Experimental observation of gapped shear waves and liquid-like to gas-like dynamical crossover in active granular matter

Abstract Unlike crystalline solids, liquids do not exhibit long-range order due to frequent structural rearrangements. When considering shorter time and length scales, molecular dynamics simulations and theoretical propositions suggest that collective shear excitations in liquids display a gap in wave-vector space, referred to as the k-gap. Above this gap, solid-like transverse waves re-emerge. However, direct experimental verification of this phenomenon in classical liquids remains elusive, with the only documented evidence from studies in two-dimensional dusty plasmas. Active granular systems provide a novel platform for exploring the emergence of collective dynamics and showcasing a rich interplay of complex phases and phenomena. Our study focuses on bi-disperse active Brownian vibrators. In this work, we demonstrate that this active system exhibits both gas-like and liquid-like phases, depending on the packing fraction, despite pure hard-disk-like repulsive interactions. Notably, within the granular liquid-like phase, we experimentally validate the existence of a k-gap in the dispersion of transverse excitations. Our results offer a direct experimental confirmation of the k-gap phenomenon, extending its relevance beyond classical thermal liquids to active granular systems, and reveal the existence of intriguing similarities between the physics of active granular matter and supercritical fluids.