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Flows involving liquid-coated grains are ubiquitous in nature (pollen capture, avalanches) and industry (air filtration, smoke-particle agglomeration, pharmaceutical mixing). In this work, three-body collisions between liquid-coated spheres are investigated experimentally using a "Stokes' cradle", which resembles the popular desktop toy known as the Newton's cradle. Surprisingly, previous work indicates that every possible outcome was observed in the wetted system except the traditional Newton's cradle (NC) outcome. Here, we are able to experimentally achieve NC via guidance from a first-principles model, which revealed that controlling the volume of the liquid bridge connecting the two target particles is the key parameter in attaining the NC outcome. By independently decreasing the volume of the liquid bridge, we not only achieved NC but also uncovered several new findings. For example, in contrast to previous work on two-body collisions, three-body experiments provide direct evidence that the fluid resistance upon rebound cannot be completely neglected due to presumed cavitation; this resistance also plays a role in two-body systems yet cannot be isolated experimentally in such systems. The herein micro-level description provides an essential foundation for macro-level descriptions of wetted granular flows., Comment: 4 pages, 5 figures, accepted into Physical Review Letters

In this work, a combination of experiments and theory is used to investigate three-body, normal collisions between solid particles with a liquid coating (i.e., "wetted" particles). Experiments are carried out using a Stokes' cradle, an apparatus inspired by the Newton's cradle desktop toy except with wetted particles. Unlike previous work on two-body systems, which may either agglomerate or rebound upon collision, four outcomes are possible in three-body systems: fully agglomerated, Newton's cradle (striker and target particle it strikes agglomerate), reverse Newton's cradle (targets agglomerate while striker separates), and fully separated. Post-collisional velocities are measured over a range of parameters. For all experiments, as the impact velocity increases, the progression of outcomes observed is fully agglomerated, reverse Newton's cradle, and fully separated. Notably, as the viscosity of the oil increases, experiments reveal a decrease in the critical Stokes number (the Stokes number that demarcates a transition from agglomeration to separation) for both sets of adjacent particles. A scaling theory is developed based on lubrication forces and particle deformation and elasticity. Unlike previous work for two-particle systems, two pieces of physics are found to be critical in the prediction of a regime map that is consistent with experiments: (i) an additional resistance upon rebound of the target particles due to the pre-existing liquid bridge between them (which has no counterpart in two-particle collisions), and (ii) the addition of a rebound criterion due to glass transition of the liquid layer at high pressure between colliding particles., Comment: 29 page, 12 figures, accepted into the Journal of Fluid Mechanics

The focus of the symposium was on multiparticle hydrodynamic interactions which lead to fluctuating motion of the particles and resulting particle migration and dispersion or diffusion. Implications of these phenomena were described for sedimentation, fluidization, suspension flows, granular flows, and fiber suspensions. Computer simulation techniques as well as experimental techniques were described. Each session had an invited leadoff talk which overviewed the session topic as well as described the speaker's own related research. Ample time for discussion was included after each talk as well as at the end of each session. The symposium started with a keynote talk on the first evening on What is so puzzling about hydrodynamic diffusion?, which set the tone for the rest of the meeting by emphasizing both recent advances and unanswered issues.

A population balance model is presented for the coarsening of the dispersed phase of liquid-liquid two-phase mixtures in microgravity due to gravity sedimentation and Marangoni migration, which lead to the collision and coalescence of droplets. The model is used to predict the evolution of the size distribution of the dispersed phase in a liquid-phase miscibility gap system, Zn-Bi, which has been used in a number of experimental microgravity processing studies in which significant phase segregation has been observed. The analysis shows that increasing the temperature gradient, gravity level, volume fraction of the dispersed phase, initial average drop radius, initial standard deviation of droplet radii, or the temperature coefficient of the interfacial tension leads to an increase in the rate of droplet growth due to collision and coalescence. Comparison of the distribution evolutions for unimodal and bimodal initial distributions shows that the latter yield significantly more rapid droplet growth. Finally, it is shown that droplet growth can be dramatically reduced with antiparallel orientation of the gravity vector and the temperature gradient, provided that the relative magnitude of these two vectors is properly chosen.