20 research outputs found
Rheology and microrheology of deformable droplet suspensions
Dense suspensions of soft colloidal particles display a broad range of
physical and rheological properties which are still far from being fully
understood. To elucidate the role of deformability on colloidal flow, we employ
computer simulations to measure the apparent viscosity of a system of droplets
of variable surface tension subjected to a pressure-driven flow. We confirm
that our suspension generically undergoes discontinuous shear thinning, and
determine the dependence of the onset of the discontinuity on surface tension.
We find that the effective viscosity of the suspension is mainly determined by
a capillary number. We present active microrheology simulations, where a single
droplet is dragged through the suspension. These also show a dynamical phase
transition, analogous to the one associated with discontinuous shear thinning
in our interpretation. Such a transition is signalled by a discontinuity in the
droplet velocity versus applied force
Non-equilibrium effects of molecular motors on polymers
We present a generic coarse-grained model to describe molecular motors acting on polymer substrates, mimicking, for example, RNA polymerase on DNA or kinesin on microtubules. The polymer is modeled as a connected chain of beads; motors are represented as freely diffusing beads which, upon encountering the substrate, bind to it through a short-ranged attractive potential. When bound, motors and polymer beads experience an equal and opposite active force, directed tangential to the polymer; this leads to motion of the motors along the polymer contour. The inclusion of explicit motors differentiates our model from other recent active polymer models. We study, by means of Langevin dynamics simulations, the effect of the motor activity on both the conformational and dynamical properties of the substrate. We find that activity leads, in addition to the expected enhancement of polymer diffusion, to an effective reduction of its persistence length. We discover that this effective "softening" is a consequence of the emergence of double-folded branches, or hairpins, and that it can be tuned by changing the number of motors or the force they generate. Finally, we investigate the effect of the motors on the probability of knot formation. Counter-intuitively our simulations reveal that, even though at equilibrium a more flexible substrate would show an increased knotting probability, motor activity leads to a marked decrease in the occurrence of knotted conformations with respect to equilibrium
Flow of Deformable Droplets:Discontinuous Shear Thinning and Velocity Oscillations
We study the rheology of a suspension of soft deformable droplets subjected
to a pressure-driven flow. Through computer simulations, we measure the
apparent viscosity as a function of droplet concentration and pressure
gradient, and provide evidence of a discontinuous shear thinning behaviour,
which occurs at a concentration-dependent value of the forcing. We further show
that this response is associated with a nonequilibrium transition between a
`hard' (or less deformable) phase, which is nearly jammed and flows very
slowly, and a `soft' (or more deformable) phase, which flows much more easily.
The soft phase is characterised by flow-induced time dependent shape
deformations and internal currents, which are virtually absent in the hard
phase. Close to the transition, we find sustained oscillations in both the
droplet and fluid velocities. Polydisperse systems show similar phenomenology
but with a smoother transition, and less regular oscillations