Liquid surface force apparatus

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. 

A novel liquid surface forces apparatus (LSFA) developed at Hull has been used to study oil-water-oil films. An oil drop of radius a few microns is supported in an aqueous phase on the tip of a fine glass micropipette that is held initially below an oil-water interface. The other thick end of the micropipette is connected to a manometer containing the same oil. The radius of the oil drop is controlled by the balance of the Laplace pressure inside the drop and the applied pressure head. When the oil drop is moved up to the oil-water interface, a thin water film is formed between the apex of the drop and the interface the disjoining pressure in the film turns out to be simply one half of the pressure head since there are two... 

Fig. VI-4. Illustration of the surface force apparatus with the crossed-cylinder geometry shown as an inset. The surface separations are determined from the interference fringes from white light travelling vertically through the apparatus. At each separation, the force is determined from the deflection in the force measuring spring. For solution studies, the entire chamber is filled with liquid. (From Ref. 29.)...

Idziak S H J ef a/1994 The x-ray surface forces apparatus structure of a thin smectic liquid crystal film under confinement Science 264 1915-8... 

Another powerful tool for investigating a rheology of liquid films on nano-scale is Surface Force Apparatus (SFA)... 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

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 surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

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. 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

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. 

As we have seen in Section 6.6.1 such confined liquids may behave quite differently from the bulk lubricant. Near the surfaces, the formation of layered structures can lead to an oscillatory density profile (see Fig. 6.12). When these layered structures start to overlap, the confined liquid may undergo a phase transition to a crystalline or glassy state, as observed in surface force apparatus experiments [471,497-500], This is correlated with a strong increase in viscosity. Shearing of such solidified films, may lead to stick-slip motions. When a critical shear strength is exceeded, the film liquefies. The system relaxes by relative movement of the surfaces and the lubricant solidifies again. 

The surface force apparatus (SFA) has been used extensively over the past 30 years to measure the force directly as a function of separation between surfaces in liquids and vapors. If the force-measuring spring is replaced with a mechanically more rigid support, the two opposing surfaces become an ideal model pore for the study of confinement effects on phase behavior [16], A detailed review can be found in reference ]. Briefly, the shift of the melting temperature AT can be related to the size h of the condensate measured with SFA according to... 

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. 

Later, in 1973, Tabor and Israelachvili developed a surface force apparatus, SFA, to measure the interaction force in a vacuum at the 1.5 nm level for the first time. In this equipment, the interaction forces between two crossed cylinders coated with freshly cleaved mica sheets having atomically smooth surfaces were measured. One of the cylinders is mounted on a piezoelectric transducer, and the other cylinder is mounted on a spring of known and adjustable spring constant with a force resolution down to 1CT8N. SFA has been further developed for performing measurements in liquids and vapors. In these developed SFA versions, the separation distance between the cylinders can be measured interfero-... 

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... 

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]. 

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