Precursor films in wetting phenomena

TL;DR

This paper reviews the experimental and theoretical understanding of precursor films in wetting phenomena, focusing on their formation, spreading dynamics, and recent developments in liquid-on-solid and solid-on-solid systems.

Contribution

It provides a comprehensive overview of experimental observations and theoretical models of microscopic precursor films, highlighting recent advances and future research directions.

Findings

Precursor films can be microscopic or mesoscopic in size.

Theoretical models help explain the formation and spreading of these films.

Recent studies reveal new insights into solid-on-solid precursor films.

Abstract

The spontaneous spreading of non-volatile liquid droplets on solid substrates poses a classic problem in the context of wetting phenomena. It is well known that the spreading of a macroscopic droplet is in many cases accompanied by a thin film of macroscopic lateral extent, the so-called precursor film, which emanates from the three-phase contact line region and spreads ahead of the latter with a much higher speed. Such films have been usually associated with liquid-on-solid systems, but in the last decade similar films have been reported to occur in solid-on-solid systems. While the situations in which the thickness of such films is of mesoscopic size are rather well understood, an intriguing and yet to be fully understood aspect is the spreading of microscopic, i.e., molecularly thin films. Here we review the available experimental observations of such films in various liquid-on-solid and solid-on-solid systems, as well as the corresponding theoretical models and studies aimed at understanding their formation and spreading dynamics. Recent developments and perspectives for future research are discussed.