Images of nanobubbles on hydrophobic surfaces and their interactions.

Imaging of hydrophobic surfaces in water with tapping mode atomic force microscopy reveals them to be covered with soft domains, apparently nanobubbles, that are close packed and irregular in cross section, have a radius of curvature of the order of 100 nm, and a height above the substrate of 20-30 nm. Complementary force measurements show features seen in previous measurements of the long-range hydrophobic attraction, including a jump into a soft contact and a prejump repulsion. The distance of the jump is correlated with the height of the images. The morphology of the nanobubbles and the time scale for their formation suggest the origin of their stability.

Interaction and deformation of viscoelastic particles: nonadhesive particles.

A viscoelastic theory is formulated for the deformation of particles that interact with finite-ranged surface forces. The theory generalizes the static approach based upon classic continuum elasticity theory to account for time-dependent effects, and goes beyond contact theories such as Hertz and that given by Johnson, Kendall, and Roberts by including realistic surface interactions. Common devices used to measure load and deformation are modeled and the theory takes into account the driving velocity of the apparatus and the relaxation time of the material. Nonadhesive particles are modeled by an electric double layer repulsion. Triangular, step, and sinusoidal trajectories are analyzed in a unified treatment of loading and unloading. The load-deformation and the load-contact area curves are shown to be velocity dependent and hysteretic.

Contact-angle hysteresis in solid-on-solid wetting.

The spreading of an elastic adhesive sphere on a substrate is calculated using continuum elasticity theory and the van der Waals interaction between the solid surfaces. The deformation and contact area are obtained self-consistently as a function of load. Hysteresis is demonstrated between the loading and the unloading cycles: the receding contact angle exceeds the advancing one. In addition, for a given deformation (flattening), or for a given applied load, the receding contact area exceeds the advancing one. This nonequilibrium phenomenon is velocity dependent and in accord with experiments on adhesion and crack propagation.

Surface Charge of Silver Iodide and Several Metal Oxides. Are All Surfaces Nernstian?

Charge titration data for silver iodide, titanium dioxide, aluminum oxide, silica dioxide and ferric hydroxide are analyzed using a generic site binding model that has the Nernst equation as a limiting form. Both the hypernetted chain and the nonlinear Poisson-Boltzmann approximations are used to relate the diffuse double-layer potential to the surface charge, and a zeroth-order Stern layer is used to give the surface potential. In all cases it is shown that in the vicinity of the point of zero charge the Nernst equation accurately gives the surface charge. Copyright 2000 Academic Press.

The attractive forces between polar lipid bilayers.

Long-range attractive forces between lipid bilayers are not well described by the Lifshitz theory of Van der Waals forces between macroscopic media. It is shown that when correlations between polar headgroups are taken into account, the predicted forces take a qualitatively different form consistent with the measured data.