Instabilities and avalanches in 2D hopper flows of soft-rigid granular mixtures

Handling granular material flows through narrow pathways can be critically important. In the pharmaceutical industry, for instance, powder ingredients or gel tablets can clog hopper exits and halt production. Previous work suggests that soft particle flows are less prone to clogging, but they also exhibit more complex flow features [1,2]. Unlike rigid particles that either flow smoothly or clog permanently, soft particles can flow intermittently by forming temporary clogs that can be spontaneously disrupted by gravity or other driving forces. This behavior is due to the deformability of soft particles that enables them to create self-destructing contacts. We study the transition between flow and clogging in mixtures of soft and rigid granular particles in a quasi-2D hourglass-shaped rotating hopper. We study the flow dynamics of soft-rigid mixtures using different particle sizes, measuring the outflow rate, avalanche size, and the time intervals between consecutive particle exits. For each particle size, we investigate a range of mixing fractions. Preliminary results suggest that mixtures containing more rigid particles flow both faster and more smoothly. Mixtures containing more soft particles exhibit slower and oscillating outflow velocities, especially when they contain smaller particles. Soft particles may become sticky when they are sufficiently small, which makes them more subject to intermittent flowing. Interestingly, our results may provide insight into the collective dynamics that can result when two species flow at different velocities.





1. Tao, Wilson, Weeks, PRE 104, 044909 (2021)

2. Alborzi, Clark, Hashmi, Soft Matter 18, 4127 (2022).