Bubble formation during the collision of a sessile drop with a meniscus

TL;DR

This study investigates how the location of coalescence during a droplet's impact on a moving meniscus affects bubble size, revealing that coalescence at the contact line produces larger bubbles than at a finite height.

Contribution

It provides experimental insights into bubble formation mechanisms during droplet impact, highlighting the influence of coalescence location on bubble size and dynamics.

Findings

Bubble size depends on coalescence location.

Coalescence at the contact line yields larger bubbles.

Bubble size is independent of drop size and speed when coalescence occurs above the substrate.

Abstract

The impact of a sessile droplet with a moving meniscus, as encountered in processes such as dip-coating, generically leads to the entrapment of small air bubbles. Here we experimentally study this process of bubble formation by looking through the liquid using high-speed imaging. Our central finding is that the size of the entrapped bubble crucially depends on the location where coalescence between the drop and the moving meniscus is initiated: (i) at a finite height above the substrate, or (ii) exactly at the contact line. In the first case, we typically find bubble sizes of the order of a few microns, independent of the size and speed of the impacting drop. By contrast, the bubbles that are formed when coalescence starts at the contact line become increasingly large, as the size or the velocity of the impacting drop is increased. We show how these observations can be explained from a balance between the lubrication pressure in the air layer and the capillary pressure of the drop.