Dr. Evans' research program is focussed on the understanding of the liquid crystalline state of matter using the methods of statistical thermodynamics. His research addresses two areas in liquid crystal (LC) physics: the equilibrium properties, such as phase transitions and accompanying phase diagrams; and non-equilibrium or dynamic properties, such as the transport coefficients, time correlation functions and the dynamic response of LC materials.
The primary goal in the static research is to derive a phase diagram starting from the microscopic molecular Hamiltonian. Using the techniques of statistical mechanics, specifically Density Functional theory, one can make significant progress in predicting a phase diagram of a material taking as input only the shape of a molecule and its long range intermolecular potential. Shown in the figure below is a hard particle liquid in the isotropic and the nematic states (the left and right figures, respectively). Typically, a phase diagram of a highly nonspherical molecule will include gas, isotropic liquid, nematic, smectic and solid phases. Even the simplest of all models, a hard body representation of molecular shape, has significant utility. Modern flat panel liquid crystal displays, typified by the so-called super twist chiral nematic, can be understood with very simple molecular models and the predictions of color from hard body theory is encouraging. Of course, real nematogens are soft and flexible and these degrees of freedom must be addressed in the theory. This is active area of concern.
Dynamics in liquid crystalline fluids occurs on many timescales. Wholesale molecular rotation is very slow, nearly in the millisecond range, whereas the timescale for momentum transfer between neighboring molecules takes place in a fraction of a picosecond. The disparity in these processes presents one of the many challenges to the understanding of molecular motion in nematogens. The research emphasis has begun with an analysis of self diffusion and viscosity of isotropic and nematic fluids. In the isotropic fluid, there is one viscosity, whereas in the nematic, there are five and one is negative. There are no reported calculations of viscosities of nematics and no fundamental understanding. The theory of dielectric response of liquid crystals focusses on the microscopic processes responsible for collective molecular rotation: an important problem from the standpoint of basic understanding of molecular physics and key to the development of LC displays that can switch faster than the millisecond range. Dynamics in polyatomic fluids have been analyzed using kinetic theory, hydrodynamics and projection operator methods.
There are several motivations for the OSU LC research: i) the physics of liquid crystals are interesting in their own right, as the formation of these phases illustrates one example of molecular self assembly and spontaneous symmetry breaking; ii) LC devices are of technological importance; and iii) the explanations of the processes occurring in LC systems have lead to many analogies applicable to other physical phenomena. Although the simple physical models used in the OSU LC research certainly do not represent every facet of complicated, torsionally soft, dipolar nematogens, these simple theoretical models play an important role in the unraveling of the elementary chemical physics of complex systems.