Sidney Nagelís lab at the University of Chicago has explored the behavior
of liquid drops---how and when they fall from a faucet---granular materials,
crumpling, and other everyday-but-difficult-to-explain phenomena. At
the APS meeting, Nagelís graduate student, Lei Xu, revealed a surprising
discovery concerning one of the commonest physical effects: the splash
a liquid drop makes when it strikes a flat surface.
Under ordinary atmospheric
conditions a liquid drop will flatten out on impact, splay sideways,
and also raise a tiara-like crown of splash droplets. Remove some of
the ambient atmosphere, and surprisingly the splash becomes less. At
about one-fifth atmosphere the splash disappears altogether, leaving
the outward going splat but no upwards splash (see movie at kauzmann.uchicago.edu
). Apparently it is the presence of the air molecules that give the
impacting liquid something to push off of; remove the surrounding atmosphere,
and the splash disappears.