Ultraslow Settling Kinetics of Frictional Cohesive Powders

K Nan and RS Hoy, PHYSICAL REVIEW LETTERS, 130, 166102 (2023).

DOI: 10.1103/PhysRevLett.130.166102

Using discrete element method simulations, we show that the settling of frictional cohesive grains under ramped-pressure compression exhibits strong history dependence and slow dynamics that are not present for grains that lack either cohesion or friction. Systems prepared by beginning with a dilute state and then ramping the pressure to a small positive value Pfinal over a time tau ramp settle at packing fractions given by an inverse-logarithmic rate law, phi settled(tau ramp) = phi settled(infinity) + A=1 + B ln(1 + tau ramp=tau slow). This law is analogous to the one obtained from classical tapping experiments on noncohesive grains, but crucially different in that tau slow is set by the slow dynamics of structural void stabilization rather than the faster dynamics of bulk densification. We formulate a kinetic free-void- volume theory that predicts this phi settled(tau ramp), with phi settled(infinity) = phi ALP and A = phi settled(0) - phi ALP , where phi ALP -:135 is the "adhesive loose packing" fraction found by Liu et al. Equation of state for random sphere packings with arbitrary adhesion and friction, Soft Matter 13 , 421 (2017).

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