My research program focuses on the physics of complex fluids, and the development of techniques to characterize such materials using x-rays.
X-Ray Studies of the Structure and Dynamics of Biomimetic Membranes
Biomimetic membranes are artificial materials which mimic some of the features of biological membranes. A simple example of such a material is a phospholipid bilayer supported on a silicon substrate. Such a system is referred to as a “supported” membrane because of the silicon support which provides a fixed orientation; crucial to x-ray scattering studies. A supported phospholipid bilayer shares some important properties of real biological membranes, which are also composed of phospholipids, but artificial bilayers are much simpler to construct and they do not have the complicated admixtures of proteins and cholesterols which constitute real cell membranes. The goal of my present work is to use a combination of techniques such as x-ray reflectivity, x-ray diffuse scattering and AFM to study thermal fluctuations in such supported phospholipid bilayers.
X-Ray Studies of the Structure and Dynamics of Polymer Films
Polymers films are important for many applications such as resists for semiconductor patterning, bio-compatible materials, optical coatings, high strength composite materials and hydrogen fuel cells. Polymers are also being actively investigated as materials that have the ability to self assemble at the nanoscale. These applications have stimulated interest in understanding how the physical properties of polymers change when they are confined. I have been using small-angle x-ray scattering and x-ray photon correlation spectroscopy (XPCS) to study the structure and dynamics of polymer-vacuum, and polymer-polymer interfaces. This has allowed the determination of important physical parameters of the films such as viscosity, compressibility, bending rigidity and interfacial tension. In particular, these measurements have revealed how these parameters change under confinement of the polymers at the nanoscale.
X-ray Diffraction Study of Supersolid 4He
In collaboration with Norbert Mulders of the University of Delaware, Clem Burns of the University of Western Michigan and Moses Chan of Penn. State University I have been investigating the unusual supersolid phase of 4He. In the supersolid phase, helium has a well defined crystalline order, yet is still able to flow without dissipation. Most theories of the supersolid state explain this behavior based on the mobility of crystal defects. In order to test such theories we are measuring the crystal defect density as a function of temperature through a careful comparison of the lattice volume measured by x-ray diffraction and the atomic density measured through the sound velocity.
Development of New Techniques for Coherent X-Ray Scattering
In collaboration with the scientists at sector-8 of the APS, I have been working to develop improved techniques for measuring dynamics in complex fluids using coherent x-rays. This work has included the development of a direct-detection x-ray camera based on CMOS technology, the development of real-time data compression software, and the development of a new method for performing x-ray fluorescence fluctuation spectroscopy in reflection geometry.