research

Nanocomposite performance fundamentally relies on reproducible dispersion and arrangement of nanoparticles, such that the dominate morphology across macroscopic dimensions is also nanoscopic. To facilitate dispersion, chemical approaches, including surfactant or macromolecular stabilization are usually employed to modify the surface of nanoparticles. However, the approach depends on the material system and usually involves trial-and-error to identify the best practice. Much less quantitative information is available on the coupling between the surface modification and external processing factors, including shear, electric or magnetic fields. In a recent work, we considered electric field on the interaction of nano-plates. For ideal dielectrics an electric field may assist (or retard) exfoliation depending on the angle between a collection of plates and the field. A critical electric field strength to promote exfoliation is predicted when the field is parallel to the surface of the plates. Structural refinement is predicted to occur by cleavage through the center of the stack. For lossy dielectrics, frequency can be tuned to cause exfoliation in all plate orientations.

Professor Rodney Ruoff and colleagues at Northwestern University and Purdue University have developed a process that promises to lead to the creation of a new class of composite materials - graphene-based materials. They reported the results of their research in Nature, 442 (2006) 282-286. This team has overcome the difficulties of yielding a uniform distribution of graphene-based sheets in a polymer matrix. Such composites can be readily processed using standard industrial technologies such as moulding and hot-pressing. The technique should be applicable to a wide variety of polymers. The graphene composites may compete with carbon nanotube-based materials in terms of mechanical properties. This new class of composites may stimulate the applied mechanics community to study the fundamental reinforcing mechanisms of graphene sheets from both experimental and theoretical approaches.

A vicinal Si (001) surface may form stripes of terraces, separated by monatomic-layer-high steps of two kinds, S_A and S_B. As adatoms diffuse on the terraces and attach to or detach from the steps, the steps move. In equilibrium, the steps are equally spaced due to elastic interaction. During deposition, however, S_A is less mobile than S_B. We model the interplay between the elastic and kinetic effects that drives step motion, and show that during homoepitaxy all the steps may move in a steady state, such that alternating terraces have time-independent, but unequal, widths. The ratio between the widths of neighboring terraces is tunable by the deposition flux and substrate temperature. We study the stability of the steady state mode of growth using both linear perturbation analysis and numerical simulations. We elucidate the delicate roles played by the standard Ehrlich-Schwoebel (ES) barriers and inverse ES barriers in influencing growth stability in the complex system containing (S_A+S_B) step pairs.

Preprint available in the attachment.

Abstract: The gating pathways of mechanosensitive channels of large conductance (MscL) in two bacteria (Mycobacterium tuberculosis and Escherichia coli) are studied using the finite element method. The phenomenological model treats transmembrane helices as elastic rods and the lipid membrane as an elastic sheet of finite thickness; the model is inspired by the crystal structure of MscL. The interactions between various continuum components are derived from molecular-mechanics energy calculations using the CHARMM all-atom force field. Both bacterial MscLs open fully upon in-plane tension in the membrane and the variation of pore diameter with membrane tension is found to be essentially linear. The estimated gating tension is close to the experimental value. The structural variations along the gating pathway are consistent with previous analyses based on structural models with experimental constraints and biased atomistic molecular-dynamics simulations. Upon membrane bending, neither MscL opens substantially, although there is notable and nonmonotonic variation in the pore radius. This emphasizes that the gating behavior of MscL depends critically on the form of the mechanical perturbation and reinforces the idea that the crucial gating parameter is lateral tension in the membrane rather than the curvature of the