Microstructured surfaces that control the direction of liquid transport are not only ubiquitous in nature, but they are also central to technological processes such as fog/water harvesting, oil–water separation, and surface lubrication. However, a fundamental understanding of the initial wetting dynamics of liquids spreading on such surfaces is lacking. Here, we show that three regimes govern microstructured surface wetting on short time scales: spread, stick, and contact line leaping. The latter involves establishing a new contact line downstream of the wetting front as the liquid leaps over specific sections of the solid surface. Experimental and numerical investigations reveal how different regimes emerge in different flow directions during wetting of periodic asymmetrically microstructured surfaces. These insights improve our understanding of rapid wetting in droplet impact, splashing, and wetting of vibrating surfaces and may contribute to advances in designing structured surfaces for the mentioned applications.

The impact of liquid drops on a rigid surface is central in cleaning, cooling, and coating processes in both nature and industrial applications. However, it is not clear how details of pores, roughness, and texture on the solid surface influence the initial stages of the impact dynamics. Here, we experimentally study drops impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We found that the difference between impact velocity and the capillary speed on a solid surface is a key factor of spreading asymmetry, where the capillary speed is determined by the friction at a moving three-phase contact line. The line-friction capillary number Ca-f = mu V-f(0)/sigma (where mu V-theta(0), and sigma are the line friction, impact velocity, and surface tension, respectively) is defined as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when Ca-f << 1, the droplet impact is asymmetric; the contact line speed in the direction against the inclination of the ridges is set by line friction, whereas in the direction with inclination, the contact line is pinned at acute corners of the ridges. When Ca-f >> 1, the geometric details of nonsmooth surfaces play little role.

Textured hydrophobic surfaces which repel liquid droplets unidirectionally are found in nature such as butterfly wings and ryegrass leaves and are also essential in technological processes such as self-cleaning and anti-icing surfaces.However, the droplet impact on such surfaces is not fully understood.Here, we study, using a high-speed camera, droplet impact on surfaces with inclined pillars.We observed directional rebound on surfaces with dense arrays of pillars and at high impact speeds.The key factor determining whether droplets rebound straight up or directionally lies in the retraction phase after the droplet impact. The retraction of the contact line in the direction against the inclination is slower than in the direction with the inclination. This is attributed to the asymmetric receding contact angles, which arise from the different wetting behavior of the structure sidewalls.The experimental observations are complemented with numerical simulations to elucidate the detailed movement of the drops over the pillars.These insights improve our understanding of droplet impact on hydrophobic microstructures and may be a useful for designing structured surfaces for controlling droplet mobility. 

In this study, we investigate the spreading of viscous drops, both Newtonian (aqueous glycerol) and shear-thinning fluids (dilute aqueous polyacrylamide and Xanthan gum solutions). The rapid spreading in the first millisecond is insensitive to shear-thinning viscosity, i.e.,  the zero-shear viscosity does not influence the rapid spreading. In contrast, the rapid spreading of aqueous glycerol is slower than water. Using numerical simulations of comparable droplets, we identify that the contact line friction of the polymeric solutions is unchanged from water. In addition, the shear-thinning effect reduces the effective viscosity during rapid spreading.Finally, we show how the spreading is insensitive to the polymer concentration using the conventional Ohnesorge number  and the line-friction Ohnesorge number  , where µ, µ_f, ρ, σ, R₀ are viscosity, the line friction parameter, density, surface tension, and droplet radius, respectively.  Due to the shear-thinning, Oh « 1 is satisfied for the shear-thinning fluid in this study, i.e., viscosity is unimportant.