Experimental results surface force measurements

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

Before equations such as Eqs. 6, 7 and 8 can be used, values for the surface energies have to be obtained. While surface energies of liquids may be measured relatively easily by methods such as the du Nouy ring and Wilhelmy plate, those of solids present more problems. Three approaches will be briefly described. Two involve probing the solid surface with a liquid or a gas, the third relies on very sensitive measurement of the force required to separate two surfaces of defined geometry. All involve applying judicious assumptions to the experimental results. 

Surface force apparatus has been applied successfully over the past years for measuring normal surface forces as a function of surface gap or film thickness. The results reveal, for example, that the normal forces acting on confined liquid composed of linear-chain molecules exhibit a periodic oscillation between the attractive and repulsive interactions as one surface continuously approaches to another, which is schematically shown in Fig. 19. The period of the oscillation corresponds precisely to the thickness of a molecular chain, and the oscillation amplitude increases exponentially as the film thickness decreases. This oscillatory solvation force originates from the formation of the layering structure in thin liquid films and the change of the ordered structure with the film thickness. The result provides a convincing example that the SFA can be an effective experimental tool to detect fundamental interactions between the surfaces when the gap decreases to nanometre scale. 

The experimental results in Fig. 26 have a further implication that the nominal value of surface energy does not associate directly with the friction level. The FFM measurements of friction force between gold surfaces and Si tips... 

Figure 9.10 Adsorbed amount of water on a silicon oxide surfaces versus relative vapor pressure at 20°C. The continuous line was calculated with the theory of Polanyi and assuming van der Waals forces only (Eq. 9.57). Experimental results as measured on Aerosil 200 were adapted from Ref. [379] (see also Fig. 9.9). The deviation at high pressure is partially due to the porosity of the adsorbent. The equilibrium vapor pressure is P0 = 3.17 kPa.

To summarize, the model used in this paper captures many important features of protein structure and dynamics and is indeed seen to reproduce many of the general trends observed in SM-FRET experiments. At the same time, we have also observed several intriguing discrepancies between the model predictions and the experimental results. One possibility is that these discrepancies originate from shortcomings of the model. For example, the SM-FRET measurements reported in Refs. [30, 33] were performed on a coiled-coil that was immobilized on a positively charged amino-silanized glass surface and involved charged dye molecules. This implies that the protein-surface and donor-acceptor interactions may be dominated by electrostatic forces. Our... 

A Models to describe microparticles with a core/shell structure. Diametrical compression has been used to measure the mechanical response of many biological materials. A particular application has been cells, which may be considered to have a core/shell structure. However, until recently testing did not fully integrate experimental results and appropriate numerical models. Initial attempts to extract elastic modulus data from compression testing were based on measuring the contact area between the surface and the cell, the applied force and the principal radii of curvature at the point of contact (Cole, 1932 Hiramoto, 1963). From this it was possible to obtain elastic modulus and surface tension data. The major difficulty with this method was obtaining accurate measurements of the contact area. 

Numerous experimental studies and computer simulations have been carried out during the last several years to check the results of these two theoretical approaches (for a recent review see [28]). The brush height can be obtained from force measurements between two brushes, since the brushes first interact when the distance between their respective grafting surfaces is 2h [18,37]. The inner structure of the brushes has been probed by small angle neutron scattering [13,14,38,39] and neutron reflectivity [21,23,24,40,41]. All these studies, as well as a number of simulations [28], give results that are consistent with the analytical SCF predictions. 

The main trends of the study of surface forces in foam films are briefly outlined here and the results obtained will be successively discussed in the next Sections. Furthermore, some earlier considerations will be commented, for instance, the first quantitative experimental verification of the DLVO-theory with the aid of foam films, since these results form the base of the further achievements in measurement and interpretation of surface forces and their role in the stability of foam films. 

These forces have been measured. A further consequence of the new theories is that inferences of surface charge, "ion-binding" to surfaces as adduced from force measurements and/or electrophoretic mobility studies or NMR which involve the crude Poisson-Boltzmann theory are wrong. If the simple analytic Poisson-Boltzmann theory is used to interpret direct force measurements, experimental results can be forced into that analytic form, but the real binding constants are quite different. 

Experimental Results. The DLVO theory, which is based on a continuum description of matter, explains the nature of the forces acting between membrane surfaces that are separated by distances beyond 10 molecular solvent diameters. When the interface distance is below 10 solvent diameters the continuum picture breaks down and the molecular nature of the matter should be taken into account. Indeed the experiment shows that for these distances the forces acting between the molecularly smooth surfaces (e.g., mica) have an oscillatory character (8). The oscillations of the force are correlated to the size of the solvent, and obviously reflect the molecular nature of the solvent. In the case of the rough surfaces, or more specifically biomembrane surfaces, the solvation force displays a mono tonic behavior. It is the nature of this solvation force (if the solvent is water, then the force is called hydration force) that still remains a puzzle. The hydration (solvation) forces have been measured by using the surface force apparatus (9) and by the osmotic stress method (10, II). Forces between phosphatidylcholine (PC) bilayers have been measured using both methods and good agreement was found. 

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