Understanding the Early Regime of Drop Spreading.

We present experimental data to characterize the spreading of a liquid drop on a substrate kept submerged in another liquid medium. They reveal that drop spreading always begins in a regime dominated by drop viscosity where the spreading radius scales as r ∼ t with a nonuniversal prefactor. This initial viscous regime either lasts in its entirety or switches to an intermediate inertial regime where the spreading radius grows with time following the well-established inertial scaling of r ∼ t(1/2). This latter case depends on the characteristic viscous length scale of the problem. In either case, the final stage of spreading, close to equilibrium, follows Tanner's law. Further experiments performed on the same substrate kept in ambient air reveal a similar trend, albeit with limited spatiotemporal resolution, showing the universal nature of the spreading behavior. It is also found that, for early times of spreading, the process is similar to coalescence of two freely suspended liquid drops, making the presence of the substrate and consequently the three-phase contact line insignificant.