  A liquid drop impacting a solid surface may splash by emitting a thin liquid
sheet that subsequently breaks apart or by promptly ejecting droplets from the
advancing liquid-solid contact line. Using high-speed imaging, we show that air
pressure and surface roughness influence both splash mechanisms. Roughness
increases prompt splashing at the advancing contact line but inhibits the
formation of the thin sheet. If the air pressure is lowered, droplet ejection
is suppressed not only during thin-sheet formation but for prompt splashing as
well. The threshold pressure depends on impact velocity, liquid viscosity and
surface roughness.
