Pascale Aussillous

Quick deposition of a fluid on the wall of a tube

We are interested in the amount of fluid left behind a drop moved inside a capillary tube. Long ago, Taylor showed that for very viscous liquids moved at small velocities, the film thickness is a monotonic increasing function of the capillary number. New data obtained with liquids of low viscosity are reported here and compared with Taylor’s law. Two successive effects are observed: above a threshold in capillary number, the film is thicker than a Taylor film; at a very high speed, the deposition law becomes a decreasing function of the drop velocity. Both behaviors are analyzed thanks to scaling arguments and shown to be consequences of inertia.

Properties of liquid marbles

Liquid marbles are liquid drops made non-wetting by the use of a powder which coats them. Because of the absence of a contact line, quick motions without leakage of small amounts of liquid are allowed, which can be of interest in microfluidic applications. After characterizing the static liquid marble, we focus on its properties and study experimentally the viscous motion of liquid marbles. Then, we describe qualitatively possible ways for putting marbles into motion and quantify the robustness of this object.

Elasticity of an interfacial particle raft

We study the collective behaviour of a close-packed monolayer of non-Brownian particles at a fluid-liquid interface. Such a particle raft forms a two-dimensional elastic solid and can support anisotropic stresses and strains, e.g. it buckles in uniaxial compression and cracks in tension. We characterise this solid in terms of Youngs modulus and Poisson ratio derived from simple theoretical considerations and show the validity of these estimates by using an experimental buckling assay to deduce Youngs modulus.

Sediment dynamics. Part 1. Bed-load transport by laminar shearing flows

We propose a two-phase model having a Newtonian rheology for the fluid phase and friction for the particle phase to describe bed-load transport in the laminar viscous regime. We have applied this continuum model to sediment transport by viscous shearing flows. The equations are shown to reduce to the momentum equation for the mixture and the Brinkman equation for the fluid velocity. This modelling is able to provide a description of the flow of the mobile granular layer. At some distance from threshold of particle motion, where the continuum approach is more realistic as the mobile layer is larger than one particle diameter, there is very little slip between the two phases and the velocities inside the mobile bed have approximately a parabolic profile. When the Poiseuille (or Couette) flow is not significantly perturbed, simple analytical results of the particle flux varying cubically with the Shields number and of the bed-load thickness varying linearly with it can then be obtained. These predictions compare favourably with experimental observations of bed-load transport in pipe flows.

Determination of the critical Shields number for particle erosion in laminar flow

We present reproducible experimental measurements for the onset of grain motion in laminar flow and find a constant critical Shields number for particle erosion, i.e., θc=0.12±0.03, over a large range of small particle Reynolds number: 1.5×10−5⩽Rep⩽0.76. Comparison with previous studies found in the literature is provided.We present reproducible experimental measurements for the onset of grain motion in laminar flow and find a constant critical Shields number for particle erosion, i.e., θc=0.12±0.03, over a large range of small particle Reynolds number: 1.5×10−5⩽Rep⩽0.76. Comparison with previous studies found in the literature is provided.

Shapes of rolling liquid drops

We describe here liquid drops in rotation, and first classify the different shapes possible. Then, we study the behaviour of liquid marbles, which are droplets coated with hydrophobic grains, running down inclines. Because these marbles roll as they move, this device allows us to achieve revolving drops. We report the different forms generated during the descent, and quantify their size by way of scaling arguments. We finally focus on the transformations which may occur between different drop shapes.

Granular collapse in a fluid: Role of the initial volume fraction

The collapse of a granular column in a viscous liquid is experimentally investigated. The morphology of the deposits is shown to be mainly controlled by the initial volume fraction of the granular mass and not by the aspect ratio of the column, an observation which differs from dry granular collapse. Two different regimes are identified corresponding to initially loose and dense packings. Loose packings give rise to thin and long deposits, the dynamics being fast. A positive liquid pressure is measured below the column. For dense packings, the runout distance is twice less, the flow is slow, and a negative pore pressure is measured during the flow. These observations suggest that the dynamics of the granular collapse in a fluid is strongly affected by the dilatancy or contractancy behavior of the granular medium.

Non-Stick Droplets

Recently, many ways (sometimes inspired by nature) for achieving super-hydrophobic surfaces have been proposed in the literature. On such surfaces, water makes a contact angle close to 180°, which produces spectacular properties: droplets do not stick and the surfaces repel water, which bounces when thrown on them. In this paper, we describe a way to reach the maximum possible contact angle, namely 180°, by texturing the liquid surface instead of the solid one, as it is done for super-hydrophobic solids. It is shown that the contact between such a marble and the solid on which it is deposited is very small, which dramatically reduces the friction when these marbles move. High speeds are thus observed. Together with the fact that the marbles roll as they move, this produces spectacular changes in shape. But the marbles resist to these changes, which can be of interest for practical applications in microfluidics.

Sediment dynamics. Part 2. Dune formation in pipe flow

We present a phase diagram of the different dune patterns observed when a bed composed of spherical particles is subjected to a pipe flow. While the threshold for incipient motion is determined by the Shields number, that for dune formation seems to be controlled by the Reynolds number. A simple linear stability analysis based on a particle flux derived by Ouriemi, Aussillous & Guazzelli (J. Fluid Mech., 2009) accounts reasonably well for the experimental observations.

Steady-state mushy layers: experiments and theory

A new facility has been developed to investigate the directional solidification of transparent aqueous solutions forming mushy layers in a quasi-two-dimensional system. Experiments have been conducted on NaCl-H 2 O solutions by translating a Hele-Shaw cell at prescribed rates between fixed heat exchangers providing a temperature gradient of approximately 1° Cmm -1 . The mush-liquid interface remained planar at all freezing velocities larger than 8 μm s -1 , while steepling occurred at lower velocities. No significant undercooling of the mush-liquid interface was detected at freezing velocities up to 12 μm s -1 . Mathematical predictions of the steady-state temperature profile and mushy-layer thickness as functions of freezing rate are in excellent agreement with experimental measurements.