The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer. In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially. These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.

The rate of growth of capillary condensates in a model wedgelike pore has been studied using the surface force apparatus. The method described allows the direct measurement of the diffusionlimited growth of liquid condensates with interface radii of curvature as small as 20 nm. The observed rates of condensation of vapors of n-pentane, n-hexane, cyclohexane, and water are close to, but consistently lower than, values calculated from a model based on Langmuir's theory of droplet growth. [S0031-9007(99)

Nucleation in a supercooled or a supersaturated medium is a stochastic event, and hence statistical analyses are required for the understanding and prediction of such events. The development of reliable statistical methods for quantifying nucleation probability is highly desirable for applications where control of nucleation is required. The nucleation of gas hydrates in supercooled conditions is one such application. We describe the design and development of a high pressure automated lag time apparatus (HP-ALTA) for the statistical study of gas hydrate nucleation and growth at elevated gas pressures. The apparatus allows a small volume (≈150 μl) of water to be cooled at a controlled rate in a pressurized gas atmosphere, and the temperature of gas hydrate nucleation, T(f), to be detected. The instrument then raises the sample temperature under controlled conditions to facilitate dissociation of the gas hydrate before repeating the cooling-nucleation cycle again. This process of forming and dissociating gas hydrates can be automatically repeated for a statistically significant (>100) number of nucleation events. The HP-ALTA can be operated in two modes, one for the detection of hydrate in the bulk of the sample, under a stirring action, and the other for the detection of the formation of hydrate films across the water-gas interface of a quiescent sample. The technique can be applied to the study of several parameters, such as gas pressure, cooling rate and gas composition, on the gas hydrate nucleation probability distribution for supercooled water samples.

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The molecular details of adhesion mechanics in phospholipid bilayers have been studied using atomic force microscopy (AFM). Under tension fused bilayers of dipalmitoylphosphatidylcholine (DPPC) yield to give non-distance dependent and discrete force plateaux of 45.4, 81.6 and 113+/-3.5 pN. This behaviour may persist over distances as great as 400 nm and suggests the stable formation of a cylindrical tube which bridges the bilayers on the two surfaces. The stability of this connective structure may have implications for the formation of pili and hence for the initial stage of bacterial conjugation. Dimyristoylphosphatidylcholine (DMPC) bilayers also exhibit force plateaux but with a much less pronounced quantization. Bilayers composed of egg PC, sterylamine and cholesterol stressed in a similar way show complex behaviour which can in part be explained using the models demonstrated in the pure lipids.

The surface tension of n-octadecane was studied in the vicinity of the bulk melting point using both the maximum bubble pressure and Wilhelmy plate methods. The bubble surfaces were found to be supercooled below the surface freezing point. The onset of surface freezing is indicated by a sharp drop in surface tension at a constant temperature. This transition is accompanied by an increased film stability resulting in longer bubble lifetimes at the liquid surface. Variations in bubble lifetime reflect changes in the interfacial mechanical properties of the film from liquidlike to solidlike.

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The memory effect in the nucleation of gas hydrates is one of the longest outstanding mysteries in the field of gas hydrates. We previously investigated the memory effect of natural gas hydrates in the presence of a solid wall using a first-generation high-pressure automated lag time apparatus (HP-ALTA MkI) and confirmed that the memory effect existed beyond statistical variation that is inherent in the nucleation of gas hydrates. The amount of the memory effect also varied depending on the sample cells used, which indicated that the presence of a solid wall was a major factor. In the present study, we used a second-generation HP-ALTA (MkII) to investigate the memory effect of the same natural gas hydrates on the surface of a quasi-free water droplet suspended in squalane. No significant increase was detected in the nucleation rate with the reduction of the superheating temperature in the quasi-free water droplet, which suggests that the presence of a solid wall is required for the memory effect. A new hypothesis based on the presence of interfacial nanobubbles and other interfacial gaseous states on solid walls is proposed to account for all known attributes of the memory effect.