Vesicles and bilayer membranes are self-assembled aggregates of surfactant molecules. Charged vesicle suspension-based formulations such as household fabric softeners constitute a multi-billion dollar business worldwide; yet a fundamental understanding of what determines suspension microstructure, and thus product stability, is lacking. Vesicles in such suspensions can spontaneously transition from having a single bilayer membrane to so-called "onion" vesicles involving multiple imbedded bilayers. These spontaneous transformations can negatively impact the suspension's shelf life and flow behavior. This work has been aimed at understanding the underlying mechanisms driving microstructural and rheological transformations that occur in concentrated vesicle suspensions over time, and relating them to key suspension parameters such as its volume fraction and the mechanical properties of the constituent vesicle membranes. Such an understanding could allow for better design of commercial formulations with predictable physical and mechanical behavior.

In this work, various steps have been taken to characterize and examine the key properties of vesicle bilayers and vesicle suspensions. Using the classical technique of micropipette aspiration coupled with a modified experimental protocol that is better suited for charged vesicles, novel measurements of mechanical properties of highly charged lipid bilayers have been made, and the effect of salt concentration on these properties is studied. The depletion-attraction induced adhesion of highly charged multilamellar vesicles is studied using a new Cantilevered-Capillary Force Apparatus, and direct measurements of hydrodynamic and non-hydrodynamic interaction forces involved in the adhesion process have been made. Furthermore, a simple dilution method to determine the hydrodynamic volume fraction of concentrated, polydisperse and multilamellar vesicle suspensions has been developed and optimized to arrive at a robust recipe for measuring this key parameter. This technique has been used in conjunction with cryo-TEM imaging to quantitatively follow both the macroscopic and microstructural time-evolution of cationic vesicle suspensions at different surfactant and salt concentrations. It was observed that charged unilamellar vesicles in a suspension can spontaneously deflate and subsequently transition to form bilamellar vesicles, even in the absence of externally applied triggers such as salt or temperature gradients. The results presented herein provide strong evidence that the driving force for this deflation-induced transition is the repulsive electrostatic pressure between charged vesicles in concentrated suspensions, above a critical effective volume fraction. Lastly, the coupling between the rheological behavior of charged vesicle suspensions and the thermotropic phase transitions of the constituent bilayer and its mechanical properties has been examined to discern mechanisms leading to the formation and aging of charged vesicle gels.