Wetting of Flat Gradient Surfaces

TL;DR

This paper develops a variational approach to analyze the wetting behavior of droplets on flat gradient surfaces, deriving a generalized Young equation and exploring contact angle hysteresis and electrowetting effects.

Contribution

It introduces a variational framework for wetting on gradient surfaces, deriving a generalized Young equation accounting for surface gradients and contact line dynamics.

Findings

Generalized Young equation valid for gradient surfaces.

Contact angle depends on contact line radius and interfacial tension derivatives.

Contact angle hysteresis is unavoidable on gradient surfaces.

Abstract

Gradient, chemically modified, flat surfaces enable directed transport of droplets. Calculation of apparent contact angles inherent for gradient surfaces is challenging even for atomically flat ones. Wetting of gradient, flat solid surfaces is treated within the variational approach, under which the contact line is free to move along the substrate. Transversality conditions of the variational problem give rise to the generalized Young equation valid for gradient solid surfaces. The apparent (equilibrium) contact angle of a droplet, placed on a gradient surface depends on the radius of the contact line and the values of derivatives of interfacial tensions. The linear approximation of the problem is considered. It is demonstrated that the contact angle hysteresis is inevitable on gradient surfaces. Electrowetting of gradient surfaces is discussed.