We present an experimental study of the tunable anisotropic wetting behavior of chemically patterned anisotropic surfaces. Asymmetric glycerol droplet shapes, arising from patterns of alternating hydrophilic (pristine SiO2) and hydrophobic (fluoroalkylsilane self-assembled monolayers) stripes with dimensions in the low-micrometer range, are investigated in relation to stripe widths. Owing to the well-defined small droplet volume, the equilibrium shape as well as the observed contact angles exhibit unique scaling behavior. Only the relative width of hydrophilic and hydrophobic stripes proves to be a relevant parameter. Our results on morphologically flat, chemically patterned surfaces show similarities with those of experiments on topographically corrugated substrates. They are discussed in terms of the energetics at the liquid-solid interface.

The equilibrium shape of droplets on surfaces, functionalized with stripes of alternating wettability, have been investigated using simulations employing a finite element method. Experiments show that a droplet deposited on a surface with relatively narrow hydrophobic stripes compared to the hydrophilic stripes adopts a strongly elongated shape. The aspect ratio, the length of the droplet divided by the width, decreases toward unity when a droplet is deposited on a surface with relatively narrow hydrophilic stripes. The aspect ratio and the contact angle parallel to the stripes show unique scaling behavior as a function of the ratio between the widths of the hydrophobic and hydrophilic stripes. For a small ratio, the contact angle parallel to the stripes is low and the aspect ratio high, while for a large ratio, the contact angle parallel is high and the aspect ratio low. The simulations exhibit similar scaling behavior, both for the aspect ratio of the droplets and for the contact angles in the direction parallel to the stripes. Two liquids with different surface tensions have been investigated both experimentally and in simulations; similarities and differences between the findings are discussed. Generally, three parameters are needed to describe the droplet geometry: (i) the equilibrium contact angles on the hydrophilic and (ii) hydrophobic areas and (iii) the ratio of the widths of these chemically defined stripes. Furthermore, we derive a simple analytical expression that proves to be a good approximation in the quantitative description of the droplet aspect ratio.

The motion of droplets under the influence of lithographically created anisotropic chemically defined patterns is described and discussed. The patterns employed in our experiments consist of stripes of alternating wettability: hydrophobic stripes are created via fluorinated self-assembled monolayers, and for hydrophilic stripes, the SiO(2) substrate is used. The energy gradient required to induce the motion of the droplets is created by varying the relative widths of the stripes in such a way that the fraction of the hydrophilic area increases. The anisotropic patterns create a preferential direction for liquid spreading parallel to the stripes and confine motion to the perpendicular direction, giving rise to markedly higher velocities as compared to nonstructured surface energy gradients. Consequently, the influence of the distinct pattern features on the overall motion as well as suggestions for design improvements from an application point of view are discussed.

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We experimentally investigate the dynamics of nanometer-high, micrometer-wide gassy layers at the interface between a hydrophobic solid and bulk water. These micropancakes grow laterally in time, on the timescale of an hour, leading to partial dewetting of the solid. The growth is directional, mediated by chemical roughness on the substrate, and transient, occurring within the first hour after liquid deposition. We use circularity to measure the roundness of a micropancake (circularity C = 2(piA)(1/2)/L, where A is the surface area and L is the perimeter). The growth is anisotropic, as demonstrated by a decrease in circularity with time. However, once a micropancake reaches size saturation, its bulk rearranges its shape in order to minimize the length of its three-phase line. We interpret this combination of growth followed by bulk rearrangement as dynamic dewetting.