Bubble dynamics in thin liquid films and breakup at drop impact†
A liquid drop impacting on a smooth surface heated above the boiling point and below the Leidenfrost temperature spreads over the surface forming a thin liquid film. We observed bubble nuclei originating at the first contact of a liquid with a heated sapphire surface, and the formation of a thin liquid film with one layer of microbubbles. In the present paper, we describe in detail the bubble dynamics for the substrate temperature ranging from 130 to 170 °C. The thickness of the liquid film is estimated to vary within the range from 40 to 80 μm at We = 76 and an advancing contact angle of 76°. We have detected how bubbles break the film, which is followed by dry patch propagation and sessile droplet formation, and elucidated the mechanism of spontaneous liquid film breakup. The bubble coalescence and waves induced by the rollback flow from the lamella periphery or by reversible bubble bursting are the reason for irreversible hole formation.