Optical blocking of microfluidic droplets through laser-induced thermocapillarity

The localized heating produced by a tightly focused infrared laser leads to surface tension gradients at the interface of microfluidic drops, resulting in a net force on the drop whose origin and magnitude are the focus of this paper. First, by co-localization of the surfactant micelles with a fluorescent dye, we demonstrate that the heating alters their spatial distribution, driving the interface out of thermodynamic equilibrium. This soluto-capillary effect opposes and overcomes the purely thermal dependence of the surface tension, leading to anomalous Marangoni flows. This sets the interface into motion and creates recirculation rolls outside and inside the drop, which we measure using time-resolved micro-Particle Image Velocimetry. Second, the net force produced on the drop is measured to be in the range of a few hundred nN by using an original microfluidic design. This micro-dynanometer further shows that the magnitude of the heating, which is determined by the laser power and its absorption in the water, sets the magnitude of the net force on the drop. On the other hand, the dynamics of the force generation is determined by the time scale for heating which is independently measured to be tau_Theta = 4 ms. This time scale sets the maximum velocity that the drops can have and still be blocked, by requiring that the interface pass the laser spot in a time longer than tau_Theta. The maximum velocity is measured at U^max = 0.7 mm/s for our geometric conditions. Finally, a simple model is derived that describes the blocking force in a confined geometry as the result of the viscous stresses produced between the drop and the lateral walls.