The Surface Force Apparatus (SFA) is reviewed. The origin of the technique, from multiple-beam interferometry, tribological studies and measurements of van der Waals forces in air to the versatile instrument of today, is traced. The underlying principles are described while the inherent simplicity of the measurements is stressed. The two chief advantages of the SFA compared to other force-measuring instruments are emphasised- (i) that the zero of separation is accurately determined, and (ii) that the shape of the surfaces and the refractive index of the intervening medium can be monitored with multiple-beam interferometry. For these reasons, the SFA is not only a device for measuring forces, it is a valuable tool for studying surface interactions and thin film behaviour in general.
Basic experimental applications of the Surface Force Apparatus (SFA) to vapours, pure liquids and mixtures of simple liquids are described. The interpretation of the simplest possible measurement, that of adhesion in air or nitrogen, is discussed with examples of possible complications. The use of the SFA to measure adsorption isotherms at isolated surfaces in vapours and to study capillary condensation of vapour between two surfaces in or close to contact is detailed. The measurement of solvation forces in pure liquids, including non-polar, dipolar and hydrogen-bonding liquids and mixtures of liquids is described. General features of the experiments and possible pitfalls are discussed throughout.
Recent progress in surface forces measurements allow us to measure forces between protein adsorbed mica surfaces directly. The force curves can be devided into two regimes due to the surface separation, a “longer distance” regime and an “adsorbed layer” regime. In a “longer distance” regime, the forces are largely due to an electrostatic double-layer force originating from the charges on the adsorbed protein layers. In the “adsorbed layer” regime, forces of steric origin owing to overlap of adsorbed layers dominate. From the analysis of the forces in an “adsorbed layer” regime, conformations of the adsorbed protein molecules can be estimated.
In this paper we describe some measurements of the interactions between interfaces coated with block copolymers of ethylene oxide (E) and butylene oxide (B). We used two types of surface force techniques for determining the forces between hydrophobic surfaces across aqueous solutions of the diblock copolymer B8E41. With the help of the interferometric surface force technique the adsorbed layer thickness could be determined as well as the range and distance dependence of the steric force. The results obtained with a non-interferometric surface force technique agreed very well with the ones obtained using the interferometric technique provided that one takes into account the compressibility of the adsorbed polymer layer. The steric interactions between two air-water inferfaces with adsorbed B8E41, are less long-ranged than the interaction between hydrophobic solid surfaces coated with the same polymer. This result is not yet understood. The range of the steric force in foam lamella stabilized by the diblock copolymer is found to be much more long ranged than that of the triblock copolymer E20B8E20. The reason for this is that the triblock copolymer adsorbs less and forms smaller tails.
This article reviews recent advances in the surface forces measurements with an overview given in a table Measurements using a surface forces apparatus and an atomic force microscope are explained in a concise manner, and detailed researches on soft comlex systems are described. 1) The elastic compressibility modulus of polyelectrolyte (ionized poly (glutamic acid)) brush layers was determined by analysing the steric repulsion of the surface forces. Abrupt change in the modules was found at a critical density, which was accounted for in terms of the transition in the counterion binding to polyelectrolytes. 2) Two-dimensionally organized protein layers was prepared utilizing LB films which could bind His-tag (poly (histidine) tagged) proteins through specific interaction between metal-chelating lipids and poly (histidine) stretches via Cu (II) ions. An atomic force microscope was employed to measure the forces between these protein monolayers. 3) Nanorheological behavior of liquid crystal thin films was studied by a shear forces apparatus. Resonance curves of the mechanical system were monitored with changing film thickness and normal load. The rheological as well as tribological parameter, local viscosity and friction coefficient, were obtained from the amplitude and the phase of response signals around the resonance frequencies.
Following talc, phlogopite and muscovite, which were reported in a previous report (J. Surface Sci. Soc. Jpn.17, pp.90-97, 1996), the cleavage surfaces of brucite and chlorite (clinochlore) are studied using AFM and LEED. The (0001) surface of natural brucite crystal cleaved in a pure argon atmosphere was placed in an AFM sample chamber with an argon gas stream. A hexagonal network image of OH- ions with a distance of 3.1Å was clearly observed. This structure was supported by the observed LEED pattern and crystallographical data. For the cleavage surface of chlorite in a pure argon atmosphere, two types (i.e. talc-like AFM image (O2--O2- distance of 2.5 Å) and brucite-like AFM image (OH--OH- distance of 2.7 Å)) were observed. However, the lattice distances from these AFM images were shorter than those from the AFM images of talc and brucite, respectively. A large-scale (1000 nm x 1000 nm) AFM image of the chlorite cleavage surface indicates that there are island-shaped Brucite-like layers existing on basal talc-like layer. The interpretation of this AFM image is supported by the observed chlorite LEED pattern, which consists of diffraction spots of talc-like and brucite-like structures.
The effects of crystal defects near the surface on the position of surface Fermi level (EFS) are investigated using photoluminescence (PL) measurements and synchrotron radiation photoelectron spectroscopy (SRPES). For the lightly Sidoped GaAs (001) surface, PL measurements reveal that after heating to 500°C a layer with lower PL peak intensities related to gallium vacancies than those of the bulk exists just under the thermal degraded layer. SRPES shows that EFS moves upward to 1.1-1.17 eV above the valence band maximum when this thermal degraded layer is removed by chemical etching. The excess arsenic on the surface, which is formed by rinsing the etched surface with deoxygenated and deionized water, is evaporated by heating in ultra-high vacuum (UHV). After evaporation of excess arsenic on the surface by heating, the etching-depth dependence of EFS for a sample pre-heated in UHV correlates with the existence of this defect concentration layer. These results suggest that the position of EFS for GaAs (001) surface is strongly influenced by crystal defects near the surface.
The surface microstructure is one of the important factors to determine functions and properties of materials. It is necessary to characterize the surface microstructure for the purpose of controlling and designing. This review deals with mobilities and structures of functional groups adsorbed on the surfaces of inorganic solids after chemical modification. I focus on the measurements and simulations on the microstructures of normal saturated alkyl chains, since these chains have few complicated interaction between inter- and/or inter-molecules, such as hydrogen bonding.
The quartz crystal microbalance (QCM) provides unique information on minute mass of nanogram range which is often difficult to obtain with other techniques. Recent success in the preparation of a Au (111) single crystal QCM electrode makes the QCM more useful, particularly for fundamental research. The QCM response or frequency change is well proportional to mass change in most of monolayer systems. However, the QCM also responds to changes in density and viscosity of solution and in viscoelasticity of modified layers. Therefore, it is recommended to examine the accuracy of QCM measurements with other techniques such as electrochemistry, spectroscopy and quartz crystal impedance method. Recently, novel QCM/spectroscopy (UV-vis and FT-IR) combined methods have been developed by us. It is demonstrated that these methods are useful to conduct accurate and detailed analysis of processes at modified electrode/electrolyte solution interfaces.