Achieving the Bidirectional Transportation of Boiling Droplets on the Functional Ratchet-Valley Array

Guiding the directional transport of droplets on heated surfaces is of critical importance for thermal management technologies. While substantial progress has been made in studying unidirectional droplet transport on heated surfaces, achieving controllable bidirectional transport across a wide temperature range remains a significant challenge. Based on laser micronano processing technology, this study fabricates a functional ratchet-valley array (RVA) on the 45 carbon steel surface to achieve bidirectional droplet self-transport across a wide temperature range. Experiments confirm that droplets exhibit three distinct boiling states on the functional RVA surface under different temperatures: nucleate boiling, transitional boiling, and film boiling. During nucleate and transitional boiling states, droplets move forward along the RVA; in contrast, when in film boiling, they move backward against the RVA. Notably, during the transitional boiling state, we observe that the impact process of droplets with the high-temperature RVA exhibits two distinct contact modes, which we define as the adhesive mode and the rebound mode. Although the two contact modes share the same direction of movement, differences in their contact modes lead to significant disparities in the motion behavior. In addition, this study focuses on investigating the effects of parameters such as the Weber number (We) and droplet volume on the directional transport behavior of droplets. It is found that by adjustment of the We and droplet volume, droplets can exhibit different boiling states at the same temperature, thereby achieving bidirectional self-transport behavior.