An engineered anisotropic nanofilm with unidirectional wetting properties


Anisotropic textured surfaces allow water striders to walk on water, butterflies to shed water from their wings and plants to trap insects and pollen. Capturing these natural features in biomimetic surfaces is an active area of research. Here, we report an engineered nanofilm, composed of an array of poly(p-xylylene) nanorods, which demonstrates anisotropic wetting behaviour by means of a pin-release droplet ratchet mechanism. Droplet retention forces in the pin and release directions differ by up to 80 μN, which is over ten times greater than the values reported for other engineered anisotropic surfaces. The nanofilm provides a microscale smooth surface on which to transport microlitre droplets, and is also relatively easy to synthesize by a bottom-up vapour-phase technique. An accompanying comprehensive model successfully describes the film’s anisotropic wetting behaviour as a function of measurable film morphology parameters.

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Figure 1: An overview of PPX nanofilm preparation and anisotropic wetting property.
Figure 2: Anisotropic wetting property of a PPX nanofilm.
Figure 3: Drop motion on a PPX nanofilm coated half-pipe.
Figure 4: Model of advancing and receding contact angles.


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We gratefully acknowledge financial support for this work from the Pennsylvania State University and the Office of Naval Research under the Naval Research Laboratory Core 6.1 Research Program and the Young Investigator Program. We thank J. Bush for a number of useful discussions.

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M.C.D. and W.J.D. planned the research, and M.C.D. supervised the research. M.C.D., N.A.M. and K.S. carried out the experiments. M.J.H. developed the theoretical model. All authors contributed to writing and revising the manuscript, and agreed on its final contents.

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Correspondence to Melik C. Demirel.

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The authors declare no competing financial interests.

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Malvadkar, N., Hancock, M., Sekeroglu, K. et al. An engineered anisotropic nanofilm with unidirectional wetting properties. Nature Mater 9, 1023–1028 (2010).

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