title: Substrate curvature gradient drives rapid droplet motion.
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.
authors: Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.
authors: Cunjing
authors: C
authors: Lv
authors: Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.
authors: Chao
authors: C
authors: Chen
authors: Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors: Yin-Chuan
authors: YC
authors: Chuang
authors: Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan and Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan.
authors: Fan-Gang
authors: FG
authors: Tseng
authors: Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China.
authors: Yajun
authors: Y
authors: Yin
authors: Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China and Centre Universitaire d'Informatique, University of Geneva, CH-1227 Carouge, Switzerland.
authors: Francois
authors: F
authors: Grey
authors: Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China and Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China.
authors: Quanshui
authors: Q
authors: Zheng