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Accurate measurements of drainage forces between an approaching sphere and flat surface have revealed the presence of boundary slip in aqueous Newtonian Solutions. The degree of slip has been characterised and is revealed to be dependent upon the shear rate and viscosity of the fluid. Hydrodynamic forces have also been utilised as a means to accurately determine the spring constant of colloid probes (very fine Atomic Force Microscope cantilever springs with a sphere placed at the end). This method is convenient, accurate and highly suitable for colloid probes used in surface force measurement. A technique has been developed to further characterise colloid probes by conveniently determining the radius of curvature of the colloid probe in the interaction region and analysis of the surface features of the colloid probe. This involves 'reverse imaging" of the probe surface using a surface composed of a series of sharp spikes ( V. Craig, C. Neto*, D. Williams)
Surface Force Measurements
The effectiveness of the recently developed Capacitance Dilatometer for the Surface Force Apparatus has been greatly enhanced by the implementation of new instrumentation and anti-vibration measures. The distance between the surfaces to be investigated is obtained from the capacitance of a parallel plate capacitor measured with a variable ratio transformer bridge. The surface separation is made to vary by means of the standard piezoelectric transducer or a magnetic force transducer. The dilatometer is able to measure adhesion of silica and of single crystal mica surfaces with high accuracy and reproducibility and has been used to measure forces between mica surfaces in electrolyte using the droplet method. Measurements have been made of the dispersion force between mica surfaces in air in the retarded regime with unprecedented accuracy and it is found that above 10 nm separation Casimir-Polder theory that takes account of retardation fits the results much better than non-retarded van der Waals theory.
A.M. Stewart, V.V. Yaminsky
Interfacial friction
The interfacial friction between single crystal mica surfaces has been studied with the friction attachment for the Surface Force Apparatus that has been constructed in the Department. In contrast to ordinary friction which involves shear and fracture of asperities in the contacting surfaces, interfacial friction occurs without observable damage between the atomically smooth surfaces. A previously unreported peak has been detected in the interfacial friction of mica after the surfaces have been separated for only a few seconds and then brought together again and this has been correlated with the humidity of the experimental atmosphere.
S. Ohnishi and A.M.Stewart
Theory of Atomic Forces and Intermolecular Interactions
It has been shown that in semi-classical electrodynamics, which describes how electrically charged particles move according to the laws of quantum mechanics under the influence of a prescribed classical electromagnetic field, only a restricted class of gauge transformations is allowed. This lack of full gauge invariance, in contrast to the situation in classical and quantum electrodynamics which are fully gauge invariant theories, is due to the requirement that the scalar potential in the Hamiltonian of wave mechanics represent a physical potential. Probability amplitudes and energy differences are independent of gauge within this restricted class of gauge transformation.
In 1957 Foreman and Lomer proposed a method of estimating the harmonic forces between parallel planes of atoms of primitive cubic crystals by Fourier transforming the squared frequencies of phonons propagating along principal directions. A generalized form of this theorem is derived in this paper and it is shown that it is more appropriate to apply the method to certain combinations of the phonon dispersion relations rather than to individual dispersion relations themselves. Further, it is also shown how the method may be extended to the hexagonal close packed and diamond lattices. Explicit, exact and general relations in terms of atomic force constants are found for deviations from the Blackman sum rule which itself is shown to be derived from the generalized Foreman-Lomer theorem.
A. M. Stewart
Observation of capillary forces in vapors of various liquids
S. Ohnishi and V. Yaminsky Forces between the fluorocarbon monolayer surfaces were measured under various vapor conditions with the interfacial gauge. The force profiles in vapors of ethanol or chloroform agree with the theory of capillary bridging by constant volume meniscus. In vapors of perfluorohexane, the interaction form changed to the capillary force for the meniscus in equilibrium with the vapor. By fitting the measured force curves with theoretical forms ,the condensate volumes and the acting values of relative vapor pressure were estimated.
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