Instability near the contact line of an evaporating volatile droplet

The evaporation of a droplet deposited on a solid surface represents an intriguing system that displays typical complexities present in many free surface and coating flows. For example, when a droplet of aqueous surfactant solution is deposited on a substrate covered with thin water film, it spreads by propagating fingers. A similar fingered spreading can also be observed during the evaporation of a binary droplet or a surfactant-laden droplet deposited on a dry substrate. In contrast, here, we establish that such patterned spreading behaviour is also exhibited by a volatile droplet of a pure substance when deposited on a dry surface. We show that the fingered spreading of an ethanol droplet deposited on a slightly heated surface can be triggered by controlling the relative humidity around the droplet. Notably, the thermal properties of the underlying substrate also play an important role in determining the extent of fingered spreading. Additionally, our investigation explores the sudden shrinkage of the evaporating droplet in its final moment that leaves myriad small droplets behind.