Non-technical Abstract Many common materials are soft jammed solids. Examples include colloidal pastes (such as toothpaste), gels (such as gelatin), foams (such as shaving cream), and emulsions (such as mayonnaise). These materials are soft: for example, toothpaste squirts out of the tube without much effort. However, they are jammed solids: you can make a pile of toothpaste, rather than having it turn into a puddle. For nearly two decades, scientists have conjectured that the solid-like properties of these soft materials are similar to the solid-like properties of window glass. Very recently, computer simulations suggested the solid-like properties of soft materials are fundamentally different than the solid-like properties of a glass. The key difference is thought to be the vibrational motions due to temperature, which are important for molecules in a glass and unimportant for the constituents of soft materials. This project uses emulsions (oil droplets in soapy water) to address these questions. Some samples are made using very tiny droplets, for which temperature-induced vibrations are crucial; and other samples are made using large droplets, for which temperature-induced vibrations are negligible. The goal is to directly compare behavior of the two types of samples with all other details held constant, thus directly testing the simulation predictions.
The PI and his lab group use concentrated emulsions as an experimental test of the universality of the jamming transition. Emulsions are liquid droplets in an immiscible liquid, where a surfactant has been added to prevent the droplets from coalescing. At low droplet concentration, emulsions flow easily. At higher droplet concentration, the droplets contact one another and the sample becomes "jammed." As a jammed material, it remains microscopically disordered but macroscopically has a yield stress and appears solid-like, albeit a quite soft solid. Over the past 20 years there have been many statements that this transition to an amorphous solid in emulsions (or other soft materials) is the same as the regular glass transition. However, recent simulations suggest that the glass transition and jamming transition are distinct, with the former occurring for thermal materials and the latter for athermal materials. The PI and his lab group use emulsions to tune from the thermal to athermal limit by varying the droplet sizes. Rheology is used to measure the relationship between stress and strain rate in the samples. The change of rheological behavior is investigated as the droplet concentration is varied, obtaining data directly testing if the glass and jamming transitions are distinct. Additionally, microscopy is used to view sheared samples to show what microscopic behaviors give rise to the rheological behavior. The lab group also hosts field trips for primary school students, giving the visiting students hands-on laboratory experiences with the squishy materials such as the dense emulsions used in the experiments. Such materials challenge ideas that materials only come as solids, liquids, or gases.
