Jamming in soft materials

Materials that have both solid and liquid-like properties are important industrially and are ubiquitous in nature. Transitions from liquid-like to solid-like mechanical properties often occur when many solid elements within a material are brought into close contact, or jammed together. This thesis is an exploration of how jamming can cause important changes in material properties. We first show how particle jamming can cause bubbles to become stable to dissolution, a result that has useful implications for industrial applications in areas such as food and cosmetics. The observed stability occurs when particles on the surface of the bubble form a jammed structure that provides mechanical stability and allows the air/liquid interface to assume a shape that is stable to dissolution. A second way to form useful solid-like particle structures is through shear flow. Shear-induced clustering of particles in suspensions can cause liquids to dramatically transform into solids, but only while they are flowing. This "cornstarch" effect is so dramatic that it can be used to enhance the resistance of body armor to ballistics. We demonstrate that when shear-jamming materials are in a shear-induced solid-like state, they possess a transient flow stiffness that we measure by analyzing the stress fluctuations that occur during the flow. We also show that this method of measuring non-linear elasticity is general enough to be applied to other materials with important non-linear behavior, including a biological systems. Finally, we show that compression, buckling, and jamming of solid-elements on a liquid surface can lead to very unusual rheological properties in the case of gallium oxide films on liquid gallium. This oxide film is extremely useful in the creation of new solid-like structures in microfluidic devices,[21] but its mechanical properties have not been characterized. We show that these films possess a rich variety of mechanical behavior that may be useful for future technologies.