Nonlinear microrheology of an aging, yield stress fluid using magnetic tweezers

The large deformation mechanical response of complex fluids and soft materials provides fundamental insight into their underlying microstructure and dynamics. Additionally, associated yielding and flow processes are often central to industrial processing and end-use. By probing nonlinear rheological properties at the microscopic scale, microstructural dynamics and flow mechanisms can be more directly elucidated. In the current work, we present a simple magnetic tweezer technique for probing the nonlinear microrheology of complex fluids and soft materials. The setup is characterized in terms of the accessible stresses, the applied magnetic fields, and the measurable viscosities and shear rates. Further, we report the first use of magnetic tweezers to determine yield stresses at the microscopic scale, as well as the first comparison between bulk and micro-scale yield stress measurements. The capabilities of the technique are demonstrated on an aqueous dispersion of Laponite (R): an aging, thixotropic colloidal clay of considerable scientific and practical interest. Probe trajectories in this material reflect the yield stress and strong shear-thinning behaviour observed on the bulk scale, and for sufficient clay concentrations we find good agreement for the shear yield stress obtained from bulk rheology and magnetic tweezer measurements. These unforeseeable observations illuminate the nature of the dispersion microstructure, including the characteristic size of microstructural features.