100microliters, by 15cm
.0001kg of water, by 0.15m
E=mgh
E=9.8(0.0001)(0.15)=0.000146 Joules of energy available.
Let's be generous and go with a red LED (2.2V) that is visible with only 5mA.
0.000147=(2.2)(0.005)t, t=0.013 seconds
One drop may be able to produce a brief flash that you could see. Neat gimmick.