In this project, the PI and his lab group will study the flow of soft materials, in particular emulsions (oil droplets in water). The goal is to connect the microscopic details of the flowing emulsions to their macroscopic properties. The project has two thrusts: first, to study the shear of dense three-dimensional emulsions; and second, to investigate the flow properties of dense two-dimensional emulsions. The 3D samples will be viewed using confocal microscopy. For these 3D samples, the importance of polydispersity will be examined using samples with droplet sizes that vary by as much as an order of magnitude. Flow of these highly polydisperse samples is qualitatively distinct from less polydisperse cases, and the data from this project will be a foundation to understand jammed materials that are highly polydisperse. For the 2D samples, the oil droplets will be confined between two parallel glass plates, so that the droplets are deformed into round quasi-two-dimensional disks. From the outlines of the droplets and knowledge of their surface tension, forces between droplet pairs will be measured and related to the rearrangements as the droplets flow. These measurements enable testing of theories of flowing amorphous materials. Microfluidic techniques will be used to create the oil droplets, allowing careful control over droplet sizes, viscosities, and surface tension.
Soft jammed materials are common, and include colloidal pastes (such as toothpaste), gels (such as gelatin), foams (such as shaving cream), and emulsions (such as mayonnaise). In contrast to regular liquids like water, the flow properties of these materials are less well understood. For example, the flow of an emulsion composed of oil droplets in water must involve rearrangements of individual droplets; additionally, the droplets can be stretched and even break up, making the flow even more complicated. The projects funded by this award will use microscopes to take movies of flowing emulsions, to see how the oil droplets move and how these motions are related to the forces required to flow them. This will lead to better understanding of how to create emulsions with specific flow properties, and how to better process emulsions in industry. Additionally, Professor Weeks will conduct at least one field trip each year to his laboratory for groups of primary school students. These students will investigate the properties of "squishy" materials such as the oil-and-water emulsions used in this project. While such materials are common and safe, they are also counterintuitive and get students curious. For example, consider shaving cream, which can sit in a pile on a table, in contrast to a puddle of soapy water which must lie flat. Such materials challenge the ideas students have that materials only come as solids, liquids, or gases.
