  The bulk acoustic modes of liquid droplets, well understood from a
theoretical perspective, have rarely been observed experimentally. Here, we
report the direct observation of acoustic vibrational modes in a
picoliter-scale droplet, extending up to ~ 40 MHz. This was achieved by
coupling the droplet to an ultra-sensitive optomechanical sensor, which
operates in a thermal-noise limited regime and with a substantial contribution
from acoustic noise in the ambient medium. The droplet vibrational modes
manifest as Fano resonances in the thermal noise spectrum of the sensor. This
is amongst the few reported observations of droplet acoustic modes, and of Fano
interactions in a coupled mechanical oscillator system driven only by thermal
Brownian motion.
