Experimental challenges of shear rheology: how to avoid bad data

A variety of measurement artifacts can be blamed for misinterpretations of shear-thinning, shear-thickening, and viscoelastic responses, when the material does not actually have these properties. The softness and activity of biological materials will often magnify the challenges of experimental rheological measurements. The theoretical definitions of rheological material functions are based on stress, strain, and strain-rate components in simple deformation fields. In reality, one typically measures loads and displacements at the boundaries of a sample, and the calculation of true stress and strain may be encumbered by instrument resolution, instrument inertia, sample inertia, boundary effects, and volumetric effects. Here we discuss these common challenges in measuring shear material functions in the context of soft, water-based, and even living biological complex fluids. We discuss techniques for identifying and minimizing experimental errors and for pushing the experimental limits of rotational shear rheometers. Two extreme case studies are used: an ultra-soft aqueous polymer/fiber network (hagfish defense gel), and an activelyswimming suspension of microalgae (Dunaliella primolecta). Randy H. Ewoldt Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA e-mail: [email protected] Michael T. Johnston Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA e-mail: [email protected] Lucas M. Caretta Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA e-mail: [email protected] 1 Author-generated preprint, to appear as: Ewoldt, R.H., M.T. Johnston, L.M. Caretta, "Experimental challenges of shear rheology: how to avoid bad data," in: S. Spagnolie (Editor), Complex Fluids in Biological Systems, Springer (2015)