Isothermal and non-isothermal spreading of a viscous droplet on a solid surface in total wetting condition

Abstract

We study the isothermal and non-isothermal spreading of viscous silicone oil droplets on a glass surface in total wetting condition. In particular, the effects of viscosity, impact velocity, and substrate temperature on the spreading dynamics are reported. We employ high-speed photography to record time-varying droplet shapes from the side. An infrared camera maps the temperature distribution on the liquid–gas interface. In the isothermal inertial-capillary or early regime, the initial spreading is driven by inertial forces, and kinetic energy converts into surface energy and gets dissipated by bulk viscosity. The later stage is governed by the balance of surface energy and viscosity dissipation, i.e., capillary–viscous or late regime. The characteristics timescales of the two regimes are obtained using scaling arguments. The measured crossover time from early to late spreading regimes for different cases of impact velocity and viscosity corroborates with a scaling analysis developed in the present work. Measurements confirm the value of exponents of established power-law spreading with time in early and late regimes r∼tn. At a larger substrate temperature, the spreading magnitude is larger for droplets with larger viscosity and is explained by the reduction of viscous dissipation by heating the droplet. However, in the case of non-isothermal spreading of a low viscosity droplet, recoiling after the early spreading reduces the spreading magnitude compared to the isothermal case. We explain the recoiling and spreading rates obtained in different cases. We analyze unsteady heat transfer between the droplet and substrate by combining measurements and a numerical model.