Theory of van der Waals forces

Due to its ubiquitous nature, the van der Waals interaction accounts for a significant part of interfacial energies between solids and liquids. In the theory of dispersion forces, we are mainly concerned with the long-wavelength part of electromagnetic fluctuations. However, recently we examined the spatial dispersion in electromagnetic fluctuations and found that the origin of orientational order of molecular assemblies on a crystalline surface is linked to the anisotropic van der Waals force [6, 7, 10].

Surfactant micelles spontaneously adsorb on a substrate with orientational order dictated by the crystal structure. In particular, on a gold surface it happens despite the screening effects of delocalized electron clouds in metallic systems. To understand the van der Waals forces that provide organization on metallic substrates, we developed a formalism wherein the dielectric response acquires directional dependence through phonon dispersion relations. In metals, ionic screening is enhanced along certain directions and a crystalline metallic substrate generates both torque and attraction on geometrically asymmetric objects. The advance could lead to a new `bottom-up' tool in nanofabrication and give scientists refined control over how nanoscale building blocks assemble on a substrate.

Hydrophobic Interaction, Development of Force Fields for Biomolecular Simulations

Hydrophobicity is the molecular driving force behind numerous important biological processes, including protein folding and the formation of biological membranes. A quantitative understanding of hydrophobic interactions is crucial for modeling protein structures and manipulation of hydrophobic nanoparticles/nanotubes in aqueous solutions.

As a model for hydrophobic association, we employed first-principles simulations to evaluate the force between two methanes dissolved in water. A constrained MD approach has been developed to efficiently calculate the free energy during the association of the two methanes. One conclusion is that the strength and shape of the (hydrophobic) potential of mean force are in conflict with earlier classical-force-field simulations but agree well with solubility experiments. We also analyzed the dynamic behavior of water in the solvation shell and found well-structured solvation shells for particular methane-methane separations [11].

Our goal has been to develop new water potentials which can describe hydrophobic interactions comparable to first-principles results.

Exfoliation of Graphite Oxide

Exfoliation of graphite down to a single graphene plate has been achieved and shows many interesting physical and chemical properties. Many questions lingered; these include the structural properties of graphite oxide (the starting material), the exfoliation process, the mechanical properties of an individual functionalized graphene sheet, etc. We are vigorously investigating these problems in collaboration with a team at Princeton University [8, 9, 12, 13].

Functionalized graphene sheets could lead to nanoscale graphite origami. Using atomic force microscopy, we found these crumpled single sheets can be folded back and forth easily. We have deduced the elastic property of the sheet by balancing the van der Waals binding energy in the folded region and the strain energy in the bending region. Several questions remain to be investigated. What are hinge lines made of? How does functionalization of graphite facilitate easier folding than a pristine graphene sheet? As is well known, a wire with wiggles is easier to bend than the one without: the short-wavelength wiggles renormalize the long-wavelength bending rigidity. A more fundamental understanding of this phenomenon paves a way to achieve graphite origami, three-dimensional forms of tailor-made graphite.