Soft matter, including polymers, proteins, colloidal particles, is a class of materials where thermal fluctuations strongly influence structure and behavior. The variety of weak interactions in soft matter make its properties difficult to predict yet they crucially determine their suitability for technological applications. Furthermore, surfaces and interfaces become especially important. Our research aims to understand the behavior of soft matter at surfaces and interfaces at small length scales. In this talk, I will show two types of interfaces: solid-air and liquid-liquid. First, solid polymers can be either crystalline or amorphous. In the first part of the talk, I will show that the interface between glassy polymers and the air is not so glassy but there exists a liquid-like layer of polymer. We directly measure the flow of this liquid-like layer while the bulk remains solid-like. Second, charged nanoparticles and oppositely charged surfactants can assemble at the water-oil interface, which enables us to structure liquids into arbitrary shapes by means of interfacial jamming. I will show how we use ionic strength to enhance the liquid structuring ability and how nanostructure affects the mechanical properties of the liquid-liquid interface. In these studies, in situ characterization enabled the direct elucidation of the nanoscale structural properties that determine macroscopic behavior.
Yu Chai is a postdoctoral researcher at the University of California, Berkeley and also affiliated with the Molecular Foundry at the Lawrence Berkeley National Laboratory with Prof. Thomas P. Russell and Dr. Paul D. Ashby. Before this, he received his Ph.D. in physics at the University of Waterloo under the supervision of Prof. James. A Forrest. His research interests include soft matter with a particular focus on interfaces.