Droplet Manipulation
Spontaneous High-Speed Transport of Subnanoliter Water Droplet on Gradient Nanotextured Surfaces （APL, 2009, also highlighted by Nature Nanotechnology, 2009, JMM 2008, JMS 2006, Sen. Act. A 2004, JMM 2007）
We present water droplets that undergo spontaneous self-directed motion upon contact with a chemically patterned nanotextured surface with wedge-shape gradient. The surface exhibits two distinct wetting properties and low hysteresis. The droplet velocity depends on the droplet position and gradient angle. A wide range of droplet volume can be transported and a droplet velocity as high as 0.5 m/s has been achieved herein. Ascension of water droplets with all-round acclivity and a subnanoliter droplet movement were also demonstrated. We conclude that it is the combination of surface tension gradient and nano-wetting actuation that governs the droplet motion.
Substrate Curvature Gradient Drives Rapid Droplet Motion (PRL 2014)
Making small liquid droplets move spon taneously 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.