Influence of the electron-phonon interfacial conductance on the thermal transport at metal/dielectric interfaces

Thermal boundary conductance at a metal-dieletric interface is a quantity of prime importance for heat management at the nanoscale. While the boundary conductance is usually ascribed to the coupling between metal phonons and dielectric phonons, in this work we examine the influence of a direct coupling between the metal electrons and the dielectric phonons. The effect of electron- phonon processes is generally believed to be resistive, and tends to decrease the overall thermal boundary conductance as compared to the phonon-phonon conductance {\sigma}p . Here, we find that the effect of a direct coupling {\sigma}e is to enhance the effective thermal conductance, between the metal and the dielectric. Resistive effects turn out to be important only for thin films of metals having a low electron-phonon coupling strength. Two approaches are explored to reach these conclusions. First, we present an analytical solution of the two-temperature model to compute the effective conductance which account for all the relevant energy channels, as a function of {\sigma}e , {\sigma}p and the electron-phonon coupling factor G. Second, we use numerical resolution to examine the influence of {\sigma}e on two realistic cases: gold film on silicon or silica substrates. We point out the implications for the interpretation of time-resolved thermoreflectance experiments

A temperature-controlled device for volumetric measurements of Helium adsorption in porous media

We describe a set-up for studying adsorption of helium in silica aerogels, where the adsorbed amount is easily and precisely controlled by varying the temperature of a gas reservoir between 80 K and 180 K. We present validation experiments and a first application to aerogels. This device is well adapted to study hysteresis, relaxation, and metastable states in the adsorption and desorption of fluids in porous media

Aggregation-fragmentation and individual dynamics of active clusters

International audienceA remarkable feature of active matter is the propensity to self-organize. One striking instance of this ability to generate spatial structures is the cluster phase, where clusters broadly distributed in size constantly move and evolve through particle exchange, breaking or merging. Here we propose an exhaustive description of the cluster dynamics in apolar active matter. Exploiting large statistics gathered on thousands of Janus colloids, we measure the aggregation and fragmentation rates and rationalize the resulting cluster size distribution and fluctuations. We also show that the motion of individual clusters is entirely consistent with a model positing random orientation of colloids. Our findings establish a simple, generic model of cluster phase, and pave the way for a thorough understanding of clustering in active matter

Helium condensation in aerogel: avalanches and disorder-induced phase transition

We present a detailed numerical study of the elementary condensation events (avalanches) associated to the adsorption of $^4$He in silica aerogels. We use a coarse-grained lattice-gas description and determine the nonequilibrium behavior of the adsorbed gas within a local mean-field analysis, neglecting thermal fluctuations and activated processes. We investigate the statistical properties of the avalanches, such as their number, size and shape along the adsorption isotherms as a function of gel porosity, temperature, and chemical potential. Our calculations predict the existence of a line of critical points in the temperature-porosity diagram where the avalanche size distribution displays a power-law behavior and the adsorption isotherms have a universal scaling form. The estimated critical exponents seem compatible with those of the field-driven Random Field Ising Model at zero temperature.Comment: 16 pages, 14 figure

Gas adsorption/desorption in silica aerogels: a theoretical study of scattering properties

We present a numerical study of the structural correlations associated to gas adsorption/desorption in silica aerogels in order to provide a theoretical interpretation of scattering experiments. Following our earlier work, we use a coarse-grained lattice-gas description and determine the nonequilibrium behavior of the adsorbed gas within a local mean-field analysis. We focus on the differences between the adsorption and desorption mechanisms and their signature in the fluid-fluid and gel-fluid structure factors as a function of temperature. At low temperature, but still in the regime where the isotherms are continuous, we find that the adsorbed fluid density, during both filling and draining, is correlated over distances that may be much larger than the gel correlation length. In particular, extended fractal correlations may occur during desorption, indicating the existence of a ramified cluster of vapor filled cavities. This also induces an important increase of the scattering intensity at small wave vectors. The similarity and differences with the scattering of fluids in other porous solids such as Vycor are discussed.Comment: 16 pages, 15 figure

Level-set simulations of a 2D topological rearrangement in a bubble assembly: effects of surfactant properties

International audienceA liquid foam is a dispersion of gas bubbles in a liquid matrix containing surface active agents. Their flow involves the relative motion of bubbles, which switches neighbours during a so-called topological rearrangement of type 1 (T1). The dynamics of T1 events, as well as foam rheology, have been extensively studied, and experimental results point to the key role played by surfactants in these processes. However, the complex and multiscale nature of the system has so far impeded a complete understanding of the mechanisms at stake. In this work, we investigate numerically the effect of surfactants on the rheological response of a 2D sheared bubble cluster. To do so, a level-set method previously employed for simulating two-phase flow has been extended to include the effects of the surfactants. The dynamical processes of the surfactants-diffusion in the liquid and along the interface, adsorption/desorption at the interface-and their coupling with the flow-surfactant advection and Laplace and Marangoni stresses at the interface-are all taken into account explicitly. Through a systematic study in Biot, capillary and Péclet numbers which characterise the surfactant properties in the simulation, we find that the presence of surfactants can affect the liquid/gas hydrodynamic boundary condition (from a rigid-like situation to a mobile one), which modifies the nature of the flow in the volume from a purely extensional situation to a shear. Furthermore, the work done by surface tension (the 2D analogue of the work by pressure forces), resulting from surfactant and interface dynamics, can be interpreted as an effective dissipation, which reaches a maximum for Péclet number of order unity. Our results, obtained at high liquid fraction, should provide a reference point, to which experiments and models of T1 dynamics and foam rheology can be compared

Local mean-field study of capillary condensation in silica aerogels

We apply local mean-field (i.e. density functional) theory to a lattice model of a fluid in contact with a dilute, disordered gel network. The gel structure is described by a diffusion-limited cluster aggregation model. We focus on the influence of porosity on both the hysteretic and the equilibrium behavior of the fluid as one varies the chemical potential at low temperature. We show that the shape of the hysteresis loop changes from smooth to rectangular as the porosity increases and that this change is associated to disorder-induced out-of-equilibrium phase transitions that differ on adsorption and on desorption. Our results provide insight in the behavior of $^4$He in silica aerogels.Comment: 19 figure

Probing helium interfaces with light scattering : from fluid mechanics to statistical physics

We have investigated the formation of helium droplets in two physical situations. In the first one, droplets are atomised from superfluid or normal liquid by a fast helium vapour flow. In the second, droplets of normal liquid are formed inside porous glasses during the process of helium condensation. The context, aims, and results of these experiments are reviewed, with focus on the specificity of light scattering by helium. In particular, we discuss how, for different reasons, the closeness to unity of the index of refraction of helium allows in both cases to minimise the problem of multiple scattering and obtain results which it would not be possible to get using other fluids.Comment: 21 page

Run and tumble particle under resetting:a renewal approach

We consider a particle undergoing run and tumble dynamics, in which its velocity stochastically reverses, in one dimension. We study the addition of a Poissonian resetting process occurring with rate $r$. At a reset event the particle's position is returned to the resetting site $X_r$ and the particle's velocity is reversed with probability $\eta$. The case $\eta = 1/2$ corresponds to position resetting and velocity randomization whereas $\eta =0$ corresponds to position-only resetting. We show that, beginning from symmetric initial conditions, the stationary state does not depend on $\eta$ i.e. it is independent of the velocity resetting protocol. However, in the presence of an absorbing boundary at the origin, the survival probability and mean time to absorption do depend on the velocity resetting protocol. Using a renewal equation approach, we show that the the mean time to absorption is always less for velocity randomization than for position-only resetting.Comment: 16 pages, 1 figure, version accepted in Journal of Physics