Intermittent flow and clogging of mixed soft-rigid granular particles in a 2D hopper

From cereal, rice and drug tablets, to sand, snow and rocks, granular materials are ubiquitous in agriculture, food, pharmaceuticals and the environment. Understanding the clogging behavior and dynamics of such materials holds great importance for improving transport and mitigating clogging in industrial flows and predicting catastrophic avalanches. Many uniform systems are well studied, as well as polydisperse sizes and shapes. We study the flow and clogging behavior of heterogeneous granular materials through the narrow exit of a 2D hopper. Here, heterogeneity refers to both size and mechanical properties of spherical particles: one species is rigid and frictional; the other is soft and frictionless. We measure clogging probability as a function of particle size and mixing fraction. The size of rigid, frictional particles impacts clogging probability much more than the size of soft, frictionless ones. Even a small fraction of soft, frictionless particles can greatly mitigate clogging. By investigating the shape of the blocking arch, we find the variability of arch geometry correlates with clogging probability. Intermittent flow and avalanches emerge when soft particles are present. Surprisingly, the identity of the first-falling particle and its neighbors is irrelevant. Instead, the maximum bond angle between successive particles in the arch determines the first-falling particle. Further studies can extend these results to determine the relative importance of both softness and friction.