(322c) Micro-Scale Nonlinear Viscoelasticity of An Aging Colloidal Gel Using a Magnetic Tweezer

Micro-scale rheological properties of complex fluids are often essential to their function and performance in both industry and nature. Examples include drilling fluids, which must entrain macro- and micro-scale rock cuttings and prevent their sedimentation, and cytoskeletons, which must provide structural stability while allowing for the mobility of biomolecules and organelles inside a cell. The field of microrheology has been developed to explore rheological properties at the microscopic scale, and studies of linear viscoelastic properties are now well-established. However, microrheological studies of nonlinear viscoelasticity are scarce in spite of the fact that nonlinear properties such as yield stress, shear thinning, elastic recoil, etc, are dominant characteristics of many complex fluids at the bulk scale. In particular, only one study has reported a yield stress measurement at the microscopic scale, using an optical tweezer (J.N. Wilking and T.G. Mason, 2008). We present a new application of a magnetic tweezer for measuring nonlinear viscoelasticity at the microscopic scale. The tweezer consists of a core of high magnetic permeability steel, machined to a sharp tip and wound with magnet wire. With the tip placed directly into the fluid of interest, large magnetic field gradients can be achieved, resulting in forces of ~ 10 nN on superparamagnetic probe particles that are embedded in the fluid. The technique, which is the microscopic equivalent of a continuous shear stress ramp, is demonstrated on an aqueous dispersion of Laponite, a synthetic discotic colloidal clay often used in industrial and consumer product applications as a rheological modifier. Micro-scale yield stress and shear-thinning observations are compared with bulk rheology results, allowing insights into the structure and rheological response of the clay dispersion at multiple length scales.