Actuating water droplets on liquid infused surfaces: A rickshaw for droplets

We investigate the dynamics of millimeter-sized droplets moved on a liquid infused surface. The motion of the droplet is driven by a small spherical bead, whose trajectory is precisely controlled, which acts as a carrier. We first characterize the strength of the contact that maintains the adhesion between the droplet and the bead as a function of the ratio R/r of their radii. When the bead is moved at a fixed velocity, the droplet follows its trajectory until a critical value of the velocity is reached at which the bead and the droplet lose contact. The critical velocity is rationalized as a balance between the capillary contact force and the friction acting on the droplet where it is in contact with the substrate. Experimental results are in good agreement with the model proposed. This study highlights a very efficient actuation method for millimetric droplets.