INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1931
667
Some Properties of Dispersions of the Quicksand Type”’ R. V. Williamson and W. W. Heckert E. I . D V PON?
DE
NEMOURS & COMPANY,
H E plastic and viscous properties of paints and varnishes have been studied quite intensively during the last few years in order to obtain a more comprehensive understanding of their flowing, brushing, settling! gelling, and other physical properties. These studies have indicated that three different types of flow-stress curves are necessary to express the viscous and plastic properties of paints and varnishes. The three types of flow-pressure curves are shown in Figure 1. Curve I is characteristic of the curve of a fluid which obeys the ordinary laws of viscous flow. Curve I1 represents the curve of what we have chosen to call “pseu-
T
I
1
wIL,MINGTON,
DEL.
It was found that these peculiar dispersions show a most striking increase in apparent viscosity under all conditions of stirring and pouring, although in a Bingham-Murray plastometer they flow like truly viscous liquids. The increase in apparent viscosity with increase in shearing stress is easily demonstrated in a modified Stormer viscometer employing rotating paddles. In such an instrument the apparent viscosity increases slightly with increasing rate of stirring up to a critical value, beyond which the rate remains practically constant. This critical rate of stirring is apparently a linear function of the distances between particles or of the volume fraction of the dispersion medium. The experimental data are not sufficiently accurate to distinguish between these two possible relations. The property of inverted plasticity as observed in starch dispersions is to be contrasted with “dilatancy” effects described by Reynolds ( 2 ) and by Mead (1) for wet sand, in that it occurs a t lower concentrations of the solid phase than those corresponding to the range between tetrahedral and cubical packing of the particles, which is the condition for dilatancy effects. The writers have observed the phenomenon with dispersions of a large number of finely divided solids in media which were found to wet the dispersed phase very well. Excellent wetting and a proper volume relationship appear to be the conditions essential to the development of inverted plasticity. 12-
5TPt54
II -
Figure 1-Types of Flow-Stress Curves I-Viscous liquids 11-Pseudoplastic dispersions, as of nitrocellulose 111-Plastic dispersions, such as clay in water IV-Dispersions showing inverted plasticity
doplastic” dispersions (S), such as certain types of nitrocellulose dispersions and well-flowing paints. Curve I11 is the type of curve obtained from a dispersion with a high degree of plasticity, such as a flat wall paint or a plastic clay. The writers have recently observed, however, that some dispersions exhibit flowing properties that do not correspond to any one of these three types; in fact, they become more viscous when they are stirred or shaken vigorously. The flow-stress curve for this type of dispersion is shown in curve IV, Figure 1. The writers have chosen the term “inverted plasticity” to designate the flowing properties of such dispersions because their rate-of-stirring vs. stress curves have a form similar to that of an inverted plasticity curve. A suspension of the proper concentration of cornstarch in water exhibits this property to a marked degree, as do also dispersions of pigments in certain paint and enamel vehicles. Dispersions of cornstarch were chosen for a semi-quantitative study of the property of inverted plasticity, because it was believed that starch particles would most easily lend themselves to particle-size measurements. 1 Received Ivfarch 25, 1931. Presented before the Division of Paint and Varnish Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930. 2 Contribution No. 36 from the Experimental Station of E. I. du Pont de Nemours & Co., Wilmington, Del.
Figure 2-Experimental Flow-Stress Curves Given by Viscous Liquids (I and XIII) Plastic Dispersions (X XI and X U ) , and Dispersions Showing Inverted Plasticit; (V, VI, VII, VIII, a n d IX)
Materials
The starch used in these experiments was an ordinary commercial grade of cornstarch. Distilled water, commercial
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absolute alcohol, technical carbon tetrachloride, technical xylene, and petroleum hydrocarbon (commonly known as turpentine substitute) were the liquids used.
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come within the range of these two curves, one may roughly estimate the viscosity of the suspensions from the slopes of the curves, although the data are expressed in relative values only. Curve I is slightly curved in spite of the fact that the Measurements with Bingham-Murray Plastometer solution is truly viscous. This curvature is due to turbulence, Attempts to measure the inverted plasticity of cornstarch which occurs in liquids of low viscosity a t comparatively and water suspensions in a modified Bingham-Murray plas- low rates of stirring. tometer were unsuccessful for the following reason: The sucCurves X, XI, and XI1 are for suspensions of cornstarch tion used to pull the suspension through the capillary caused in alcohol. These curves are typical of false-bodied paints, the water to be drawn through the starch suspension, with which show decided plastic properties. The curves appear the result that the concentration of starch in the capillary to cut the axis of abscissas a t increasingly greater distances decreased as the experiment proceeded. It was possible with from the origin as the concentration of starch is increased. They are straight lines over a considerable range of shearing 0ISTA.Kt B t l W t t N PAPlICLL3 IX NlCpOll5 1.0 0.9 08 07 Q6 0.5 rates, but a t high shearing rates the curves for some of the r more concentrated suspensions bend toward the axis of 6ordinates. As the concentration of starch is reduced, the curves approach the shape of curve I, for an ordinary viscous liquid. 5Curves I to IX are for starch suspensions in water, in which the concentration of starch increases in the order enumerated. Curves V to IX show that, after a certain rate of stirring is attained, it remains practically constant with f4Y increasing load on the stirring device, These curves show L 1 also that a rather abrupt change in the rate of stirring neces5 sary to produce the flat portion of the curves occurs a t a 53concentration of starch represented by curve IV. This feature appears more clearly in Figure 3, where the rates of stirring corresponding to the flat portions of curves V to I X are fi v plotted against the concentrations of starch in the suspensions. u 2The curve shows that the critical rate of stirring increases linearly with decreasing concentration of starch until the concentration is so low that the property of inverted plasticity begins to disappear. The sharp break in the curve indicates that a marked change in the character of the dispersion with respect t o this property occurs a t a concentration of 40.5 per cent starch by volume. The extrapolated curve appears to cut the concentration axis a t a starch concentration of 45 per cent by volume (55 per cent voids). Evidently the critical rate of stirring is practically zero even when the volume of voids exceeds that corresponding to close cubical this instrument to obtain several flow-pressure measure- packing of the particles. The curve also indicates that the ments within a few seconds after the beginning of an experi- property is still detectable when the volume of the voids is ment. Probably very little change in concentration of 10 per cent greater than that for cubical packing; thus the starch occurred during such a short interval, so that these property of inverted plasticity appears only within a small measurements were more significant than those made after concentration range. the dispersion had been flowing for a longer time. These measurements indicated, however, that the suspension was Distance between Particles flowing in a manner characteristic of a viscous liquid and d n attempt was made to determine the relationship of the gave no evidence of inverted plasticity. average distance between particles to the property of inMeasurements with Stirring Device verted plasticity. The average diameter of the starch grains A modified Stonner viscometer was then tried as a means of was determined microscopically to be 10.3 microns. The measuring inverted plasticity and was found to give very distance between particles was calculated on the assumption satisfactory results. An ordinary Stormer viscometer was that the particles were all of this diameter and equally spaced modified by replacing the rotating cylinder with a two-bladed throughout the suspension. Figure 3 shows that the critical stirrer, in order to reproduce the stirring action of the prac- rate of stirring is likewise approximately a linear function of the distance between particles until the distance becomes tical painter when he stirs paint with a ~ p a t u l a . ~ sufficiently great to cause the property to disappear. Results The critical rate of stirring cannot be a linear function of both the percentage of volume concentration and the distance Figure 2 shows a series of rate-of-stirring us. stress curves between particles. It is not possible from the experimental for various concentrations of starch by volume. Curve I, representing the behavior of a 60 per cent sucrose data to say which is linear or whether either is exactly linear, solution as well as of a 35.7 per cent starch suspension, and but since the experimental data cover such a short range of curve XIII, for commercial brand of sirup, show the type of concentrations and such short distances between particles, curve obtained with this apparatus for ordinary viscous both relationships are probably linear within the limits of liquids, The viscosities in poises of these two solutions a t the errors of the experiments. From the curve in Figure 3 25" C. are, respectively, 0.44 and 73. Since the other curves the critical rate of stirring appears to be zero when the distance between particles is 0.54 micron. When this distance a This apparatus was designed by J. E. Booge and E. S. Steinbring in reaches 0.9 micron, the curve shows an abrupt rise and apthe du Pont laboratories.
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INDUSTRIAL A N D ENGINEERlNG CHEMISTRY
June, 1931
pears to approach an infinite slope when the particles are approximately 1.O micron apart. The distance between particles when this condition exists is roughly one-tenth the average diameter of the particles. Relation of Wetting t o Inverted Plasticity
Casual observation of the phenomenon during the preparation of a large number of paint and enamel mill bases has indicated that inverted plasticity occurs only when the pigment is well wet by the vehicle; that is, it is observed only in vehicles which promote excellent dispersion, slow settling, and small sediment volumes. These facts were well illustrated by experiments with starch dispersions. Dispersions of starch in ethyl alcohol, petroleum hydrocarbon, or carbon tetrachloride do not exhibit inverted plasticity. These dispersions settle (or cream) rapidly to less dense sediments than in the case of water. The settling experiments are graphically illustrated in Figure 4. They were performed with dispersions of‘20 grams of starch in 100 cc. of medium. I n Figure 4 the height of the surface of the settling particles was plotted against the expression: Time X difference in densitv between starch and medium Viscosity of medium The values for viscosity were taken from the handbooks. In the absence of varying degrees of flocculation and swelling, the initial slanting selections of these curves should be superimposed. The curve pertaining to carbon tetrachloride was obtained by subtracting the readings from 10.0, since this dispersion creamed rather than settled.
(TlNL < DIFTLPLKt
Figure 4-Rate
in RN51TY)/
VlSCO5i?-.(
of Sedimentation of Suspensions of Starch i n Various Liquids
After a constant sediment volume had been obtained by gravitational settling, the samples were centrifuged. The difference in volume of the sediment under these two conditions may be taken as a measure of the flocculation. Thus flocculation was greatest in carbon tetrachloride and least in water (Table I). Table I-Variation
of Flocculation with Dispersion Medium HEIGHTOF SEDIMENT
DISPERSION MEDIUM Gravity
Water Ethyl alcohol Petroleum hydrocarhon Carbon tetrachloride
cenk:;t&g Difference
Cm.
Cm.
Cm.
2.7 3.1 4 6 7 0
2 6
0.1 0.9 2.2 4.4
2 2 2 4
2 6
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The absence of flocculation (poor wetting) in the case of water was shown by the cloudy appearance of the supernatant liquid during the settling process. An interesting feature of these experiments is the fact that the volume of the starch grains in the water after centrifuging was approximately 15 per cent greater than in alcohol. This was probably due to a swelling of the starch in the water. Swelling of Starch Grains
An attempt was made to determine microscopically whether or not the starch grains were swollen by water during the time necessary to make the flow-stress experiments. Photomicrographs were made of individual starch grains after being submerged in water for internn vals varying from several minu t e s t o 48 h o u r s , a n d the diameters were m e a s u r e d . No swelling was observed by this method, but the experimental error was of the order of 5 per cent corresponding to the 15 per cent volume difference noted above. u
Discussion of Results
The property of inverted plasticity shown by cornstarch and water a n d b y v a r i o u s other suspensions appears to Figure 5-Cubical and Tetrahedral Packing be related to the familiar rigidity of sand and water. Band flows readily when wet wit,h:a large excess of mater but, if only enough water is present to fill the voids in closely packed sand, it is quite rigid and resists deformation to a marked degree. It also appears to become dry when subjected to deformation. The rigidity of sand and water mixtures has been explained by Reynolds (Z) and by Mead (1) as follows: Figure 5 represents a mass of spheres arranged to show the form of tetrahedral and cubical packing. The two middle rows are packed in the form of cubical packing and the remainder in the form of tetrahedral packing. The voids between the cubically packed particles are obviously considerably larger than those between the tetrahedrally packed ones. If a mass of moistened sand is arranged in the form of the tetrahedral grouping, any deformation necessarily enlarges the voids and causes an infiltration of water if an excess is present. When the surface of the water coincides with the surface of the sand, the surface tension of the water becomes active in binding the particles together. Further deformation requires work to be done against the surface tension of the water as well as in overcoming greater friction between the particles; consequently, the mass resists stirring or other forms of deformation more than when it is either dry or wetted by an excess of water. Reynolds applied the term “dilatancy” to the property of expansion upon deformation which closely packed assemblages of particles display. I n attempting to apply the principles of dilatancy to the phenomenon of inrerted plasticity, one finds (1) that, unlike the case of sand and water, the phenomenon can be observed when there is an excess of dispersion medium over that required to fill the voids for cubical packing; ( 2 ) that there is no obvious explanation for the almost constant rate of flow attained above a certain shearing stress or for the linear relationship between the critical rate of stirring and the average distance between particles; and ( 3 ) that dilatancy is not restricted to dispersions in which there is a high attraction between the phases-i. e., excellent wetting. One might postulate that the water in excess of that re~~
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quired for cubical packing tends to flow into the track made by the stirrer, more or less of the remaining paste assuming a condition of tetrahedral packing. Inverted plasticity might appear under such circumstances when the shearing stress was great enough to cause relative movement of the particles in the regions where they are packed more closely than those in cubical piling. This critical shearing stress might also be expected to increase with decrease in concentration of the dispersed phase. As a matter of fact, curve I, Figure 2, obtained with a purely viscous liquid (60 per cent sucrose), bends slightly convex to the stress axis, so that the unique portion of the inverted plasticity curves is that part where the rate of stirring remains essentially constant, independent of the shearing stress. One of the writers has attempted to account for this in the following manner: It is known that the particles in starch-water suspensions are well dispersed and closely assembled. It would not seem improbable that initial motion of the paddles would cause most of the particles in its immediate vicinity to arrange themselves in more or less concentric shells, with most of the shear taking place in the liquid between “shells.” It is difficult t o conceive of any appreciable rate of shear in a closely packed dispersion of particles, separated from each other by less than one-tenth of their diameter, unless some orderly arrangement of particles is obtained. As the rate of shear is increased, the shearing stress between shells would also increase, until finally it might be expected to reach a value capable of pulling particles from the adjoining layer. This would disrupt the orderly flow. The innermost “shells” would lose their identity and seize to the stirrer. The zone of shearing would then spread to outer shells of increased diameter and require an increase in total shearing stress to maintain the same rate of shear between shells.
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The postulate of a critical disrupting force between ‘‘shells,” together with the laws of viscous flow, would demand a linear relationship between the critical rate of stirring and the average distance between particles. This is what was actually found by experiment. Furthermore, this mechanism may account for the failure to observe the property of inverted plasticity with the Murray-Bingham plastometer. It is a well-established fact that laminar flow takes place when liquids are passed through small capillaries. Both the manner in which shearing stress is applied and the shape of the container conspire t o preserve this orderly, laminar type of flow. Under these conditions “seizure” of adjoining layers would probably only occur a t much higher rates of shearbeyond the range of these experiments. A method for testing this mechanism has been planned but as yet has not been carried out. Recently a somewhat different type of flow has been observed which deserves a little description. When needleshaped CaSO4.2H20 was stirred into a small amount of water, a dispersion was obtained which alternately “seized” and then “released” a spatula which was stirred through the mass. It appears quite certain that this modified form of inverted plasticity is definitely connected with the needle shape of the particles. The phenomenon was only observed in certain cases where the needle-shaped characteristics were very marked (a sample of Baker’s precipitated CaS04.2H20). More finely divided calcium sulfate did not show the phenomenon. Literature Cited (1) Mead, W, J., J . Geol., 33, No. 7 (1925). (2) Reynolds, Osborne, “Scientific Papers, Vol. 11, p. 217 (1901); Phil. M a g . , Dec., 1885; Proc. Roy. I n s ! . , Feb., 1886; Nature, 33, 30 (1901). (3) Williamson, R . V , I N D . E N D . CHEM., 21, 1108 (1929).
Speed of Crystallization of Lactose, Galactose, Glucose, and Sucrose from Pure Solution’ E. 0. Whittier and S. P. Gould RESEARCH LABORATORIES, BUREAUOF DAIRYINDUSTRY,
N T H E course of work on the manufacture of lactose it became necessary to know the effect of differences of temperature on the s p e e d of its crystallization. Galactose and glucose were included in the investigation because of their constituent relationship t o lactose; sucrose, because it is not a mutarotating sugar, was included for purposes of cornprison.
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20”
c.
The most striking conclusions drawn from previous work on speed of crystallization of sucrose are: (1) that it is imPossible to correlate velocity and viscosity (3); (2) that the Velocity Of CryStalliZatiOn increases with speed Of agitation UP to 400 r. p. m., above which it is no longer iduenced by December 6, 1930.
D. C.
change in this factor (8); and (3) that the velocity of cryp t a l l i z a t i o n v a r i e s as the s q u a r e of the a m o u n t of supersaturation (4). These conclusions are based on observations on the growth of individual crystals. No work appears to have been published on the crystallization rates of galactose and glucose. The course of crystallization of lactose from a 60 per cent s o l u t i o n a t s e v e r a l temperatures has been calculated and plotted by Sharp (7) for the later stages on the assumption that the rate of p + change is the sole determining factor at this time. If the amount of ,plactose initially representing supersaturation to the alpha form crystallized instantaneously a t the time plotted by Sharp as 0 hours, his calculated values should agree with experimentally determined values, provided that viscosity and rate of stirring could be disregarded
The courses of crystallization Of lactose, galactose, glucose, and sucrose from pure solution have been followed by refractometric methods at temperatures from 0’ to 30’ C. and the velocity constants calculated. In the earlier stages Of CrYStallization of lactose, galactose, and glucose the rate of crystallization of the form separating is the principal contro!ling factor. Subsequently the rate is diminished considerably as the rate of attainment of equilibrium among the isomeric forms becomes the major determinant. The most rapid crystallization of lactose takes Place if the solution is maintained at or slightly above 30” C. for 3 hours and is then allowed to cool to approximately
Previous Work
1 Received
WASHINGTON,
