Andrew Marian Homola

Liquid to solidlike transitions of molecularly thin films under shear

We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very co...

Measurements of and Relation Between the Adhesion and Friction of Two Surfaces Separated by Molecularly Thin Liquid Films

A new technique is described for sliding (shearing) two molecularly smooth surfaces laterally past each other in liquids while monitoring their exact contact area, the normal and transverse forces, and the surface separation. First, we show that the elastic deformations of two initially curved surfaces in adhesive contact are the same under static and dynamic (i.e., sliding) conditions. Detailed results are then presented of how the shear properties of thin films of water and a simple nonpolar liquid are “quantized” with the number of layers. Results with water as the intervening liquid, as well as the effects of humidity on sliding in air, reveal that more complex mechanisms are operating than with simple liquids which appear to be related to the complex “hydration” forces between two surfaces in water or in aqueous salt solutions. The results suggest a close correlation between the static forces and shear properties of very thin liquid films, and the molecular structure of the liquids confined within such films.

Influence of monomer architecture on the shear properties of molecularly thin polymer melts

The effective viscosity and shear behavior of ultrathin films of two structurally different perfluoropolyether fluids have been investigated. The materials used were Fomblin Z, a linear random copolymer of fluoroethylene oxide and fluoromethylene oxide, and Fomblin Y, a branched random copolymer with fluoropropylene oxide and fluoromethylene oxide monomer units. The shearing experiments were conducted with the fluids confined between molecularly smooth surfaces at shear rates ranging from 200 to 4×103 s−1. It was found that when the thickness of the fluid films decreases from 10 to 2 nm, both perfluoropolyethers exhibited a sharp increase in viscosity, from bulk values to ‘‘surface viscosity’’ values that are many orders of magnitude larger. With increasing shear rates, the Z‐type fluid showed a gradual decrease in the shear stress indicating an apparent ordering of the molecules due to the applied shear forces. On the other hand, with the Y‐type copolymer, the shear stresses were significantly lower and ...

Ultrathin particulate magnetic recording media (abstract)

This paper presents a novel method of producing an ultrathin and highly oriented particulate magnetic recording medium. The process involves the laying down of a monolayer of magnetically aligned particles where the thickness of the layer is limited only by the diameter of the magnetic particles. Thus, using commercially available magnetic particles, a 50–100‐nm‐thick magnetic layer can be produced. The particle deposition process depends on electrostatic interactions of charged particles with an oppositely charged disk surface in an aqueous medium. In order to reduce the tendency of the magnetic particles to aggregate, the particles are stabilized by adsorbed colloidal silica. Particle volume concentration (PVC) is higher than usually realized with conventional particulate media and it was measured in excess of 50%. A typical particulate monolayer has an Ms of 1.6×10−3 emu/cm2 and the orientation ratio (Mr/Ms) of 4–4.5. Resolution, at 150‐nm flying height, was measured at 1200 fc/mm with a high signal‐to...

Measurements of Static and Dynamic Interactions of Molecularly Thin Liquid Films Between Solid Surfaces

New experimental techniques are described for measuring both the static and dynamic interactions of very thin liquid films between two molecularly smooth surfaces as they are moved normally or laterally relative to each other. Film thickness can be measured and controlled to IA. Results are presented of how the properties of liquid films only one, two, or three molecular layers thick differ from those of bulk liquids. When close together, two molecularly smooth surfaces slide (shear) past each other while separated by a discrete number of liquid layers. The critical shear stress required to initiate motion for simple spherical molecules is “quantized” with the number of layers. Another important characteristic of molecular thin liquid films undergoing shear is the phenomenon of “stick-slip” or “intermit-tent” motion. Both of these phenomena have been studied as a function of driving velocity, area of contact, and number of molecular layers.

Fluorocarbon/hydrocarbon interactions and frictional surface forces

Abstract Alumina-adsorbed hydridosiloxane polymers are used as a “base” for further chemical modification and determination of intersurface frictional coefficients. The stability of this base layer is found to be particularly sensitive to the presence of moisture. Sedimentation experiments suggest that the final densities of compacts produced from hydridosiloxane-coated alumina particles are independent of the range of molecular weights used in this study. Subsequent reaction of the base layer with silanol-terminated trifluoropropylsiloxane and polydimethylsiloxane tails yielded considerable differences in sediment density and character in spite of similar molecular weights. Use of a frictional surface forces apparatus suggests that this phenomenon is not due to a frictional or lubricative effect induced by the fluorine-containing component.