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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