Electrothermally Assisted Surface Charge Density Gradient Printing to Drive Droplet Transport.

Surface 2019, surface charge density (SCD) gradient printing-driven droplet transport, has attracted considerable attention as a novel and effective approach, which adopts the water droplet impacting a nonwetting surface to create a reprintable SCD gradient pathway conveniently and realizes the high-velocity and long-distance transport of droplets. In the present work, we further investigated the effects of electrothermal behavior on SCD gradient printing on hydrophobic surfaces by considering the droplet impact dynamics. After the electrothermal function was activated, the wettability of the hydrophobic surface improved in terms of the spreading factor history and the infiltration depth, which increased the probability of solid/liquid contact electrification to generate a more favorable SCD gradient. Since the hydrophobic surface was negatively charged by droplet impact, polarized droplets rolled forward along the preprinted SCD gradient pathway due to opposite charge attraction. Based on these results, we designed a SCD gradient printer with an electrothermal function for hydrophobic surfaces. Subsequently, the kinematic parameters of rolling droplets on hydrophobic surfaces were observed and quantified to evaluate the improvements resulting from the electrothermal function.