Elastic response of wire frame glasses. II. Three-dimensional systems

We study the elastic response of rigid wire frame particles in concentrated glassy suspensions to a step strain by applying the simple geometric methods developed in Paper I. The wire frame particles are comprised of thin rigid rods of length L, and their number density, ρ, is such that ρL³ ≫ 1. We specifically compare rigid rods to L-shapes made of two equal length rods joined at right angles. The behavior of wire frames is found to be strikingly different from that of rods. The linear elasticity scales like ρ³L⁶ for L-shaped particles, whereas it scales proportional to ρ for rods and the non-linear response shows a transition from shear hardening to shear softening at a critical density ρ_c∼K/k_BTL⁶, where K is the bending modulus of the particles. For realistic particles made of double stranded DNA, this transition occurs at densities of about ρL³ ∼ 10. The reason for these differences is that wire frames can be forced to bend by the entanglements with their surroundings, whereas rods always remain straight. This is found to be very important even for small strains, with most particles being bent above a critical strain γ_c∼ρL^{3 -1}.