Using experiments and numerical simulations, we investigate the spontaneous spread-ing of droplets of aqueous glycerol (Newtonian) and aqueous polymer (shear-thinning) solutions on smooth surfaces. We find that in the first millisecond the spreading of the shear-thinning solutions is identical to the spreading of water, regardless of the polymer concentration. In contrast, aqueous glycerol solutions show a different behavior, namely, a significantly slower spreading rate than water. In the initial rapid spreading phase, the dominating forces that can resist the wetting are inertial forces and contact-line friction. For the glycerol solutions, an increase in glycerol concentration effectively increases the contact-line friction, resulting in increased resistance to wetting. For the polymeric solutions, however, an increase in polymer concentration does not modify contact-line friction. As a consequence, the energy dissipation at the contact line cannot be controlled by varying the amount of additives for shear-thinning fluids. The reduction of the spreading rate of shear-thinning fluids on smooth surfaces in the rapid-wetting regime can only be achieved by increasing solvent viscosity. Our results have implications for phase-change applications where the control of the rapid spreading rate is central, such as anti-icing and soldering.

Textured hydrophobic surfaces that 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. In many occasions, surface textures are oriented to direct rebounding droplets. Surface macrostructures (>100 μm) have often been explored to induce directional rebound. However, the influence of impact speed and detailed surface geometry on rebound is vaguely understood, particularly for small microstructures. Here, we study, using a high-speed camera, droplet impact on surfaces with inclined micropillars. We observed directional rebound at high impact speeds on surfaces with dense arrays of pillars. We attribute this asymmetry to the difference in wetting behavior of the structure sidewalls, causing slower retraction of the contact line in the direction against the inclination compared to with the inclination. 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 useful for designing structured surfaces for controlling droplet mobility. 

Wetting of a liquid on a solid surface is ubiquitous in everyday life, such as morning dew on a plant leaf and raindrops hitting on a glass window. However, this phenomenon is far away from being completely understood. Its dynamics are complicated due to its physics which involves multi-length scales from molecular motions to macroscopic fluid motions as well as numerous parameters such as viscosity, surface tension, contact angles, and so on. Moreover, the situation becomes more complicated in rapid situations -  initial rapid wetting and droplet impact. 

Not only the understanding of contact line physics and wetting on complete surfaces from a theoretical point of view, but surface properties are also of importance in most practical situations. In real life, non-smooth surfaces are found more often than smooth surfaces. Natural organisms take advantage of surface structures to manipulate the wetting behavior. Numerous examples such as lotus leaves, the pitcher plant, water strider, ryegrass and bamboo leaves, and butterfly wings have attracted attention and their properties are replicated by man-made materials and utilized in technical applications such as anti-icing, self-cleaning, drug delivery, water-oil separation, microreactors, and transport in microfluidic channels. However in previous studies on such applications, the detailed mechanisms of how the surface details such as the orientation of the structures and their spacing determine the wetting are scarcely understood. To design surface structures better and optimize for different applications, the understanding of spreading/wetting mechanisms is fundamental.   

This work intends to elucidate the underlying mechanisms of droplet motion on structured surfaces. The surfaces studied are arrays of inclined microridges and micropillars. The rapid wetting and droplet impact situations are studied through droplet spreading experiments using a high-speed camera and supplemental numerical simulations of comparable droplets.  This thesis first reveals the spreading mechanisms on asymmetric microstructures combining experiments and numerical simulations, and presents a theoretical model to predict the influence of the surface details to answer the question - how much is the contact line speed influenced?   Together with the microstructures, the influence of additives on the droplet spreading is studied to investigate the influence of complex fluid properties. 

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.

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.

Wetting phenomena, i.e. the spreading of a liquid over a dry solid surface, are important for understanding how plants and insects imbibe water and moisture and for miniaturization in chemistry and biotechnology, among other examples. They pose fundamental challenges and possibilities, especially in dynamic situations. The surface chemistry and micro-scale roughness may determine the macroscopic spreading flow. The question here is how dynamic wetting depends on the topography of the substrate, i.e. the actual geometry of the roughness elements. To this end, we have formulated a toy model that accounts for the roughness shape, which is tested against a series of spreading experiments made on asymmetric sawtooth surface structures. The spreading speed in different directions relative to the surface pattern is found to be well described by the toy model. The toy model also shows the mechanism by which the shape of the roughness together with the line friction determines the observed slowing down of the spreading.

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.