  Liquid drops that are pinned to solid surfaces by contact-angle hysteresis
can be dislodged by wind forcing. When this occurs at high Reynolds numbers,
substantial drop-interface oscillations precede depinning. It has been
hypothesized that coupling between drop interface oscillations and unsteady
airflow vortices are important to the depinning process. This possibility is
investigated using simultaneous high-speed side-view drop images and airflow
fluctuation measurements. The results show no evidence of coupling across a
range of drop volumes and wind speeds for water drops in air. When properly
scaled by drop volume, drop interface fluctuation frequencies are not affected
by wind speed. Airflow vortex shedding occurs as if the drop were a solid
surface protuberance. For the air/water system in these experiments, vortex
shedding frequencies are substantially higher than drop interface frequencies
and the disparate frequencies may make the hypothesized coupling impossible.
