Direct Evidence for an Absorbing Phase Transition Governing Yielding of a Soft Glass

We present the first direct experimental evidence showing that yielding of a prototypical soft solid - a colloidal glass - is a non-equilibrium 'absorbing' phase transition. By simultaneously quantifying single-particle dynamics and bulk mechanical response, we extracted critical exponents for the order parameter and the relaxation time and found that this transition belongs to the conserved directed percolation universality class. In addition, the observed critical slowing down is accompanied by a growing correlation length associated with the size of regions of high Debye-Waller factor which are precursors to yield events in glasses. Our results show unambiguously that yielding of soft solids falls squarely in the realm of non-equilibrium critical phenomena.