Francis W. Starr
Associate Professor of Physics
Computational soft condensed matter, biophysics and nanotechnology: functionalized nanoparticles, complex liquids, water, glass formation, and polymeric materials.
Self-Assembled DNA-based Materials and Nanostructures
The complementary binding of base pairs, combined with the ability to directly control the base sequence, makes DNA an ideal candidate for the development of network-based, nanostructured materials. Simulating the hybridization of DNA to form complex structures is infeasible from an atomistic approach, so we have developed coarse-grained models for single-stranded DNA that reproduce the selective lock-and-key binding of DNA base-pairs. By linking single strands to a core nanoparticle, we create a designed molecule that can be used as a node for a complex higher order structure.
Our studies explore the various structures that these DNA base units can spontaneously form, as well as the underlying physics needed to comprehend the thermodynamics and dynamics of these new materials. Our research has shown that the highly cooperative nature of bonding in double-stranded DNA allows for a direct connection between the mobility in these systems and the fraction of bonded strands. We have also shown that these materials can serve as model systems to gain insights into traditional molecular systems. In particular, nanoparticles with four tethered single strands of DNA have been helpful to understand the emergence of liquid-liquid critical points in systems like water and silica.
While water may be ubiquitous, it is still relatively poorly understood in comparison to "simple" liquids. Not surprisingly, this is largely due to hydrogen bonding, which gives rise to the well-known fact that ice floats in liquid water. These odd behaviors become even more pronounced in the supercooled region of liquid water --- that is, when the liquid is cooled below its equilibrium freezing point.
Our research focuses on the behavior of water in this supercooled region, focusing on the interplay between thermodynamics and dynamics that is not apparent at higher temperatures. More specifically, simulations suggest that there is a low temperature phase transition between two distinct liquid states. As a result, it appears that the dynamic properties in the vicinity of that transition differ significantly from other liquids. In addition, we also study the behavior of water confined to nanoscale regions. The properties of this confined water can differ greatly from the behavior of ordinary water, which has important implications in nanofluidic and biological systems.
Polymer Nanocomposite Materials and Self-Assembly
The addition of organic or inorganic material to polymers has long been exploited to change material properties. With the emergence of custom designed nanoparticles, it has been shown that dramatic changes in material properties with only a fraction of added nanoparticles. To achieve these improvements, it is critical to control the clustering behavior of the added nanoparticles.
Our recent work has shown that the clustering of nanoparticles appears to be described by the same physics controlling equilibrium polymerization and thermoreversible gelation. Unfortunately, the minimal criteria for a reversible gelation and the detailed physics of gelation are not nearly as well understood as simple phase transitions, which are well described by statistical physics.
We are studying models that reveal insights into the basic phenomena of gelation, as well as models that elucidate practical connections between interfacial nanoparticle-polymer behavior and bulk properties. We are also using the insights from these systems to improve the modeling of nanocomposite materials.
The process by which an amorphous liquid becomes an amorphous solid (as opposed to a crystalline solid) has been a subject of study for millennia. While the understanding of the gradual slowing of liquid dynamics has greatly improved, there remain many open questions.
Computational studies are particularly useful to identify the molecular level processes that are involved in the relaxation of liquids near their glass transition. Computational models have played a major role in developing the "energy landscape" approach of liquid dynamics, the phenomena of dynamic heterogeneity, and in showing the connection between local structure of the liquid and its dynamics. Our research aims at developing each of these inter-related approaches.