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

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