Water spring: a model for bouncing drops

– It has been shown that a water drop can bounce persistently, when thrown on a super-hydrophobic substrate. We present here scaling arguments which allow us to predict the maximal deformation and the contact time of the drop. This approach is completed by a model describing the flow inside the drop, and by original experimental data. Introduction. – A liquid ball is a drop which remains quasi-spherical when brought into contact with a solid surface. Different examples of such objects have recently been described: let us quote pearl drops, which result from the extreme hydrophobicity of the solid [1, 2, 3, 4], liquid marbles, achieved by texturing the surface of the liquid [5, 6, 7], and (more classically) Leidenfrost drops, obtained by putting a small amount of volatile liquid on a very hot plate [8]. In such cases, quick transportation of tiny amounts of liquid becomes possible without any leak, which can be of great interest in microfluidics applications. At the same time these systems realize pure capillary devices, and are worth being studied for their original properties. For example, a liquid ball (with a typical diameter 1 mm) impinging onto a solid substrate bounces off, as if it were a tennis ball on the ground [9,10]. The rebounds are persistent (the restitution coefficient can be very high, of the order of 0.9), and observed in a large window of impact velocity. If the velocity is too small (typically smaller than a few centimeters per second), the drop gets stuck on the substrate. If too large (above around 1 m/s), the drop endures extreme deformation during the shock, which results in breaking it into several pieces. In between, a more detailed analysis of the shock itself shows that the contact time of the drop with the substrate does not depend on the impact velocity V over a large interval of velocity [11]. (∗) E-mail: [email protected] (∗∗) E-mail: [email protected] (∗∗∗) E-mail: [email protected]