Hamza Chraibi

Influence of contact angle on slow evaporation in two-dimensional porous media

We study numerically the influence of contact angle on slow evaporation in two-dimensional model porous media. For sufficiently low contact angles, the drying pattern is fractal and can be predicted by a simple model combining the invasion percolation model with the computation of the diffusive transport in the gas phase. The overall drying time is minimum in this regime and is independent of contact angle over a large range of contact angles up to the beginning of a transition zone. As the contact angle increases in the transition region, the cooperative smoothing mechanisms of the interface become important and the width of the liquid gas interface fingers that form during the evaporation process increases. The mean overall drying time increases in the transition region up to an upper bound which is reached at a critical contact angle thetac. The increase in the drying time in the transition region is explained in relation with the diffusional screening phenomenon associated with the Laplace equation governing the vapor transport in the gas phase. Above thetac the drying pattern is characterized by a flat traveling front and the mean overall drying time becomes independent of the contact angle. Drying time fluctuations are studied and are found to be important below thetac, i.e., when the pattern is fractal. The fluctuations are of the same order of magnitude regardless of the value of contact angle in this range. The fluctuations are found to die out abruptly at thetac as the liquid gas interface becomes a flat front.

Thermocapillary flows and interface deformations produced by localized laser heating in confined environment

The deformation of a fluid-fluid interface due to the thermocapillary stress induced by a continuous Gaussian laser wave is investigated analytically. We show that the direction of deformation of the liquid interface strongly depends on the viscosities and the thicknesses of the involved liquid layers. We first investigate the case of an interface separating two different liquid layers while a second part is dedicated to a thin film squeezed by two external layers of same liquid. These results are predictive for applications fields where localized thermocapillary stresses are used to produce flows or to deform interfaces in presence of confinement, such as optofluidics.

Simulation of an optically induced asymmetric deformation of a liquid–liquid interface

Deformations of liquid interfaces by the optical radiation pressure of a focused laser wave were generally expected to display similar behavior, whatever the direction of propagation of the incident beam. Recent experiments showed that the invariance of interface deformations with respect to the direction of propagation of the incident wave is broken at high laser intensities. In the case of a beam propagating from the liquid of smaller refractive index to that of larger one, the interface remains stable, forming a nipple-like shape, while for the opposite direction of propagation, an instability occurs, leading to a long needle-like deformation emitting micro-droplets. While an analytical model successfully predicts the equilibrium shape of weakly deformed interface, very few work has been accomplished in the regime of large interface deformations. In this work, we use the Boundary Integral Element Method (BIEM) to compute the evolution of the shape of a fluid-fluid interface under the effect of a continuous laser wave, and we compare our numerical simulations to experimental data in the regime of large deformations for both upward and downward beam propagation. We confirm the invariance breakdown observed experimentally and find good agreement between predicted and experimental interface hump heights below the instability threshold.

Optohydrodynamics of soft fluid interfaces: Optical and viscous nonlinear effects

Abstract.Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state.

Universal morphologies of fluid interfaces deformed by the radiation pressure of acoustic or electromagnetic waves

We unveil the generation of universal morphologies of fluid interfaces by radiation pressure regardless of the nature of the wave, whether acoustic or optical. Experimental observations reveal interface deformations endowed with steplike features that are shown to result from the interplay between the wave propagation and the shape of the interface. The results are supported by numerical simulations and a quantitative interpretation based on the waveguiding properties of the field is provided.

Eddies and interface deformations induced by optical streaming

We study flows and interface deformations produced by the scattering of a laser beam propagating through non absorbing turbid fluids. Light scattering produces a force density resulting from the transfer of linear momentum from the laser to the scatterers. The flow induced in the direction of the beam propagation, called “optical streaming“, is also able to deform the interface separating the two liquid phases and to produce wide humps. The viscous flow taking place in these two liquid layers is solved analytically, in one of the two liquid layers with a stream function formulation, as well as numerically in both fluids using a Boundary Integral Element Method. Quantitative comparisons are shown between the numerical and analytical flow patterns. Moreover, we present predictive simulations dedicated to the effects of the geometry, of the scattering strength and of the viscosities, on both the flow pattern and the deformation of the interface. Theoretical arguments are finally put forth to explain the robustness of the emergence of secondary flows in a two-layer fluid system.

Convection flows driven by laser heating of a liquid layer.

When a fluid is heated by the absorption of a continuous laser wave, the fluid density decreases in the heated area. This induces a pressure gradient that generates internal motion of the fluid. Due to mass conservation, convection eddies emerge in the sample. To investigate these laser-driven bulk flows at the microscopic scale, we built a setup to perform temperature measurements with a fluorescent-sensitive dye on the one hand, and measured the flow pattern at different beam powers, using a particle image velocimetry technique on the other hand. Temperature measurements were also used in numerical simulations in order to compare predictions to the experimental velocity profiles. The combination of our numerical and experimental approaches allows a detailed description of the convection flows induced by the absorption of light, which reveals a transition between a thin and a thick liquid layer regime. This supports the basis of optothermal approaches for microfluidic applications.

Sedimentation of granular columns in the viscous and weakly inertial regimes.

We investigate the dynamics of granular columns of point particles that interact via long-range hydrodynamic interactions and fall under the action of gravity. We investigate the influence of inertia using the Greens function for the Oseen equation. The initial conditions (density and aspect ratio) are systematically varied. Our results suggest that universal self-similar laws may be sufficient to characterize the temporal and structural evolution of the granular columns. A characteristic time above which an instability is triggered (which may enable the formation of clusters) is also retrieved and discussed.

Excitation of fountain and entrainment instabilities at the interface between two viscous fluids using a beam of laser light.

We report on two instabilities, called viscous fountain and viscous entrainment, triggered at the interface between two liquids by the action of bulk flows driven by a laser beam. These streaming flows are due to light scattering losses in turbid liquids, and can be directed either toward or forward the interface. We experimentally and numerically investigate these interface instabilities and show that the height and curvature of the interface deformation at the threshold and the jet radius after interface destabilization mainly depend on the waist of the laser beam. Analogies and differences between these two instabilities are characterized.