Capillary-like Fluctuations of a Solid-Liquid Interface in a Non-Cohesive Granular System

TL;DR

This study investigates the fluctuations of a solid-liquid interface in a vibrated granular system, revealing capillary-like behavior and energy equipartition despite the system being far from equilibrium.

Contribution

It demonstrates that granular interface fluctuations can be described by capillary wave theory, allowing measurement of surface tension and mobility in a non-cohesive granular system.

Findings

Interface fluctuations follow capillary wave theory

Energy equipartition observed at the interface

Surface tension and mobility estimates align with atomic systems

Abstract

One of the most noticeable collective motion of non-cohesive granular matter is clustering under certain conditions. In particular, when a quasi-two-dimensional monolayer of mono-disperse non-cohesive particles is vertically vibrated, a solid-liquid-like transition occurs when the driving amplitude exceeds a critical value. Here, the physical mechanism underlying particle clustering relies on the strong interactions mediated by grain collisions, rather than on grain-grain cohesive forces. In average, the solid cluster resembles a drop, with a striking circular shape. We experimentally investigate the coarse-grained solid-liquid interface fluctuations, which are characterized through the static and dynamic correlation functions in the Fourier space. These fluctuations turn out to be well described by the capillary wave theory, which allows us to measure the solid-liquid interface surface tension and mobility once the granular "thermal" kinetic energy is determined. Despite the system is strongly out of equilibrium and that the granular temperature is not uniform, there is energy equipartition at the solid-liquid interface, for a relatively large range of angular wave-numbers. Furthermore, both surface tension and mobility are consistent with a simple order of magnitude estimation considering the characteristic energy, length and time scales, which is very similar to what can be done for atomic systems. Please find the supplementary material of this article here: arXiv:1304.2646