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Letter

Lotus Effect Amplifies Light-Induced Contact Angle Switching

Rohit Rosario,^† Devens Gust,^‡ Antonio A. Garcia,^† Mark Hayes,^‡ J. L. Taraci,^§ T. Clement,^§ J. W. Dailey,^§ and S. T. Picraux*^§

Harrington Department of Bioengineering, Department of Chemistry and Biochemistry, and Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona 85287

J. Phys. Chem. B, 2004, 108 (34), pp 12640–12642

DOI: 10.1021/jp0473568

Publication Date (Web): July 29, 2004

†

Harrington Department of Bioengineering.

‡

Department of Chemistry and Biochemistry.

Department of Chemical and Materials Engineering.

To whom correspondence should be addressed. E-mail: picraux@asu.edu.

Abstract

A rough surface morphology is shown to significantly amplify the light-induced change in water contact angle of a photoresponsive surface. Smooth Si surfaces and fractally rough Si nanowire surfaces grown on a Si substrate were studied, both coated with a hydrophobic monolayer containing photochromic spiropyran molecules. Under visible irradiation the spiropyran is in a closed, hydrophobic form, whereas UV irradiation converts the spiropyran to a polar, hydrophilic form, reducing the contact angle. The superhydrophobic nanowire surface both amplifies the light-induced contact angle change by a factor of 2 relative to a smooth surface and reduces the contact angle hysteresis. As a result the UV-induced advancing contact angle is lower than the receding contact angle under visible irradiation, allowing water drops to be moved solely under the influence of a UV−visible light gradient. The amplification of the reversible light-induced wetting angle change was predicted using the Wenzel model for fractally rough surfaces. The model and amplification effects are expected to apply to other types of stimuli-induced contact angle changes such as that by heat or electrical potentials.

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