Droplet adhesion to hydrophobic fibrous surfaces

Abstract

A water droplet can exhibit a high apparent contact angle on a hydrophobic fibrous surface. However, a high contact angle does not guarantee droplet mobility on the surface. The reasons behind droplet adhesion to a hydrophobic fibrous surface has not yet been analyzed or formulated. In this work, the force required to detach a droplet from a hydrophobic fibrous surface is investigated experimentally and computationally. Electrospun Polystyrene mats are considered for this study as they exhibit high contact angles coupled with poor droplet mobility. To better isolate the effects of microstructural properties of the mats and study their effects on droplet detachment, randomness of the fiber orientation is minimized by producing highly oriented fibers in orthogonal layers. As the earth gravity is not strong enough to detach small droplets from such surfaces, aqueous ferrofluid droplets are used in a controllable magnetic field to enhance the effect of gravity. The detachment process is recorded via a high-speed camera and the images are used to detect the moment of detachment and to analyze droplet shape before and during detachment. Numerical simulations are also conducted to provide additional insight into the physics of droplet detachment, and more importantly, to develop an equation for estimating droplet detachment force from a fibrous surface. In this paper, we discuss the effects of fiber properties, e.g., Young-Laplace contact angle or fiber spacing, on the force needed to detach a droplet from a fibrous surface.