Drop bouncing dynamics on ultra-thin films*

Drops impacting liquid films tend to irreversibly coalesce and spread on a film made of the same substance. However, in this work, new dynamics of drops initiating contact yet carrying enough momentum to completely lift off of the substrate are called contact bouncing drops. An ultra-thin film is generated by making the ratio of the liquid films to the drop diameters to be less than 0.1 by either creating a smooth draining or flowing film via a syringe pump. High-speed interferometry is also used to visualize the interfacial gas layer over various film thicknesses and Weber numbers. While the increased mobility of the thin liquid films increases the gas entrainment early on, it concomitantly accelerates the rate of the gas purging underneath the drops, enhancing the probability of contact before the gas film retraction and the subsequent drop lift-off sequence. Drops which contact the liquid film during the retraction stage are able to bounce with negligibly small mass loss where the residual drop deposition may be of use to various controlled nanoliter range drop deposition applications as well as understanding the physics of dropwise contamination routes on various surfaces.