Drops on soft surfaces learn the hard way
Year: 2013
Type: article
Abstract: A liquid drop sits on a solid surface and slides away when the surface is tilted. Few physical systems seem so mundane or would appear to be governed by such plain, predictable, and long-understood principles. Indeed, Young’s fundamental understanding of the equilibrium gas/liquid/solid contact line (Fig. 1A) has remained intact for over two centuries (1). Nonetheless, static and dynamic contact lines continue to provide surprises (2), yielding qualitatively new insights, motivating new materials, and highlighting the mysteries that can remain overlooked within even the simplest of systems. In PNAS, Style et al. reveal yet another surprise hidden within this classic system: droplets spontaneously migrate along substrates that are chemically homogeneous, but inhomogeneously deformable (3). The Young (or Young–Dupre) equation,relates the equilibrium angle formed at the solid/liquid/gas contact line to the respective surface energies γ, which act as tensions pulling along each interface (Fig. 1A). Any nonzero vector sum of these three tensions would cause the contact line to move in response, unless some chemical or topographical inhomogeneity held it in place. Young’s equilibrium contact angle, then, arranges the interfaces to balance these forces, giving Eq. 1.
Author: Todd M. Squires
Institution: University of California, Santa Barbara
Language: English
Subfield: Computational Mechanics
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2092910324