Surface force apparatus solids

Chapter 1 is a view of the potential of surface forces apparatus (SFA) measurements of two-dimensional organized ensembles at solid-liquid interfaces. At this level, information is acquired that is not available at the scale of single molecules. Chapter 2 describes the measurement of surface interactions that occur between and within nanosized surface structures—interfacial forces responsible for adhesion, friction, and recognition. 

When two such surfaces approach each other, layer after layer is squeezed out of the closing gap (Fig. 6.12). Density fluctuations and the specific interactions then cause an exponentially decaying periodic force the periodic length corresponds to the thickness of each layer. Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [168], Periodic solvation forces across confined liquids were first predicted by computer simulations and theory [168-171], In this case, however, the experimental proof came only few years afterwards using the surface forces apparatus [172,173]. Solvation forces are not only an important factor in the stability of dispersions. They are also important for analyzing the structure of confined liquids. 

Between hydrophobic surfaces a completely different interaction is observed. Hydrophobic surfaces attract each other [184], This attraction is called hydrophobic interaction. The first direct evidence that the interaction between solid hydrophobic surfaces is stronger than the van der Waals attraction was provided by Pashley and Israelachvili [185,186], With the surface force apparatus they observed an exponentially decaying attractive force between two mica surfaces with an adsorbed monolayer of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Since then the hydrophobic force has been investigated by different groups and its existence is now generally accepted [189]. The origin of the hydrophobic force is, however, still under debate. 

Experimentally, surface forces between solids surfaces are commonly determined using the atomic force microscope or the surface force apparatus. 

As shown in Equation 10.4, the depression of the melting point of a given confined solvent is related to the geometry of the pores of the confining material. In principle, measurement of AT can give access to the pore size. Three main techniques have been developed to measure porosity in solids via the use of the Gibbs-Thomson equation thermoporosimetry, NMR cryporometry and surface force apparatus. These techniques are secondary methods since they require pre-... 

The surface force apparatus is now being used routinely to study the equation of state of solutions confined between opposed, molecularly thin solid films. The apparatus is also used in one laboratory to study electrochemistry of thin films at electrodes a few nanometers thick and in a few other laboratories to study the behavior of molecularly thin films subjected to shear and flow [7]. 

Scheludko et al. [13,15,73,89,229,230] derived the Reynolds relation in a slightly generalised form and tested it experimentally. The agreement between experiment and theory was very reasonable. More recently, Chan and Horn [231] have used the surface force apparatus (SFA) and found that the Reynolds approach to hydrodynamic lubrication is very successful in describing the drainage of liquid films between smooth solid surfaces. 

In the last 40 years, techniques to directly measure surface forces and force laws (force vs. separation distance between surfaces) have been developed such as the surface forces apparatus (SFA) [6] and AFM. Surface forces are responsible for the work required when two contacting bodies (such as an AFM tip in contact with a solid surface) are separated from contact to infinite distance. Although the physical origin of all relevant surface forces can be derived from fundamental electromagnetic interactions, it is customary to group these in categories based on characteristic features that dominate the relevant physical behavior. Thus, one speaks of ionic (monopole), dipole—dipole, ion—dipole interactions, electrostatic multipole forces (e.g., quadrupole), induced dipolar forces, van der Waals (London dispersive) interactions, hydrophobic and hydrophilic solvation, structural and hydration forces,... 

The effect of polymer brushes on the reduction of sliding friction was also observed for solid friction, using surface force apparatus (SEA) measurements [83-86]. For example, Klein et al. reported a massive lubrication between mica surfaces modified by repulsive polyelectrolyte brushes in water [83]. These results show that polymer dangling chains on solid or gel surfaces can dramatically reduce the surface friction if the polymer brush has a repulsive interaction with the sliding substrate. 

For the sake of concreteness of the following developments, we consider a fluid confined to a slit-pore such that the solid surfaces representing the pore walls are planar, parallel to one another, and perpendicular to the 2-axis of a Cartesian coordinate system. The separation between the pore walls will be denoted s. In addition, the two solid surfaces can be manipulated by external agents normal to the fluid-solid interface sucli that s, may be altered. EventuaJl), these planar surfaces will come to rest at some equilibrium separation Sj,. As we shall see later in Section 5.3.1, the situation just described is akin to laboratory experiments in which the rheology of confined fluids is investigated by means of the so-called surface forces apparatus (SFA). 

There is a whole body of experiments on polymer mediated interactions between solid surfaces performed using the surface forces apparatus (mostly by the group of J. Klein [38], but also by other techniques. The theoretical pictrue presented here is in good agreement with most of the results. 

Figure 6.5. A force-distance profile obtained using the surface forces apparatus for the force between two mica surfaces each bearing a physically adsorbed polystyrene brush, of relative molecular mass 140000, in toluene. The solid line is a fit to equation (6.1.14). After Taunton et al. (1990).

Chen et al. [301] used a surface forces apparatus (SFA) to measure the force between two hard, solid-like polybutadiene (PB = 10 Da) layers, about 110 nm thick, immersed in liquid PDMS. The measured force was found to be independent of the approach velocity in the range V = 0.01-50 nm s . Four stages were identified (i) an initial smooth approach, (ii) a jump when the two layers were about 250 nm apart, (iii) an approximately 4-nm thick layer of PDMS was trapped between the two PBD surfaces (1 or 2 molecular layers), and (iv) coalescence. Large normal and lateral deformations were consistent with the calculated van der Waals forces. The results imply that liquid droplets (or biological cells) can sense each other at relatively large distances. 

The effect of solid surface modification with polymer brushes on sliding friction has been investigated using surface force apparatus (SFA) measurements [31-43]. 

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