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Uni-directional liquid spreading on asymmetric nanostructured surfaces


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 lubrication1,2,3,4,5,6,7,8. Advancements in surface engineering, with the fabrication of various micro/nanoscale topographic features9,10,11,12,13, and selective chemical patterning on surfaces14,15, have enhanced surface wettability3,16,17 and enabled control of the liquid film thickness18 and final wetted shape19. In addition, groove geometries and patterned surface chemistries have produced anisotropic wetting, where contact-angle variations in different directions resulted in elongated droplet shapes20,21,22,23,24,25,26. In all of these studies, however, the wetting behaviour 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.

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Figure 1: Comparison of wetting behaviour on symmetric and asymmetric nanostructured surfaces.
Figure 2: Scanning electron micrographs with uniform arrays of asymmetric nanostructured surfaces.
Figure 3: Time-lapse images of uni-directional spreading of a liquid droplet.
Figure 4: Experimental results and the theoretical curves predicting uni-directional liquid spreading.

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The authors gratefully acknowledge financial support from the National Science Foundation (under Award EEC-0824328), the DARPA Young Faculty Award and the Northrop Grumman New Faculty Innovation Grant. The authors would also like to acknowledge the Intel Higher Education Grant for a generous computer donation, and the MIT Microsystems Technology Lab. The authors thank B. E. Polat from D. Blankschtein’s group for help with surface tension measurements, and especially, M. E. Alf and S. Baxamusa from K. K. Gleason’s group for the initiated chemical vapour polymer deposition, all in the Department of Chemical Engineering, MIT.

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All authors contributed to designing and conducting the experiments, model development and preparing the manuscript.

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Correspondence to Evelyn N. Wang.

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The authors declare no competing financial interests.

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Chu, KH., Xiao, R. & Wang, E. Uni-directional liquid spreading on asymmetric nanostructured surfaces. Nature Mater 9, 413–417 (2010).

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