Molecular Motion in Polymers, Colloids, and Complex Fluids
The transport behavior of complex fluids differs remarkably from that of simple liquids. For example,
in polymers and colloids, motion becomes highly cooperative near the glass transition, resulting in
dramatic changes to transport and rheology. My group is developing theory and simulation tools to
understand this phenomenon, and elucidate the nature of supercooled liquids, vitrification and crystallization.
We have discovered that particles (in the case of colloids) and molecules (in the case of molecular liquids)
move cooperatively in “string-like” structures near their glass transition. We are searching for similar structures
in several different types of fluids to determine how universal the phenomena is, and are developing a statistical
mechanical framework to describe this “dynamical heterogeneity“. Our work in this area has broad application ranging
from pharmaceuticals and drug design, micro- and nanofluidics, information technology, nanolubrication,
food preservation and processing, to sporting goods.
Cooperative molecular motion and spatially heterogeneous dynamics in supercooled liquids and glasses
Cooperative molecular motion and spatially heterogeneous dynamics in supercooled liquids and glasses
Over the past several years, we have performed extensive molecular dynamics simulation studies of the behavior of liquids cooled
below their crystallization temperature to the supercooled liquid state. As these metastable liquids approach their glass transition
temperature, the dynamics of the individual molecules comprising the liquid become increasingly spatially heterogeneous. Our group
discovered in 1998 that this heterogeneity involves the cooperative motion of molecules into highly coordinated, one-dimensional
string-like objects in which molecules follow one another like dancers in a conga line. These strings aggregate into clusters whose
transient behavior appears to dominate much of the bulk dynamics of supercooled and glass-forming liquids. Through MD simulations and
the development of a statistical mechanical description of correlated dynamics, we continue to study these remarkable features of the
liquid state in order to ascertain how and why they form, and to unlock further secrets of the nature of the glass transition.
Role of Spatially Heterogeneous Dynamics and Polydispersity in the Early Stages of Homogeneous Nucleation: Application to the Promotion and Suppression of Crystallization and Vitrification
Role of Spatially Heterogeneous Dynamics and Polydispersity in the Early Stages of Homogeneous Nucleation: Application to the Promotion and Suppression of Crystallization and Vitrification
Supercooled and supersaturated liquids are metastable liquids that typically crystallize after sufficient time has passed.
Rapid cooling or densification can suppress homogeneous nucleation of the crystal phase, leading to vitrification and the
transformation of the liquid into a glass. Polydispersity in the sizes of the atoms or particles comprising the liquid can
also inhibit crystallization. We are carrying out molecular dynamics simulations to study the local structure and dynamics
of supercooled atomic and molecular liquids and supersaturated colloidal liquids prior to and during nucleation. In collaboration
with the Solomon group, who use confocal microscopy to directly view particle motion in supersaturated colloidal suspensions,
we are determining the fundamental issues controlling vitrification vs. homogeneous nucleation in model liquids approaching their
glass transition.
*Funded by the National Aeronautics and Space Administration.