Wetting on the microscale: shape of a liquid drop on a microstructured surface at different length scales.

Describing wetting of a liquid on a rough or structured surface is a challenge because of the wide range of involved length scales. Nanoand micrometersized textures cause pinning of the contact line, reflected in a hysteresis of the contact angle. To investigate contact angles at different length scales, we imaged water drops on arrays of 5 μm high poly(dimethylsiloxane) micropillars. The drops were imaged by laser scanning confocal microscopy (LSCM), which allowed us to quantitatively analyze the local and large-scale drop profile simultaneously. Deviations of the shape of drops from a sphere decay at two different length scales. Close to the pillars, the amplitude of deviations decays exponentially within 1−2 μm. The drop profile approached a sphere at a length scale 1 order of magnitude larger than the pillars’ height. The height and position dependence of the contact angles can be understood from the interplay of pinning of the contact line, the principal curvatures set by the topography of the substrate, and the minimization of the air−water interfaces. ■ INTRODUCTION Wetting is of immense importance in technology and everyday life. It plays a key role in the morphology and stability of a liquid on a given surface. The wetting behavior controls the dynamics of spreading of drops on leaves or insect wings as well as the surface properties of a substrate after spray-coating or drop-casting. Some processes require fast and homogeneous wetting, whereas in other applications antiwetting or selfcleaning is required. Primarily, the wetting properties depend on the surface energy. The contact angle Θ on an ideally flat, homogeneous, and inert surface can be quantified by Young’s equation cos Θ = (γSV − γSL)/γLV. Here, γLV is the liquid−vapor, γSL is the solid− liquid, and γSV is the solid−vapor interfacial tensions. However, real surfaces are chemically inhomogeneous and rough. Wenzel accounted for roughness by introducing a roughness factor, defined as the ratio of the actual to the projected surface area. It has been shown that the contact angle of rough surfaces is only affected by the surface properties in the vicinity of the liquid−substrate−vapor interface, the socalled three-phase contact line. AFM measurements of nonevaporating drops (such as polymers) provide a contact angle with a resolution of a few nanometers. However, these studies are limited to low contact angles θ < 90°. Inhomogeneities and roughness, even of a few nanometers, lead to pinning of the contact line. Pinning prevents drops from reaching their global equilibrium shape. The macroscopic contact angle just before a droplet advances, θadv, exceeds the macroscopic contact angle just before it recedes, θrec. The difference defines the contact angle hysteresis, Δθ = θadv − θrec. It quantifies the degree to which the macroscopic contact angle of a drop depends on its history rather than thermodynamic equilibrium. To improve insight into the interplay between the contact angle, hysteresis, and pinning, previous studies investigated substrates patterned with chemical stripes or topographic ridges. Asymmetric patterns can cause deformation of drops. Recent numerical studies calculated pinning points and contact angles with a resolution higher than the length scale of the roughness pattern, but at the expense of spatial extension. Also, experimental studies suffer from limited spatial resolution. To the best of our knowledge, the contact angles close to a pinning site and the three-dimensional drop shape have never been quantitatively measured simultaneously. However, this information is required to understand the relationship between the microscopic structure of a solid surface and the macroscopic contact angle. In most cases the contact line is studied by optical microscopy. Recently, confocal imaging experiments aimed at clarifying this discussion have been carried out. Wu et al. investigated the contact angle on a smooth substrate patterned with hydrophilic and hydrophobic patches. Using water dyed with rhodamine B, they visualized the contact line with a resolution of about 10 μm. However, rhodamine B Received: January 28, 2012 Revised: May 10, 2012 Published: May 11, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 8392 dx.doi.org/10.1021/la300379u | Langmuir 2012, 28, 8392−8398 adsorbed at the air−liquid and solid−liquid interfaces and caused a reduction of the interfacial tension and pinning. Here, we employed a model substrate composed of an array of micropillars, in order to quantify drop shape variations at a resolution higher than the length scale of roughness. To investigate the shape of the liquid surface, we imaged water drops on smooth and microstructured hydrophobic surfaces with vertical resolution of 0.7 μm with a laser scanning confocal microscope (LCSM). The microstructured surface consisted of micropillars (5 μm radius, 5 μm height) arranged in a square lattice with 20 μm lattice distance. We mapped the drop contour surrounding a pinning point and simultaneously the macroscopic three-dimensional drop shape. These results are compared to conventional contact angle measurements on the