Spreading Dynamics of Volatile Droplets Deposited on Immiscible Liquid Layers

Spreading dynamics and interfacial instabilities of a volatile liquid released on an immiscible liquid bath creates a highly complex flow for both solutal and thermal Marangoni stresses appear simultaneously. We investigate the development of a droplet of methanol deposited onto an oil bath. We show that the spreading dynamic of the methanol droplet of radius R(t) obeys a R(t)~t^0.25±0.02 scaling. This exponent is found to be independent of the oil bath depth and the initial drop volume. Neglecting the evaporation effects at the early stage of the spreading, a similarity solution based on lubrication approximation is developed, predicting a R(t)~t^1/4 consistent with the experimental observations. It indicates that solutal Marangoni and viscosity are the main ingredient for the drop spreading. Thermal Marangoni, on the other hand, play a prominent role in destabilizing the rim at the periphery of the methanol droplet, and breaking it into hundreds of small droplets. Measuring different parameters of the instability, the underlying physics behind the bursting phenomenon is explained in detail. Using thermal imaging, effect of evaporation on the recession of methanol droplet is also studied.