Research

Ion channels allow ions to pass through the cell membrane. They are central to nerve function and are involved in many cellular processes. Many ion channels only permit certain ions to pass through, yet maintain high passage rates. The mechanism that gives rise to this behavior is hotly debated. Experimental 2D IR spectroscopy coupled with theoretical spectroscopic predictions have been used in the past to determine the mechanism, but the current experiments do not distinguish all proposed mechanisms. We have taken the reverse approach, where we use the theoretical spectroscopy to propose experiments that will discriminate between the proposed mechanisms. These experiments are currently underway in the Zanni group. This work was featured on the cover of the Biophysical Journal and highlighted in a blog post by the Biophysical Society.

Supercritical fluids have wide ranging applications from hazardous waste cleanup to decaffeination. The diversity of applications comes from their principal physical property: tunable density. Below the critical point, only small areas of the density-temperature phase diagram are accessible because the density is discontinuous across the liquid-gas phase transition. Above the critical point, the density can be tuned continuously, and with it, many other important physical properties. The structure and dynamics of supercritical fluids at a molecular level, however, are poorly understood, especially in non-trivial fluids like water, where hydrogen bond networks dominate the liquid state. We use computer simulations and theoretical spectroscopy to understand how the behavior of these fluids on the molecular scale controls their macroscopic properties. This approach not only deepens our current understanding of the properties of supercritical fluids, but also paves the way to new applications.

Rovibrational spectroscopy is usually understood from a quantum mechanical perspective. We have found a classical analog of this effect due to the classical rotational correlation function. Surprisingly, this classical analog even captures quantum mechanical rovibrational selection rules. For example, in a quantum rigid rotor approximation for HCl, ΔJ=±1, but for H₂O, ΔJ=0,±1. Classically, these selection rules are still obeyed, and originate from the dynamical stability and instability of rotations about the different moments of inertia as described by the “Tennis Racket Theorem” or “Intermediate Axis Theorem.” Because these effects are purely classical, they can be observed by tossing  everyday objects like a smartphone or tennis racket, permitting a more hands on approach to understanding these selection rules. A particularly stunning demonstration comes from the International Space Station.

On microscopic length and time scales, statistical mechanics underpins the molecular dynamics methods for systems at thermal equilibrium. On macroscopic scales, continuum hydrodynamics can describe fluids driven away from equilibrium. Conversely, microscopic MD simulations only generate rigorously accurate dynamics for closed and isolated systems and continuum hydrodynamic approaches inherently break down on atomic scales.We developed a rigorous method for the simulation of atomistic systems undergoing steady-state flow. This method has been instrumental in studying nanoporous graphene membranes with applications as reverse osmosis membranes. We have implemented our method in the open-source molecular dynamics package LAMMPS.