Why a droplet can contact smooth surface so rapidly?
Lei Xu, Physics Department, The Chinese University of Hong Kong
When a droplet gently lands on an atomically-smooth substrate, it will most likely contact the underlying surface in about 0.1 second. However, theoretical estimation from fluid mechanics predicts a contact time of 10 to 100 seconds. What causes this large discrepancy and how does nature speed up contact by two orders of magnitude? To probe this fundamental question, we prepare atomically-smooth substrates by either coating a liquid film on glass or using a freshly-cleaved mica surface, and visualize the droplet contact dynamics with 30nm resolution. Interestingly, we discover two distinct speed-up approaches: (1) droplet skidding due to even minute perturbations breaks rotational symmetry and produces early contact at the thinnest gap location, and (2) for the unperturbed situation with rotational symmetry, a previously-unnoticed boundary flow around only 0.1mm/s expedites air drainage by over one order of magnitude. Together these two mechanisms universally explain general contact phenomena on smooth substrates. The fundamental discoveries shed new light on the contact and drainage research.