612
W. D. KUMLER
FIGURES IN THIN LAYERS OF GREASE AND VISCOUS LIQUIDS
W. D. KUMLER College of Pharmacy, University of California, San Francisco, California Received September $0, 1080
Many chemists have observed that the grease on a vacuum desiccator usually becomes opaque when air enters the evacuated vessel. Some time ago, when some high-vacuum stopcock grease was used on a desiccator, it was observed that, instead of the usual smudge, large figures of great beauty (figure 1) were formed in the grease when the pressure wag released. Since we found nothing in the literature on the subject, an investigation of the nature of these figures wag undertaken. The grease was made according to Birch's method (2), by taking the residue that was left after distilling yellow petrolatum at a pressure of 10-3 to 10-4 mm. of mercury and at a bath temperature of 300" to 320°C. This petrolatum residue is not only a good high-vacuum stopcock grease but also an excellent material for use on a vacuum desiccator. Figure 1 is a photograph taken by reflected light of the figures formed in this grease on a Pyrex desiccator. The largest figures were about 1 cm. long. The kind of glass from which the desiccator is made does not affect the figures, fQr similar figures were formed on Pyrex and on soft-glws desiccators. The kind of grease, however, does affect them. Six greases ordinarily used in greasing an automobile were obtained from a service station and the characteristic figures were obtained with the three most viscous greases. The other three gave only a translucent smudge. The figures disappear when the desiccator is reevacuated and are distorted or destroyed when the top is slid on the base. Turning the desiccator top when the two surfaces are in contact ejther before or after the vessel is evacuated does not change the direction or shape of the figures that are subsequently formed when air enters the desiccator. The figures have the same general configuration when observed from either the top or the bottom. When air is allowed to enter the desiccator slowly, small spots forming in the grease are first observed. The spots then seem to push out branches which grow into the completed figures. The spots form a t some distance from the edges of the ground surfaces. Only rarely does a branch from one figure extend into another figure. The fact that the figures did not extend to the edges of the ground surface suggested that the vacuum as such had no effect on the figures but that the resulting pressure was the important factor. If that was the case, then the figures should be formed by the mechanical application
FIGURES IN THIN LAYERS OF GREASE
613
and release of pressure. To test this the grease was placed between two glass plates about 1 in. square and the plates were squeezed together in a vise made for the purpose. Upon releasing the pressure the characteristic figures were obtained. The figures were likewise obtained when two pieces of celluloid were treated in the same way. The necessary conditions for the formation of the figures appear to be two reasonably smooth surfaces, a substance of the proper viscosity and stickiness, and the application and subsequent release of pressure. Although the general appearance of the figures is similar to that of some crystals, the evidence indicates that they are definitely not crystalline but, on the contrary, that the figures are holes in the grease. First, the photographs (figures 1 to 8) show that the edges of the figures are curved and not straight, as would be expected for crystals. Second, the figures are invariably dark under crossed Xcols (figures 3, 6, 7 ) , even when the grease itself is anisotropic. Third, it was found possible to separate the plates without disturbing the general pattern of the figures that previously had been formed thereon. An examination under the microscope revealed that the figures were places where the grease did not make a continuous medium from one plate to the other. By rubbing a pointed instrument on the glass where the figures appeared it was found that a thin film of grease was still on the glass in these areas, so that apparently grease pulls away from grease and not grease from glass during the formation of the figures. There is a possibility that the figures may result from trapped air, which upon application of pressure decreases in volume or dissolves in the grease and then reappears upon release of the pressure to form the figures. To investigate this possibility an apparatus was constructed from a filter flask by which the operations of putting the greased plates together, applying pressure, and then releasing it could all be carried out in a vacuum. Several experiments were carried out a t a pressure of 0.01 to 0.02 mm. of mercury and in each case the characteristic figures were obtained; this rules out the possibility that trapped air is responsible for their formation. With the petrolatum residue grease the field was almost entirely dark when viewed under crossed Nicols. both without and with pressure being applied to the plates'. However, if pressure is applied and then released 1 The vise used consisted of two blocks of wood held together a t the four corners with bolts and nuts. There was a $-in. hole through the center of both blocks. With this arrangement pressure could be applied to the plates and the whole arrangement could be placed on the stage of a polarizing microscope for observation either while the pressure was applied or later when i t was released. Lead washers were used between the glass and the wood. The glass plates have t o be thick, otherwise they will bend and not produce the desired pressure within the $-in. hole where the figures are to be observed. Glass in. thick was satisfactory.
+
61.1
W ' . D. KlTMLER
su tliat figure8 are iomicd, theii the figures arc dark arid ttie gir?a~crelatively bright, (figure 3). This we irit,erprct as indicating that under thew cimim.itanct~san orimtation of tlrc moict&% mciirs, whioh gives au anisotropic rhrai%cr to tlic grraw in mms. 'l'lio individual mi)/cCuIes
616
W. D. KUMLER
It was discovered that, by repeated application and release of pressure on the plates, the grease could be squeezed out thin enough so that interference colors would form in the figures themselves. This phenomenon not only substantiates the previous conclusion that the figures are holes but it also confirms the order of magnitude of the above determination of the thickness of the film. To give interference figures the distance between surfaces must be ofothe order of magnitude of a wave length of visible light (4000 tooSOOO A.); this compares favorably with the thickness of 2500 to 6500 A. previously determined by an entirely different method. It is apparent that the plates must spring apart when the pressure is released, because the substance makes a continuous medium (a disc of a certain diameter, both faces of which are in contact with the glass plates) between the plates while the pressure is applied and in only a small part of this area does the substance touch both plates when the pressure is released and the figures are formed. Hence the plates must be farther apart after figure formation. It does not seem to make any difference how long the pressure is applied before it is released, for the figures formed on a desiccator that had been evacuated for 3 weeks. Since the materials thus far studied were all petrolatum greases it seemed desirable to try other substances that differed chemically from these greases. Materials that gave figures include ammonium oleate, oleic acid, n-butyl stearate, lanolin, benzoinated lard, triethanolamine, non-crystalline glucose, glycerol, polymerized glycerol, concentrated sulfuric acid, polymerized butene, and three lubricants made from the following materials : glycerol-water-sodium alginate, castor oil-carnauba wax, and aluminum stearate-vaseline. Thus the figures are formed in a wide variety of chemical substances from the very polar water-soluble substances such m glycerol, triethanolamine, and concentrated sulfuric acid to the non-polar hydrocarbons such as the petrolatum residue. There appears to be no correlation between ability to form the figures and ability to form hydrogen bonds. Some of the compounds that give the figures, such as ammonium oleate, form liquid crystals. The conditions favorable to the formation of liquid crystals have been defined by Bernal and Crowfoot (1) as follows: First, the molecule must be anisometric and rod- or lath-shaped; second, it must contain not more than one highly polar group and may contain additional mildly polar groups. Ammonium oleate shows birefringence, which is a property common to all liquid crystals and is the property most commonly used in identifying the state. The polymerized butene possibly has rod-shaped molecules, but it lacks the polar group. This substance showed no evidence of birefringence, the field being entirely dark with crossed Nicols. Polymerized glycerol likewise gave no evidence
FIGURES IN THIN LAYERS OF GREASE
617
of birefringence. In this case, although the possibility of rod-shaped molecules and dipoles is present, the even distribution of polar groups throughout the molecule would prevent any regular arrangement in one or two coordinate directions,-a condition which is characteristic of liquid crystals. Some of the other materials that show birefringence, such as lanolin, benzoinated lard, castor oil-carnauba wax, and aluminum stearatevaseline, do so not because they are in the liquid crystalline state, but because soft anisotropic solid crystals are present in these substances. Compounds such as glycerol, sulfuric acid, and triethanolamine, which do not have the molecular characteristics favorable to liquid crystal formation and give no evidence of such formation, give the figures. There seems to be no parallel between compounds that form the figures and compounds that form liquid crystals. The viscosity and a less well-defined property, the “stickiness” of the substance, are the only properties thus far observed to be connected with the formation of the figures. The substances that give the most elaborate figures are viscous and sticky, such as the petrolatum residue (figures 1, 2, 3) and the polymerized butene (figure 4). The effect of viscosity can be noticed by examining the figures given by materials made up of molecules of the same type but of different size. For instance, a free-flowing petroleum oil (Triton S.A.E. 70) gives figures (figure 8) without any apparent symmetry which lose their original shape a few minutes after they are formed. The petrolatum residue gives figures of a high degree of symmetry that retain their original shape for several months. A hard grease made mostly of paraffin gives figures that lack symmetry and have the appearance of cracks. Their shape is very much the same as that of the figures in figure 7 , which is a picture of a hard aluminum stearatevaseline grease. Thus three substances similar in regard to the type of molecules but varying in the size of the molecules and in viscosity give figures that are different. The effect of viscosity was investigated also by observing the type of figures that were formed when the vise was immersed in carbon dioxide snow. With some substances, such as the petrolatum residue, the shape of the figures was changed very markedly a t the low temperature; with others, such as glycerol, the effect was less noticeable. Figures 2 to 8 are photomicrographs of 12X. They were all taken of the various substances when placed on glass plates which were squeezed together in the special vise. Figures 2 and 3 are of the same figures, the only difference being that in figure 2 the Nicols were parallel, while in figure 3 they were crossed. Not enough substances have been investigated to show what value the figures may have as a means of analysis. However, the possibility that they may have some use in this connection is evident. The cause of the formation of the figures is a question of much interest.
618
W. D. KUMLER
The fact that the petrolatum residue gives a bright field under crossed Nicols after the figures are formed (figure 3), although the field is dark before they are formed, indicates that some kind of orientation takes place during their formation. Ammonium oleate shows the same effect, although in this case the material is visible under crossed Nicols before figure formation but there is a marked increase in brightness after figure formation, indicating additional orientation. These are the only two cases in which this effect was noticed. With the other substances the brightness of the field under crossed Nicols did not change after figure formation, so that any orientation occurring in these cases was not sufficient to give rise to birefringence. The greases and liquids act as if they were in an unstable state when squeezed out into a thin film. When the pressure is released they tend to assume a more stable condition, which involves bunching up with the resulting formation of the figures. This might be thought of as a kind of surface tension effect, such as gives rise to the formation of drops. Another view is that the molecules are squeezed out of their normal shape or out of their normal orientation with respect to one another. When the pressure is released they assume their more normal orientation and the figures result. The general shape of the figures might be worthy of a mathematical investigation. Also, it would be interesting to study the figures formed when the plates were made of other transparent substances such as quartz, mica, and various resins. SUMMARY
Figures are formed in greases or liquids of the proper viscosity when the substance is placed between smooth plates and pressure is applied and subsequently released. The figures are holes in the material, and their shape depends upon the viscosity and composition of the material used. The figures are formed in a wide variety of chemical compounds, including the very polar and the very non-polar. There is no parallel between compounds that form the figures and compounds that form liquid crystals. The viscosity and “stickiness” of the material were the only properties that were observed to be associated with ability to form the figures.
I wish to express my appreciation to Dr. Frank M. Goyan for several helpful discussions during the course of this work. REFERENCES (1) BERNAL AND CROWFOOT: Trans. Faraday SOC. ‘29, 1032 (1933). (2) BIRCH:Nature 122, 729 (1928).
