Nonlinear signatures in active microbead rheology of entangled polymer solutions

Abstract

We present experimental data and numerical modeling of a nonlinear phenomenon in active magnetic microbead rheology that appears to be common to entangled polymer solutions (EPS). Dynamic experiments in a modest range of magnetic forces show: 1. a short-lived high viscosity plateau, followed by 2. a bead acceleration phase with a sharp drop in apparent viscosity, and 3. a terminal steady state that we show resides on the shear-thinning slope of the steady-state flow curve from cone and plate data. This latter feature implies a new protocol to access the nonlinear steady-state flow curve for many biological EPS only available in microliter-scale volumes. We solve the moment-closure form of the Rolie-Poly kinetic model for EPS hydrodynamics, together with a decoupling approximation that obviates the need for a full 3D flow solver, and show that the model qualitatively reproduces the dynamic experimental sequence above. In this way, we explain the phenomenon in terms of entangled polymer physics, and show how the nonlinear event (acceleration and termination on the shear-thinning response curve) is tunable by the interplay between molecular-scale mechanisms (relaxation via reptation and chain retraction) and magnetic force controls. The experimental conditions mimic movement of cilia tips, bacteria, and sperm in mucus barriers, implying a physiological relevance of the phenomenon, and compelling further development of the fully coupled, 3D flow-microstructure model to achieve quantitative accuracy.

Nonlinear Microrheology Reveals Entanglement-Driven Molecular-Level Viscoelasticity of Concentrated DNA

“…2(d)]. While LW suggest that entanglement reduction is due primarily to chain stretching, not accounted for in SS theory [24,36], it also shows that significant chain stretch is the source of stress overshoots experimentally seen in macrorheology studies on entangled flexible polymers, including DNA [16,23]. We find no such overshoot, in accord with previous rheology studies on filamentous actin solutions and gelatins as well as SS predictions [13,26,42].…”

“…Results of the SS model are consistent with previous extensions to DE theory that introduce CCR dynamics [28,29] as well as experimental and simulation results for flexible polymers [33][34][35][36]. Conversely, Lu, Wang, and co-workers (LW) [24,36] propose that chain stretching is the dominant mechanism underlying nonlinear features such as a stress overshoot during shear [16,23]. As such, experiments that directly elucidate the underlying molecular dynamics leading to widely observed nonlinear effects are needed [31,32].…”

“…Linear microrheology measurements, which use passive probe motion or small force probe oscillations, can readily determine microscale linear rheological properties via generalized Stokes-Einstein relations [3], provided the probe is larger than a characteristic length scale of the material [11]. While numerous studies have provided critical insight into the microscale linear viscoelastic properties of entangled polymers and other soft materials [1][2][3][4][5][6][7][8][12][13][14][15], far fewer have investigated the nonlinear regime in which the probe is actively driven (via magnetic or optical tweezers) to significantly deform the material from equilibrium [13,16,17]. Here, interpretations within a macrorheology framework are complicated by the inherently inhomogeneous flow fields induced [6,18].…”

Transient dynamics of a colloidal particle driven through a viscoelastic fluid

“…More recent developments of active microrheology have aimed to probe nonlinear responses of complex fluids by perturbing their microstructure far away from equilibrium. This has been achieved by dragging or rotating the probe through the fluid at sufficiently large constant force or torque [19][20][21][22][23], or at constant velocity [24][25][26][27], thus inducing, e.g., thinning.…”

Rheological Study of Soft Matters: A Review of Microrheology and Microrheometers

“…In addition, this technique is highly selective for magnetic particles while most biological samples or polymer solutions are weakly responsive to magnetic fields. Therefore, magnetic tweezers are widely used in complex and heterogeneous environments, including the studies of DNA topology, cell mechanics, and polymer networks …”