Understanding and Controlling Janus Droplet at High Temperature for Efficient Heat Transfer

Description

Controlled propulsion of liquid droplets on a solid surface not only generates fascinatingscience, but also promises viable applications in various heat transfer and microfluidictechnologies. Despite extensive advances in this field of research, our understanding and theability to control droplet dynamics at high temperature remain limited, in part due to theemergence of complex wetting states intertwined by the phase change process at the triple-phaseinterfaces. When the substrate supporting a liquid droplet is above a critical(Leidenfrost) temperature, a continuous vapor layer separates the droplet from the hotsurface, naturally obviating the contact line pinning and thus the associated friction.However, the vapor layer beneath the droplet that allows for the elimination of the interfacialhydrodynamic resistance lends itself to a rather large heat transfer resistance as well. Apartfrom this, on conventional surfaces, neither a Leidenfrost nor a boiling state is very effectivefor heat transfer, as droplets can move away from local hotspots due to vigorous shapefluctuation, aided by small but non-negligible thermal Marangoni forces. The problem isfurther complicated by the spray-induced aerodynamic drift coupled with coalescenceoccurring with other droplets. Thus, controlled vectoring of droplets at high temperaturewithout affecting the integrity of the coating remains an unsolved problem.In this project, we propose to design novel surfaces with one-dimensional or two-dimensionalgradient in structural morphology that enable the manifestation of two contrasting wettingstates (Leidenfrost and contact boiling) in a single droplet above its boiling point for efficientheat transfer. The presence of the so-called Janus thermal state rectified by asymmetrictextures engenders a preferential motion of droplet towards the contact boiling region, whichis associated with higher heat transfer coefficient. Moreover, the vectoring and confinementof droplet to the contact boiling region also suppresses the vigorous shape fluctuation andthermal Marangoni flow, thereby facilitating the alleviation of local hotspots. We willsystematically investigate the droplet vectoring, confinement and boiling dynamics on as-proposedsurfaces under different temperatures, and develop analytical models to elucidatethe effect of surface topography on the droplet dynamics. We envision that our fundamentalunderstanding and the ability to control the droplet dynamics at high temperatures representan important advance and enable the rational design of various surfaces for multifunctionalapplications, especially in high temperature thermal systems where high energy efficiency,security and stability are preferred.