Contact line propagation beneath an impacting drop

EMSI lab has shown the very initial stages of contact propagation beneath an impacting drop. Using a nano-meter sensitive microscopy modality developed by EMSI lab head Kolinski, images of the liquid-air interface were recorded that show how the contact line velocity is set beneath the impacting droplet. The wetting front advances with a universal feature, called a `halo' due to its appearance in the images, that regulates the capillary stress in the very thin layer of air beneath the droplet. In this work, the researchers show that the feature is in fact a capillary wave, and that the wetting front advances by `surfing' on this capillary wave - at a velocity an order of magnitude slower than expected from a simple inertial-capillary balance. 

Finally, the effect of liquid viscosity was explored, and was demonstrated to have a much weaker effect on the contact line velocity than one might assume if the wetting front were advancing at its `speed limit,' or the capillary velocity, which decreases inversely with viscosity. This work establishes a fundamentally different modality of contact line propagation than is expected, and can account for anomalously slow wetting velocities.

The article was recently published in Physical Review Fluids, and can be found at the link below.