arXiv subjects

Lydéric Bocquet

Publications and source records attributed to Lydéric Bocquet.

64 records · Page 4Linked to original sources

arXiv2011

Inhomogeneous shear flows in soft jammed materials with tunable attractive forces

We perform molecular dynamics simulations to characterize the occurrence of inhomogeneous shear flows in soft jammed materials. We use rough walls to impose a simple shear flow and study the athermal motion of jammed assemblies of soft particles, both for purely repulsive interactions and in the presence of an additional short-range attraction of varying strength. In steady state, pronounced flow inhomogeneities emerge for all systems when the shear rate becomes small. Deviations from linear flow are stronger in magnitude and become very long-lived when the strength of the attraction increases, but differ from permanent shear-bands. Flow inhomogeneities occur in a stress window bounded by the dynamic and static yield stress values. Attractive forces enhance the flow heterogeneities because they accelerate stress relaxation, thus effectively moving the system closer to the yield stress regime where inhomogeneities are most pronounced. The present scenario for understanding the effect of particle adhesion on shear localization, which is based on detailed molecular dynamics simulations with realistic particle interactions, differs qualitatively from previous qualitative explanations and ad-hoc theoretical modelling.

cond-mat.soft↗

arXiv2007

Polymer and surface roughness effects on the drag crisis for falling spheres

We make time resolved velocity measurements of steel spheres in free fall through liquid using a continuous ultrasound technique. We explore two different ways to induce large changes in drag on the spheres: 1) a small quantity of viscoelastic polymer added to water and 2) altering the surface of the sphere. Low concentration polymer solutions and/or a pattern of grooves in the sphere surface induce an early drag crisis, which may reduce drag by more than 50 percent compared to smooth spheres in pure water. On the other hand, random surface roughness and/or high concentration polymer solutions reduce drag progressively and suppress the drag crisis. We also present a qualititative argument which ties the drag reduction observed in low concentration polymer solutions to the Weissenberg number and normal stress difference.

physics.flu-dyn↗

arXiv2006

Flow boundary conditions from nano- to micro-scales

The development of microfluidic devices has recently revived the interest in "old" problems associated with transport at, or across, interfaces. As the characteristic sizes are decreased, the use of pressure gradients to transport fluids becomes problematic, and new, interface driven, methods must be considered. This has lead to new investigations of flow near interfaces, and to the conception of interfaces engineered at various scales to reduce flow friction. In this review, we discuss the present theoretical understanding of flow past solid interfaces at different length scales. We also briefly discuss the corresponding phenomenon of heat transport, and the influence of surface slip on interface driven (e.g. electro-osmotic) flows.

cond-mat.mtrl-sci↗

arXiv1999

Influence of Wetting Properties on Diffusion in a Confined Fluid

We briefly discuss how the wetting properties of a fluid/solid interface can indirectly influence the diffusion properties of fluid confined between two solid walls. This influence is related to the variability of the hydrodynamic boundary conditions at the interface, which correlates to the wetting properties.

cond-mat.soft↗

arXiv1999

Nucleation Theory for Capillary Condensation

This paper is devoted to the thermally activated dynamics of the capillary condensation. We present a simple model which enables us to identify the critical nucleus involved in the transition mechanism. This simple model is then applied to calculate the nucleation barrier from which we can obtain informations on the nucleation time. These results are compared to the numerical simulation of a Landau-Ginzburg model for the liquid-vapor interface combined with a Langevin dynamics.

cond-mat.stat-mech↗

arXiv1998

Influence of wetting properties on the hydrodynamic boundary condition at a fluid-solid interface

It is well known that, at a macroscopic level, the boundary condition for a viscous fluid at a solid wall is one of "no-slip". The liquid velocity field vanishes at a fixed solid boundary. In this paper, we consider the special case of a liquid that partially wets the solid, i.e. a drop of liquid in equilibrium with its vapor on the solid substrate has a finite contact angle. Using extensive Non-Equilibrium Molecular Dynamics (NEMD) simulations, we show that when the contact angle is large enough, the boundary condition can drastically differ (at a microscopic level) from a "no-slip" condition. Slipping lengths exceeding 30 molecular diameters are obtained for a contact angle of 140 degrees, characteristic of Mercury on glass. On the basis of a Kubo expression for $δ$, we derive an expression for the slipping length in terms of equilibrium quantities of the system. The predicted behaviour is in very good agreement with the numerical results for the slipping length obtained in the NEMD simulations. The existence of large slipping lentgh may have important implications for the transport properties in nanoporous media under such "nonwetting" conditions.

cond-mat.soft↗

arXiv1997

Phenomenological theory of friction in the quasistatic limit : collective pinning and memory effects

When a elastic body is moved quasistatically back and forth over a surface, the friction of the interface is experimentally observed to circulate through a hysteretic loop. The asymptotic behaviour of the hysteresis loop is approached exponentially. We describe how this behaviour is connected to the collective properties of the elastic instabilities suffered by the elastic body as it is displaced quasistatically. We express the length scale of the exponential in terms of the elasticity of the surface and the properties of the rough substrate. The predicted scalings are confirmed numerically.

cond-mat.stat-mech↗

arXiv1996

Microscopic Derivation of Non-Markovian Thermalization of a Brownian Particle

In this paper, the first microscopic approach to the Brownian motion is developed in the case where the mass density of the suspending bath is of the same order of magnitude as that of the Brownian (B) particle. Starting from an extended Boltzmann equation, which describes correctly the interaction with the fluid, we derive systematicaly via the multiple time-scale analysis a reduced equation controlling the thermalization of the B particle, i.e. the relaxation towards the Maxwell distribution in velocity space. In contradistinction to the Fokker-Planck equation, the derived new evolution equation is non-local both in time and in velocity space, owing to correlated recollision events between the fluid and particle B. In the long-time limit, it describes a non-markovian generalized Ornstein-Uhlenbeck process. However, in spite of this complex dynamical behaviour, the Stokes-Einstein law relating the friction and diffusion coefficients is shown to remain valid. A microscopic expression for the friction coefficient is derived, which acquires the form of the Stokes law in the limit where the mean-free in the gas is small compared to the radius of particle B.

cond-mat↗

arXiv1996

Binary friction tensor for Brownian particles : overcoming spurious finite-size effects

Starting from a careful analysis of the coupled Langevin equations for two interacting Brownian particles, we derive a method for extracting the binary friction tensor from the correlation function matrix of the instantaneous forces exerted by the bath particles on the fixed Brownian particles, and from the relaxation of the total momentum of the bath in a {\it finite} system. The general methodology, which circumvents the pitfalls associated with the inversion of the thermodynamic and long time limits, is applied to the case of two Brownian hard spheres in a bath of light spheres.

cond-mat↗

arXiv1996

High friction limit of the Kramers equation : the multiple time-scale approach

The purpose of the paper is to give a pedagogical introduction to the multiple time-scale technique, on the example of the high friction limit of the Kramers equation. We begin with a discussion of the standard perturbation technique as presented in van Kampen's reference book \refto{VK}, which will be shown to fail in the long-time limit. Application of the multiple time-scale technique avoids these difficulties and leads to a uniform expansion in powers of the inverse of the friction. Analogy with the Chapman-Enskog expansion is discussed.

cond-mat↗