Description:

Many rheology modifiers are composed of rod-like particles or worm-like surfactant aggregates. Real world applications often expose these solutions to a very wide range of shear and temperature conditions. In this talk I will discuss a microcapillary device used to measure the shear rate dependent viscosity of microliter scale volumes. The device uses pneumatically driven flow through a microcapillary attached to a glass capillary, mounted on an optical linear sensor, where the air-liquid meniscus is tracked in real time. The solutions measured with the device display strong shear-thinning behavior due to the microstructural changes upon application of shear, which are measured up to shear rates of 105 s-1. Flow birefringence measurements are made on the microcapillary flow to obtain structural measurements and can also characterize relaxation after flow cessation. Measurement of wormlike micelle solutions using this device has motivated the development of a two-species model that describes the rheological transitions observed in experiments. A main feature of the model is a description of stress-induced micelle breakage using kinetic equations for the average micelle length. The model predicts a decrease from several micrometer length at low shear rates to a few hundred nanometers at high shear rates. The model is used to better understand the rheological features observed in experiments in context of a structural transition from entangled wormlike behavior to a dilute rod rheology with increasing shear rate. The capillary rheometer is also used to measure solutions of fd bacteriophage as a model monodisperse rod system in which hydrodynamic interactions between rods contribute significantly to the viscosity at high shear rates. 

Paul Salipante, Mechanical Engineer, National Institute of Standards and Technology, Gaithersburg, MD

Host: Michelle Driscoll