Surface force apparatus dimensions

The development of a host of scanning probe devices such as the atomic force microscope (AFM) [13-17] and the surface forces apparatus (SFA) [18-22], on the other hand, enables experimentalists to study almost routinely the behavior of soft condensed matter confined by such substrates to spaces of molecular dimensions. However, under conditions of severe confinement a direct study of the relation between material properties and the microscopic structure of confined phases still remains an experimental challenge. 

Until fairly recently, the theories described in Secs. II and III for particle-surface interactions could not be verified by direct measurement, although plate-plate interactions could be studied by using the surface forces apparatus (SFA) [61,62]. However, in the past decade two techniques have been developed that specifically allow one to examine particles near surfaces, those being total internal reflection microscopy (TIRM) and an adapted version of atomic force microscopy (AFM). These two methods are, in a sense, complementary. In TIRM, one measures the position of a force-and torque-free, colloidal particle approximately 7-15 fim in dimension as it interacts with a nearby surface. In the AFM method, a small (3.5-10 jam) sphere is attached to the cantilever tip of an atomic force microscope, and when the tip is placed near a surface, the force measured is exactly the particle-surface interaction force. Hence, in TIRM one measures the position of a force-free particle, while in AFM one measures the force on a particle held at a fixed position. 

For surfaces that deform plastically, the contact area A is proportional to the applied load. A single elastic contact deforms as as load increases, and would not be expected to follow this rule. However, when considering an exponential surface height distribution, which leads to a multiplicity of elastic asperity contacts, a linearity between load and contact area is recovered. Using instrumentation developed in the last 15yr, notably the atomic force microscope (AFM) and surface forces apparatus (SFA), researchers have explored the universality of friction-load proportionality over a much wider range of dimensions and surface characteristics. Indeed, SFA experiments have shown friction-load proportionality between atomically smooth mica surfaces in dry air over square micrometers of contact area. A contact mechanics expression for elastic contacts that incorporates the effects of adhesion was used. Similarly, AFM experiments of... 

The surface forces apparatus measures the forces between surfaces of macroscopic dimensions. The requirement that large areas be brought to very small separations imposes demanding requirements on the flatness of the surfaces used and by far the majority of experiments with the SFA have used freshly cleaved mica as a surface that can be relied upon to be atomically smooth over macroscopic areas. The technique of scanning force microscopy allows the force between a tip of small radius of eurvature and virtually any kind of surface to be measured. In addition, the tip can be scanned over the surface to provide an image, which can be of very high resolution, of the force as a function of position. The drawbacks of the technique are that it is difficult to... 

Can the observed behaviour on the macroscopic and microstructural scales be reconciled with what we know about frictional sliding under these conditions To answer this question, we turn to other reported work with mica in which the real and apparent areas of contact coincide. Johnson reports that experiments with an atomic friction microscope (AFM), on mica in which the contact dimension is 2 to 10 nm, indicate a frictional shear stress of 1 GPa. Other measurements performed with a surface force apparatus (SFA), in which the contact dimension is in the order... 

Surface-induced layering corresponds to oscillations of the disjoining pressure in the molecular range of thickness (see also chapter 1). Well-known saw-tooth force profiles are obtained with surface forces apparatus (SFA) setups. In the case of microdroplets, where the thickness is free to adjust, an unstable part in the disjoining pressure behaves just like the usual pressure at three dimensions a Maxwell construction leads to a horizontal part in n(f), corresponding to the coexistence of films with different thickness (see chapter 3). 

Abstract The structure and mechanics of very thin hquid crystal films depend on the intermolecular interactions in confined dimensions. The rheology of such films has been investigated by a shear force apparatus constructed as an attachment to the surface forces apparatus. The novelty of this method is that the rheological parameters are extracted from the amplitude and the phase of the output signal as a function of the resonance frequency. The apparent viscosity of the liquid crystal film is calculated from the damping coefficient by using a simple theoretical model. The viscosity of nanometer thin films of 4-cyano-4-... 

In an effort to create model surfaces to correlate the conformations of the polymers with the measured interactions, we have been examinming the interactions between layers of 2-vinylpyridine(PVP)-styrene(PS) block copolymers adsorbed onto mica substrates (in the surface force apparatus). " In these systems, we observed repulsive forces at distances many ( 10) times larger than the dimension of those polymers in solutions. In particular, we found that the distance at which the repulsive force becomes significant is a linear function of PS-block length. 

Although the ratio of catalyst volume to mercury volume may be made quite small, as long as liquid mercury is kept in complete contact with the tubes in the catalyst zone, the ratio of catalyst volume to tube surface and the maximum distance of catalyst from tube surface must be controlled within limits. As the maximum distance which heat has to travel in passing from the reaction zone to the heat absorbing surface increases, the temperature differential or driving force" between this maximum distance point and the absorbing surface must increase in order for the same amount of heat to be transferred. In this type of apparatus the rate at which heat reaches the tube wall controls the rate at which it may be dissipated by the mercury. Also it is imperative that the catalyst at no point exceed a definite maximum temperature, which depends in value upon the activity of the catalyst, to prevent undue losses by complete combustion. It is therefore necessary to restrict the cross sectional dimension of the catalyst tubes. The patentfor the apparatus claims a minimum ratio of three square inches of tube surface per each cubic inch of catalyst volume. With square tubes inch inside... 

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