Understanding droplet motion on solid surfaces is essential to many natural and industrial processes. Yet conventional theories cannot fully explain why droplets can require more force to start moving than to keep sliding, or why friction can decrease as sliding velocity increases.
A collaborative research team led by Professor Junichiro Shiomi at The University of Tokyo, together with Professor Takeshi Fukuma at Kanazawa University, has demonstrated that droplets on molecularly coated solid surfaces become harder to move the longer they remain at rest. On certain surfaces, the team also observed a surprising phenomenon in which friction decreases as droplets move faster, contrary to the conventional expectation that friction increases with speed. Using precise friction measurements and atomic force microscopy, the researchers linked these behaviors to the interactions between the droplet’s contact line and nanoscale surface defects over time. They propose that, while a droplet rests, thermal fluctuations allow its contact line to gradually become trapped at surface defects, increasing the force needed to initiate motion. This process, termed “contact-line aging”, competes with the renewal of contact-line pinning sites during sliding, giving rise to an unusual dependence of friction on sliding velocity. They subsequently developed a physical model that quantitatively explains both the time-dependent increase in static friction and the velocity-dependent behavior of dynamic friction. These findings provide a new physical framework for understanding and controlling macroscopic droplet motion through nanoscale surface design, with potential applications in coatings, printing, and microfluidic devices.
Figure 1. History-dependent droplet friction driven by contact-line aging at nanoscale surface defects. The inset illustrates how defect-mediated pinning strengthens with resting time.
Papers
Journal: Proceedings of the National Academy of Sciences（PNAS）
Title: History-Dependent Droplet Friction Driven by Contact-line Aging
Authors: Yuanzhe Li, Yaerim Lee, Kaito Hayashi, Haohui Zhang, Kazuki Miyata, Jiangyang Liu, Takuro Hosomi, Takeshi Yanagida, Takeshi Fukuma, Timothée Mouterde, Junichiro Shiomi*
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