Optohydrodynamics of soft fluid interfaces : Optical and viscous nonlinear effects

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

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 regarding the effects of the geometry, of the scattering strength and of the viscosities, on both the flow pattern and the deformation of the interface. Finally, theoretical arguments are put forth to explain the robustness of the emergence of secondary flows in a two-layer fluid system

Thermocapillary valve for droplet production and sorting

Droplets are natural candidates for use as microfluidic reactors, if active control of their formation and transport can be achieved. We show here that localized heating from a laser can block the motion of a water-oil interface, acting as a microfluidic valve for two-phase flows. A theoretical model is developed to explain the forces acting on a drop due to thermocapillary flow, predicting a scaling law which favors miniaturization. Finally, we show how the laser forcing can be applied to sorting drops, thus demonstrating how it may be integrated in complex droplet microfluidic systems.Comment: Five pages, four figure

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

Bistabilité d'une surface liquide induite par la pression de radiation acoustique et application à l'atténuation d'ondes capillaires

En focalisant une onde acoustique sur l'interface entre deux fluides, on déforme cette interface par pression de radiation. Si l'interface est totalement transparente, la déformation de l'interface croît linéairement avec l'énergie acoustique incidente. Si l'interface est totalement réfléchissante, l'onde acoustique est confinée dans la cavité limitée par la surface de l'émetteur et l'interface fluide, impliquant un comportement bistable de cette interface. Ceci se traduit par une hystérésis de la hauteur de la déformation en fonction de la fréquence ou de l'amplitude de l'onde incidente. Un modèle à une dimension d'un résonateur de Fabry-Pérot déformable permet de reproduire fidèlement ce comportement. Ce phénomène ouvre d'intéressantes perspectives en terme de contrôle d'interfaces, comme l'atténuation d'ondes capillaires par la cavité acoustique

Liquid Transport Due to Light Scattering

Using experiments and theory, we show that light scattering by inhomogeneities in the index of refraction of a fluid can drive a large-scale flow. The experiment uses a near-critical, phase-separated liquid, which experiences large fluctuations in its index of refraction. A laser beam traversing the liquid produces a large-scale deformation of the interface and can cause a liquid jet to form. We demonstrate that the deformation is produced by a scattering-induced flow by obtaining good agreements between the measured deformations and those calculated assuming this mechanism.Comment: 4 pages, 5 figures, submitted to Physical Review Letters v2: Edited based on comments from referee

Simulation numérique de l'hydrodynamique d'interfaces liquide-liquide contrôlées par laser

Le contrôle de la déformation sans contact des interfaces liquides à l'échelle micro-métrique est un enjeu majeur pour toute une série d'applications en micro-fluidique. Une technique originale récemment développée au Laboratoire CPMOH (Bordeaux), consiste à employer la pression de radiation d'une onde laser continue, pour déformer des interfaces liquides à l'échelle du micron. Cela conduit pour de faibles intensité, à des formes en cloche indépendantes du sens de propagation du faisceau puis pour des intensités plus élevées, à des formes surprenantes de tétines quand le faisceau se propage du milieu le plus réfringeant vers le moins réfringeant, ou de jet de micro-gouttes pour une direction de propagation inverse. Afin de mieux comprendre la physique de ces écoulements et en maîtriser les applications, nous avons developpé un outil numérique basé sur la méthode des élements de frontière couplant à la fois l'hydrodynamique et l'électromagnétisme. Les résultats numériques obtenus avec cette méthode sont comparés aux résultats expérimentaux en régime de déformation linéaire et non-linéaire et ceci pour les deux sens de propagation

Contrôle tout-optique du cycle de la vie d'une gouttelette en microcanal

La formation et le transport d'une gouttelette dans un microcanal sont généralement imposés par la géométrie figée de ce microcanal. Pour surmonter cette contrainte, nous présentons une nouvelle approche basée sur la génération par laser de contraintes thermocapillaires localisées. Nous parvenons ainsi à contrôler sans contact ni microfabrication dédiée le débit et la taille des gouttelettes émises (vanne et calibreur), à trier ces gouttes (aiguillage), à les forcer optiquement à coalescer (mélangeur) ou au contraire à forcer la rupture en deux gouttes filles de taille contrôlée (diviseur). Utilisés en combinaison, ces nouveaux microcomposants hydrauliques élémentaires ouvrent la voie vers une approche tout-optique du laboratoire sur puce

Stretching and squeezing of sessile dielectric drops by the optical radiation pressure

We study numerically the deformation of sessile dielectric drops immersed in a second fluid when submitted to the optical radiation pressure of a continuous Gaussian laser wave. Both drop stretching and drop squeezing are investigated at steady state where capillary effects balance the optical radiation pressure. A boundary integral method is implemented to solve the axisymmetric Stokes flow in the two fluids. In the stretching case, we find that the drop shape goes from prolate to near-conical for increasing optical radiation pressure whatever the drop to beam radius ratio and the refractive index contrast between the two fluids. The semi-angle of the cone at equilibrium decreases with the drop to beam radius ratio and is weakly influenced by the index contrast. Above a threshold value of the radiation pressure, these "optical cones" become unstable and a disruption is observed. Conversely, when optically squeezed, the drop shifts from an oblate to a concave shape leading to the formation of a stable "optical torus". These findings extend the electrohydrodynamics approach of drop deformation to the much less investigated "optical domain" and reveal the openings offered by laser waves to actively manipulate droplets at the micrometer scale