47 research outputs found
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
ContrĂ´le 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