Surface force apparatus tension

Israelachvili, J. N., Intermodular and Surface Forces, 2d ed., Academic Press, New York, 1991. (Graduate and undergraduate levels. An excellent source for the relation between molecular-level van der Waals interactions and macroscopic properties and phenomena such as surface tension, cohesive energies of materials, adhesion, and wetting. Also discusses direct measurement of van der Waals forces using the surface force apparatus.)... 

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. 

Several properties of surfaces have origins that are very sensitive to the surface, but are essentially macroscopic in character. Examples include adhesion, surface tension and the contact angle. Techniques to measure these properties are not discussed in this chapter, but rather are mentioned in connection with discussions of the property involved elsewhere in this book. However, surface properties can also be probed at a microscopic level for example the forces between surfaces at microscopic separations may be probed using the surface forces apparatus (SFA). The scanning force microscope... 

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. 

The apparatus used in the present study is based on the axi-symmetric bubble shape analysis, i.e., a firmly established technique for the measurement of static and dynamic surface tension as well as of the geometrical properties of the bubble (Loglio et al. 1996, 2001, Kovalchuk et al. 2001, Miller et al. 2000, Rusanov and Prokhorov 1996, Neumann and Spelt 1996, Cheu et al. 1998). In essence, the shape of a bubble (or of a drop) is determined by a combination of surface tension and gravity effects. Surface forces tend to make drops and bubbles spherical whereas gravity tends to elongate them. 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

TENSTOMETER. An apparatus for measuring the surface tension of a liquid by registering the force necessary to detach a metal ring from the surface. 

There are two established methods for measuring the distribution of pore volumes. The mercury-penetration method depends on the fact that mercury has a significant surface tension and does not wet most catalytic surfaces. This means that the pressure required to force mercury into the pores depends on the pore radius. The pressure varies inversely with a 100 psi (approximately) is required to fill pores for which a = 10,000 A, and 10,000 psi is needed for a — 100 A. Simple techniques and equipment are satisfactory for evaluating the porervolume distribution down to 100 to 200 A, but special high-pressure apparatus is necessary to go below a = 100 A, where much of the surface resides. In the second method, the nitrogen-adsorption experiment (described in Sec. 8-5 for surface area measurement) is continued until the nitrogen pressure approaches the... 

While determining the liquid height, it is better to measure with a falling (or receding) meniscus, so that the liquid level is initially raised above its equilibrium value by a slight suction above the capillary tube, and then left to equilibrate. On the other hand, two-armed capillary tubes, connected with a cross tube above the liquid level, are also used to ensure that the pressure in both arms of the glass apparatus is the same. An interesting modification of the capillary rise method is to measure the pressure, AP, that is required to force the meniscus down until it is on the same level as the plane surface of liquid outside the capillary tube. This method is useful to compare the surface tension of water and its dilute solutions. 

As the neighboring cells of smooth muscle tissues are mechanically coupled, the contractile apparatus of each individual cell exerts tension on its neighbors. Force transmission across the sarcolemma of smooth muscle cells occurs at membrane-associated dense plaques found over the entire cell surface. When isolated smooth muscle cells contract, the points of attachment of contractile filaments are drawn into the cell, resulting in out-pouching of the membrane areas between the plaques (Harris and Warshaw 1991, Draeger et al 1990). 

Water possesses the largest cohesive forces and consequently, the largest surface tension of the commonly encountered liquids. The molecules which form biological membranes have unique properties which allow them to reduce the surface tension at a cell s aqueous interface, and thereby increase the surface area of the cell and its internal organs, e.g., the Golgi apparatus. 

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