  A liquid drop impacting a smooth solid substrate splashes by emitting a thin
liquid sheet from near the contact line of the spreading liquid. This sheet is
lifted from the substrate and ultimately breaks apart. Surprisingly, the splash
is caused by the ambient gas, whose properties dictate when and if the sheet is
created. Here I focus on two aspects of this process. Using high-speed imaging
I find that the time of thin-sheet creation displays a different quantitative
dependence on air pressure if the sheet is created during the early stages of
spreading, rather than when the liquid has already spread to a large radius.
This result sheds light on previously observed impact velocity regimes.
Additionally, by measuring impacts of drops on surfaces comprised of both rough
and smooth regions, I identify a new threshold velocity that limits the times
at which the thin sheet can be created. This velocity determines the threshold
pressure below which splashing is suppressed.
