Discontinuous shear-thinning in adhesive dispersions

We present simulations for the steady-shear rheology of a model adhesive dispersion. We vary the range of the attractive forces $u$ as well as the strength of the dissipation $b$. For large dissipative forces, the rheology is governed by the Weisenberg number $\text{Wi}\sim b\dot\gamma/u$ and displays Herschel-Bulkley form $\sigma = \sigma_y+c\text{Wi}^\nu$ with exponent $\nu=0.45$. Decreasing the strength of dissipation, the scaling with $\text{Wi}$ breaks down and inertial effects show up. The stress decreases via the Johnson-Samwer law $\Delta\sigma\sim T_s^{2/3}$, where temperature $T_s$ is exclusively due to shear-induced vibrations. During flow particles prefer to rotate around each other such that the dominant velocities are directed tangentially to the particle surfaces. This tangential channel of energy dissipation and its suppression leads to a discontinuity in the flow curve, and an associated discontinuous shear thinning transition. We set up an analogy with frictional systems, where the phenomenon of discontinuous shear thickening occurs. In both cases tangential forces, frictional or viscous, mediate a transition from one branch of the flowcurve with low tangential dissipation to one with large tangential dissipation

Universal nature of particle displacements close to glass and jamming transitions

We examine the structure of the distribution of single particle displacements (van-Hove function) in a broad class of materials close to glass and jamming transitions. In a wide time window comprising structural relaxation, van-Hove functions reflect the coexistence of slow and fast particles (dynamic heterogeneity). The tails of the distributions exhibit exponential, rather than Gaussian, decay. We argue that this behavior is universal in glassy materials and should be considered the analog, in space, of the stretched exponential decay of time correlation functions. We introduce a dynamical model that describes quantitatively numerical and experimental data in supercooled liquids, colloidal hard spheres and granular materials. The tails of the distributions directly explain the decoupling between translational diffusion and structural relaxation observed in glassy materials.Comment: 5 pages; 4 fig

Confinement and crowding control the morphology and dynamics of a model bacterial chromosome

Motivated by recent experiments probing shape, size and dynamics of bacterial chromosomes in growing cells, we consider a polymer model consisting of a circular backbone to which side-loops are attached, confined to a cylindrical cell. Such a model chromosome spontaneously adopts a helical shape, which is further compacted by molecular crowders to occupy a nucleoid-like subvolume of the cell. With increasing cell length, the longitudinal size of the chromosome increases in a non-linear fashion to finally saturate, its morphology gradually opening up while displaying a changing number of helical turns. For shorter cells, the chromosome extension varies non-monotonically with cell size, which we show is associated with a radial to longitudinal spatial reordering of the crowders. Confinement and crowders constrain chain dynamics leading to anomalous diffusion. While the scaling exponent for the mean squared displacement of center of mass grows and saturates with cell length, that of individual loci displays broad distribution with a sharp maximum.Comment: 12 pages, 12 figure