Toward Unveiling the Anomalies Associated with the Spontaneous Spreading of Droplets

The liquid droplet spreads over a solid surface to minimize the surface energy when brought in direct contact with the surface. The spreading process is rapid in the early stages, tends to slow down during its progress, and has resulted in peculiarity due to the experimental difficulties in the accurate determination of the contact line radius. In the present numerical study, we found that drop spreading begins with a viscosity-dominated Stokes regime, where contact radius scales as r ∼ t for a wide range of drop liquid viscosities. Subsequent to the Stokes regime, the inertial regime is observed where contact radius scales as r ∼ t^0.5 for low- to medium-viscous droplets, whereas for very high viscous drops, the spreading dynamics is completely dominated by the viscous regime. It is also found that the equilibrium wetting condition does not affect the power-law scaling for the contact radius of the drop. The amplitude of capillary waves induced across the interface of the drop is observed to be sufficiently high to cause necking and ejection of satellite drops from the main drop during its spreading for low-viscous liquids from complete wetting to partial wetting conditions. A regime plot between the Ohnesorge number and advancing contact angle of the substrate is presented to demarcate the regions of damped waves without pinch-off and drop spreading with satellite drops.