ABSTRACT
When a droplet gently lands on a smooth surface, a thin layer of air is always trapped under the droplet, and it then drains away. Simple as it may seem, there is a large discrepancy between theory and observation. Basic fluid mechanics predicts that the droplet takes about 10 to 100 s to contact the surface. Almost all contacts, however, actually occur in about one-tenth of a second—a mystery that lacks a satisfactory explanation. What causes this huge discrepancy up to 1000 times? Understanding this discrepancy can impact a wide range of applications, including inkjet printing, protection of surfaces from freezing rain, and cleanups of large-scale oil spills. We combine advanced optical measurements, surface profile measurements, and high-speed photography to reveal two mechanisms responsible for speeding up contact [1].
When a droplet impacts a smooth surface with high enough speeds, it always splashes. This common phenomenon is crucial in many important fields such as agriculture, printing, surface coating, and spray cooling. However, despite extensive studies over one century, the origin of splashing remains a big mystery. Combining experiment with model, we show that the air trapped under the liquid drop forms a special flow within a nanoscale gap. This airflow produces a stress 10 times stronger than the common airflow and generates small Kelvin–Helmholtz instabilities that trigger splash. Our model agrees quantitatively with the experimental verifications and brings a fundamental understanding to the general phenomenon of drop splashing on smooth surfaces [2].
[1] "Mechanism of Contact between a Droplet and an Atomically Smooth Substrate", H. Y. Lo, Y. Liu and L. Xu, Phys. Rev. X 7, 021036, 2017. [2] "Kelvin– Helmholtz instability in an ultrathin air film causes drop splashing on smooth surfaces", Y. Liu, P. Tan and L. Xu, Proc. Natl. Acad. Sci. USA (PNAS) 112, 3280- 3284, 2015.
BIOGRAPHY
Prof Xu got his PhD in The University of Chicago and was a Postodc Fellow at Harvard University. His main research interests include Fluid mechanics, Complex fluids, Fluid flow in porous media. He received awards in “2014 Young Researcher Award, The Chinese University of Hong Kong” and “Higher Education Outstanding Scientific Research Output Awards 2014, Natural Science Award Class II (2014 年度高等学校科学研究优秀成果奖, 自然科学奖二等奖), Ministry of Education of China.”