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Phys Rev Lett. 2014 Jul 11;113(2):026101. Epub 2014 Jul 10.

Substrate curvature gradient drives rapid droplet motion.

Author information

1
Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.
2
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan.
3
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan and Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan.
4
Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China.
5
Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China and Centre Universitaire d'Informatique, University of Geneva, CH-1227 Carouge, Switzerland.

Abstract

Making small liquid droplets move spontaneously on solid surfaces is a key challenge in lab-on-chip and heat exchanger technologies. Here, we report that a substrate curvature gradient can accelerate micro- and nanodroplets to high speeds on both hydrophilic and hydrophobic substrates. Experiments for microscale water droplets on tapered surfaces show a maximum speed of 0.42  m/s, 2 orders of magnitude higher than with a wettability gradient. We show that the total free energy and driving force exerted on a droplet are determined by the substrate curvature and substrate curvature gradient, respectively. Using molecular dynamics simulations, we predict nanoscale droplets moving spontaneously at over 100  m/s on tapered surfaces.

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