  When a droplet hits a surface fast enough, droplet splashing can occur:
smaller secondary droplets detach from the main droplet during impact. While
droplet splashing on smooth surfaces is by now well understood, the surface
roughness also affects at which impact velocity a droplet splashes. In this
study, the influence of the surface roughness on droplet splashing is
investigated. By changing the root mean square roughness of the impacted
surface, we show that the droplet splashing velocity is only affected when the
droplet roughness is large enough to disrupt the spreading droplet lamella and
change the droplet splashing mechanism from corona to prompt splashing.
Finally, using Weber and Ohnesorge number scaling models, we also show that the
measured splashing velocity for both water and ethanol on surfaces with
different roughness and water-ethanol mixtures collapse onto a single curve,
showing that the droplet splashing velocity on rough surfaces scales with the
Ohnesorge number defined with the surface roughness length scale.
