Fragmentation of granular droplets on impact.

Abstract

The spreading dynamics of a liquid droplet impacting a solid surface depend on the surface tension of the liquid and the dissipation phenomena at work in the droplet. In this study, we investigate the final state after the impact of a granular droplet in a pendular state, i.e., a droplet composed of solid grains held together by liquid capillary bridges, where the dissipation and fragmentation processes during impact are not well understood. We measure the total surface area and perimeter of all the fragments formed after impact, as seen from above. We investigate how the final surface area and perimeter of the granular droplet after impact depend on the impact speed, grain diameter, and the properties of the interstitial liquid. Our observations show that the final perimeter and surface area of the granular droplets can be rationalized by scaling laws expressed in terms of a Weber number based on the diameter of grains that constitute the droplet. Compared with the impact of a pure liquid droplet, the impact of a granular droplet is more prone to fragmentation, which affects the evolution of the final perimeter with the Weber number. In addition, we study the number of fragments and their size distribution as a function of the Weber number and observe similarities with the fragmentation of pure liquid droplets.