The bouncing threshold in silica nanograin collisions

Abstract

Using molecular dynamics simulations, we study collisions between amorphous silica nanoparticles. Our silica model contains uncontaminated surfaces, that is, the effect of surface hydroxylation or of adsorbed water layers is excluded. For central collisions, we characterize the boundary between sticking and bouncing collisions as a function of impact velocity and particle size and quantify the coefficient of restitution. We show that the traditional Johnson-Kendall-Roberts (JKR) model provides a valid description of the ingoing trajectory of two grains up to the moment of maximum compression. The distance of closest approach is slightly underestimated by the JKR model, due to the appearance of plasticity in the grains, which shows up in the form of localized shear transformation zones. The JKR model strongly underestimates the contact radius and the collision duration during the outgoing trajectory, evidencing that the breaking of covalent bonds during grain separation is not well described by this model. The adhesive neck formed between the two grains finally collapses while creating narrow filaments joining the grains, which eventually tear.

Collision‐Induced Melting in Collisions of Water Ice Nanograins: Strong Deformations and Prevention of Bouncing

“…At v = 150 m/s, strain is maximum in an oval region surrounding the contact plane; no sign of strain localization is observed that points at the occurrence of shear transformation zones that otherwise govern the plasticity of amorphous materials (Falk & Langer, 1998;Huang et al, 2013;Schuh & Lund, 2003). Since such shear transformation zones are actually observed in collisions of silica grains of comparable sizes (Nietiadi et al, 2017), we conclude that it is the softness of amorphous ice that precludes the existence of these zones in this material.…”

“…The value of the constant C depends strongly on the assumptions of energy dissipation during the collision and has been discussed to assume values between 0.3 and 57.9 (Krijt et al, ; Nietiadi et al, ). Even with the highest value of C , equation predicts bouncing to occur above v b =91 m/s.…”

“…(1) The value of the constant C depends strongly on the assumptions of energy dissipation during the collision and has been discussed to assume values between 0.3 and 57.9 (Krijt et al, 2013;Nietiadi et al, 2017).…”

“…MD has been used previously to characterize the result of collisions of nanometer-sized grains, mostly for generic interactions such as modeled by the Lennard-Jones potential (Tanaka et al, 2012;Millán et al, 2016aMillán et al, , 2016b. Recently, simulations of silica grain collisions confirmed the gross trends predicted by JKR theory in the nanoscale regime but pointed at systematic deviations caused by intergrain attractions (Nietiadi et al, 2017).…”

Bouncing of Hydroxylated Silica Nanoparticles: an Atomistic Study Based on REAX Potentials

“…This approach is non-trivial in the case of polar materials such as silica, where adsorbates may strongly influence interactions at the surface [14]. Thus, it has been found that when modeling silica collisions with clean surfaces, the colliding NPs stick rather than bounce over a wide range of velocities [15], in contrast to experiments [16][17][18]. One may presume [13] that the surface passivation by water-that is, the surface hydroxylationis responsible for this behavior.…”