiency.

Chiral Molecules Split Light: Reflection and Refraction in a Chiral Liquid

Ambarish Ghosh — 2006-11-14T23:28:28-00:00

Physical Review Letters, Vol. 97, No. 17. (2006)

A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.

Anti-Stokes Laser Cooling in Bulk Erbium-Doped Materials

Joaquin Fernandez — 2008-05-04T18:58:31-00:00

Physical Review Letters, Vol. 97, No. 3. (2006)

We report the first observation of anti-Stokes laser-induced cooling in the Er3+:KPb2Cl5 crystal and in the Er3+:CNBZn (CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.

Capillary Origami: Spontaneous Wrapping of a Droplet with an Elastic Sheet

Charlotte Py — 2008-05-04T18:50:09-00:00

Physical Review Letters, Vol. 98, No. 15. (2007)

The interaction between elasticity and capillarity is used to produce three-dimensional structures through the wrapping of a liquid droplet by a planar sheet. The final encapsulated 3D shape is controlled by tailoring the initial geometry of the flat membrane. Balancing interfacial energy with elastic bending energy provides a critical length scale below which encapsulation cannot occur, which is verified experimentally. This length is found to depend on the thickness as h3/2, a scaling favorable to miniaturization which suggests a new way of mass production of 3D micro- or nanoscale objects.

Multiferroicity Induced by Dislocated Spin-Density Waves

Joseph Betouras — 2008-05-04T00:09:06-00:00

Physical Review Letters, Vol. 98, No. 25. (2007)

We uncover a new pathway towards multiferroicity, showing how magnetism can drive ferroelectricity without relying on inversion symmetry breaking of the magnetic ordering. Our free-energy analysis demonstrates that any commensurate spin-density-wave ordering with a phase dislocation, even if it is collinear, gives rise to an electric polarization. Because of the dislocation, the electronic and magnetic inversion centers do not coincide, which turns out to be a sufficient condition for multiferroic coupling. The novel mechanism explains the formation of multiferroic phases at the magnetic commensurability transitions, such as the ones observed in YMn2O5 and related compounds. We predict that in these multiferroics an oscillating electrical polarization is concomitant with the uniform polarization. On the basis of our theory, we put forward new types of magnetic materials that are potentially ferroelectric.

Limited Path Percolation in Complex Networks

Eduardo — 2008-05-03T23:43:59-00:00

We study the stability of network communication after removal of a fraction q=1-p of links under the assumption that communication is effective only if the shortest path between nodes i and j after removal is shorter than a[script-l]ij(a>=1) where [script-l]ij is the shortest path before removal. For a large class of networks, we find analytically and numerically a new percolation transition at p-tildec=(kappa0-1)(1-a)/a, where kappa0[equivalent]<k2>/<k> and k is the node degree. Above p-tildec, order N nodes can communicate within the limited path length a[script-l]ij, while below p-tildec, Ndelta (delta<1) nodes can communicate. We expect our results to influence network design, routing algorithms, and immunization strategies, where short paths are most relevant.

Guiding superconducting vortices with magnetic domain walls

Vlasko Vlasov — 2008-05-03T16:28:20-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 13. (2008)

We demonstrate a unique prospect for inducing anisotropic vortex pinning and manipulating the directional motion of vortices by using the stripe domain patterns of a uniaxial magnetic film in the superconducting/ferromagnetic hybrid. Our observations can be described by a model, which considers interactions between magnetic charges of vortices and surface magnetic charges of domains resulting in the enhanced pinning of vortices on domain walls.

Ghost-imaging experiment by measuring reflected photons

Ron Meyers — 2008-05-03T16:01:55-00:00

Physical Review A (Atomic, Molecular, and Optical Physics), Vol. 77, No. 4. (2008)

A CCD array is placed facing a chaotic light source and gated by a photon counting detector that simply counts all randomly scattered and reflected photons from an object. A “ghost” image of the object is then observed in the gated CCD. Differing from all published ghost-imaging experiments, this setup captures ghosts from scattered and reflected light of an object, instead of the transmitted ones. This new feature is not only useful for practical applications, but is also important fundamentally. It further explores the nonclassical interference nature of thermal light ghost imaging.

Crumpled surface structures

Douglas — 2008-04-16T22:33:43-00:00

Refraction of Shear Zones in Granular Materials

Tamás Unger — 2008-03-19T17:28:17-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

We study strain localization in slow shear flow focusing on layered granular materials. A heretofore unknown effect is presented here. We show that shear zones are refracted at material interfaces in analogy with refraction of light beams in optics. This phenomenon can be obtained as a consequence of a recent variational model of shear zones. The predictions of the model are tested and confirmed by 3D discrete element simulations. We found that shear zones follow Snell's law of light refraction.

Critical Fluctuations of Tense Fluid Membrane Tubules

Jean Fournier — 2008-03-19T17:24:33-00:00

Physical Review Letters, Vol. 98, No. 1. (2007)

We show that, contrary to planar membranes under tension, tense membrane tubules exhibit important critical fluctuations originating from unidimensional Goldstone modes. The latter yield unexpected behavior, such as correlations extending over the whole tube length and the increase of the fluctuating area over the projected area with increasing tension.

Controlled Contact to a C60 Molecule

Néel — 2008-03-19T16:47:42-00:00

Red Blood Cells and Other Nonspherical Capsules in Shear Flow: Oscillatory Dynamics and the Tank-Treading-to-Tumbling Transition

JM Skotheim — 2008-03-19T16:44:43-00:00

Physical Review Letters, Vol. 98, No. 7. (2007)

We consider the motion of red blood cells and other nonspherical microcapsules dilutely suspended in a simple shear flow. Our analysis indicates that depending on the viscosity, membrane elasticity, geometry, and shear rate, the particle exhibits either tumbling, tank-treading of the membrane about the viscous interior with periodic oscillations of the orientation angle, or intermittent behavior in which the two modes occur alternately. For red blood cells, we compute the complete phase diagram and identify a novel tank-treading-to-tumbling transition as the shear rate decreases. Observations of such motions coupled with our theoretical framework may provide a sensitive means of assessing capsule properties.

Topological Changes in Bipolar Nematic Droplets under Flow

Fernández-Nieves — 2008-03-19T16:26:33-00:00

Kinetic Pathways for the Isotropic-Nematic Phase Transition in a System of Colloidal Hard Rods: A Simulation Study

Alejandro Cuetos — 2008-03-19T16:18:47-00:00

Physical Review Letters, Vol. 98, No. 9. (2007)

We study the kinetic pathways for the isotropic-to-nematic transition in a fluid of colloidal hard rods. In order to follow the formation of the nematic phase, we develop a new cluster criterion that distinguishes nematic clusters from the isotropic phase. Applying this criterion in Monte Carlo simulations, we find spinodal decomposition as well as nucleation and growth depending on the supersaturation. We determine the height of the nucleation barrier and we study the shape and structure of the cluster. More specifically, we find ellipsoidal nematic clusters with an aspect ratio of about 1.7 and a homogeneous nematic director field. Our results are consistent with theoretical predictions on the shape and director field of nematic tactoids. Classical nucleation theory gives reasonable predictions for the height of the nucleation barrier and the critical nucleus size.

Patterning Colloidal Films via Evaporative Lithography

Daniel Harris — 2008-03-19T01:18:09-00:00

Physical Review Letters, Vol. 98, No. 14. (2007)

We investigate evaporative lithography as a route for patterning colloidal films. Films are dried beneath a mask that induces periodic variations between regions of free and hindered evaporation. Direct imaging reveals that particles segregate laterally within the film, as fluid and entrained particles migrate towards regions of higher evaporative flux. The films exhibit remarkable pattern formation that can be regulated by tuning the initial suspension composition, separation distance between the mask and underlying film, and mask geometry.

Granular Rayleigh-Taylor Instability: Experiments and Simulations

Jan Vinningland — 2008-03-18T21:42:16-00:00

Physical Review Letters, Vol. 99, No. 4. (2007)

A granular instability driven by gravity is studied experimentally and numerically. The instability arises as grains fall in a closed Hele-Shaw cell where a layer of dense granular material is positioned above a layer of air. The initially flat front defined by the grains subsequently develops into a pattern of falling granular fingers separated by rising bubbles of air. A transient coarsening of the front is observed right from the start by a finger merging process. The coarsening is later stabilized by new fingers growing from the center of the rising bubbles. The structures are quantified by means of Fourier analysis and quantitative agreement between experiment and computation is shown. This analysis also reveals scale invariance of the flow structures under overall change of spatial scale.

Spontaneous Separation of Charged Grains

Amit Mehrotra — 2007-08-29T15:18:03-00:00

Physical Review Letters, Vol. 99, No. 5. (2007)

In 1867, Lord Kelvin described an experiment in which two streams of water droplets were connected so that each stream amplified the charge on the second stream [W. Thomson, Proc. R. Soc. London 16, 67 (1867).]. We present here a complementary effect in flowing grains that spontaneously separates similar and well-mixed grains into two charged streams of demixed grains. This effect has important consequences for industrial and natural processes.

Local Structure of Liquid Carbon Controls Diamond Nucleation

LM Ghiringhelli — 2008-03-18T21:12:19-00:00

Physical Review Letters, Vol. 99, No. 5. (2007)

Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process, i.e., diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that by increasing the pressure the local coordination of the liquid changes from threefold to fourfold, and we show that the free-energy cost to create a diamond-liquid interface is lower in the fourfold than in the threefold liquid. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. We conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.

Colloidal Crystal Growth at Externally Imposed Nucleation Clusters

Sven van Teeffelen — 2008-03-17T22:23:57-00:00

Physical Review Letters, Vol. 100, No. 10. (2008)

We study the conditions under which and how an imposed cluster of fixed colloidal particles at prescribed positions triggers crystal nucleation from a metastable colloidal fluid. Dynamical density functional theory of freezing and Brownian dynamics simulations are applied to a two-dimensional colloidal system with dipolar interactions. The externally imposed nucleation clusters involve colloidal particles either on a rhombic lattice or along two linear arrays separated by a gap. Crystal growth occurs after the peaks of the nucleation cluster have first relaxed to a cutout of the stable bulk crystal.