(387d) Simulations of Splashing Micro-Scale Droplets on a Dry Surface

The physics of dynamic contact line behavior is an important, but still poorly understood topic, with applications in, for example, coating processes, inkjet printing, and aerosol printing. When a liquid spreads too fast on a surface, the contact line becomes unstable, resulting in wetting failure, i.e. defects in coating films, and the splashing of droplets when they hit a surface. It has been known for some time that reducing the ambient pressure of the system results in a more stable contact line, and thus suppresses wetting failure and splashing. However, current theories are not able to explain this effect. Other open questions include the nature of the stress and pressure singularity at the contact line, and the effect of wall roughness and contact angle hysteresis on spreading.

Results are presented for the full simulations of micro-scale droplets splashing on a dry surface. The simulations are performed using a Volume Of Fluid approach and a Finite Volume technique. The contact line is described using the generalized Navier boundary condition. Both the gas phase and the liquid phase are assumed to be incompressible.

The results of these simulations show good agreement with experiments. A lamella forms upon impact, with an apparent contact angle appraching 180 degrees. This lamella breaks up into smaller droplets as the spreading progresses. When the simalation is repeated with a reduced ambient pressure the droplet does not break up, and splashing is suppressed. Moving beyond splashing at high speeds, results will be presented for the spreading of liquid droplets on rough or patterned substrates.