Mechanics of biopolymer networks, stimuli responsive particle suspensions, and their combinations

The key aim of this thesis is to demonstrate new paradigms in designing stiffness changing soft materials. The systems developed and studied in this work have salient and unprecedented features such as (1) the ability to controllably stiffen up to 100 times (10,000 %) when exposed to an external stimulus of temperature or magnetic field, (2) the ability to uncontrollably assemble into a ultra-soft hydrogel by undergoing 10,000 fold volume expansion within 0.4 s, and (3) transformation from a repulsive colloidal glassy state to a particulate gel thus undergoing change in the dynamics and mechanical properties. With a combination of rigorous experiments and mathematical models, this thesis offers novel ways to achieve functionality in soft materials and may have numerous applications in the fields of soft robotics, defense, and direct-write additive manufacturing. In part one of this thesis, a naturally produced biomaterial, hagfish slime is studied to understand its design principles. Hagfish slime is a unique predator defense material containing a network of long fibrous threads each 10 -15 cm in length. Hagfish releases the threads in a condensed coiled state known as skeins (∼ 100 µm), which must unravel within a fraction of a second and form a soft hydrogel to thwart a predator attack. The mechanisms of how the hagfish controls the unraveling rates, and the properties of the resulting gel are not well understood. The combined experimental and theoretical approach adopted in this thesis address these questions. First, the hypothesis that the viscous hydrodynamics may be responsible for the rapid unravelling rates is considered, and the scenario of a single skein unspooling as the fiber peels away due to viscous drag is discussed. As a result, its is shown that under reasonable physiological conditions viscous-drag-induced unravelling can occur within a few hundred milliseconds, comparable with the physiological time scales. Subsequently, through the rheological study on slime networks it is shown that key rheological and structural features of hagfish slime are insensitive to its concentration, in spite of the uncontrolled gelation process, and this peculiar characteristic may be vital for its physiological use. In part two, the linear and nonlinear rheology of a model system of soft microgel suspensions is investigated. The interaction pair-potential between the microgel is temperature-dependent. By increasing concentration of the suspension, a transition from a viscous liquid to an entropic glass to a soft jammed state at low temperatures where the microgels interact via a repulsive potential. Increasing the temperature of the suspension beyond the Lower Critical Solution Temperature [LCST], introduces additional attractive interactions, and results in the formation of particulate gels. The competition between repulsive and attractive interactions gives a rich temperature-dependent rheological response that is also concentration-dependent. An integrated experimental and quantitative theoretical approach is presented to understand the key linear and nonlinear of the suspensions in various regimes. In part three, two novel soft composite systems capable of unprecedented change in their mechanical properties in response to magnetic and thermal excitation are developed. The composites were formed by integrating stimuli-responsive particles (thermoresponsive microgels and magnetic particles) into the strain stiffening network of biopolymer fibrin. The interactions between the stimuli-responsive particles and biopolymer mesh is hypothesized to induces local stresses in the mesh, which inherently stiffens under the stress owing to its semiflexible nature. This helps achieve a higher sensitivity to the external field in the fabricated composites compared to the traditional flexible-polymer matrix of composite systems. Phenomenological models are developed that quantify this hypothesis, and the derived predictions are qualitatively consistent with the experimental data. This approach of using composites based on semiflexible polymers with strong inherent nonlinearity offers a promising method for developing functional materials with actively tunable mechanical propertie