Towards probing velocity distributions in dense granular fluids: Utilizing fiber Bragg gratings.

Granular gases are commonly characterized through their velocity distribution, which provides access to the granular temperature. In experiments, velocity distributions are typically obtained by particle tracking, which becomes, however, limited at moderate and high particle densities. As a way forward, we propose a new technique for measuring particle velocities in situ by using a fiber Bragg grating (FBG) sensor, which remains applicable at significantly higher particle densities. The FBG sensor detects strain pulses induced by particle-fiber collisions, from which the velocity of the impacting particle can be derived. Applying this method to an ensemble of granular particles allows us to extract its velocity distributions, as we present for a granular system excited by a vibrational shaker. We validate the extracted velocity distribution against conventional particle-tracking measurements, confirming the reliability of the FBG-based technique.