Experimental results disjoining pressure

In this Chapter we introduce some of the theoretical approaches for studying thin nematic liqnid crystalline systems, both on the microscopic and macroscopic level. In the former, one models the microscopic interactions between the constituing molecules, leaves the system to evolve, and then determines its macroscopic properties. If the obtained macroscopic behaviour is in agreement with the experimental evidence the modeled interaction is considered appropriate. On the other hand, the macroscopic description takes into account the universal properties of systems in the vicinity of phase and structural transitions. This means that they are based on the fact that in the vicinity of phase changes the macroscopic properties of the system do not depend on the details of the microscopic interactions but on the symmetry properties and dimensionality of the system in question. Most of our attention is focused on the effects of confinement on to liquid crystalline order. Finally, we will be interested in the resulting disjoining pressure. The evidences in experiments will be briefly mentioned. 

One more experimental result, which is important for PT is as follows. Only polar liquids fill conical capillaries from both sides. We used various penetrants to fill conical defects Pion , LZh-6A , LZhT , LUM-9 etc. It was established that only the penetrants containing polar liquid as the basic liquid component (various alcohols, water and others) manifest two-side filling phenomenon. This result gives one more confirmation of the physical mechanism of the phenomenon, based on liquid film flow, because the disjoining pressure strongly depends just on the polarity of a liquid. 

To illustrate the above approach, a set of values for Ym and Xd derived from the experimental results of a direct measurement of film disjoining pressure on the film thickness dependence was used (see Fig. 3.44,a). These values are relevant to foam films from aqueous solutions of a non-ionic surfactant and an electrolyte (KC1) of concentration 51 O 3 and 31 O 3 mol dm 3, respectively. 

Fig. 12.5. The disjoining pressure as a function of the interparticle distance of separation for spheres sterically stabilized by poly(oxyethylene) curve 1, the experimental results of Homola and Robertson (1976) curve 2, constant segment density model. The crosses (x) show the theoretical results for a softened elastic potential (after Evans and Napper, 1977).

Experiments have clearly shown the formation of a dry spot at the central area of the layer (Gaver and Grotberg 1990 Ahmad and Hansen 1972 Fraaije and Cazabat 1989). This is also one of our findings and it is a part of the results reported here. An attempt to explain the dry spot formation due to the action of surface forces appeared in ref. (Jensen and Grotberg 1992), where the disjoining pressure is taken a function of the film thickness as for non-wettable surfaces. However this is not our case for we have complete wetting. We provide the appropriate explanation also accounting for the expected limited precision of experimental measurements, and for evaporation. 

In a liquid medium, for which there is a liquid interlayer between the contiguous bodies, and the effects of capillary, electric, and Coulomb forces are excluded (see 11-13), adhesion is due solely to the molecular forces fthe disjoining pressure opposes adhesion). The value of the molecular forces is directly proportional to the dimensions of the particles [see Eqs. (1 47) and (1.49)]. For an aqueous medium the experimental results relating to the dependence of the adhesive forces on particle dimensions coincide with the theoretical data, and Deryagin s thermodynamic theory of adhesion is practically confirmed (see 5). 

At Cei>Cei cr the double layer repulsion is practically suppressed and this is manifested by the fact that the film thickness no longer changes with increasing Cel above Cei.cr (Figure 6.7). Under these conditions, flei = 0, and the experimental results cannot be explained in the framework of DLVO theory only. If one assumes that the only contribution to n is there are very large deviations from the experimental data. This directly implies that there is an additional repulsive contribution to the disjoining pressure, namely flsf... 

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