title: Super liquid-repellent layers: The smaller the better.
abstract: Super liquid-repellent layers need to have a high impalement pressure and high contact angles, in particular a high apparent receding contact angle. Here, we demonstrate that to achieve both, the features constituting the layer should be as small as possible. Therefore, two models for super liquid-repellent layers are theoretically analyzed: A superhydrophobic layer consisting of an array of cylindrical micropillars and a superamphiphobic layer of an array of pillars of spheres. For the cylindrical micropillars a simple expression for the apparent receding contact angle is derived. It is based on a force balance rather than a thermodynamic approach. The model is supported by confocal microscope images of a water drop on an array of hydrophobic cylindrical pillars. The ratio of the width of a pillar w to the center-to-center spacing a is a primary factor in controlling the receding angle. Keeping the ratio w/a constant, the absolute size of surface features should be as small as possible, to maximize the impalement pressure.
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany. Electronic address: butt@mpip-mainz.mpg.de.
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Butt
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Doris
authors.initials: D
authors.lastname: Vollmer
authors.affiliation: Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Periklis
authors.initials: P
authors.lastname: Papadopoulos
keywords: Cassie state; Superamphiphobicity; Superhydrophobicity; Superoleophobicity; Wenzel state; Wetting