Immersion force

Flotalion forces disappear forR[Pg.190]

To distinguish between the capillary forces in the case of floating particles and in the case of partially immersed particles on a substrate, the former are called lateral flotation forces and the latter, lateral immersion forces. - " These two kinds of force exhibit similar dependence on the interparticle separation but very different dependencies on the particle radius and the surface tension of the liquid (see References 35 and 212 for comprehensive reviews). The flotation and immersion forces can be both attractive (Figures 5.17a and b) and repulsive (Figures 5.17c and d). This is... 

Hence, the flotation force decreases, while the immersion force increases, when the interfacial tension a increases. Besides, the flotation force decreases much more strongly with the decrease of R than the immersion force. Thus, F(,o tion is negligible for F < 10 pm, whereas can be... 

The immersion force appears not only between particles in wetting films (Figure 4.21b and d), but also in symmetric fluid films (Figure 4.21f). The theory provides the following asymptotic expression for calculating the lateral capillary force between two particles of radii / , and / 2 separated by a center-to-center distance L [37,288-293] ... 

As seen in Figure 4.22, the immersion force can be significant between particles whose radii are larger than few nanometers. It has been found to promote the growth of 2D crystals from colloid particles [294-297], viruses, and globular proteins [298-304]. Such 2D crystals have found various applications in nanolithography [305], microcontact printing [306], as nanostructured materials in photo-electrochemical cells [307], in photocatalytic films [308], photo- and electroluminescent semiconductor materials [309], as samples for electron microscopy of proteins and viruses [310], as immunosensors [311], etc. (for reviews see Refs. [37,312]). 

C.D. Dushkin, P.A. Kralchevsky, V.N. Paunov, H. Yoshimura, and K. Nagayama, Torsion balance for measurement of capillary immersion forces, Langmuir, 12, 641-651 (1996). 

Figure 5.18 Capillary immersion forces between particles in a liquid film on a solid support can be attractive (left) or repulsive (center) depending on fti and ft2- In freestanding films (right), immersion forces are always attractive.

Lateral capillary forces also occur when the particles are partially immersed in a thin liquid film on a solid support (Figure 5.18) [590,595]. In this case, they are called immersion forces. Immersion forces on solid surfaces occur always when suspensions of solid particles are dried. In the last state of evaporation, the particles are only partially immersed in the thinning liquid film and attractive immersion forces lead to an aggregation of particles (Figure 5.19). Immersion forces can be used to self-assemble particles in two-dimensional arrays [595, 596]. The deformation of the liquid surface is related to the wetting properties of the particles, that is, to the position of the contact line and the contact angle rather than the weight. For this reason, also very small particles such as proteins are affected. 

Immersion forces can also be calculated with Eq. (5.43). They can be attractive or repulsive, depending on the meniscus angles and 112 The same rule applies as for flotation forces attraction for sin sin 2 > 0 and repulsion for sin sin 2 < 0. Usually, the particles attract each other since the contact angles are low if the particles were not wetted by the liquid it would be difficult to keep them dispersed in solution. One example of a force versus distance measurement for two immersed spheres is shown in Figure 5.20. 

Figure 5.19 Polystyrene (PS) particles of 0.64 im radius dried from aqueous suspension on a solid surface and imaged by scanning electron microscopy. Due to immersion forces the particles tend to aggregate ratherthan being isolated as in solution. Bare PS particles are...

Figure 5.20 Lateral immersion force versus center-to-center distance for two glass spheres of Ri = 0.6 mm radius measured with a torsion balance [601]. The two spheres are partially immersed in aqueous surfactant solution with Yl = 0.0368 Nm and Xc = 1.94 mm. Each particle is kept at the liquid surface by a support...

Fig. V-5. The repulsive force between crossed cylinders of radius R (1 cm) covered with mica and immersed in propylene carbonate solutions of tetraethylammonium bromide at the indicated concentrations. The dotted lines are from double-layer theory (From Ref. 51).

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

The composite conductor is typically wound in the form of a cable, which can be cooled either internally by a forced belium flow or externally by immersion in a pool of belium. Large electromagnetic body forces, up to 500 t/m, are experienced by the conductor during operation. These are contained by a massive external stmcture, although designs have been proposed in which the conductor itself serves as its own force containment stmcture (126). 

Tank Cells. A direct extension of laboratory beaker cells is represented in the use of plate electrodes immersed into a lined, rectangular tank, which may be fitted with a cover for gas collection or vapor control. The tank cell, which is usually undivided, is used in batch or semibatch operations. The tank cell has the attraction of being both simple to design and usually inexpensive. However, it is not the most suitable for large-scale operation or where forced convection is needed. Rotating cylinders or rotating disks have been used to overcome mass-transfer problems in tank cells. An example for electroorganic synthesis is available (46). 

The motion oF Fine particles immersed in a moving tliiid is more greatly aFFected by tliiid drag Forces than that For similar large particles, For vei v small particles in a tliiid, particle motion approximates... 

SFA has been traditionally used to measure the forces between modified mica surfaces. Before the JKR theory was developed, Israelachvili and Tabor [57] measured the force versus distance (F vs. d) profile and pull-off force (Pf) between steric acid monolayers assembled on mica surfaces. The authors calculated the surface energy of these monolayers from the Hamaker constant determined from the F versus d data. In a later paper on the measurement of forces between surfaces immersed in a variety of electrolytic solutions, Israelachvili [93] reported that the interfacial energies in aqueous electrolytes varies over a wide range (0.01-10 mJ/m-). In this work Israelachvili found that the adhesion energies depended on pH, type of cation, and the crystallographic orientation of mica. 

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