Strain-induced interface reconstruction in epitaxial heterostructures

We investigate in the framework of Landau theory the distortion of the strain fields at the interface of two dissimilar ferroelastic oxides that undergo a structural cubic-to-tetragonal phase transition. Simple analytical solutions are derived for the dilatational and the order parameter strains that are globally valid over the whole of the heterostructure. The solutions reveal that the dilatational strain exhibits compression close to the interface which may in turn affect the electronic properties in that region.Comment: 7 pages, 5 figures, to be published in Physical Review

Wrinkles, folds and plasticity in granular rafts

We investigate the mechanical response of a compressed monolayer of large and dense particles at a liquid-fluid interface: a granular raft. Upon compression, rafts first wrinkle; then, as the confinement increases, the deformation localizes in a unique fold. This characteristic buckling pattern is usually associated to floating elastic sheets and as a result, particle laden interfaces are often modeled as such. Here, we push this analogy to its limits by comparing the first quantitative measurements of the raft morphology to a theoretical continuous elastic model of the interface. We show that although powerful to describe the wrinkle wavelength, the wrinkle-to-fold transition and the fold shape, this elastic description does not capture the finer details of the experiment. We describe an unpredicted secondary wavelength, a compression discrepancy with the model and a hysteretic behavior during compression cycles, all of which are a signature of the intrinsic discrete and frictional nature of granular rafts. It suggests also that these composite materials exhibit both plastic transition and jamming dynamics.Comment: 10 pages, including Supplementary Information. Submitted to Physical Review Material

Compression under pressure: physiological and methodological factors influencing the effect of compression garments on running economy

Evidence for the effects of compression garments on sports performance and physiological responses to dynamic exercise remains equivocal. Contradictory findings within the sporting literature are confounded by methodological heterogeneity in terms of; intensity and modality of exercise, type of garment worn, and the interface pressure produced by the garment. The interface pressure applied by compression clothing is an important measure in evaluating the bio-physical impact of compression. Interface pressure values obtained in vivo with two portable pressure devices (PicoPress and Kikuhime) were compared against a reference standard (HOSY). The PicoPress satisfied the a priori thresholds for acceptable validity at the posterior and lateral orientation with calf stockings and tights, confirming its future use to assess interface pressure. A small, likely beneficial improvement in running economy was observed with correctly fitted (95%:5%:0%; η2 = 0.55) but not oversized compression tights, indicating that a certain level of interface pressure is required. Compression tights improved running economy only at higher relative exercise intensities (77.7 - 91.5% V̇O2max). The absence of any improvement at lower intensities (67.1 - 77.6 % V̇O2max) suggest that changes in running economy from compression are dependent on relative exercise intensity when V̇O2max (%) is used as an anchor of exercise intensity. Comparing measures from two portable, wireless near-infrared spectroscopy (NIRS) devices (PortaMon and MOXY) we found that the low-cost and light-weight MOXY device gave tissue oxygen saturation values at rest and during exercise that were physiologically credible and suitable for future research. Compression tights did affect ground contact time but not tissue oxygen saturation, cardiovascular or other kinematic parameters during running at intensities equivalent to long-distance race speed. Compression tights can produce small improvements in running economy, but effects are restricted to higher intensity exercise and appear dependent on garment interface pressure. It remains unlikely that this small positive effect on running economy, in very specific conditions, is enough to result in a meaningful impact on running performance

A quasi-steady modeling approach of a heat pump water heater system with experimental validation and analysis

In a heat pump water heater system, the dynamic interdependence between the vapor compression system and the water in the storage tank poses a modeling challenge for steady-state refrigeration system models. The aim of this project is to develop a model and methodology for coupling steady-state vapor compression system modeling with a dynamic condenser cooling medium. Consequently, a quasi-steady transient model is developed involving a steady-state vapor compression system model in Engineering Equation Solver (EES) and a computational fluid dynamics (CFD) model in ANSYS Fluent for simulating the heat transfer and fluid dynamics in the water tank. The respective models are connected at the interface of the tank wall and the water in the storage tank. The linked modeling process, described in the manuscript, involves iteration between the CFD model of the water tank and the vapor compression system model around a quasi-steady warm up of a heat pump water heating system. The model is developed around a physical heat pump water heater system. An experimental investigation is also conducted to aid with model validation

Membrane mechanics as a probe of ion-channel gating mechanisms

The details of conformational changes undergone by transmembrane ion channels in response to stimuli, such as electric fields and membrane tension, remain controversial. We approach this problem by considering how the conformational changes impose deformations in the lipid bilayer. We focus on the role of bilayer deformations in the context of voltage-gated channels because we hypothesize that such deformations are relevant in this case as well as for channels that are explicitly mechanosensitive. As a result of protein conformational changes, we predict that the lipid bilayer suffers deformations with a characteristic free-energy scale of 10kBT. This free energy is comparable to the voltage-dependent part of the total gating energy, and we argue that these deformations could play an important role in the overall free-energy budget of gating. As a result, channel activity will depend upon mechanical membrane parameters such as tension and leaflet thickness. We further argue that the membrane deformation around any channel can be divided into three generic classes of deformation that exhibit different mechanosensitive properties. Finally, we provide the theoretical framework that relates conformational changes during gating to tension and leaflet thickness dependence in the critical gating voltage. This line of investigation suggests experiments that could discern the dominant deformation imposed upon the membrane as a result of channel gating, thus providing clues as to the channel deformation induced by the stimulus

Lipid aggregate formation at an oscillating bubble surface: A simulation study

We perform a molecular dynamics simulation study of the behavior of a lipid coating layer on an oscillating bubble surface. Micrometer sized bubbles, stabilized with a lipid monolayer coating, are used in acoustic imaging as a contrast agent. The coating layer is expected to be strongly influenced by the oscillation of the bubble in the high frequency sound field, with a period of a microsecond. The typical time scale of molecular motion, however, is of the order of femtoseconds. One of the challenges is to bridge this nine decade gap in time scales. To this end we have developed a model that is highly coarse grained, but still features the essential mechanisms determining lipid dynamics, with time scales of picoseconds. This approach allows us to severely restrict the computing times, although we make use of very modest computing equipment. We show in our simulation that the amphiphilic monolayer folds upon contraction of the bubble, and forms micellar aggregates at the air-water interface. Some micellar structures survive consecutive re-expansion and indeed remain persistent over several cycles. These structures may add to the anisotropic behavior of the bubbles under oscillating conditions. We also investigated temperature and frequency dependenc

Forced desorption of nanoparticles from an oil-water interface.

While nanoparticle adsorption to fluid interfaces has been studied from a fundamental standpoint and exploited in application, the reverse process, that is, desorption and disassembly, remains relatively unexplored. Here we demonstrate the forced desorption of gold nanoparticles capped with amphiphilic ligands from an oil-water interface. A monolayer of nanoparticles is allowed to spontaneously form by adsorption from an aqueous suspension onto a drop of oil and is subsequently compressed by decreasing the drop volume. The surface pressure is monitored by pendant drop tensiometry throughout the process. Upon compression, the nanoparticles are mechanically forced out of the interface into the aqueous phase. An optical method is developed to measure the nanoparticle area density in situ. We show that desorption occurs at a coverage that corresponds to close packing of the ligand-capped particles, suggesting that ligand-induced repulsion plays a crucial role in this process. © 2011 American Chemical Society

Deformation of Silica Aerogel During Fluid Adsorption

Aerogels are very compliant materials - even small stresses can lead to large deformations. In this paper we present measurements of the linear deformation of high porosity aerogels during adsorption of low surface tension fluids, performed using a Linear Variable Differential Transformer (LVDT). We show that the degree of deformation of the aerogel during capillary condensation scales with the surface tension, and extract the bulk modulus of the gel from the data. Furthermore we suggest limits on safe temperatures for filling and emptying low density aerogels with helium.Comment: 8 pages, 5 figures, submitted to PR