Frédérique Giorgiutti-Dauphiné

Modelling film flows down a fibre

Consider the gravity-driven flow of a thin liquid film down a vertical fibre. A model of two coupled evolution equations for the local film thickness h and the local flow rate q is formulated within the framework of the long-wave and boundary-layer approximations. The model accounts for inertia and streamwise viscous diffusion. Evolution equations obtained by previous authors are recovered in the appropriate limit. Comparisons to experimental results show good agreement in both linear and nonlinear regimes. Viscous diffusion effects are found to have a stabilizing dispersive effect on the linear waves. Time-dependent computations of the spatial evolution of the film reveal a strong influence of streamwise viscous diffusion on the dynamics of the flow and the wave selection process.

Dynamical Janssen effect on granular packing with moving walls.

Apparent mass (M(app)) measurements at the bottom of granular packings inside a vertical tube in relative motion are reported. They demonstrate that Janssens model is valid over a broad range of velocities v. The variability of the measurements is lower than for static packings and the theoretical exponential increase of M(app) with the height of the packing is precisely followed (the corresponding characteristic screening length is of the order of the tube diameter). The limiting apparent mass at large heights is independent of v and significantly lower than the static value.

Powder flow down a vertical pipe: the effect of air flow

The dynamics of dry granular flows down a vertical glass pipe of small diameter have been studied experimentally. Simultaneous measurements of pressure proles, air and grain flow rates and volume fractions of particles have been realized together with spatio-temporal diagrams of the grain distribution down the tube. At large grain flow rates, one observes a stationary flow characterized by high particle velocities, low particle fractions and a downflow of air resulting in an underpressure in the upper part of the pipe. A simple model assuming a free fall of the particles slowed down by air friction and taking into account nite particle fraction eects through Richardson{ Zaki’s law has been developed: it reproduces pressure and particle fraction variations with distance and estimates friction forces with the wall. At lower flow rates, sequences of high-density plugs separated by low-density bubbles moving down at a constant velocity are observed. The pressure is larger than outside the tube and its gradient reflects closely the weight of the grains. Writing mass and momentum conservation equations for the air and for the grains allows one to estimate the wall friction, which is less than 10% of the weight for grains with a clean smooth surface but up to 30% for grains with a rougher surface. At lower flow rates, oscillating-wave regimes resulting in large pressure fluctuations are observed and their frequency is predicted.

Structural anisotropy of directionally dried colloids

Aqueous colloidal dispersions of silica particles become anisotropic when they are dried through evaporation. This anisotropy is generated by a uniaxial strain of the liquid dispersions as they are compressed by the flow of water toward a solidification front. Part of the strain produced by the compression is relaxed, and part of it is stored and transferred to the solid. This stored elastic strain has consequences for the properties of the solid, where it may facilitate the growth of shear bands, and generate birefringence.

Spatial evolution of a film flowing down a fiber

We report the response to a forcing at the inlet of a film flowing down a vertical fiber. Parameters are chosen in order to examine the effects of both inertia and surface tension. The spatial response of the film to inlet forcing depends on the ratio of the forcing frequency ffor to the frequency fM corresponding to the maximum linear growth rate. At ffor≈fM, the primary instability leads directly to the formation of a saturated wavetrain at the forcing frequency, whereas at low forcing frequencies ffor<fM, this formation is preceded by a sequence of periodic coalescence events. The amplitude, speed, profiles, and inner flow pattern of traveling waves have been characterized and compared to the solutions to the two-equation model obtained by Ruyer-Quil et al. [J. Fluid Mech. 603, 431 (2008)], showing a remarkable agreement. A steepening of the waves is observed when inertia becomes dominant. An excellent correlation between data is observed when the amplitude and speed of the waves are made dimensionless...

Elapsed time for crack formation during drying

The drying of colloidal films usually leads to mechanical instabilities that affect the uniformity of the final deposit. The resulting patterns are the signature of the mechanical stress, and reveal the way the system consolidates. We report experimental results on the crack patterns induced by the drying of sessile drops of concentrated dispersions. Crack patterns exhibit a well-defined spatial order, and a regular temporal periodicity. In addition, the onset of cracking occurs after a well-defined elapsed time that depends on the mechanical properties of the gel, and on the drying kinetics. The estimation of the time elapsed before cracks form is related to the elastic properties of the material. This is supported by quantitative measurements using indentation testing and by a simple scaling law derived from poro-elastic theory.Graphical abstract

Effect of a non-volatile cosolvent on crack patterns induced by desiccation of a colloidal gel

The consolidation of colloidal gels results in enormous stresses that are usually released by the formation of undesirable cracks. The capacity of a gel network to crack during drying depends on the existence and significance of a pressure gradient in the pore liquid; in addition it depends on the way the gel relaxes the resulting drying stresses. In this paper the effect of a binary mixture of solvents saturating the gel network on the crack patterns formation is investigated. Indeed, incorporation of a small quantity of non-volatile cosolvent, i.e. glycerol, inhibits drying-induced cracks; moreover addition of a concentration greater than 10% to a colloidal dispersion leads to a crack free coating under room conditions. Mass variation with time reveals that both evaporation rate and cracking time are not affected by glycerol, in the range of added glycerol contents studied. In addition measurements of mechanical properties show that the elastic modulus is reduced with glycerol content. The decrease of the number of cracks with the glycerol content is related to the flattening of the pressure gradient in the pore liquid. The mechanism is shown to be due to the combination of two processes: flow driven by the pressure gradient and diffusion mechanisms in accordance with Scherers work (1989).

Instability and morphology of polymer solutions coating a fibre

We report an experimental study on the dynamics of a thin film of polymer solution coating a vertical fibre. The liquid film has first a constant thickness and then undergoes the Plateau‐Rayleigh instability, which leads to the formation of sequences of drops, separated by a thin film, moving down at a constant velocity. Different polymer solutions are used, i.e. xanthan solutions and polyacrylamide (PAAm) solutions. These solutions both exhibit shear-rate dependence of the viscosity, but for PAAm solutions, there are strong normal stresses in addition to the shear thinning effect. We characterize experimentally and separately the effects of these two non-Newtonian properties on the flow on the fibre. Thus, in the flat film observed before the emergence of the drops, only the shear-thinning effect plays a role, and tends to thin the film compared to the Newtonian case. The effect of the nonNewtonian rheology on the Plateau‐Rayleigh instability is then investigated through the measurements of the growth rate and the wavelength of the instability. Results are in good agreement with linear stability analysis for a shear-thinning fluid. The effect of normal stress can be taken into account by considering an effective surface tension, which tends to decrease the growth rate of the instability. Finally, the dependence of the morphology of the drops on normal stress is investigated, and a simplified model including the normal stress within the lubrication approximation provides good quantitative results on the shape of the drops.

Intermittent dry granular flow in a vertical pipe

The intermittent compact flow of glass beads in a vertical glass pipe of small diameter is studied experimentally by combining particle fraction, pressure, and air, and grain flow rates measurements with a spatiotemporal analysis of the flow. At the onset of the flow, a decompaction front is observed to propagate from the bottom to the top of the tube at a velocity much larger than that of the grains. The blockage front also propagates upward and at a still higher velocity. The decompaction induces a decreasing pressure wave strongly amplified as it propagates upward toward the top of the tube. Pressure variations of 3000 Pa or more are detected in this region while particle fraction variations are of the order of 0.02. Grain velocities during the flow period also increase strongly at the top of the tube while the corresponding fraction of total time decreases. A one-dimensional numerical model based on a simple relation between the effective acceleration of the grains and the particle fraction variations reproduces the amplification effect and provides predictions for its dependence on the permeability of the packing.

Direct observation of concentration profiles induced by drying of a 2D colloidal dispersion drop

We report experimental results on the drying of a colloidal dispersion drop in a circular thin cell. This confined geometry is well adapted to quantify concentration profiles inside the drop using fluorescence microscopy. Two stages have been identified in the drop evolution. In the first one the drop is shrinking such as if a pure drop, keeping axisymmetry. In the second one strong distortions occur and result in the appearance of a local depression at the drop surface. This process results in the spontaneous formation of a complex drop shape with both concave and convex interfaces. The influence of the interface concavity on the concentration profiles inside the drop and the drying kinetics are investigated. Particularly, concentration profiles are related to the nonuniform evaporation rate at the distorted drop surface.