The bouncing dynamics of droplets with various viscosities on superamphiphobic surfaces is experimentally investigated. It is shown that contact number T ≡ We Re−1/2 can characterize both the maximum spreading factor and the contact time of viscous liquid droplets. Using energy conservation and contact number T, a new theoretical model of the maximum spreading factor for various viscous liquid droplets is proposed. The predictions of this model agree with the experimental results and data from previous studies. Liquid viscosity has a significant effect on the impact process and leads to an increase in contact time. For low-viscosity droplets, contact time is independent of impact velocity, whereas for high-viscosity droplets, contact time increases with increasing impact velocity. Therefore, the new time scaling τ ∼ D0/U0T=ρD03μU0/σ21/2 proposed in our previous work is adopted to characterize the effects of viscosity on contact time. Excellent agreement between the scaling and experimental results is found. These findings should therefore help understand how to design surfaces for a variety of applications, such as anti-icing or reducing heat transfer with impacting liquids.

The bouncing dynamics of microdroplets with various viscosities on a superhydrophobic surface is numerically investigated. An axisymmetric lattice Boltzmann method is developed on the basis of Zheng et al. capable of handling multiphase flows with a large density ratio, which is implemented to simulate the impact. It is shown that in the low-viscosity regime, the contact time tc remains constant over a wide Weber number range (10 < We < 120), which is consistent with macro-scale bouncing. Nevertheless, in the high-viscosity regime, tc increases with impact velocity. A contact number T≡WeRe−1/2=ρD0 ηU03/σ21/2 is proposed to describe the viscosity effect; meanwhile, a new scaling τ ∼ D0/U0T=ρηD03U0/σ21/2 is deduced to characterize the contact time for this regime, and the simulated results for such droplets agree well with the new scaling. To find out the internal physical mechanism, the evolution of kinetic energy, dissipated energy, and velocity vector fields is studied, which quantifies the impact dynamics. Also, simulation data demonstrate that viscous dissipation is not negligible even for relatively low-viscosity fluids. These findings are highly useful for fundamental understanding of microdroplet dynamics with various viscosities, and it can be used to precisely control the contact time.