Interfacial forces and spectroscopic study of confined fluids

In this chapter we discuss three specific issues which are relevant for liquids in intimate contact with solid surfaces. (1) Studies of the hydrodynamic flow of simple and complex fluids within ultra-narrow channels show the effects of flow rate, surface roughness and fluid-surface interaction on the determination of the boundary condition. We draw attention to the importance of the microscopic particulars to the discovery of what boundary condition is appropriate for solving continuum equations and the potential to capitalize on slip at the wall for purposes of materials engineering. (2) We address the long-standing question of the structure of aqueous films near a hydrophobic surface. When water was confined between adjoining hydrophobic and hydrophilic surfaces (a Janus interface), giant fluctuations in shear responses were observed, which implies some kind of flickering, fluctuating complex at the water-hydrophobic interface. (3) Finally we discuss recent experiments that augment friction studies by measurement of diffusion, using fluorescence correlation spectroscopy (FCS). Here spatially resolved measurements showed that translation diffusion slows exponentially with increasing mechanical pressure from the edges of a Hertzian contact toward the center, accompanied by increasingly heterogeneous dynamical responses. This dynamical probe of how liquids order in molecularly thin films fails to support the hypothesis that shear produces a melting transition.