Jamming and Shear Yielding of Ultra-Low Density Packings of Brownian Rods

Jamming describes a transition from a freely flowing to an arrested amorphous state with rigidity. At critical values of the packing fraction large aspect ratio rods undergo a jamming transition. Prior work on granular rod-like systems has demonstrated the existence of a maximally, isotopically packed state with a universal packing fraction. The influence of thermal fluctuations on rod packings remains unclear, with much experimental work limited to low aspect ratio regimes. We experimentally study the jamming of ultra-low density Brownian rigid rods with millimeter persistence length. To overcome the previous limits on generating high-density isotropic suspension, we polymerize rods in situ from monomers. Beyond a critical density or aspect ratio, the rods arrest into a solid, jammed structure that lacks local orientational order. Using optical fluorescence microscopy, we study single filament dynamics in jammed environments. Due to strong local confinement, both the diffusive and bending dynamics of these filaments are strongly suppressed. The dense isotropic packing yields above a critical strain, generating a locally aligned, fluidized state with a dramatically lower shear modulus. We correlate this yielding transition with the microscopic dynamics of entangled rods, finding reduced confinement effects due to the yielding induced dilation of the reptation tube.