Dynamics, Mechanics and Stability of Physical Gels

Citation

Omar, Ahmad Khalid (2019) Dynamics, Mechanics and Stability of Physical Gels. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3F0A-4S95. https://resolver.caltech.edu/CaltechTHESIS:06062019-082100687

Abstract

From the commercial products that we encounter in our daily lives to the mucous that lines our gut, gels assembled by the reversible association of polymers or colloids are a ubiquitous, important and fascinating class of soft materials. The dual solid and fluid-like (viscoelastic) properties of associative polymer gels render them useful in a number of applications including as tissue-regeneration scaffolds, drug delivery vectors and organic electronics and batter technologies. However, there remains a number of open questions regarding the microscopic origins of many of the dynamical and mechanical properties that make these materials so appealing. The wide range of length and timescales in physical gels present a formidable challenge towards the formulation of a complete microscopic dynamical and rheological portrait. My work has focused on the development of microscopically-informed and experimentally verifiable explanations for some of the fundamental dynamical and mechanical properties of associative gels. I first present our viewpoint, informed by computer simulation and experiment, on the origin of the long-time self-diffusivity of telechelic polymer gels. Our perspective and resulting theory compare favorably with experiments. Shearing an associative polymer gel is found to result in the emergence of new diffusive modes with applied shear that are can destabilize homogeneous flow for gels sufficiently close to the two phase boundary. This finding motivates the idea that nonequilibrium forcing may promote the relaxation of arrested colloidal materials, such as a colloidal gel, closer to their thermodynamic ground state. The driving force need not be externally applied. The induced collective motion in colloidal gels subject to internal driving forces (such as the presence of a small fraction of self-propelling colloids) can drive the system from a state of arrested metastablity to a state of lower free energy. I conclude by showing that the internal stress generated by the self-propelling particles -- the active stress -- is not a "true" stress, but rather an equivalent stress analogous to the dynamic pressure of fluids in a gravitational field. The importance of this finding is demonstrated in resolving the perplexing finding of a negative surface tension in phase separated active materials.

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