I am an experimental physicist at Johns Hopkins University working in the field of condensed matter. Condensed matter physics is the discipline within physics that seeks to explain the material world around us. Our understanding of materials' properties relies crucially on the tools of statistical mechanics that allow us to predict the average behavior of a system with many particles, and the fluctuations about that average, without knowing the detailed behavior of every single particle. Disorder and out-of-equilibrium conditions in a condensed matter system can profoundly affect its behavior, creating novel material properties that pose unique challenges for statistical mechanics. Much of my research at Johns Hopkins has been directed at understanding the physics of disordered and out-of-equilibrium materials. 

My primary research interests are in experimental soft condensed matter physics with particular focus on disordered and out-of-equilibrium systems such as glass-forming liquids, liquid crystal-colloid composites, and gels.

Most of the materials on which I focus my research can be described as complex fluids. Complex fluids, such as colloidal suspensions and liquid crystals, are soft materials that possess liquid-like properties but that differ from simple liquids due to internal structure on the nanometer or micrometer scale. Often the properties of complex fluids derive from a delicate balance of interactions at the microscale including entropic, electrostatic, and interfacial forces. The fragile nature of the states that complex fluids assume, as well as their experimental accessibility, make these systems particularly well suited for exploring the consequences of disorder and out-of-equilibrium behavior. Among the disordered systems I have been studying recently include liquid-crystal/colloid composites, gels, and emulsions. 

Much of my research involves x-ray and neutron scattering techniques. I am a frequent user of Sector 8-ID of the Advanced Photon Source and also perform experiments at the National Synchrotron Light Source and at the NIST Center for Neutron Research. I am also a member of the Materials Research Science and Engineering Center (MRSEC) at Johns Hopkins. In collaboration with the group of Dan Reich, I have been pursuing a research program that employs state-of-the-art magnetic nanostructure fabrication to create custom-synthesized particles for microrheology. Our strategy is to optimize the probes' geometry as well as their magnetic and surface properties to match specific complex fluid environments and measurements objectives. We have applied this approach to explore the novel elastic forces experienced by anisotropic particles in nematic liquid crystals and to investigate the shear rheology of thin fluid films.

When a colloidal particle is suspended in a structured fluid such as a liquid crystal, distortions in the fluid’s order can engender particle interactions that have unique properties. These effects have proven valuable for exploring fundamental issues in physics of complex fluids and have motivated efforts to employ the interactions as a mechanism for colloidal manipulation and self-assembly. In collaboration with the Reich group, we explore this topic through a combination of table-top experiments and synchrotron x-ray studies. A particular recent focus of our research on colloids in liquid crystals has been the influence of the interplay of elastic and hydrodynamic forces on dynamics.

Disordered soft solids such as gels, pastes, and emulsions will change their shape and flow when subjected to sufficient forcing. At the microscopic level, this deformation and flow involves spatially heterogeneous re-arrangements of the constituents that are often difficult to identify and characterize. Understanding the connections between these microscopic motions and the macroscopic deformation and flow remains a central challenge for the fields of mechanics and colloid science. In collaboration with scientists at the Advanced Photon Source and with the groups of Jim Harden at U. Ottawa and Simon Rogers at UIUC, We are developing advanced x-ray scattering methods, specifically those involving coherent x-ray scattering, to interrogate the microscopic motions in disordered soft solids with unprecedented resolution and are employing these methods to uncover the salient microscopic behavior underlying the solids’ macroscopic properties.

A gel is formed when particles suspended in solution become unstable to aggregation, ultimately forming a system spanning network that imparts mechanical rigidity to the system. As gels are examples of out-of-equilibrium systems, their properties evolve in time, a process called aging. While the loss by particle mobility and the development of rigidity during gel formation and aging are intimately linked, the quantitative connections between these microscopic and macroscopic phenomena remain unresolved. Collaborating with groups from the Advanced Photon Source, Florida State University, and University of Ottawa, we are employing advanced x-ray scattering techniques in concert with mechanical measurements in efforts to establish such connections, which potentially impact related, challenging scientific problems such as the formation, stability, and dynamics of glasses.

Biofilms, assemblies of microbes in a self-produced conglomeration of extracellular polymeric substances, play a crucial role in the growth, protection, and dispersal of many bacteria species. Biofilms are abundant in nature; an estimated that 99% of all bacteria cells reside within biofilms, making them fundamentally important to the life cycle of nearly all bacterial species. In collaboration with the group of Kate Stebe at U Penn, we have investigated the formation, evolution, and mechanical behavior of biofilms forming at oil-water interfaces. The evolution and properties of these films are strongly influenced by the swimming motion of the bacteria that form them, making the biofilms a fruitful model for exploring the physics of such “active” systems in an interfacial context.

Full List of Publications are on Google Scholar