Mechanism of Contact between a Droplet and an Atomically Smooth Substrate

TL;DR

This study investigates why droplets contact atomically smooth surfaces much faster than fluid mechanics predicts, revealing two universal mechanisms—droplet skidding and boundary flow—that accelerate contact and air drainage.

Contribution

The paper uncovers two previously unrecognized mechanisms that significantly speed up droplet contact on smooth surfaces, resolving a longstanding discrepancy with fluid mechanics predictions.

Findings

Droplet skidding causes early contact at the thinnest gap.

Boundary flow around 0.1 mm/s accelerates air drainage.

Two mechanisms universally explain contact phenomena on smooth substrates.

Abstract

When a droplet gently lands on an atomically smooth substrate, it will most likely contact the underlying surface in about 0.1 s. However, theoretical estimation from fluid mechanics predicts a contact time of 10 to 100 s. What causes this large discrepancy, and how does nature speed up contact by 2 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 30 nm 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 0.1 mm/s expedites air drainage by over 1 order of magnitude. Together, these two mechanisms universally explain general contact phenomena on smooth substrates. The fundamental discoveries shed new light on contact and drainage research.