In this talk, we will discuss recently discovered collective phenomena in nonequilibrium soft
matter systems and their links to basic notions of quantum matter—particularly quasiparticles,
flat bands, and exotic topology.
First, we will examine a classical system of hydrodynamically interacting particles in two
dimensions. In the disordered phase of this matter, measurements show a population of long-
lived particle pairs. Analysis of the ordered crystalline phase identifies the pairs as effective
quasiparticles, emerging at the Dirac cones of the spectrum and inducing the melting of the
crystal. When the intrinsic threefold symmetry of the hydrodynamic interaction matches that of
the crystal, the cones connect into a nearly-flat band of slow excitations whose divergent density
drives a much sharper melting transition.
In the second part of the talk, we will discuss how adding elastic interactions among the
hydrodynamically coupled particles gives rise to non-Hermitian topological phenomena. The
interplay of hydrodynamics and elasticity splits the Dirac cones into bulk Fermi arcs, pairing
exceptional points with opposite half-integer topological charges. The bulk Fermi arc is a generic
hallmark of the system exhibited in all lattice and flow symmetries.
Altogether, these findings demonstrate the usefulness of concepts from quantum matter theory in
understanding many-body physics in classical dissipative settings—nonequilibrium phase
transitions and exceptional topology. Moreover, analogs of collective phenomena in quantum
systems are observed in a broad class of ordinary soft matter, suggesting dissipative systems as
accessible playgrounds to investigate these phenomena and opening avenues for technological
applications in this regime.
Saeed et al. (2021) Quasiparticles, Flat Bands, and the Melting of Hydrodynamic Matter, in
TT (2021) Exceptional Topology in Ordinary Soft Matter, Phys Rev E.