Directional Liquid Transport from the Cold Region to the Hot Region on a Topological Surface

Bioinspired functional SLIPSs and wettability gradient surfaces and their synergistic cooperation and opportunities for enhanced condensate and fluid transport

Bioinspired smart functional surfaces have received increasing attention in recent years owed to their tunable wettability and enhanced droplet transport suggesting them as excellent candidates for industrial and nanotechnology-related applications. More specifically, bioinspired slippery lubricant infused porous surfaces (SLIPSs) have been proposed for their low adhesion enabling continuous dropwise condensation (DWC) even of low-surface tension fluids. In addition, functional surfaces with chemical and/or structural wettability gradients have also been exploited empowering spontaneous droplet transport in a controlled manner. Current research has focused on the better understanding of the mechanisms and intimate interactions taking place between liquid droplets and functional surfaces or on the forces imposed by differences in surface wettability and/or by Laplace pressure owed to chemical or structural gradients. Nonetheless, less attention has been paid to the synergistic cooperation of efficiently driving droplet transport via chemical and/or structural patterns/gradients on a low surface energy/adhesion background imposed by SLIPSs, with the consequent promising potential for microfluidics and condensation heat transfer applications amongst others. This review provides a detailed and timely overview and summary on recent advances and developments on bioinspired SLIPSs and on wettability gradient surfaces with focus on their synergistic cooperation for condensation and fluid transport related applications. Firstly, the fundamental theory and mechanisms governing complex droplet transport on homogeneous, on wettability gradient surfaces and on inclined SLIPSs are introduced. Secondly, recent advances on the fabrication and characterization of SLIPSs and functional surfaces are presented. Then, the condensation performance on such functional surfaces comprising chemical or structural wettability gradients is reviewed and their applications on condensation heat transfer are summarized. Last a summary outlook highlighting the opportunities and challenges on the synergistic cooperation of SLIPSs and wettability gradient surfaces for heat transfer as well as future perspective in modern applications are presented.

Regular Self-Actuation of Liquid Metal Nanodroplets in Radial Texture Gradient Surfaces

Liquid metal movement in microfluidic devices generally requires an external stimulus to achieve its motion, which results in many difficulties to precisely manipulate its motion at a nanoscale. Therefore, there is an attempt to control the motion of a liquid metal droplet without the input of an external force. In this paper, we report an approach to achieve the self-actuation of a gallium nanodroplet in radial texture gradients on substrates. The results have proved the validity of this method. It is suggested that there are four stages in the self-motion of the droplet and that the precursor film forming on the second stage plays a pivotal role in the motion. Furthermore, how the impact velocity affects the self-actuation of the nanodroplet on the gradient surface is also studied. We find that the moderate impacting velocity hinders the self-actuation of the gallium nanodroplet. This study is very helpful to regulate the self-actuation on patterned substrates and facilitate their applications in the fields of microfluidics devices, soft robots, and liquid sensors.