Experimental and numerical simulation on the dual role of liquid and gas viscosity in the droplet spreading dynamics

The dynamics of droplet impact on a free-slip surface are studied experimentally and numerically. Experimental results and numerical predictions demonstrate good consistency in the dimensionless droplet maximum spreading diameter βmax. Non-monotonic droplet maximum spread rate is observed and ascribed to the dual role of liquid viscosity at high Weber number (We≥30, We=ρD0V02/σ), which is different from the small Weber number (We< 30). For droplet spreading under relatively small Reynolds number Re=ρD0V0/μ (indicating high liquid viscosity), the initial kinetic energy dissipates efficiently, allowing surface energy to dominate the energy budget. Liquid viscosity dominates the viscous dissipation rate, which decreases as Re increases. However, for relatively large Re, the initial kinetic energy cannot be fully utilized, leading to substantial strain rate in the gas phase. The strain rate increases with Re increasing and in turn dominates viscous dissipation. Since gas viscous dissipation becomes more significant under large Re conditions, the impact of gas viscosity on droplet spreading was examined. The dual role of gas viscosity is also observed, where the energy dissipation in the gas phase exhibits a non-monotonic trend concerning Re. A practical model for estimating βmax under small Weber numbers is proposed, demonstrating good consistency with previous experimental results.