  Droplets, particularly water, are abundant in nature and artificial systems.
Thermal fluctuations imply that droplet interfaces behave like a stormy sea at
the sub-nanometer scale.Thermal capillary-waves have been widely studied since
1908 and are of key importance in surface science. Here we use an optical mode
of a micro-droplet to probe its radius fluctuation. Our droplet benefits from a
finesse of 520 that accordingly boosts its sensitivity inrecording Brownian
capillaries at 100-kHz rates and 1+-0.025 angstom amplitudes, in agreement with
natural-frequency calculation and the equipartition theorem. A fall in the
fluctuation spectrum is measured below cutoff at the
drop'slowest-eigenfrequency. Our device facilitates resonantly-enhanced
optocapillary interactions that might enable optical excitation (/cooling) of
capillary droplet-modes, including with the most-common and important liquid -
water.
