Study of early time dynamics of drop spreading in different surrounding pressure

Abstract

The initial spreading phenomena of oil droplets on a partially wettable surface under various surrounding pressures was conducted through an experimental investigation outlined in this study. The gauge pressure of the surrounding medium was varied stepwise in the range from atmospheric pressure to 30 MPa. Upon contact with the surface, spreading alters the total energy of the system. The timescale for this process is very fast and the spreading involves the motion of the three-phase contact line established at the junction of the droplet, the substrate and the surrounding medium. In this study, the early time dynamics of different grades of oil drops spreading on PTFE substrates were examined utilizing high-speed imaging and drop shape analysis. Results show that the temporal evolution of the spreading radius closely resembles a power law where the empirically obtained exponent, α, was calculated for different surrounding pressures. Observations of instantaneous drop spreading reveal that spreading occurs through three distinct regimes: an initial spreading regime, a short transition regime and a viscous regime. In the inertial regime of the early time dynamics, spreading radius scales as r~tα where the spreading exponent, α, decreases with the increase of pressure. In addition, pressure influences the onset of the transition regime, the contact line speed, and the contact angle during the initial wetting process.