Cohesion of Amorphous Silica Spheres: Toward a Better Understanding of the Coagulation Growth of Silicate Dust Aggregates

TL;DR

This study revises the surface energy of amorphous silica spheres, showing it is higher than previously thought, which aligns theoretical models with experiments and supports the idea that silicate grains can grow into planetesimals in protoplanetary disks.

Contribution

The paper provides a revised estimate of the surface energy of amorphous silica, improving the accuracy of models for dust coagulation and planetesimal formation.

Findings

Surface energy for hydrophilic amorphous silica in vacuum is ten times higher than previously assumed.

Revised models now match laboratory experiments on silica sphere adhesion.

Silicate grains of 0.1 μm radius can grow into planetesimals via coagulation in protoplanetary disks.

Abstract

Adhesion forces between submicrometer-sized silicate grains play a crucial role in the formation of silicate dust agglomerates, rocky planetesimals, and terrestrial planets. The surface energy of silicate dust particles is the key to their adhesion and rolling forces in a theoretical model based on the contact mechanics. Here we revisit the cohesion of amorphous silica spheres by compiling available data on the surface energy for hydrophilic amorphous silica in various circumstances. It turned out that the surface energy for hydrophilic amorphous silica in a vacuum is a factor of 10 higher than previously assumed. Therefore, the previous theoretical models underestimated the critical velocity for the sticking of amorphous silica spheres, as well as the rolling friction forces between them. With the most plausible value of the surface energy for amorphous silica spheres, theoretical models based on the contact mechanics are in harmony with laboratory experiments. Consequently, we conclude that silicate grains with a radius of $0.1~\mu$m could grow to planetesimals via coagulation in a protoplanetary disk. We argue that the coagulation growth of silicate grains in a molecular cloud is advanced either by organic mantles rather than icy mantles or, if there are no mantles, by nanometer-sized grain radius.