Recent Progress in Research on “Liquid-Like” Surfaces Showing Low Contact Angle Hysteresis and Excellent Liquid Sliding Behavior

Abstract

Most studies on (super)hydrophobic/oleophobic materials/surfaces have particularly focused on the effects of surface textures and surface modifications with long-chain perfluorinated compounds (LCPFCs) to achieve maximum static contact angle (CA, θS) and minimize the contact area between the probe liquid and the solid surface. However, it has recently been recognized that such θS value alone does not faithfully reflect the actual surface wetting properties. Hence, the importance of dynamic wettability evaluation, such as dynamic (advancing (θA) and receding (θR)) CAs, CA hysteresis (θA−θR (Δθ) or cosθR−cosθA (Δθcos)), and substrate tilt/sliding angle (α), has recently been recognized as a better way to estimate the actual surface wettability of the target materials. For example, if the apparent CA is small, as long as the CA hysteresis is small enough or negligible, excellent liquid sliding properties can be realized not only with aqueous liquids but also with organic liquids having low surface tensions. To reduce CA hysteresis, the use of "liquid-like" surfaces, on which surface-tethered functional groups can rotate freely and provide a surface with liquid-like nature, is promising because such surfaces can be prepared without relying on either fragile surface micro/nanostructures or subsequent perfluorination with harmful LCPFCs, in addition to excellent liquid sliding behavior. In this mini-review, we focused on the recent progress in research on liquid-like surfaces with low CA hysteresis demonstrating excellent sliding of various probe liquids. We highlighted some of the representative studies and concluded with a brief discussion of future directions in this promising research field.