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.

We report an experimental study of a "gas" of inelastically colliding particles, excited by vibrations in low gravity. In the case of a dilute granular medium, we observe a spatially homogeneous gaslike regime, the pressure of which scales like the 3͞2 power of the vibration velocity. When the density of the medium is increased, the spatially homogeneous fluidized state is no longer stable but displays the formation of a motionless dense cluster surrounded by low particle density regions. 81.70.Ha, 83.10.Pp, 83.70.Fn Vibrated granular media display striking fluidlike properties: convection and heaping [1,2], period doubling instabilities [3], and parametric extended [4] or localized [5] surface waves. When the vibration is strong enough, the granular medium undergoes a transition to a fluidized state. It looks like a gas of particles that can be described using kinetic theory [6]. The "granular temperature," i.e., the mean kinetic energy per particle, is determined by the balance between the power input due to the external vibration and dissipation by inelastic collisions. Fluidization by vibrations has been studied experimentally [7,8] and numerically [8,9], but no agreement has been found so far for the dependence of the granular temperature on the amplitude and the frequency of external vibrations [10][11][12].One of the most interesting properties of such "granular gases" is the tendency to form clusters. Although this has probably been known since the early observation of planetary rings [13], there exist only a few recent laboratory experiments. One experiment, with a horizontally shaken two-dimensional layer of particles, displayed a cluster formation, but the coherent friction force acting on all the particles was far from being negligible [14]. We performed a similar three-dimensional experiment in the laboratory and observed clustering, but we could not rule out the possibility of a resonance mechanism between the time of flight under gravity and the excitation frequency [15]. Various cluster types in granular flows have also been observed numerically [16]. The mechanisms of cluster formation are an active subject of research that still deserves more study because of its relevance to technical, astrophysical [17], or geophysical [18] applications of granular media. At a more fundamental level, it is of a primary interest to understand the new qualitative behaviors due to inelasticity of collisions, i.e., nonconservation of energy, in kinetic theory.In this Letter, we report a study of the kinetic regimes of a granular medium, fluidized by vibrating its container in a low gravity environment. The motivation for low gravity is to achieve an experimental situation in which inelastic collisions are the only interaction mechanism. The aim of the experiment is to observe new phenomena which result from the inelasticity of the collisions and are thus absent in a usual gas. In the dilute case, we show that the pressure of a granular gas scales like the 3͞2 power of the vibration velocity. When the density o...

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The development of microfluidic devices is still hindered by the lack of robust fundamental building blocks that constitute any fluidic system. An attractive approach is optical actuation because light field interaction is contactless and dynamically reconfigurable, and solutions have been anticipated through the use of optical forces to manipulate microparticles in flows. Following the concept of "optical chip" advanced from the optical actuation of suspensions, we propose in this survey new routes to extend this concept to microfluidic two-phase flows. First, we investigate the destabilization of fluid interfaces by the optical radiation pressure and the formation of liquid jets. We analyze the droplet shedding from the jet tip and the continuous transport in laser-sustained liquid channels. In a second part, we investigate a dissipative light-flow interaction mechanism consisting in heating locally two immiscible fluids to produce thermocapillary stresses along their interface. This opto-capillary coupling is implemented in adequate microchannel geometries to manipulate two-phase flows and propose a contactless optical toolbox including valves, droplet sorters and switches, droplet dividers or droplet mergers. Finally, we discuss radiation pressure and opto-capillary effects in the context of the "optical-chip" where flows, channels and operating functions would all be performed optically on the same device.

We report on the direct experimental observation of laser-induced flows in isotropic liquids that scatter light. We use a droplet microemulsion in the two-phase regime, which behaves like a binary mixture. Close to its critical consolute line, the microemulsion undergoes large refractive index fluctuations that scatter light. The radiation pressure of a laser beam is focused onto the soft interface between the two phases of the microemulsion and induces a cylindrical liquid jet that continuously emits droplets. We demonstrate that this dripping phenomenon takes place as a consequence of a steady flow induced by the transfer of linear momentum from the optical field to the liquid due to light scattering. We first show that the cylindrical jet guides light as a step-index liquid optical fiber whose core diameter is self-adapted to the light itself. Then, by modelling the light-induced flow as a low-Reynolds-number, parallel flow, we predict the dependence of the dripping flow rate on the thermophysical properties of the microemulsion and the laser beam power. Satisfying agreement is found between the model and experiments.

We used thermocapillary stresses induced locally by laser on flowing drops to build high throughput drop switchers and sorters for digital microfluidics. Since the laser is disconnected to the chip, the method does not require dedicated micropatterning. We show switching efficiencies of 100% for drop velocities up to 1.3 cm s , demonstrate the involved mechanism and apply laser switching for sorting droplets of different nature for lab-on-a-chip applications.

We experimentally study the deformations of liquid-liquid interfaces induced by a high-intensity focused ultrasonic beam. We quantitatively verify that small-amplitude deformations of a transparent chloroform-water interface are well described by the theory of Langevin acoustic radiation pressure, in both static and dynamic regimes. The large-amplitude deformations depend on the direction of propagation of the beam and are qualitatively similar to those induced by electromagnetic radiation pressure.

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 interface deformation on the scale of the experimental setup 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.