Surface force apparatus experimental studies

Experimental techniques based on the application of mechanical forces to single molecules in small assemblies have been applied to study the binding properties of biomolecules and their response to external mechanical manipulations. Among such techniques are atomic force microscopy (AFM), optical tweezers, biomembrane force probe, and surface force apparatus experiments (Binning et al., 1986 Block and Svoboda, 1994 Evans et ah, 1995 Israelachvili, 1992). These techniques have inspired us and others (see also the chapters by Eichinger et al. and by Hermans et al. in this volume) to adopt a similar approach for the study of biomolecules by means of computer simulations. 

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. 

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. 

In spite of the strong economic importance of friction and wear and the resulting scientific effort, our understanding of the fundamental processes is still rudimentary. This results from the complexity of these topics. In addition, this complexity demands a multi-disciplinary approach to tribology. In recent years the development of new experimental methods such as the surface forces apparatus, the atomic force microscope, and the quartz microbalance made it possible to study friction and lubrication at the molecular scale. However, this new wealth of information does not alter the fact, that there are no fundamental equations to describe wear or calculate friction coefficients. Engineers still have to rely largely on their empirical knowledge and their extensive experience. 

The interferometric surface forces apparatus (SFA) has played a major role in the discovery and experimental investigation of a variety of surface forces since the early seventies. It has now served its scientific purpose in surface chemistry for more than 30 years. The SFA was envisaged by Tabor and Winterton [1] and later developed by Tabor and Israelachvili [2] and Israelachvili and Adams, and Klein [3-5]. Considerable development of the basic technique has occurred since then, and this has opened new perspectives in studying matter in confinement. 

The above studies have benefited from a fertile exchange with experimental groups using the Surface Force Apparatus (SFA). The SFA allows the mechanical properties of fluid Aims to be studied as a function of thickness over a range from hundreds of nanometers down to contact. The fluid is confined between two atomically flat surfaces. The most commonly used surfaces are mica, but silica, polymers, and mica coated with amorphous carbon, sapphire, or aluminum oxide have also been used. The surfaces are pressed together with a constant normal load, and the separation between them is measured using optical interferometry. The fluid can then be sheared by translating one surface... 

As two bodies approach each other, at very close distances (nanometer range), there exists attractive (van der Waals) and repnlsive (Conlombic) forces, van der Waals forces acting between two different phases have attracted the attention of various investigators however, experimental measurement of these forces at very small distances (mn) has not been easy. These are measured by the surface-force apparatus (SFA) by using direct force. In the literature are studies using two cnrved mica surfaces. 

Basically two different types of experimental approaches have been used to study the boundary shp local (direct) [45,60] and effective (indirect) methods [49-52,61]. The first group of methods is based on apphcation of optical techniques using tracer particles or molecules to determine the flow field. These techniques have a resolution of less than lOOnm, so they cannot distinguish small differences in slip lengths. The effective methods assume the boundary conditions (Eq. 18) or similar ones to hold at the substrate surface and infer the slip length by measuring macroscopic quantities. These methods have been the most popular so far and they include atomic force microscopy (AFM), surface force apparatus (SEA), capillary techniques, and QCM. 

The surface forces apparatus (SFA) measures forces between atomically flat surfaces of mica. Mica is the only material that can be prepared with surfaces that are atomically flat across square-millimetre areas. The SFA confines liquid films of a few molecular layers thickness between two mica surfaces and then measures shear and normal forces between them (figure C2.9.3)b)). In essence, it measures the rheological properties of confined, ultra-thin fluid films. The SFA is limited to the use of mica or modified mica surfaces but can be used to study the properties of a wide range of fluids. It has provided experimental evidence for the formation oflayered structures in fluids confined between surfaces and evidence for shear-induced freezing of confined liquids at temperatures far higher than their bulk freezing temperatures [14. 15]. 

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. 

First observations of forces in liquid crystal were performed in the 80 s by Horn et al. [59]. They studied the force between two mica plates separated by a liquid crystal 5CB using the surface force apparatus. Later on, the forces due to the structure were briefly discussed by Poniewierski and Sluckin [57] and a detailed theoretical study of forces in paranematic systems was performed by Borstnik and Zumer [43]. In the meantime, much attention was paid to the fluctuation-induced forces [11,12,60,61]. Theoretical studies were followed by renewed experimental interest, studies being performed with surface forces apparatus [46] and atomic force microscopes [62]. 

PNIPAM brushes are considered promising surface modifiers to accurately tune cell-substrate interactions. The structure of such brushes in water has been the subject of several experimental studies. On the one hand, force measuring techniques (atomic force microscopy and surface forces apparatus) have been used to probe the repulsive forces resulting from brush compression (Ishida and Biggs 2007, Kidoaki et al. 2001, Kaholek et al. 2004, Plunkett... 

Atomic-level lubrication has been investigated both experimentally and theoretically. Experimental techniques, such as the surface force apparatus (SFA), have been used to study thin molecular films. It has been shown that behaviours of thin films, of a few molecular diameters thick, are generally quite different fi-om their... 

A significant breakthrough in the experimental study of surface forces was the introduction of the surface forces apparatus, which will be described in the next section. 

Experimentally, electrostatic double-layer forces versus distance were first quantitatively measured in foam films [444—446]. Aqueous foam films with adsorbed charged surfactant at air-liquid interfaces are stabilized by double-layer forces, at least for some time. Voropaeva ef al. measured the height of the repulsive barrier between two platinum wires at different applied potentials and in different electrolyte solutions [447]. U sui et al. [448] observed that the coalescence of two mercury drops in aqueous electrolyte depends on the applied potential and the salt concentration. Accurate measurements between solid-liquid interfaces were first carried out between rubber and glass with a special setup [449]. In the late 1970s, DLVO force could be studied systematically with the surface forces apparatus [424,450,451]. With the introduction of the atomic force microscope, DLVO forces between dissimilar surfaces could be measured [198, 199, 452, 453]. 

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