Role of Surface Chemistry in Grain Adhesion and Dissipation during Collisions of Silica Nanograins

Abstract

The accretion of dust grains to form larger objects, including planetesimals, is a central problem in planetary science. It is generally thought that weak van der Waals interactions play a role in accretion at small scales where gravitational attraction is negligible. However, it is likely that in many instances, chemical reactions also play an important role, and the particular chemical environment on the surface could determine the outcomes of dust grain collisions. Using atomic-scale simulations of collisional aggregation of nanometer-sized silica (SiO2) grains, we demonstrate that surface hydroxylation can act to weaken adhesive forces and reduce the ability of mineral grains to dissipate kinetic energy during collisions. The results suggest that surface passivation of dangling bonds, which generally is quite complete in an Earth environment, should tend to render mineral grains less likely to adhere during collisions. It is shown that during collisions, interactions scale with interparticle distance in a manner consistent with the formation of strong chemical bonds. Finally, it is demonstrated that in the case of collisions of nanometer-scale grains with no angular momentum, adhesion can occur even for relative velocities of several kilometers per second. These results have significant implications for early planet formation processes, potentially expanding the range of collision velocities over which larger dust grains can form.

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

“…Experimental silanol densities are in the range of 2.6-4.6 OH nm −2 [24]. Other simulation studies gave lower values, 2.0-2.5 OH/nm 2 [25,27] or even only 1.3-1.8 OH/nm 2 [19], but also higher values of 6.6 OH/nm 2 [26,28]. This large variety is due to (i) possibly incomplete identification of under-coordinated surface atoms and (ii) atomic surface roughness resulting in a larger effective surface area than the calculated area [26,28].…”

“…Quadery et al [19] simulated nanometer-sized hydroxylated a-SiO 2 NPs (radii of 1 and 2 nm) with the REAX potential and demonstrated that such NPs exhibit reduced adhesion. They report bouncing velocities of around 0.6 and 0.3 km/s for R = 1 and 2 nm, respectively.…”

“…Their bouncing velocities are again described with Eq. 2, if only the [19] simulation data of hydroxylated silica spheres compared to simulation data of clean silica spheres [15]. Symbols, simulation results.…”

“…Surface hydroxylation is performed similar to previous studies on the silica-water interface [19,[25][26][27][28]. We identify under-coordinated O and Si atoms at the NP surface; H is added to under-coordinated O and OH to under-coordinated Si; in both cases, thus terminal silanol groups (-SiOH) are created.…”

“…One may presume [13] that the surface passivation by water-that is, the surface hydroxylationis responsible for this behavior. Up to now, simulations of passivated silica NP collisions could be performed only with small (≤ 2 nm) grains [19], which show a considerable spread in collisional outcomes due to their nanoscopic size.…”