Uni-directional liquid spreading on asymmetric nanostructured surfaces

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Title: Uni-directional liquid spreading on asymmetric nanostructured surfaces

Reference: E.N. Wang, K.-H. Chu, R. Xiao G.C. Rutledge, G.H. McKinley, “Science” 318, 1618 (2007).

Soft matter keywords

asymmetry, contact angle, hydrophocitiy, hysteresis , nanopillars, wetting

Abstract from the original paper

Controlling surface wettability and liquid spreading on patterned surfaces is of significant interest for a broad range of applications, including DNA microarrays, digital lab-on-a-chip, anti-fogging and fog-harvesting, inkjet printing and thin-film lubrication. Advancements in surface engineering, with the fabrication of various micro/nanoscale topographic features, and selective chemical patterning on surfaces have enhanced surface wettability and enabled control of the liquid film thickness and final wetted shape. In addition, groove geometries and patterned surface chemistries have produced anisotropic wetting, where contact-angle variations in different directions resulted in elongated droplet shapes. In all of these studies, however, the wetting behavior preserves left–right symmetry. Here, we demonstrate that we can harness the design of asymmetric nanostructured surfaces to achieve uni-directional liquid spreading, where the liquid propagates in a single preferred direction and pins in all others. Through experiments and modelling, we determined that the spreading characteristic is dependent on the degree of nanostructure asymmetry, the height-to-spacing ratio of the nanostructures and the intrinsic contact angle. The theory, based on an energy argument, provides excellent agreement with experimental data. The insights gained from this work offer new opportunities to tailor advanced nanostructures to achieve active control of complex flow patterns and wetting on demand.

Soft matter example

Side view comparison of wetting behavior on symmetric and asymmetric nanostructured surfaces. The above image shows the symmetric liquid spreading of a 1 µl droplet of water on typical vertical nanopillars. The below image shows the uni-directional liquid spreading droplet a droplet of the same size on the similar nanostructures as the top image, but with a 12◦ deflection angle. Insets are the SEM images of the upright and bent posts, respectively, with scale bars at 10 µm.

Time-lapse images showing uni-directional spreading of a droplet of water atop nanopillars bent at a 12◦ deflection angle, the left image showing the side view, and the right image showing the above view. The parameters of the nanopillars are 500 nm diameters, 3.5 µm spacings, heights of 10 µm deflected at 12◦ as shown in Fig. 1.