Swelling cholesteric liquid crystal shells to direct colloids at the interface

Cholesteric liquid crystals can exhibit spatial patterns in molecular alignment at interfaces that can be exploited for particle assembly. These patterns emerge from the competition between bulk and surface energies, tunable with the system geometry. In this work, we use the osmotic swelling of cholesteric double emulsions to assemble colloidal particles through a pathway-dependent process. Particles can be repositioned from a surface-mediated to an elasticity-mediated state through dynamically thinning the cholesteric shell at a rate comparable to that of colloidal adsorption. By tuning the balance between surface and bulk energies with the system geometry, colloidal assemblies on the cholesteric interface can be molded by the underlying elastic field to form linear aggregates. The transition of adsorbed particles from surface regions with homeotropic anchoring to defect regions is accompanied by a reduction in particle mobility. The arrested assemblies subsequently map out and stabilize topological defects. These results demonstrate the kinetic arrest of interfacial particles within definable patterns by regulating the energetic frustration within cholesterics. This work highlights the importance of kinetic pathways for particle assembly in liquid crystals, of relevance to optical and energy applications.Comment: 21 pages, 9 figures total, including 4 supplemental figure

Electric winds driven by time oscillating corona discharges

We investigate the formation of steady gas flows—so-called electric winds—created by point-plane corona discharges driven by time oscillating (ac) electric fields. By varying the magnitude and frequency of the applied field, we identify two distinct scaling regimes: (i) a low frequency (dc) regime and (ii) a high frequency (ac) regime. These experimental observations are reproduced and explained by a theoretical model describing the transport and recombination of ions surrounding the discharge and their contribution to the measured wind velocity. The two regimes differ in the spatial distribution of ions and in the process by which ions are consumed. Interestingly, we find that ac corona discharges generate strong electric forces localized near the tip of the point electrode, while dc corona discharges generate weaker forces distributed throughout the interelectrode region. Consequently, the velocity of the electric winds (>1 m/s) generated by ac discharges is largely independent of the position of the counter electrode. The unified theoretical description of dc and ac electric winds presented here reconciles previous observations of winds driven by dc corona and ac dielectric barrier discharges; insights from the model should also prove useful in the design of other plasma actuators.Chemistry and Chemical Biolog

Externally Applied Electric Fields up to 1.6 × 10 5 V/m Do Not Affect the Homogeneous Nucleation of Ice in Supercooled Water

The freezing of water can initiate at electrically conducting electrodes kept at a high electric potential or at charged electrically insulating surfaces. The microscopic mechanisms of these phenomena are unknown, but they must involve interactions between water molecules and electric fields. This paper investigates the effect of uniform electric fields on the homogeneous nucleation of ice in supercooled water. Electric fields were applied across drops of water immersed in a perfluorinated liquid using a parallel-plate capacitor; the drops traveled in a microchannel and were supercooled until they froze due to the homogeneous nucleation of ice. The distribution of freezing temperatures of drops depended on the rate of nucleation of ice, and the sensitivity of measurements allowed detection of changes by a factor of 1.5 in the rate of nucleation. Sinusoidal alternation of the electric field at frequencies from 3 to 100 kHz prevented free ions present in water from screening the electric field in the bulk of drops. Uniform electric fields in water with amplitudes up to (1.6 ± 0.4) × 105 V/m neither enhanced nor suppressed the homogeneous nucleation of ice. Estimations based on thermodynamic models suggest that fields in the range of 107−108 V/m might cause an observable increase in the rate of nucleation.Chemistry and Chemical Biolog

AC Electric Fields Drive Steady Flows in Flames

We show that time-oscillating electric fields applied to plasmas present in flames create steady flows of gas. Ions generated within the flame move in the field and migrate a distance δ before recombining; the net flow of ions away from the flame creates a time-averaged force that drives the steady flows observed experimentally. A quantitative model describes the response of the flame and reveals how δ decreases as the frequency of the applied field increases. Interestingly, above a critical frequency, ac fields can be used to manipulate flames at a distance without the need for proximal electrodes.Chemistry and Chemical BiologyEngineering and Applied Science

Bubbles navigating through networks of microchannels

This paper describes the behavior of bubbles suspended in a carrier liquid and moving within microfluidic networks of different connectivities. A single-phase continuum fluid, when flowing in a network of channels, partitions itself among all possible paths connecting the inlet and outlet. The flow rates along different paths are determined by the interaction between the fluid and the global structure of the network. That is, the distribution of flows depends on the fluidic resistances of all channels of the network. The movement of bubbles of gas, or droplets of liquid, suspended in a liquid can be quite different from the movement of a single-phase liquid, especially when they have sizes slightly larger than the channels, so that the bubbles (or droplets) contribute to the fluidic resistance of a channel when they are transiting it. This paper examines bubbles in this size range; in the size range examined, the bubbles are discrete and do not divide at junctions. As a consequence, a single bubble traverses only one of the possible paths through the network, and makes a sequence of binary choices (“left” or “right”) at each branching intersection it encounters. We designed networks so that, at each junction, a bubble enters the channel into which the volumetric flow rate of the carrier liquid is highest. When there is only a single bubble inside a network at a time, the path taken by the bubble is, counter-intuitively, not necessarily the shortest or the fastest connecting the inlet and outlet. When a small number of bubbles move simultaneously through a network, they interact with one another by modifying fluidic resistances and flows in a time dependent manner; such groups of bubbles show very complex behaviors. When a large number of bubbles (sufficiently large that the volume of the bubbles occupies a significant fraction of the volume of the network) flow simultaneously through a network, however, the collective behavior of bubbles—the fluxes of bubbles through different paths of the network—can resemble the distribution of flows of a single-phase fluid.Chemistry and Chemical Biolog

Prior mucosal exposure to heterologous cells alters the pathogenesis of cell-associated mucosal feline immunodeficiency virus challenge

<p>Abstract</p> <p>Background</p> <p>Several lines of research suggest that exposure to cellular material can alter the susceptibility to infection by HIV-1. Because sexual contact often includes exposure to cellular material, we hypothesized that repeated mucosal exposure to heterologous cells would induce an immune response that would alter the susceptibility to mucosal infection. Using the feline immunodeficiency virus (FIV) model of HIV-1 mucosal transmission, the cervicovaginal mucosa was exposed once weekly for 12 weeks to 5,000 heterologous cells or media (control) and then cats were vaginally challenged with cell-associated or cell-free FIV.</p> <p>Results</p> <p>Exposure to heterologous cells decreased the percentage of lymphocytes in the mucosal and systemic lymph nodes (LN) expressing L-selectin as well as the percentage of CD4+ CD25+ T cells. These shifts were associated with enhanced ex-vivo proliferative responses to heterologous cells. Following mucosal challenge with cell-associated, but not cell-free, FIV, proviral burden was reduced by 64% in cats previously exposed to heterologous cells as compared to media exposed controls.</p> <p>Conclusions</p> <p>The pathogenesis and/or the threshold for mucosal infection by infected cells (but not cell-free virus) can be modulated by mucosal exposure to uninfected heterologous cells.</p