The microstructure of colloidal suspensions, both at rest and under flow, is a function of the particle and fluid properties, interparticle potential, and processing or flow history. Indeed, complex, nonlinear rheological phenomena, such as thixotropy and shear thickening, are associated with significant changes in microstructure during flow and processing. A modern understanding of colloidal suspension rheology thus necessitates measurement of colloidal suspension microstructure under flow as well as at rest. Two popular classes of experimental methods for microstructure measurement are introduced and explained, namely confocal microscopy and scattering of light, neutrons, and x-rays.

This chapter applies the fundamental framework for colloidal forces and rheology to biocolloids. We define biocolloids broadly as colloidal assemblies with primary applications that are biomedical in nature, e.g., (i) block copolymers used in pharmaceutical formulations and biomaterials applications, and (ii) biomacromolecules that can be reasonably described with colloidal descriptions for the interparticle interactions; namely globular proteins and protein assemblies such as casein micelles. Our discussion mainly focuses on systems where concepts from colloidal interactions prove useful in interpreting the rheological behavior. The chapter briefly discusses the importance of colloidal rheology to applications in drug delivery, biomolecular therapeutics, and foods. Examples from both classic publications and recent literature are provided, along with models to describe the rheological behavior. Specific systems discussed include thermoresponsive micellar block copolymers, associative polymers, biomimetic block copolymer assemblies with stereocomplexes and crystalline domains as well as globular proteins.

Blood is a concentrated suspension of deformable, aggregating, red blood cells within a medium of other cells and proteins. It is a complex colloidal system with a non-Newtonian rheology that is characterized by viscoplasticity, thixotropy, and viscoelasticity. After reviewing some of the key biological characteristics of human blood, and after presenting a short historical review of the subject, we present some recent accomplishments. These range from the development of a parameterized Casson model, based on the hematocrit and fibrinogen levels, to the discussion of several recent structural models that are able to capture several of the time-dependent rheological effects of blood. A comparison is also offered between model predictions and the results of recent transient measurements, some involving a newly proposed variant of LAOS: the Unidirectional LAOS. The latter experiment is especially appropriate for the study of blood rheology as it follows roughly the flow experienced by blood in the arterial circulation. It consists of a superposition of steady and large amplitude oscillatory flow in such a way that flow reversal is avoided. Some additional models are discussed along with the challenges and opportunities for future research.