Shearing of nanoscopic bridges in two-component thin liquid layers between chemically patterned walls

Fluid bridges confined between chemically nanopatterned solid substrates

We discuss equilibrium properties of classical fluids confined to nanoscopic volumes by solid substrates. The substrates themselves are endowed with wettable chemical patterns of variable symmetry. We develop a thermodynamic description suitable for these highly anisotropic systems. Based upon a combination of Monte Carlo simulations in the grand canonical ensemble and lattice density functional theory at mean-field level we analyze the structure and phase behaviour of the confined fluid. Under suitable thermodynamic conditions the fluid may condense partially in regions controlled by the wettable nanopatterns. The resulting fluid bridges are established as thermodynamic phases and exhibit unique rheological features.

The interaction of patterned solutes in binary solvent mixtures

Mean solute-solute forces and solute-induced solvent structure are investigated for pairs of chemically patterned (patched) solutes in binary mixtures near demixing coexistence. The isotropic and anisotropic hypernetted-chain integral equation theories as well as a superposition approximation are solved and compared. The patched solutes consist of one end that favors the majority species in the mixture while the other end favors the minority species. A wide range of patch sizes is considered. The isotropic and anisotropic theories are found to be in good agreement for most orientations, including the most attractive and most repulsive configurations. However, some differences arise for asymmetrical orientations where unlike ends of the solute particles face each other. In contrast, superposition often gives a rather poor approximation to the mean force, even though the results obtained for the solvent densities agree qualitatively with the anisotropic theory. The mean force is sensitive to small differences in the densities particularly near demixing. For patched solutes the influence of demixing-like behavior is evident both in the orientational dependence and in the range of the mean force acting between solutes.

Nanoscopic Liquid Bridges between Chemically Patterned Atomistic Walls

A binary liquid mixture, containing the Lennard-Jones molecules A and B, in equilibrium with a bulk liquid reservoir near the point of phase separation, confined between atomistic chemically patterned walls, is studied by grand canonical Monte Carlo simulations. In the bulk, the B-rich phase is stable and the A-rich phase is metastable. The walls bear patches attractive to A; when the walls are close, A-rich liquid bridges condense between the patches. The normal and lateral forces on the walls are measured as a function of the wall separation and of the lateral displacement between the patches on opposite walls. When there are one or two molecular layers in the bridge and the wall lattice constant is close to that of crystalline A, the normal and lateral forces depend strongly on the registry of the wall lattices, varying in an oscillatory manner.