title: Nonlinear signatures of entangled polymer solutions in active microbead rheology.
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.
authors: University of North Carolina at Chapel Hill, North Carolina 27599, USA.
authors: J A
authors: JA
authors: Cribb
authors: University of North Carolina at Chapel Hill, North Carolina 27599, USA.
authors: P A
authors: PA
authors: Vasquez
authors: Boston University, Boston, Massachusetts 02215, USA.
authors: P
authors: P
authors: Moore
authors: University of Michigan, Ann Arbor, Michigan 48109, USA.
authors: S
authors: S
authors: Norris
authors: University of California Los Angeles, California 90024, USA.
authors: S
authors: S
authors: Shah
authors: University of North Carolina at Chapel Hill, North Carolina 27599, USA.
authors: M G
authors: MG
authors: Forest
authors: University of North Carolina at Chapel Hill, North Carolina 27599, USA.
authors: R
authors: R
authors: Superfine