Probing complex fluids with microbeads

This thesis aims to provide numerical tools examining the properties of viscoelastic fluids through the recoil dynamics of microscopic probes. Specifically, the goal is to characterize the relaxation of a model fluid through the trajectories of a probe dragged at constant velocity before a sudden removal of the external forcing. The polymeric complex fluid is modeled as an ensemble of linear chains composed of coarse-grained Gaussian soft core units linked with linear springs and enclosed in a two-dimensional box with periodic boundaries. The system is evolved using LAMMPS with constant-velocity dragging until a steady state is reached, then the forcing is stopped and the trajectories of the probe and monomers are recorded until equilibrium is reached. A systematic analysis of the probe recoils is carried out via repeated independent realizations of recoils spanning various dragging velocities and polymer lengths. We observe general phenomena that closely match observations of various real complex fluids such as solutions of wormlike micelles or λ-DNA. Two relaxation timescales independent of dragging velocity and polymer length are identified. The recoil amplitudes are found to saturate at a dragging velocity that is consistent with the onset of a nonlinear phase in the rheology, where the probe is fast enough to affect the microstructure of the fluid by “hopping” through a polymer bond with Poisson statistics. Finally, scaling relationships enable a unified representation of timescale–velocity and amplitude–velocity curves with respect to polymer size, confirming the broad applicability of our numerical approach.