Study On The Effect Of Liquid Viscosity And Surface Wettability On Droplet Impact Dynamics

In this dissertation,we experimentally,theoretically and numerically investigated the impact dynamics of different viscous droplets on solid surfaces with diverse wettabilities.We show that the outcome of an impinging droplet is dependent on the physical property of the droplet and the wettability of the surface.Whereas only deposition was observed on lyophilic surfaces,more impact phenomena-bubble entrapment,partial rebound,complete rebound and so on-were identified on lyophobic and superlyophobic surfaces.The maximum spreading ratio,which was proved to be dependent on impact velocity and liquid viscosity,is one of the most parameters to characterize the impact results.However it was found that none of the existing theoretical models can well describe the maximum spreading factor under different impact conditions.Combining the energy analysis and simulation study,we found that the theoretical model always overestimates the maximum spreading ratios for low-viscosity liquid due to the neglected left kinetic energy and the inaccurate deformation assumption,while the break-down of boundary layer theory for highly viscous liquids provides underestimated maximum spreading ratios.By using the modified capillary-inertial time,which considers the effects of liquid viscosity and surface wettability on droplet spreading,a universal scaling law describing the spreading time was obtained.The left kinetic energy inside the droplet after reaching the maximum spreading causes the drop to oscillate.We analyzed the post-impact droplet oscillation with the theory for damped harmonic oscillators and interpreted the effects of liquid viscosity and surface wettability on the oscillation by simple scaling analyses.Further,numerical simulation is performed to study the influence of impact velocity,liquid viscosity and surface wettability on the energy variation with time.It revealed that viscous dissipation increases with the impact velocity and liquid velocity.The left kinetic energy and surficial energy increase with the increase of the impact velocity and the decrease of liquid velocity.For low-viscosity liquid,surface wettability plays role on the variation of energy variation.Its influence is less significant with the increase of liquid viscosity.