Collective properties of cohesive frictionless granular aggregates

Abstract

In this thesis, I present my experimental work on the collective properties of frictionless, cohesive particles. Our main question is how lack of friction as well as a well-distributed, well-controlled cohesive interaction among the particles give rise to collective properties that might or might not differ from conventional granular materials with interparticle friction, and cohesion due to capillary bridges. This is a “sandwich” thesis, in which each project is presented as a standalone manuscript in a separate chapter. In Project 1, inspired by the pendant drop experiment, we extrude dense particle aggregates from an orifice. The aggregate breaks into clusters due to interparticle cohesion, much like a dripping faucet. We analyze the cluster volume while varying the cohesion, orifice size and particle size. Our results show that the volume is proportional to the orifice area multiplied by a characteristic length that balances cohesion and gravity, known as the granular capillary length. This finding indicates that the aggregate behaves more like a soft solid than a liquid, as the volume of a classic pendant drop is proportional to the orifice perimeter rather than the orifice area. In Project 2, we investigate how geometrical constraints influence the spreading of frictionless, cohesive particles. Conducting the spreading experiment in a cylinder, we unexpectedly observe the formation of a conical pile, as the angle of repose in conventional granular materials is attributed to interparticle friction. We vary the cohesive force, particle size, and cylinder size to examine how these factors affect the angle of repose. Our findings indicate that the angle of repose is proportional to the granular capillary length divided by the particle size, and remains independent of the cylinder size within the experimental range. These results underscore the significant role of cohesion and geometrical constraints in aggregate stability.