Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Christine M. Hrenya

Second Advisor

Paul D. Beale

Third Advisor

Robert H. Davis

Abstract

The addition of a small amount of liquid to a granular system can dramatically change the flow dynamics including the flowability, tensile strength, and segregation. Such liquid-coated particles coated are common in nature (e.g. avalanches, pollen capture) and in industry (e.g. granulation, particle filtration). Despite their ubiquity, predicting macro-scale (bulk) flows of liquid-coated particles is still elusive. A microscale (particle-level) investigation of the interactions of few wetted particles will lead to the identification of the dominant physical mechanisms which feeds into the understanding and modeling of bulk flows of wetted systems.

Previous micro-scale studies of wetted particles have included experimental and theoretical efforts to study particle-wall collisions (both oblique and head-on) and particle-particle collisions (head-on only). Before using such micro-scale models to describe macro-scale flows, more general cases need to be first considered such as collisions between more than two particles and the rotational motion of agglomerates. The goal of this work is to address these two issues through a combination of experiments using a pendulum apparatus and theory.

To investigate collisions between more than two particles, this work focuses on the normal (head-on) collision of three spheres. The foundation for such work is provided by first investigating analogous dry (non-wetted) systems. Experimental results are compared to soft-sphere models, which simultaneously account for all collisions, and a hard-sphere model, which treats the three-body collision as a series of two-body collisions. While the soft-sphere models generally predicts the post-collisional velocities better, the hard-sphere model exhibits a good comparison overall.

In the wetted three-particle collisions, the pendulum apparatus is coined Stokes’s cradle for the Stokes flow in the liquid gap between the particles. In two-body collisions, only two outcomes exist, namely stick and bounce. But in three-particle collisions, four possible geometrical outcomes exist and using the model as a guide, all four outcomes are experimentally observed. Furthermore, this combination of experiments and theory led to the identification of the dominant physical mechanisms. First, due to the large pressures in the liquid gap, the fluid may undergo a glass transition at which point the particles reverse direction. Additionally, previous theories neglect the viscous resistance of the fluid as the particles move away from one another, since cavitation was assumed to occur. However, three-body experiments show definitively that the outbound resistance cannot be neglected.

To investigate how rotational motion influences agglomeration, oblique collisions between two particles are performed. Whereas in normal collisions particles rebound only due to solid deformation, so-called centrifugal forces in oblique collisions produce a new outcome in which the particles initially form a rotating agglomerate, and then de-agglomerate at a later time. Furthermore, capillary forces play an essential role in oblique collisions even when the capillary number (viscous over capillary forces) is high. This recognition leads to the introduction of a dimensionless number, the centrifugal number (centrifugal over capillary forces), which together with the previously established Stokes number characterizes the regime map of outcomes for two-particle collisions.

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