Surface force apparatus interfaces

Luckham P F and Manimaaran S 1997 Investigating adsorbed polymer layer behaviour using dynamic surface forces apparatuses—a review Adv. Coiioid interface Sc/. 73 1 -46... 

Figure C2.9.3 Schematic diagrams of the interfaces reaiized by (a) tire atomic force microscope, (b) tire surface forces apparatus and (c) tire quartz crystai microbaiance for achieving fundamentai measurements of friction in weii defined systems.

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. 

P. M. Claesson, Surface Forces Apparatus Studies of Polymers, Polyelectrolytes and Polyelectrolyte-Surfactant Mixtures at Interfaces (P. Dubin and R. Farinato, eds.), Colloid-Polymer Interactions From Fundamentals to Practice, Wiley, New York, 1999, p. 287. 

A more recently developed force measurement technique, coined the liquid surface force apparatus (LSFA), brings a drop made from a micropipette close to a flat liquid/liquid interface [29-32]. A piezo electric drive is used to change the position of the micropipette while the deflection of the pipette and the radius of the drop are recorded with piezo motion. The drop radius and thus the film thickness between the two liquid/liquid interfaces are recorded using interferometry. The method requires a calibration of the interferometer, where the drop must come into contact with the other liquid interface. The distance resolution of the film is about 1 nm at a 50-nm separation and 5 nm at a separation of 10 nm. This is a very robust technique where the authors have proposed attaching a particle to the end of the pipette instead of a drop [29]. In comparing this method to AFM, the only drawback of the LSFA is the weaker distance resolution. It is important point out that both methods required a contact point for distance calibration. 

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). 

The mutual attraction through the slit gap affects liquid film stability, and at a certain critical vapor pressure (or film thickness) the two films form a liquid bridge (Fig. 1-1 c) followed by a spontaneous filling up of the slit (assuming the film is in contact with the bulk liquid phase). The liquid-vapor interface moves to the plate boundaries. This phase transition from dilute vapor to a dense liquid is known as capillary condensation and was observed experimentally with the surface force apparatus by Christenson (1994) and Curry and Christenson (1996). Extensive theories for this phenomenon and its critical points are provided by Derjaguin and Chu-raev (1976), Evans et al. (1986), Forcada (1993), and Iwamatsu and Horii (1996). In general, slit-shaped pores fill up at a film thickness of about HI3, or when <) l(H,h)/dh = 0, such that... 

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. 

The surface force apparatus (SFA) operates in a manner similar to that of the atomic force microscope (AFM). However, it provides an atomically smooth interface over a large ( 100 m ) area that permits the spectroscopic scrutiny of molecules adsorbed at the buried interface. Discuss one study that was recently reported in the literature [56]. 

It is important to note that the lamellar phase is thus stabilized by the balance of a negative interfacial tension (of the free oil/water interface covered by an amphiphilic monolayer), which tends to increase the internal area, and a repulsive interaction between interfaces. The result, Eq. (48), indicates that the scattering intensity in a lamellar phase, with wave vector q parallel to the membranes, should have a peak at nonzero q for d > d due to the negative coefficient of the q term in the spectrum of Eq. (40). just as in the microemulsion phase. This effect should be very small for strongly swollen lamellar phases (in coexistence with excess oil and excess water), as both very small [96]. Very similar behavior has been observed in smectic liquid crystals (Helfrich-Hurault effect) [122]. Experimentally, the lamellar phase under an external tension can be studied with the surface-force apparatus [123,124] simultaneous scattering experiments have to be performed to detect the undulation modes. 

The results obtained by the EQCM contain information relevant to the understanding of phenomena in the area of nano-tribology, where techniques such as surface force apparatus and atomic force microscopy are used. In both cases the results carry information regarding the properties of a nano-scale layer of liquid at the interface. 

Luckham, P. F. and Manimaaran, S., Investigating adsorbed polymer layer behaviour using dynamic surface force apparatuses - a review, Adv. Colloid Interface ScL, 73, 1 46 (1997). 

Historically the measurement of forces between solid and fluid interfaces began at approximately the same time. However, with the advent of the Surface Force Apparatus (SFA) and Atomic Force Microscope (AFM) over the last few decades, studies concerning the direct measurement of interactions between solid interfaces have received more attention than those concentrated on fluid interfaces (i.e. thin-liquid films - films which have at least one fluid-fluid interface such as foam, emulsion... 

Some adhesion measuring equipment, notably the Surface Force Apparatus, comprises thin films of polymer mounted on an elastic substrate. The equivalent interface fracture problem to that considered above (Figure 6) is shown in Figure 10, where each elastic solid is covered by a viscoelastic layer of thickness h. The case of the opening crack has been examined by Huntley (75). He shows that the effective work of adhesion, given for viscoelastic solids by equation (18), is modified by the thickness of the layer approximately to ... 

Not until the development of the surface forces apparatus (SFA) in the late 1960s could experiments be undertaken to study the interaction forces resulting form the liquid structure at these interfaces. The development of the scanning probe microscope in the 1980s finally allowed the study of the liquid structure at solid-liquid interfaces with nanometer-scale lateral resolution. 

In order to facilitate the calculation of capillary forces, several approximations on the meniscus shape have been proposed. They are mainly applied for experimental conditions where the radius of curvature of the meniscus interface is much smaller than the radius of curvature of the solid surfaces. This is relevant for the surface force apparatus where the surface has centimetric radius, while the meniscus is typically tens of hundreds of nanometers. The most used approximation is the toroidal approximation assuming the liquid interface has a circular profile. Obviously, such a meniscus does not exhibit a constant curvature. Nevertheless, this approximation gave good results, in particular for small contact angles, and is therefore widespread (see Ref. 15 for its application in various geometries and section 9.3.1.1 for an example of its application in atomic force microscopy [AFM]). In the case of capillary condensation between a plane and a sphere with a large radius of curvature R, in contact, the tension term of the capillary force is negligible and the Laplace term leads to the simple formula F = AnRy cos 9 A parabolic profile is also sometimes used to eliminate some numerical difficulties inherent in circle approximation. 

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