Left: Experimental measurement of colloidal particles driven in a thin microfluidic channel. The particles form stable, hydrodynamically coupled pairs moving at the same velocity (arrows). These pairs are the fundamental quasiparticles of the system. Right: Simulation of a hydrodynamic crystal, showing a quasiparticle pair (leftmost yellow and orange particles) propagating in a hydrodynamic crystal, leaving behind a supersonic Mach cone of excited quasiparticles. Colors denote the magnitude of the pair excitation, and the white background denotes their velocity. Credit: Institute for Basic Science