Self-Propelled Polymeric Droplet in Leidenfrost State on a Superheated Ratchet Surface

The use of milli-scale droplets has gained considerable attention in an effort to substantially reduce process size for process intensification. In order to develop a novel droplet reactor, this study investigates self-propelled Leidenfrost droplets on a superheated ratchet surface. In particular, a polymeric droplet (an aqueous solution of xanthan gum) was utilized, aiming at the application of a highly viscous fluid process. The motions of the two types of droplets (distilled water and an aqueous solution of xanthan gum) were observed using a high-speed camera. Based on the recorded movie, the velocity development after dropping was obtained by using an image-processing technique. By fitting the developing velocity to the equation of motion, three important parameters were obtained: the terminal velocity, the driving force, and the friction coefficient. The terminal velocity depended on the boiling pattern, and nucleate boiling accelerated the self-propelled velocity compared to that of film boiling. At relatively low temperatures, the terminal velocity of the polymeric droplets was slightly higher than that of the water droplets. To investigate the effect of polymer addition on droplet dynamics, the driving force and friction coefficient were discussed in more detail. The driving force increased with polymer addition because the higher viscosity suppressed deformation during motion. Consequently, a more stable motion, maintaining a smooth interface of the droplet, was achieved. In addition, the friction coefficient increased with the addition of the polymer. This phenomenon can be attributed to the decrease in vapor file thickness. Therefore, the dynamics of self-propelled droplets were significantly affected by the higher viscosity of the polymeric droplets. Furthermore, one of the most beneficial findings of this study is that even a high-viscosity droplet can be transported on a superheated ratchet. Therefore, in chemical processes involving high-viscosity fluids, self-propelled Leidenfrost droplets can potentially replace microreactors.