arXiv subjects

Thomas Frisch

Publications and source records attributed to Thomas Frisch.

13 recordsLinked to original sources

arXiv2026

Superflows around corners

We investigate analytically and numerically the dynamics of a two-dimensional superflow governed by the Gross-Pitaevskii equation passing over finite-size rectangular obstacles: an impenetrable wall and an impenetrable rectangular well. Extending classical studies of vortex nucleation around smooth obstacles, we focus on the role of sharp corners in determining the onset of vortex nucleation. Using a combination of analytical techniques based on the Schwarz-Christoffel methods for potential flow and on numerical simulations, we show that local velocity amplification near sharp corners crucially controls the critical flow velocity for vortex nucleation. For both wall and well configurations, we identify analytically and theoretically the critical velocities as a function of the obstacle width and its height or depth, finding an excellent agreement between the theory and our numerical simulations. Our results provide a simple framework for understanding superflow stability past finite-size obstacles with sharp features and are directly relevant to experimentally realizable configurations in atomic Bose-Einstein condensates and related superfluid systems.

cond-mat.quant-gas↗

arXiv2017

Dynamics of elastically strained islands in presence of an anisotropic surface energy

The equilibrium solutions and coarsening dynamics of strained semi-conductor islands are investigated analytically and numerically. We develop an analytical model to study the effect of surface energy anisotropy on the dynamics coarsening of islands. We propose a simple model to explain the effect of this anisotropy on the coarsening time. We find that the anisotropy slows down the coarsening. This effect is rationalised using a quasi-analytical description of the island profile.

cond-mat.soft↗

arXiv2016

Shape and coarsening dynamics of strained islands

We investigate the formation and the coarsening dynamics of islands in a strained epitaxial semi-conductor film. These islands are commonly observed in thin films undergoing a morphological instability due to the presence of the elasto capillary effect. We first describe both analytically and numerically the formation of an equilibrium island using a two dimensional continuous model. We have found that these equilibrium island-like solutions have a maximum height $h_0$ and they sit on top of a flat wetting layer with a thickness $h_w$. We then consider two islands and we report that they undergo a non-interrupted coarsening that follows a two stage dynamics. The first stage may be depicted by a quasi-static dynamics, where the mass transfers are proportional to the chemical potential difference of the islands. It is associated with a time scale $t_c$ that is function of the distance $d$ between the islands, and leads to the shrinkage of the smallest island. Once its height becomes smaller than a minimal equilibrium height $h_{0}^{*}$, its mass spreads over the entire system. Our results pave the way for a future analysis of coarsening of an assembly of islands.

cond-mat.mtrl-sci↗

arXiv2013

Two dimensional Leidenfrost Droplets in a Hele Shaw Cell

We experimentally and theoretically investigate the behavior of Leidenfrost droplets inserted in a Hele-Shaw cell. As a result of the confinement from the two surfaces, the droplet has the shape of a flattened disc and is thermally isolated from the surface by the two evaporating vapor layers. An analysis of the evaporation rate using simple scaling arguments is in agreement with the experimental results. Using the lubrication approximation we numerically determine the shape of the droplets as a function of its radius. We furthermore find that the droplet width tends to zero at its center when the radius reaches a critical value. This prediction is corroborated experimentally by the direct observation of the sudden transition from a flattened disc into an expending torus. Below this critical size, the droplets are also displaying capillary azimuthal oscillating modes reminiscent of a hydrodynamic instability.

physics.flu-dyn↗

arXiv2013

Room temperature water Leidenfrost droplets

We experimentally investigate the Leidenfrost effect at pressures ranging from 1 to 0.05 atmospheric pressure. As a direct consequence of the Clausius-Clapeyron phase diagram of water, the droplet temperature can be at ambient temperature in a non-sophisticated lab environment. Furthermore, the lifetime of the Leidenfrost droplet is significantly increased in this low pressure environment. The temperature and pressure dependance of the evaporation rate are successfully tested against a recently proposed model. These results may pave a way to reach efficient Leidenfrost micro-fluidic and milli-fluidic applications.

cond-mat.soft↗

arXiv2012

The Leidenfrost effect: from quasi-spherical droplets to puddles

In the framework of the lubrication approximation, we derive a set of equations describing the steady bottom profile of Leidenfrost drops coupled with the vapor pressure. This allows to derive scaling laws for the geometry of the concave bubble encapsulated between the drop and the hot plate under it. The results agree with experimental observations in the case of droplets with radii smaller than the capillary length Rc as well as in the case of puddles with radii larger than Rc.

physics.flu-dyn↗

arXiv2012

Take-off of small Leidenfrost droplets

We put in evidence the unexpected behaviour of Leidenfrost droplets at the later stage of their evaporation. We predict and observe that, below a critical size $R_l$, the droplets spontaneously take-off due to the breakdown of the lubrication regime. We establish the theoretical relation between the droplet radius and its elevation. We predict that the vapour layer thickness increases when the droplets become smaller. A satisfactory agreement is found between the model and the experimental results performed on droplets of water and of ethanol.

cond-mat.soft↗

arXiv2009

Dynamics and ordering of the nucleationless island formation in heteroepitaxy

We study the morphological evolution of strained islands in growing crystal films by use of a continuum description including wetting, elasticity and deposition flux. Wetting breaks translational invariance, allowing the flux to tune different nonlinear regimes. Increasing the flux, we find first an annealing-like dynamics, then a slower but non-conventional ripening followed by a steady regime, while the island density continuously increases. The islands develop spatial correlations and ordering with a narrow two-peaked distance distribution and ridge-like clusters of islands for high flux.

cond-mat.mes-hall↗

arXiv2007

Nonlinear evolution of a morphological instability in a strained epitaxial film

A strained epitaxial film deposited on a deformable substrate undergoes a morphological instability relaxing the elastic energy by surface diffusion. The nonlinear and nonlocal dynamical equations of such films with wetting interactions are derived and solved numerically in two and three dimensions. Above some critical thickness, the surface evolves towards an array of islands separated by a wetting layer. The island chemical potential decreases with its volume, so that the system experiences a non-interrupted coarsening described by power laws with a marked dimension dependence.

cond-mat.stat-mech↗

arXiv2007

On the role of step-flow advection during electromigration-induced step bunching

We propose a one-dimensional model based on the Burton-Cabrera-Frank equations to describe the electromigration-induced step bunching instability on vicinal surfaces. The step drift resulting from atomic evaporation and/or deposition is explicitly included in our model. A linear stability analysis reveals several stability inversions as the evaporation rate varies, while a deposition flux is shown to have a stabilizing effect.

cond-mat.mtrl-sci↗

arXiv2006

Electromigration-Induced Step Meandering on Vicinal Surfaces: Nonlinear Evolution Equation

We study the effect of a constant electrical field applied on vicinal surfaces such as the Si$(111)$ surface. An electrical field parallel to the steps induces a meandering instability with a nonzero phase shift. Using the Burton-Cabrera-Frank model, we extend the linear stability analysis performed by Liu, Weeks and Kandel (Phys. Rev. Lett. {\bf 81}, p.2743, 1998) to the nonlinear regime for which the meandering amplitude is large. We derive an amplitude equation for the step dynamics using a highly nonlinear expansion method. We investigate numerically two limiting regimes (small and large attachment lengths) which both reveal long-time coarsening dynamics.

cond-mat.mtrl-sci↗

arXiv2006

Nonlinear evolution of the step meandering instability of a growing crystal surface

The growth of crystal surfaces, under non-equilibrium conditions, involves the displacement of mono-atomic steps by atom diffusion and atom incorporations into steps. The time-evolution of the growing crystal surface is thus governed by a free boundary value problem [known as the Burton--Cabrera--Franck model]. In the presence of an asymmetry of the kinetic coefficients [Erlich--Schwoebel barriers], ruling the rates of incorporation of atoms at each step, it has been shown that a train of straight steps is unstable to two dimensional transverse perturbations. This instability is now known as the Bales-Zangwill instability (meandering instability). We study the non-linear evolution of the step meandering instability that occurs on a crystalline vicinal surface under growth, in the absence of evaporation, in the limit of a weak asymmetry of atom incorporation at the steps. We derive a nonlinear amplitude equation displaying spatiotemporal coarsening. We characterize the self-similar solutions of this equation.

cond-mat.mtrl-sci↗

arXiv2006

Effect of step stiffness and diffusion anisotropy on the meandering of a growing vicinal surface

We study the step meandering instability on a surface characterized by the alternation of terraces with different properties, as in the case of Si(001). The interplay between diffusion anisotropy and step stiffness induces a finite wavelength instability corresponding to a meandering mode. The instability sets in beyond a threshold value which depends on the relative magnitudes of the destabilizing flux and the stabilizing stiffness difference. The meander dynamics is governed by the conserved Kuramoto-Sivashinsky equation, which display spatiotemporal coarsening.

cond-mat.stat-mech↗