  In this fluid dynamics video, we show thermocapillary actuation of a binary
drop of water and heptanol where the binary drop in motion takes on a UFO-like
shape. On a parylene-coated silicon surface subjected to a linear temperature
gradient, a pure heptanol droplet quickly moves to the cold side by the
Marangoni stress, while a pure water droplet remains stuck due to a large
contact angle hysteresis. When the water droplet was encapsulated by a thin
layer of heptanol and thermally actuated, the binary droplet takes on a
peculiar shape resembling an UFO, i.e. an "unidentified" floating object as the
mechanism is not yet completely understood. Our finding suggests that pure
liquid droplets (e.g. aqueous solutions) that are not conducive to
thermocapillary actuation can be made so by encapsulating them with another
judiciously chosen liquid (e.g. heptanol).
