  Inspired by the observation of intricate and beautifully dynamic patterns
generated by food coloring on clean glass slides, we have investigated the
behavior of propylene glycol and water droplets on high energy surfaces. In
this fluid dynamics video we show a range of interesting behaviors including
long distance attraction, and chasing/fleeing upon contact. We present
explanations for each of these behaviors including a mechanism for the long
distance interactions based on vapor facilitated coupling. Finally we use our
understanding to create several novel devices which: spontaneously align
droplets, drive droplets in circles, cause droplets to bounce on a vertical
surface, and passively sort droplets by surface tension. The simplicity of this
system lends it particularly well to application as a toy model for physical
systems with force fields and biological systems such as chemotaxis and
motility.
