Lubrication with liquid crystals

J. Cognard. Lubrication with liquid crystals. In G. Biresaw, ed. Tribology and the Liquid-Crystalline State. Washington, DC American Chemical Society, 1990, pp. 1 7. 

The field of composite liquids has not received much attention outside the industries associated with specific liquid products (e.g., the petroleum industiy). In areas such as lubrication, the United States has clear technological leadership. The situation is less clear for liquid crystals and adhesives, where there is greater competition from Europe and Japan. 

In contrast to the case of the 8CB liquid crystal, no contact potential differences between first and second layers were observed with these lubricants. This indicates that there is no special orientation of the dipole active end groups, or perhaps that the end groups form hydrogen-bonded pairs with neighboring molecules so as to give no net dipole moment. 

Electrostatic interactions resulting from the polarity of the carbon-fluorine bond play an important role in the binding of fluorinated biologically active compounds to their effectors [22] (discussed in detail in Sections 4.5 and 4.6) and for the me-sophase behavior of fluorinated liquid crystals [23] (Section 4.4). The consequences of the low polarizability of perfluorinated molecular substructures have been put into commercial use for chlorofluorocarbon (CFG) refrigerants, fire fighting chemicals, lubricants, polymers with anti-stick and low-friction properties, and fluorosur-factants. 

There are some special cases for which it is unnecessary to add a lubricant. Magnetic bearings and bearings that are suspended by a pressurized gas have no metal-to-metal contact so that wear and heat buildup are avoided. Several companies sell oil-free vacuum pumps that eliminate the trouble and expense of changing the oil, disposal costs, and contamination of process chambers, which is especially important in the manufacture of semiconductors and liquid crystal crystal displays.40 Plastic gears of nylon or polytetrafluoroethylene are self-lubricating and need no added lubricant. They reduce the noise associated with ma-... 

Their thermal stability is low and there is little chance that they will find applications in lubrication, except maybe, in biological systems. We think, however, that they are implicated in the orientation of nematic (or cholesteric) liquid crystals by their interactions with the superficial atmospheric layer, which explains the role of the salts of fatty acids as lubricant additives. 

A reason for this different behavior became clear when the film thickness measurement was attempted. The film thickness was less than 1500 A once rotation was started and remained that way as the speed was increased. The liquid crystal must have changed to an isotropic liquid of low viscosity. Unfortunately, the film thickness was too thin to yield adequate infrared emission spectra by the methods we had used successfully before on normal liquid lubricants in EHD contacts However, the absence of polarization was consistent with the pressure of a liquid. 

While the EHD apparatus simulates industrial bearings and provides information on lubricants performance, it has serious drawbacks for the spectroscopic studies of liquid crystal behavior under shear. The temperatures in the conjunction region vary with position and depend on load, speed, viscosity, etc., but cannot be controlled arbitrarily. The gap widths are similarly determined by the same parameters and cannot be maintained arbitrarily. Therefore shear rates (speed/gap width) are not easily controlled. As Fig. 7 shows, film thicknesses and tractions for a typical liquid lubricant vary with spe because more fluid is pumped in the EHD contact at high speeds while higher temperatures are generated, which cause the viscosity to decrease in ordinary fluids. As was shown, liquid crystals can behave differently because of phase changes. 

For background information on infrared spectral changes with changes of phase such as might occur in the lubricant under shear, infrared emission spectra were obtained under static conditions at different temperatures. For this purpose, a small amount of liquid crystal was sandwiched between a KBr window and a stainless steel plate and emission spectra were obtained at different temperatures. The thickness of the layer was not determined but judging from the spectra, must have been about 1.0 pm. Stainless steel had to be used to avoid chemical reaction with acidic liquid crystal. 

Our tribological experiments with the liquid crystals can be summarized as follows. LCl, which is in the nematic phase at the room temperature was subjected to a traction test. The traction of this sample is almost half of a normal liquid lubricant under the same conditions. Note that the traction did not change with die increasing speed after the initial sharp drop. These results are consistent with a quasi-solid lubricant shearing between the bounding surfaces. It should be mentioned here that a surface active material was put on the surface of the steel ball before the LCl was inserted. It was used to prevent the liquid crystal from slipping on the smooth steel surface. In any case, the traction data were unusual and would put liquid crystals of this type into a special class of lubricants. 

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