Sedimentation Volume, Dilatancy, Thixotropic and Plastic Properties of

SEDIMENTATION VOLUME, DILATANCY, THIXOTROPIC AND PLASTIC PROPERTIES O F CONCENTRATED SUSPENSIONS H. FREUNDLICH

AND

A. D. JONES

The S i r W i l l i a m Ramsay Laboratories of Inorganic and Physical Chemistry, University College, London, England Received July 81,2996

Concentrated paste-like suspensions of fine powders in liquids manifest certain properties, which depend strongly on the degree of packing. The rule given in table 1 was put forward, and was shown to hold in a few cases (2, 3). This rule was considered to be rather a rough guide, since factors such as particle shape, etc., were not taken into account. It would obviously be necessary to investigate the physical side of each of these phenomena more thoroughly, because very little is known about them. But, before doing so, it seemed desirable to compare the behavior of a large number of powders of different kinds to see whether the rule really was a first approximation to the truth, and to detect cases which might be specially fit for more exact investigation. This was the aim of the following paper. The substances used, most of which are minerals, are listed in table 2. The objection could be raised that they might be impure, and that this would impair the results. We selected the minerals in as pure a condition as possible, and any obvious foreign matter was removed; there were two mixtures,-chalk and Solnhofen slate. I n addition, the results seemed to show that small amounts of impurities, less than 1 per cent or so, did not change fundamentally the pmperties under investigation.1 Minerals have, on the other hand, several advantages. Many of them are highly insoluble, and they are harder than the solid products of the same constitution chemically which have been precipitated from solution. A thixotropic and plastic behavior was observed only if the solid particles in the suspension were sufficiently fine. There should be present a fairly high percentage with a diameter of between 1 and 10 p . The original substances were first coarsely ground in a .porcelain mortar, or in an iron percussion mortar in the case of hard substances. These powders were then ground to the correct size, in most cases in a porcelain 1 Cf. the discussion on the behavior of different forms of calcium carbonate, at the end of this paper.

1217

1218

H. FREUNDLICH AND A. D. JONES

ball-mill, containing 55 hard porcelain balls of a diameter of 1.50 cm.; generally, about 200 g. of the coarse powder was put into the mill with 0.25 1. of water, the lid then clamped on, and the whole lot rotated a t 60 t o 80 R.P.M. until the powder was sufficiently fine, as determined by sedimentation. Very hard substances, such as quartz and rutile, were powdered finely in a mechanical agate mortar, because they would be contaminated to a large extent with porcelain powder if the ball-mill were used.2 The powders were fractionated by sedimentation in water, and the fraction with particles of diameters between 1 and 10 1.1 was selected for the experiments. In some cases the particles stuck together in water; then they sedimented in a clear line, either because they were actually homodisperse, or because the larger particles formed a network whichenmeshed the smaller ones, and caused the whole lot t o sediment together. Since such powders could not be fractionated under these conditions, they were ground in the ball-mill until the rate of fall of the sedimenting level corresponded t o the correct particle size. The diameter of the particles, as calTABLE l Properties of paste-like suspensions PROPERTIES DEPENDENT UPON CLOSE PACKINQ

Small volume of sedimentation Dilatancy No thixotropy No plast,icity

1

1

PROPERTIES DEPENDENT CPON LOOSE PACXINQ

Large volume of sedimentation

No dilatancy Thixotropy Plasticity

PROPERTIES OF CONCENTRATED SUSPENSIONS

1219

TABLE 2 Description of substances studied SUBSTANCE

CEEMICAL COYPOBXTION

IARDNEBe

1. Diamond . . . . . . . . . . . . . . . . . . . C 2. Graphite .................... C

10. 11. 12. 13.

10 0.5-1 9 5.5-6 2-2 5 5-5 5 7 6-6 5

Titanium dioxide.. . . . . . . . . . Ti02 Vanadium pentoxide.. . . . . . . VZOS Tin pyrites.. . . . . . . . . . . . . . . . SnSz.CutS.FeS Galena.. . . . . . . . . . . . . . . . . . . . PbS

3-4 2-3 4 3-3.5 1 5-2 1 5-2 1.5-2

Calcium carbonate (pure). . . CaC08 Aragonite.. .. CaCOa Iceland spar. CaCOs Limestone.. . CaCOs Marble.. . . . . . . . . . . . . . . . . . . . CaCOs Complex silicate Mica (muscov Complex silicate Monax glass. . Porcelain. . . . . . . . . . . . . . . . . . . Complex silicate Kaolin.. .................... Complex silicate Steatite.. . . . . . . . . . . . . . . . . . . .Complex silicate (H~MgaSi4Od 29. Chalk. ..................... CaCOa, mixed with claj 30. Solnhofen slate.. . . . . . . . . . . . CaCOa, mixed with claj

19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

3 5-4 3

3 3-4 2.5-3 6 7 2-2 5 1

CRYBTALLINE STRUCTURE

Cubic Hexagonal Amorphous Hexagonal Hexagonal Rhombic Amorphous Hexagonal Tetragonal Tetragonal Rhombic Tetragonal Cubic Hexagonal Cubic Rhombic Rhombic Monoclinic Monoclinic Hexagonal Rhombic Hexagonal Hexagonal Hexagonal Monoclinic Amorphous Amorphous Monoclinic Monoclinic Hexagonal Hexagonal

* Jet is an intermediate compound between wood and coal, form from petrified wood under high pressure, and it contains vegetable structures. It is homogeneous, and contains carbon, some 8 per cent of water, and 20 per cent of hydrocarbons, which appear on distillation. I t shows a subconchoidal fracture, and is amorphous. t After the gypsum had been ground to the correct size in water in the ball-mill, the excess mater was removed by filtration and half of the residue was dried a t about 90°C. in a steam oven, and half by exposure to the air for several days. The ovendried sample had lost some of its water of crystallization. SEDIMENTATION VOLUME

'

This was determined by allowing a known weight of powder to sediment uniformly in a known volume of water or aqueous solution, in graduated tubes having an internal radius of 0.58 cm., calibrated in cubic centimeters

1220

H. FRBUNDLICH AND A. D. JONES

up to 20 cc., and fitted with ground glass stoppers. The weight of the substance taken, in all cases, was that weight, as calculated from the density of the material, which had the same volume as had 3.00 g. of quartz, Le., 1.13 cc. By using a constant volume, the results for the individual substances could be directly compared. The tubes were first carefully cleaned. Then.a small known volume of the liquid was poured in, and this was followed by the weighed quantity of the powder, and finally the rest of the liquid was admitted, and the whole lot shaken up until all the solid was in suspension. When the sediment had commenced to settle, and a layer of clear liquid had appeared orf the surface, the solid which remained on the walls of the tube above the liquid was pushed down into the liquid by means of a rubber plunger. Then the tube remained undisturbed for at least twenty-four hours, and the volume of the sediment was read a t the end of that time. The sediment was shaken into suspension again after this, and the experiment repeated until the sedimentation volume had reached a constant figure. The sedimentation volume proved to be a most intricate phenomenon, depending strongly upon many factors; it evidently needs a thorough examination. The absolute value of the volume of sedimentation was less in wider tubes, and, in consequence, all the tubes had to be of the same internal cross section, in this case, 1.06 sq. cm., and of constant bore. It also depended, to a certain extent, on the volume of the liquid; it decreased slightly as the volume of the liquid decreased below a certain limiting liquid volume, which in this case was about 9.0 cc., so that 15.0 cc. of liquid was used in all cases, in order to be well above this limiting volume. I n many cases, as the determinations were repeated, the sedimentation volumes decreased gradually until the final value was reached. This was probably due to the removal of entrapped air by repeated shaking. This behavior, or when there was only a very small change a t all, was considered to be the normal behavior. S n increase in sedimentation volume which was observed in a number of cases was anomalous, and most likely due to a chemical action between solid and liquid. VELOCITY OF SEDIMENTATION

Measurement of thd sedimentation volume naturally led to a determination of the velocity of sedimentation, this being obtained by observing the limiting line between sediment and clear liquid a short time after shaking, and at intervals of a few minutes until there was praotically no further fall; the time for the fall of the majority of particles was from two to three hours. Sedimentation velocity proved to be valuable in characterizing the behavior of the different suspensions. With many substances, the sedimentation volume decreased markedly


1221

if the tube was rolled to and fro between the palms of the hands after the solid had settled, and the particles allowed to settle without any further shaking. The standard time of rolling was 30 sec., the time of settling was, as before, about twenty-four hours, and this procedure was repeated until a constant volum,e was reached. A strong decrease, more than 25 per cent, was only observed with particles which were expected to be anisometric, i.e., plates or needles; for instance, mosaic gold in water (decrease 33 per cent), mica in water (decrease 40 per cent), and graphite in N hydrochloric acid solution (decrease 30 per cent). So the effect was probably due to the fact that the particles were oriented by the gentle movement. They were able to settle after orientation to a smaller sedimentation volume, the plates lying like sheets of a book on each other, while so long as they were lying irregularly, they enclosed a much larger volume of liquid, as was the case when they settled a t random without being oriented (9). DILATANCY

Dilatancy (8) was investigated only in a qualitative way, by mixing known amounts of the powders and liquids in a small basin into pastes, and observing the effect of a disturbance with a spatula. Normal or strong dilatancy was shown, for example, by a mixture of 5.0 g. of quartz and 2.25 cc. of water. This paste, if left to itself, appeared quite fluid, but when it was disturbed with a spatula, it immediately became dry and very resistant to the spatula. It was extremely difficult to pick up a portion of the paste on the spatula, but, once there, it became fluid again, and ran off in the manner of a tgeacly liquid. After the disturbance, if the paste wfts left alone again, it rapidly became liquid, and this could be accelerated by tapping the basin. Non-dilatant pastes showed no change in consistency when they were disturbed with a spatula. No liquid exuded when they were spread out, nor when they were left to themselves, nor when the basin was tapped. An intermediate behavior was observed between these extreme cases in a number of pastes, best described as a passive or weak dilatancy. The pastes concerned did not show any obvious change in consistency on mechanical treatment, nor did liquid exude when they were left to themselves, or a t least, this happened very slowly. But when the vessel containing the paste which had been mixed with the spatula was gently tapped, liquid was soon seen to come out in quantity. THIXOTROPY

Thixotropy was also observed in somewhat of a qualitative fashion, in a tube 18.0 cm. long and 0.93 cm. in internal diameter, fitted with a ground glass stopper. A series of pastes of increasing liquid concentrations waa

1222


made, beginning, frequently, with a mixture of 3.0 g. of the solid powder and 1.0 cc. of liquid, and the liquid content was increased in increments of 0.25 cc. at a time. For each thixotropic paste, there was a region of concentrations where the mass became liquid on shaking, but solidified when left to rest. With a certain minimum amount of liquid, the paste could not be heard or felt to move when shaken; the only change was a change in its position in the tube before and after shaking. This concentration of solid was taken as the upper limit of the thixotropic region. As more liquid was added, the state was reached where the paste could be heard to flow when the tube was shaken, but i t solidified immediately after shaking ceased. With still more liquid, the time of solidification became finite, and increased gradually; determinations ceased after it had reached ten minutes. The lower limit of the thixotropic region was generally chosen as that when the time of solidification was about a minute or so. On adding an excess of liquid, the paste did not solidify at all, but separated into two phases on standing. In non-thixotropic pastes, the liquid masses did not solidify a t all. As soon as the amount of liquid was sufficiently large, generally the whole mass flowed down when the tube was inverted, and the liquid separated from the solid when the tube was allowed to remain undisturbed, the sediment not being thixotropic. In some cases, mostly with coarser powders, when the tube was inverted, the solid remained as a hard clod at the top, and clear liquid ran down the walls of the tube. Quite a number of pastes were found to be rheopectic, that is, they solidified more quickly if they were subjected to a regular gentlemotion, as, for instance, by tapping the tube on the bench, or by rolling it to and fro between the palms of the hands. Rheopexy could only be observed in a region of concentrations where the time of thixotropic solidification was fairly long.3 PLASTICITY

Plasticity has not, so far as we know, been determined by any standard method. In order to compare plasticity with the other properties under consideration, an isothermal method of characterizing plastic behavior was essential. The method used was practically identical with that used for investigating flow pressure (6). The pastes were pressed against a disc, through the center of which was a small hole. The shearing took place at the hole. If the paste issued from the hole unchanged and well deform3 In some pastes a stable foam mas formed, which was liable to cause apparent thixotropy and rheopexy by preventing the liquid part from flowing in the tube. This foam could be broken by a gentle t a p on the outside of the tube, and then i t could be seen whether thixotropy or rheopexy existed or not.


1223

able, it was plastic;‘ it was not plastic if it was changed in any way by the shearing forces. This “plastometer” is shown in figure 1. The brass cylinder, AA, 7.6 em. long, and 1.91 cm. in internal diameter, was fitted with a brass screw cap B at one end, to which was attached an inextensible rubber tube to convey the air pressure from a cylinder of air; and with another cap C on the other end with a hole in its center, 0.91 om. in diameter. The jet, abcd, was housed in a piece of brass, whose vertical cross section was Tshaped, and which fitted through the hole in C, and could be clamped tight by screwing C on to the cylinder, and an air-tight joint obtained by interposing a lead washer. The iet was 0.16 cm. in diameter, drilled centrally and ground smooth with emery paste; the top, ab, w&s slightly countersunk, and the bottom edges were rounded off, else the threads tended to

FIG.1. The plastometer

curl up and stick to the brass and so form a loop instead of a nice straight thread. It was found necessary to communicate the air pressure to the paste by means of a brass piston E, of weight 60.6 g. The paste was put into the space F. Air pressure was applied on to the piston from a cylinder of compressed air. The pressure was controlled by means of a reducing valve on the cylinder. Pressure was measured by means of a simple mercury manometer in centimeters of mercury, and all pressures are given as such. This valve also controlled the rate of rise of pressure. The requisite amounts of liquid and powder were mixed in a small basin We would thus agree with the following definition of plasticiky: “Plasticity is that property which enables a material to be deformed continuously and permanently without rupture during the application of a force which exceeds the yield value of the material” (H. Wilson, Ceramics; Clay Technology, p. 55. New York (1927).)

1224

H. FFtEUNDLICH AND A. D. JONES

with a flat spatula for a measured time, usually 30 seconds, and the homogeneous paste was transferred to the cylinder, also by means of the spatula, in a measured time, again usually 30 seconds. Then the cylinder and its ends were screwed together. After noting whether the weight of the piston alone was sufficient to cause any extrusion, pressure was applied slowly. The reducing valve was opened slightly to give a rate of rise of pressure of 1 or 2 cm. per minute, and this was continued for a few centimeters rise. Then the pressure was allowed to rise more quickly, and so on, in stages, always keeping the rate constant for a few centimeters rise before the next stage, and observing the effect of the pressure on the paste, until the final pressure rise of from 50 to 70 cm., which took place a t about 60 cm. per minute. TABLE 3 Behavior of a plastic paste of chalk EXPERIMENT

1

PREBBURE

cm.

No. 1. 5 g. of chalk was mixed with 2.0 cc. of water by means of a flat spatula, in a small basin, for 30 sec., and transferred t o the cylinder in 40 sec. No. 2. 5 g. of chalk was mixed with 2.0 cc. of water by means of a flat spatula, in a small basin, for 30 sec., and transferred to the cylinder in 40 sec.

0 1-3 3-6 6-10 16-20

RATE OF RIBB OF PRESSURE

cm. per

THREAD

LENQPH

om.

mlnule

1 1-2 6 20

Nothing Threads Threads Threads Threads

1-2 2-3 3-4 5-6

1 2 7 24

Nothing Threads Threads Threads Threads

1-2 2-3 3-5 6-7

0

1-2 2-3 3-9 9-22

RESULT

In each experiment all the paste was expressed.

If the paste was plastic, the mass was extruded in threads, which broke off as soon as they had reached a certain well-defined length. This length was measured on a scale below the jet. Table 3 gives an example of the behavior of a plastic paste of chalk. The measurements on the plasticity of these pastes were remarkably reproducible, provided that the above-men tioned precautions were taken. The behavior of the pastes in this plastometer classified them very definitely and very siinply into three groups: (i).Plastic pastes, which were extruded in threads of characteristic lengths. Non-plastic pastes, in which the extrusion took the nature of: (ii). Drops of clear liquid, there being left in the cylinder a hard and rather dry cake of the solid powder. (iii). Sticky or pasty drops, consisting of a mixture of the solid and liquid phases, containing an excess of liquid compared with the original mixture.

1225


The rule mentioned above was found to hold for a great number of pastes. Firstly, there are the cases agreeing with close packing; these are the suspensions which showed a small sedimentation volume, generally less than 4.0 cc.,for the standard amount of substance (cf.page 1229), normal dilatancy, no thixotropy, and no plasticity, the extrusion in this case being sticky

LIQUID PEAMI SUBBTANCD

Water

NHCI

4.0

4.6 3.75 2.7

-

Jet. ....................................... Corundum.. ................................ Quartz Vanadium pentoxide. ...................... Fluorspar .................................. Monax glass.. ...........................

.........

.....

2.0 3.35 3.1 2.0 2.0

2.1 2.65 2.7

NNaOH

NNaCl -

4.6

3.1

3.2

3.65

3.4

~~~~~

TABLE 5 Sedimentation volumes i n cubic Centimeters of loosely packed pastes, which are strongly thixotropic and plastic

I

LIQUID PHASI

N NaOH N NaCl -N HC1

6.3

Kaolin.. ................................... Chalk. . . . . . . . . . . . . . . . . Solnhofen slate ............

6.3 5.9

5.85 7.0 8.6 10.6 8.8 6.5

7.1 10.4 8.3 10.1

6.6

5.4

7.5 5.95 7.4 5.9 5.9

6.8 9.5 11.2

9.5 9.75

drops of liquid. (The figures in the ensuing tables are the final volumes of sedimentation in cubic centimeters, previous to any intentional orientation.) The pastes shown in table 4 belong to this group. Then there are cases agreeing with loose packing; these are the suspensions which showed a large sedimentation volume, generally greater than 6.0 cc., no dilatancy, strong thixotropy over a broad range of concentrations (for instance, thixotropic pastes of graphite in water contained from

~

1226


32 to 52 per cent of solid and thixotropic pastes of hematite in water from 38 to 67 per cent of solid), and plasticity, the paste being extruded in welldefined long threads. To this group, the pastes listed in table 5 belong. In addition to these cases in which the rule holds in its entirety, a number of examples were found where portions of the rule held quite strictly; generally, the properties connected in these cases were correlated in one direction only, and could not be reversed: ( 1 ) . There was practically no case of normal dilatancy which did not correspond to a small volume of sedimentation, that is, less than 4.0 cc. I n this respect, there may be added t o the pastes mentioned in table 4, those in table 6. These differ from those in table 4 by being weakly thixotropic; the range of thixotropy for corundum in water was from 63 to 67 per cent of solid, and that for Monax glass in N sodium chloride was from 5 i to 63 per cent of solid. These pastes were not plastic. TABLE 6 Sedzmentation volumes i n cubic centimeters of closely packed pastes which are ddatant, but also weakly thixotropic5 BUBSTANCE

Diamond.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corundum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quartz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monax glass., . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

-I

LIQUID PEASE

Water

1

NHCI

4.1 4.45 3 2

1

NNaOH

[

1 I 3.4 43 45

NNaCl

3.3 34 .155

The reverse does not hold true. Pastes with a very small volume of sedimentation need not show normal dilatancy; they may be passively dilatant. Examples of this are aragonite in water arid N sodium chloride solution, and marble in water and N sodium chloride solution (see table 9). ( 2 ) . Suspensions which Fhowed a large volume of sedimentation were also thixotropic. Examples are those in table 5 , and to these, those in table i can be added. Table 7 contains pastes which have a large volume of sedimentation and, correlated with it, strong thixotropy, but which are 6 Litharge (in water and in sodium hydroxide solution) and rutile (in all four liquids) have probably to be added to this group. They both show certain aniomalies. With litharge the volume of sedimentation is rather large (5.6 and 6.4, respectively), but as the latter increases in course of time, its behavhr is anomalous and would need further investigation. Rutile in water is partly peptized, the sediment consisting of a brown layer. This is most likely an iron compound, and this impurity may be the reason why rutile pastes show no dilatancy, though this ought to be expected on the ground of their behavior as to the other properties. Titanium dioxide, as it is used for technical purposes and consisting of anatase, gives pastes whirh are very loosely packed (cf. table 5 ) .

1227


nevertheless passively dilatant and not plastic. Ih several cases the behavior of the sedimentation volume was anomalous; it increased markedly on repeating the experiment, but this was most likely due to some chemical reaction. This is obvious with the oven-dried gypsum. But this fact does not weaken the force of the argument that a large sedimentation volume favors thixotropy. (In table 7 the values given for the sedimentation volumes are the maximum values obtained.) (3). Pastes which were markedly plastic were invariably thixotropic; they were also not dilatant. Table 8 contains pastes of this kind, but they do not fit into table 5 because the sedimentation volumes were medium or small. TABLE 7 Sedimentation volumes in cubic centimeters of loosely packed pastes which are strongly thixotropic, passively dilatant, and not plastic

1 Mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gypsum (oven-dried). ...................... Pure CaCOs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iceland spar.. . . . . . .. Galena, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Water

8.45 7.5 6.7

1

NHC1

8.5 6.9

1

NNaOH

1

NNaCl

8.25

8.0

7.05 8.4 10.9

7.1

1228


showed passive dilatancy, distinct thixotropy, and no plasticity, clear liquid being pressed out of them. Of these suspensions, gypsum (air-dried sample) has a fairly wide thixotropic range in water, 46 to 67 per cent solid, but the rest showed quite a limited range, for instance, mosaic gold in water, 52 to 60 per cent solid, and aragonite in water, 60 to 67 per cent solid. The rule under discussion obviously simplifies matters too strongly. The following points have still to be considered. TABLE 8 Sedimentation volumes in cubic Centimeters of strongly plastic pastes which are also strongly thixotropic6 8UBBTANCE

Pyrolusite .................................

I

LIQUID PHASI1

1

Water

I

4.6

1

1

NHCl

4.4

1 NNaOH 4.4

I

NNaCl

4.6

TABLE 9 Sedimentation volume in cubic centimeters of thixotropic pastes which have a medium OT small volume of sedimentation, and which are passively dilatant and not plastic LIQUID P H A 0 l

SUBSTANCE

Water N RCl ~

Mosaic gold.. .............................. Gypsum (air-dried). . . . . . . . . . . . . . . . . . . . . . . . Gypsum (oven-dried) . . . . . . . . . . . . . . . . . . . . . . . Pure CaCOa.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limestone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aragonite., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marble. ....................................

6.0 5.3 5.3 5.3 3.65 3.4

5.2 5.9

N NsCl -

N NaOH

4.8 4.7 5.5 6.4 5.75 4.55

5.8 4:8 6.35 5.4 5.6 3.4 4.2

In order that the pastes should show normal dilatancy, the particles should be quite independent of each other; then they are able to return spontaneously to the state of closest packing after they have been displaced. Particles of pastes with normal dilatancy (tables 4 and 6) are Its sedimentation volume could 8 Steatite must probably be added t o this group. not be determined with certainty, owing to the very stiff foam which was formed and could not be broken. The plasticity measurements were also not well defined, obviously owing to the poor wettability of the powder. Pastes of selenite also gave only with high pressures well-defined threads when plasticity was measured

-

1229

PROPERTIES OF CONCENTRATED BUBPENBIONB

indeed very independent of each other. This can be seen in their behavior in the liquid medium under the microscope. They move about independently, and there is no attraction between them; if they come to rest by collision with each other, a slight disturbance can separate them and set them in motion again. The independent nature of the particles belonging to this group of pastes is also shown by their velocity of sedimentation. The particles settle quite independently, according to Stokes' law :

U = velocity of fall, g = acceleration due to gravity, a = radius of particles, p = density of particles, u = density of liquid, and q = viscosity of liquid. If the suspension is sufficiently homodisperse, a, and thus U , is constant.

MARBLE IN WATER

0

20

40 60 80 100 TlflE IN MINUTES

I20

140

w z

0

20

40 60 80 TlMC IN MINUTES

100

110

140

FIG. 2 FIG. 3 FIG.2. Sedimentation curves of porcelain, corundum, and fluorspar FIG.3. Sedimentation curves of marble and aragonite

'

In figure 2 the abscissa is the time in minutes, andLthe ordinate the volume of sedimentation in cubic centimeters, which is proportional to the space through which the particles have fallen. In the figure there are three curves for substances belonging to this group, porcelain and corundum in N hydrochloric acid solution and fluorspar in water. They are all straight line? approaching the abscissa; Le., the tangent of the angle between the line and the abscissa, which is proportional to the velocity of sedimentation, is constant. But the fact that a suspension has a small volume of sedimentation, that is, less than 4.0 cc., is not always an absolute proof that the particles are fully independent of each other, and that the packing is vem close. Particles of aragonite and marble showed an anomalous behavior, in so far as they have a small volume of sedimentation in water and sodium chloride solution, but no normal dilatancy and a distinct thixotropy. It was found that their particles were not absolutely independent of each other when

1230

H. FREUNDLICH AND A . D. JONES

they were observed under the microscope. They formed clusters, generally with a large particle in the center, and smaller particles stuck round it. These clusters appeared fairly independent of any neighboring clusters. Their curves of sedimentation were also not so simple as in the first case mentioned (cf. figure 3). This tendency to form clusters was found with the particles of all pastes iiientioned in tables 7 and 9. I t is readily understood why these pastes showed passive dilatancy. The liquid in the system is probably associated in part with the clusters, and the rest is free; evidence for this is obtained from experiments with the plastometer, when iu all cases with the pastes under consideration clear liquid was pressed out; that is, the free liquid was expressed, and this aniounted to about 50 per cent of the original amount. In the passively dilatant paste$, the clusters are buoyed up by the liquid associated with them to such an extent that vrtien determining dilatancy the clusters do not assume a w r y close packing, and 110 liquid appears

j 7

I14

m

-

K TIN YY2ITES IX N NaCl

6

0

23

40

63 60 TIME IN MINUTES

130

I20

If40

FIG.4. Sedimentation curves of pastes which are not dilatant but strongly thixotropic and plastic

when the past? is left to itself. But when the vessel containing the paste is tapped, the solid phaw is encouraged to separate from the liquid, and the free liquid appears on the a r f a c e . The clusters do not break up, and t,he phenomenon is reversible and may be repeated. In pastes which are not dilatant but strongly thixotropic and plastic (tables 5 and 8), the particles aggregate to a much higher degree, ?s is shown in the microscope. They form a net,mork through the system, and are all stuck together; there is no separation into clusters, and no motion.? With systems of this kind there are very irregular curves of sedimentation velocity (cf. figure 4). These are the cases in which the particles cannot 7 This difference in behavior--coherent network of clusters and separate clusters, respectively-is also shown by “short” and “long” suspensions of carbon black in hydrocarbons (cf. R. 0. Neal and G. St. Perrott, “Carbon Black, its Manufacture, Properties and Vses,” Rulletin 192 of the T3urPau of Mines, p. 77 e t seq., Washington

r(19.22)).


1231

be separated by fractionation. It is obvious that this strong aggregation favors the enclosure of large amounts of liquid, and thus causes a large volume of sedimentation, and thixotropy. There 9 indeed no exception to the rule that pastes with a large volume of sedimentation are always strongly thixotropic. But the sedimentation volume need not be very large to cause a certain degree of thixotropy. If the sedimentation volume of the suspension is only a small amount greater than that of the most closely packed sediment, there appears a certain degree of thixotropy. Corundum, for instance, was not thixotropic in N hydrochloric acid solution, and its sedimentation volume was 3.75 cc.; but in water it was thixotropic and had a sedimentation volume of 4.45 cc. This small numerical increase of 0.60 cc. goes parallel with an increase in thixotropy. The following observation on the variation of behavior of quartz parti cles in different liquid media assists this assertion. Quartz in water was not thixotropic. Quartz particles in water suspension on a microscope slide and viewed under a microscope were quite independent; they moved about, and gradually came to rest, and the smaller particles kept on moving long after the larger ones had ceased. By gently moving the cover glass, the system was disturbed and broken up, and it had to settle down all over again. Quartz in N sodium hydroxide solution was thixotropic; quartz particles in this medium under the microscope moved about, but came to rest in a very much shorter time than in water, and there were no small particles left floating about. The system manifested attracting forces between the particles] and they seemed to cluster together. After the sysdem had been disturbed by gently moving the cover glass, small particles rapidly made their way to the larger particles, and some could be seen to move with an acceleration, and attach themselves with a click. This small change in behavior is sufficient to produce thixotropy; according to tables 4 and 6, the corresponding change in sedimentation volume for quartz is 'from 2.0 cc. in water to 3.4 cc. in N sodium hydroxide solution. Here is then a further point, not expressed in the rule as originally specified, that although a large volume of sedimentation is always correlated with strong thixotropy, a large excess of liquid is not necessary to cause the latter phenomenon. Distinct thixotropic behavior may appear if the amount of liquid between the particles is not much larger than when the packing is very close. The volume of sedimentation need not be large. That is why a certain degree of thixotropy is so extremely frequent. The dilatancy of these pastes and the manner in which they behave in the plastometer show that the liquid may be bound to the particles in a way varying strongly from paste to paste. If the liquid exudes spontaneously as in strongly dilatant pastes, it is bound very loosely, or only to a

1232


rather small amount of the particles. The fact that, in the experiments on plasticity, the liquid is pressed out of the paste more or less easily proves that the liquid is readily separated from the particles. If the pastes show passive dilatancy, the liquid is part free, and part bound on to the clusters, and only the free liquid is squeezed out. Here the particles have a marked affinity for each other, they allow clear liquid to be expressed (tables 7 and 9), and the residue is a hard, rather dry cake. In the case of the pastes mentioned in tables 4 and 6, the affinity between the particles is small, with the result that the particles pass out with the liquid without keeping up a coherent structure, and the nature of the extrusion is pasty drops. In the plastic pastes (tables 5 and 8), the coherence is probably due to a mutual affinity both between the particles themselves and between the particles and the liquid. It is a drawback in the discussion of this problem that the behavior of sedimentation volume is so very complex and not sufficiently known. Some points on this theme have been already mentioned; a further one should not be neglected. There is perhaps a certain tendency to assume that, at least in aqueous pastes, the electrical charge on the particles outweighs all other influences in determining the sedimentation volume.* If the particles repel each other because they are markedly electrically charged or have a high r-potemial, they remain independent and are able to glide over each other until they reach the closest packing. If the particles do not repel each other strongly, they stick to each other, that is, they are coagulated; in consequence they enclose large amounts of liquid, and this causes loose packing. The t-potential generally has a maximum in pure water, or in a small excess of OH- ions, if the particles are naturally negatively charged, or of H+ iom, if the particles are naturally positively charged; it decreases more or less strongly in solutions of neutral electrolytes, or in more concentrated solutions of acids and alkalis. We actually frequently find a minimum of sedimentation volume in pure water, for instance, with the following substances,-jet, quartz, rutile, vanadium pentoxide, Monax glass, porcelain, pure calcium carbonate, limestone, Iceland spar, marble, mica, kaolin, steatite, chalk, and Solnhofen slate. But there are also marked deviations from this normal behavior. A maximum of sedimentation volume in pure water was found in the case of the following substances,-corundum, mosaic gold, gypsum, selenite, and pyrolusite. A smaller volume in N acid solution was found with diamond and fluorspar, and a smaller volume in N sodium chloride solu8 The following reasoning is especially due to Ehrenberg (Bodenkolloide, p. 83 e t seq., Dresden (1918)) and yon BuzAgh (Kolloidchern. Beihefte 32, 114 (1930)), but they also take into account the influence of hydration.


1233

tion was found for aragonite. This behavior needs thorough investigation. It is known that the volume of sedimentation may vary strongly with quartz in organic liquids where the electrical charge is of small importance (7,5). Probably the question of solvation can also not be neglected in the case of aqueous solutions. There appears to be no correlation between the f-potential of particles of different chemical nature, and their behavior concerning sedimentation volume, etc. For a number of particles, the f-potential was determined by measuring the electrophoretic velocity, using the well-known microscopic meth~d.~ Particles whose volume of sedimentation was small and whose pastes were dilatant and not thixotropic had large values of f, e.g., quartz, -54 millivolts; diamond, -50; corundum, -58; but also in the case of particles which had a large volume of sedimentation, and which formed strongly thixotropic and plastic pastes, large f values were obtained, e.g., graphite, -52 millivolts; hematite, -35. Most of the particles when observed under the microscope were isometric, i.e., roughly spherical or cubical in general outline. A number of substances were definitely anisometric. The particles were plate: in mosaic gold, mica, graphite, kaolin, selenite, and steatite, and were needles in hematite; the particles of gypsum which had been dried in the oven recrystallized from the water in a few minutes into long needles also. The special shape of these particles was also shown by other properties. As was mentioned above, mosaic gold, mica, graphite, and to a certain extent, kaolin also, changed their sedimentation volumes strongly when the pprticles were oriented. In a number of dilute suspensions of mica, selenite, steatite, graphite, and mosaic gold, beautiful streaks were observed on stirring, a phenomenon which is known to be characteristic of suspensions of non-spherical particles (1). All pastes with anisometric particles were thixotropic. None showed normal dilatancy, but some (mosaic gold and mica) showed passive dilatancy, and were not plastic. The hardness1° of the particles proved to be undoubtedly important in the determination of their behavior. Nearly all the substances having a strong tendency to be closely packed (tables 4 and 6) were hard; for example, diamond, 10;corundum, 9;quartz, 7; porcelain, 7; glass, 6, or were fairly hard, litharge, 5-5.5; and fluorspar, 4. Most of the substances whose pastes were loosely packed, and which, therefore, were thixotropic and plastic (tables 5 and 8) were soft, such as graphite 0.5-1;selenite 1.5-2; and steatite 1; or not very hard, galena 2-3; pyrolusite 2-2.5; 9 These experiments are perhaps not quite conclusive, as dilute suspensions are used, whereas all the properties we are investigating are determined with very concentrated suspensions. 10 Hardness is given in the scale of M o b .

1234

H. FREUNDLICH AND A: D. JONES

kaolin 2-2.5; and barytes 3-3.5. An exception was hematite, which was fairly hard (5.5-6.5), but whose pastes were loosely packed, thixotropic, and plastic. A number of substances containing chiefly calcium carbonate but of different crystalline structure and of different degrees of purity were compared: pure calcium carbonate (hexagonal), aragonite (rhombic), limestone, Iceland spar, marble (all three hexagonal), chalk, and Solnhofen slate. The last two contained a certain percentage of impurities insoluble in hydrochloric acid, mainly of a clayey nature; they amounted to 1.23 per cent in chalk (average of two analyses), and to 5.89 per cent in Solnhofen slate (also two analyses). The fairly pure substances behaved very similarly; all showed passive dilatancy, distinct thixotropy, and no plasticity, pure water being extruded. The presence of clay in chalk and Solnhofen slate made the pastes of these two strongly thixotropic and plastic. The volume of sedimentation varied to a certain extent; it was of medium value with pure calcium carbonate, limestone, chalk, and Solnhofen slate, smaller with aragonite and marble, and rather larger with Iceland spar. Pastes of gypsum are known to be strongly rheopectic (4). This was confirmed. We found further, that a number of other pastes closely related to gypsum were also rheopectic; for instance, pastes of selenite, all sorts of calcium carbonate (pure calcium carbonate, limestone, Iceland spar, marble, Solnhofen slate), and steatite. It is obvious that rheopexy can only be observed if the time of thixotropic solidification is sufficiently long, at least several minutes; rheopexy cannot be distinguished from thixotropy when the time of solidification is too short. In the way the range of thixotropy was determined, the range of rheopexy extends almost right up to,the lower limit of thixotropy; for instance, the range of thixotropy in water for air-dried gypsum was from 46 t o 67 per cent solid, and the range of rheopexy was from 40 to 46 per cent solid; for pure calcium carbonate in water, the range of thixotropy was from 54.5 to 67 per cent solid, and that of rheopexy from 50 to 52 per cent solid; for Solnhofen slate in water, the range of thixotropy was from 40 to 60 per cent solid, and that of rheopexy from 37.5 t o 39 per cent solid. The lower limit of the thixotropic range was chosen so that the paste solidified spontaneously in a short time, one minute or so. Some of the more dilute pastes which fell into the rheopectic range also solidified spontaneously if left for a sufficiently long while, whereas in other pastes the two phases separated before the system solidified. SUMMARY

In order to test the rule that close packing is joined to a small volume of sedimentation and t o dilatancy, but not to thixotropy and plasticity,

PROPERTIES O F CONCENTRATED SUSPENSIONS

1235

whilst loose packing is joined to a large volume of sedimentation, to thixotropy and plasticity, but not to dilatancy,a large number of pastes, made of finely powdered solid substances (thirty different substances, mostly minerals) mixed with water or a few aqueous solutions, were investigated. Sedimentation volume, dilatancy, and thixotropy were determined by the methods generally applied: plasticity by a device similar to the one used in measuring flow pressure. Pastes were considered to be plastic if they were pressed out of a fine hole in coherent threads; in non-plastic pastes the liquid was pressed out in drops. The following results were obtained: 1. The rule holds in a large number of cases, but there are a number of exceptions in so far as thixotropy may be observed when the volume of sedimentation has medium values or is even rather small, and further when there is no marked plasticity. 2. There appears to be a strict correlation in so far as normal dilatancy is observed only when the volume of sedimentation is small and as plastic pastes are always thixotropic. 3. It was not sufficient to distinguish dilatant pastes from non-dilatant pastes. Some pastes showed the following behavior: they did not become distinctly harder when mechanically treated with a spatula and they did not let liquid exude when left to themselves. In so far they appear to be non-dilatant. But liquid did exude when the basin which contained the mechanically treated paste was gently tapped. This kind of dilatancy was called passive dilatancy. 4. There is a group of pastes uniting the following properties: generally a medium volume of Sedimentation, passive dilatancy, distinct thixotropy, no plasticity. 5 . The degree of independence of the particles is the best criterion for judging the probable behavior of their pastes. Independent particles have no tendency to unite into clusters when seen under the microscope; such particles always form systems according to the closely packed systems of our rule. If the particles are not independent, they are seen under the microscope to unite in clusters. These clusters may unite into continuous groups. Particles which are not independent always give pastes which are to a certain extent thixotropic, but they need not be plastic and their volume of sedimentation may even be fairly small. 6. The degree of independence makes itself felt also in the velocity of sedimentation. Fairly concentrated suspensions of independent particles of uniform size follow Stokes law for a long time of settling. If the particles have a tendency to cluster, a strong anomaly of the velocity of sedimentation is shown during the process of settling. 7. Markedly anisometric, especially plate-like, particles (mosaic gold, mica, graphite, etc.) always give thixotropic pastes. 8. Particles of very hard substances (diamond, quartz, corundum, etc.) TEE JOURNAL OF PEYSICAL CEEYISTRY, VOL.

40. NO. 0

1236


are always strongly independent, and therefore give systems with closely packed particles. REFERENCES

FRIUNDLICH, AND LEONRARDT: Elster-Geitel-Festschrift, p. 453 (1) DIESSELHORST, (1915). (2) FREUNDLICH: J. Soc. Chem. Ind. 63, 218T (1934). AND JULIUSBURGER: Trans. Faraday Soc. 30,333 (1934). (3) FREUNDLICH (4) FREUNDLICH AND JULIUSBURGER: Trans. Faraday Soc. 31, 920 (1935). ( 5 ) HALLER: Kolloid-Z. 46, 366 (1928). AND SHEMTSCHUSHNY: J. Russ. Phys. Chem. Soo. (6) Cf., for instance, KURNAKOW 46, 1004 (1913); Chem. Zentr. 1913, 11, 1725. (7) OSTWALD, Wo., AND HALLER: Kolloidchem. Beihefte 29, 354 (1929). (8) REYNOLDS, OSBORNE: Phil. Mag. [5] 20, 469 (1885); Nature 33,429 (1886). (9) YON BUZAGH: Kolloidchem. Beihefte 32, 124 et seq. (1930).

Sedimentation Volume, Dilatancy, Thixotropic and Plastic Properties of

Recommend Documents