Heterogeneous nucleation on a completely wettable substrate

TL;DR

This paper revises the understanding of heterogeneous nucleation on completely wettable surfaces, showing that critical nuclei and energy barriers can exist and be observed, contrary to previous beliefs.

Contribution

It demonstrates that critical nuclei and activation processes are possible on fully wettable substrates if certain conditions are met, revising prior assumptions.

Findings

Critical nuclei can exist on fully wettable surfaces.

A free energy barrier for nucleation can be present.

The critical nucleus character changes across coexistence.

Abstract

It is widely believed that heterogeneous nucleation occurs without an activation process when the surface is completely wettable. In this report, we review our previous work [J.Chem.Phys {\bf 134}, 234709 (2011)] to show that the critical nucleus (droplet) can exist and the activation process may be observable. In fact, a critical nucleus and a free energy barrier always exist if the surface potential or the disjoining pressure allows for the first-order pre-wetting transition on a completely wettable substrate where the contact angle is zero. Furthermore, the critical nucleus changes character from the critical nucleus of surface phase transition below bulk coexistence (undersaturation) to the critical nucleus of bulk heterogeneous nucleation above the coexistence (oversaturation) when it crosses the coexistence. In this paper the morphology and work of formation of a critical nucleus on a completely-wettable substrate are re-examined to point out the possibility of observing a critical droplet on a completely wettable substrate.