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Radhakrishna Sureshkumar, Bamin Khomami
Date of Award
Restricted Access Dissertation
Doctor of Science (DSc)
Structure and rheology of surfactant solutions are very sensitive to electrostatic interactions and flow deformation. The objectives of this experimental study have been to uncover the essential physics of structure transitions in surfactant solutions under simple shear as well as porous medium flows, and how they are influenced by the presence of added salt First, we study the effect of salt concentration on the shear rheology of ionic surfactant solutions in simple shear flow. It is shown that for a given surfactant concentration, low salt concentrations induce shear-thickening behavior, while higher salt concentrations induce shear thinning. The shear thickening transition occurs due to the formation of shear induced structures (SIS), which are qualitatively different from simple micellar aggregates. It is shown that the shear-thickening transition occurs when a critical amount of strain is applied on a solution that is sheared beyond a critical shear rate [Special characters omitted.] . We provide robust scaling laws for the onset of shear thickening, and for the relaxation time of the shear-induced phase λ. Experimental data suggest a self-similar behavior of [Special characters omitted.] and λ as a function of the salt concentration such that an effective Weissenberg number for the onset of shear thickening is practically independent of salt concentration. However, SIS are found to be extremely shear sensitive, and instantaneously disintegrate upon removal of applied strain.
Based on this understanding, we design an experiment to induce and study SIS in microfluidic channels, which forms the second part of the thesis. For the first time, it has been shown that irreversible SIS can be produced. We discuss the factors responsible for this irreversibility. Moreover, unlike conventional sol-gel processes, the irreversible gels are formed without addition of alcohols, making the process completely bio-compatible. Different surfactants were tested for the robustness of the process. Finally, AFM images of the irreversible structures suggest a highly entangled network of micelles with morphology that is ideal for nano-manufacturing applications.
Vasudevan, Mukund, "Flow-Induced Self Assembly in Micellar Fluids with Applications to Nanomanufacturing" (2008). Retrospective Theses and Dissertations. 66.