  We describe a system for interrogating the acoustic properties of
sub-nanoliter liquid samples within an open microfluidics platform. Sessile
droplets were deposited onto integrated optomechanical sensors, which possess
ambient-medium-noise-limited sensitivity and can thus passively sense the
thermally driven acoustic spectrum of the droplets. The droplet acoustic
breathing modes manifest as resonant features in the thermomechanical noise
spectrum of the sensor, in some cases hybridized with the sensor's own
vibrational modes. Excellent agreement is found between experimental
observations and theoretical predictions, over the entire ~ 0 - 40 MHz
operating range of our sensors. With suitable control over droplet size and
morphology, this technique has the potential for precision acoustic sensing of
small-volume biological and chemical samples.
