SOME OBSERVATIONS CPON WETTING POWER* BY E. L . GREEX**
Introduction Xhen an orchard is to be sprayed with a liquid insecticide the readiness of wetting becomes an important consideration. If the spray is intended t o cover plant organs with poison and kill or repel insects seeking to bite them, the liquid must wet the plant. If the purpose is to place a toxic substance upon the insect's body, the insect itself must be wetted. This has been discussed a t some length by English ( I ) , * * * Stellwaag ( z ) , and others. Since spraying operations are finished for a given area in an extremely short time, the spray liquid must wet its object rapidly. Insects will escape and do their customary mischief, if for any reason areas are not covered by the spray, even though the insecticide possesses satisfactory toxicity.
FIG.I . Taken from Freundlich
(11)
among others.
I n the studies on the chemistry of oil sprays carried on at the Washington hgricultural Experiment St'ation since I 923, the importance of wetting power has often recurred, but it mas not thought possible to measure it. Within the present year, however, English ( I ) has published a description of a method, by which, with a simple apparatus, a measurement can be taken on a liquid and a solid which appears to define the intensity of wetting. The measurement is of the angle of contact. A treatment of the theory of wetting and the angle of contact that has been used in various other texts is given by Freundlich ( I I , p. I 5 7) and may be reviewed briefly. Fig. I illustrates a drop of liquid resting upon a solid. The edge of the drop may be considered a series of ultimate particles of liquid, as at P, which are subject to tension in three directions; due to the surface tension of the liquid, TI, the surface tension of the solid, T2, and the liquid-solid interfacial tension, TI?. * Published with the approval of the Director of the TVashington Agricultural Experiment Station as Scientific Paper S o . 154, College of Agriculture and Experiment Station, State College of Kashington, Pullman, Xash. * * Assistant Chemist, Washington Agricultural Experiment Station, Pullman, Wash. * * * Xumbers in the text refer to the bibliography at the end of the paper.
922
E. L. GREEN
I t is now to be assumed, or conditions so regulated that the position of P upon the surface of the solid is determined by these tensions and no others. Then for equilibrium: T, = T,, T~cos e
+
Assuming, as do many authors, among whom Quincke (14) appears to have been the first, that these three tensions define the tendency of the drop to spread over the solid; the angle 8 may be used to measure that tendency. 0 may not be less than zero nor greater than r ( I~o’),and over this range the wetting power is great for small angles, being greatest when 8 is zero; and small for large angles. It reaches a minimum when 8 is 180°, for which it has been said to be zero and theoretically to touch the solid a t only one point. Many authors associate “perfect wetting” and a zero angle of contact, in which case the drop spreads out in a thin film over the solid surface.
FIG.2. (Taken from English)
Fig. z illustrates two hypothetical cases. The adherence of the drop to the solid is manifestly greater in h than in B. To be valid for discussions based on this derivation the angle of contact must be measured Khen all other forces are prevented from affecting it. Also that portion of the liquid-air interface that is affected by no other forces in any case and displays the contact angle is extremely narrow. The rest of the surface of the drop is a smooth curve. To avoid some of these difficulties the method of the rotable solid surface was devised (6). The apparatus includes a wide-mouthed container for the liquid; a means for holding, raising, immersing and turning a specimen of the solid; and a device for measuring the angle which the surface of the solid makes with the horizontal free surface of the liquid. A complete description of a suitable apparatus is given by Stellu-aag ( 2 ) and by English ( I ) . For the purpose of the method, it is assumed that when a solid is partially immersed in a liquid, the free surface of the liquid always meets the surface of the solid in a definite angle of contact, fixed by the properties of the liquid and solid, no matter what angle the solid surface may make with the horizontal. Thus a portion of the surface of the liquid is generally compelled to
OBSERVATIOXS O S WETTING POWER
923
incline itself to the horizontal, according to this relation; and the surface joining this portion to the horizontal free surface assumes a smooth curve. Then if the solid is turned an inclination of the solid can be found for which the portion of the liquid surface constrained to turn with it will be horizontal. KOwarping would occur in the surface of the liquid, since it is now horizontal throughout, and this condition discloses the endpoint. The inclination of the solid surface to the horizontal is now the angle of contact, and is to be measured by any convenient means. The following diagram illustrates the operation.
FIG.3.
(Taken from English)
(I).
I n h is shown the condition when the angle of contact is 90'. In B it is greater than 90' and for the measurement the solid must be rotated to a position such as is shown in C. I n D is shown the endpoint for an angle of contact less than 90'. I t is to be added that it is best t o arrange the apparatus so that the axis of rotation coincides with the line of intersection of the solid surface and the horizontal plane determined by the free surface of the liquid
Experimental
As a part of the study of oil spray materials, an extension of the data and observations of English and Stellwaag to the materials used in Wenatchee was undertaken. An apparatus was constructed as illustrated in the accompanying photographs, after IZ study of the two reports available and of the experimental conditions. Some departures are evident, but the principle was carefully retained. The principal part of the apparatus is an axis long enough to turn steadily and without lateral motion. A handle by which to turn it was fastened at one end, and a t the other was attached an arm by which an ordinary object clamp could be carried across the side of a vessel high enough to interfere with the axis itself. The axis itself was made hollow and cross-hairs were fixed in the object end. Thus with a mirror behind the jar the object could be shifted in the clamp and the height of the axis regulated with the rack and pinion of the microscope stand so that the line of sight, now also the axis of rotation, passed near the line of contact of the solid and liquid. This adjustment completed, the mirror was removed and a white background placed behind the jar for the observation. The liquid was illuminated by lamps so placed that the surface was thrown into sharp relief and the observation was made by means of a three times magnifying telescope. To measure
924
E. L. GREEN
OBSERVATIONS ON WETTING POWER
925
the angles, a graduated circle was attached to the support and a pointer was fastened to the handle so as to travel over the circle as the object was turned. These arrangements made the location of the end point possible to within a degree or so. There remained, however, some difficulty in the location of the beginning-point, namely, the reading of the protractor when the surface of the solid stands exactly vertical. Due to irregularities of the object this could not always be depended upon to be 90 degrees. The cross hairs and other arrangements mentioned mere able to assure a minimum of displacement of the object during rotation, but could not fix its direction. Since it had already been decided to use apple twigs and glass rods for objects and these have approximately parallel sides, it seemed reasonable to simply read the end point first on one side and then on the other, and determine theta as half of I 80 degrees minus the difference between these readings. When the apparatus had been constructed it was turned over to Mr. James hlarshall, a graduate student in entomology, to be used in the collection of data for comparison with field trials and with the surface tensions of the same solutions. The data remain in his hands and may be published later. From the first, however, he claimed that the end-point lacked definiteness. Improvements in the visibility and in the mechanical precision of the apparatus did not remove the objection. He finally stated that, although there appeared to be an end-point, he could still see a thin sheet of liquid above the line of contact on the object, whether glass or twig, and could do nothing to prevent its presence there. This is, of course, altogether contrary to the behavior required by the theory at the end point.
Review of Literature This statement was found to be true and a study of the present state of the theory upon which this method is based was begun. A curious survival of the method in spite of very serious objections was disclosed. For example, the angle of contact of water with glass has been definitely decided to be zero and has hardly been a subject of controversy for several years. Yet this set-up I t was noticed, however, indicates for it a value in the neighborhood of 30'. that whenever an end-point was established, examination of the water surface under still higher magnification would discover yome curvature remaining in the water surface. A brief review of the literature on the angle of contact and related discussions leads back to the classical discourses on surface energy and surfaces by Laplace, well over a hundred years ago. h review occurs in Poynting and Thomson (8 pp. 173 ff). A s long as these treatments are found to be valid, a consideration of the surfaces that form the angle of contact would be incomplete without due regard for them. The experimental work on the angle of contact may be thought of as beginning with Quincke (14). He devised the method of measuring the angle of contact of liquids in drops and bubbles resting against solids, and it is believed that he originally the - published derivation, sketched above, of the equation: Cos 8 = Tz - T I ~ TI ~
926
E . L. GREEN
His method, however, seems not to be altogether successful, as shown by Magie (I j). In certain cases it has contributed to the misunderstandings as well as helped to dispel them, and it seems to require more study before it can become very useful. Ramsay and Shields (16)and Lord Rayleigh (I 7 ) became interested in the angle of contact because it introduced an unknown into their determinations of surface tension by the capillary elevation method. Their work definitely indicated zero as the angle of contact of water with glass. But the work of Richards and Carver (9) in 1921 and that of Bosanquet and Hartley (13) in the same year has surely disposed of that question permanently. Taking precautions against evaporation, the one work describes the search for a break in the narrow beam of light reflected from the region about the line of contact. Bosanquet and Hartley allowed a little water to spread over a large glass plate and examined the edge of the sheet of water for a break in refraction. Had a real angle of contact occurred that was greater than 30 minutes of arc they believe they would have found it. Some difficulty has been encountered in tracing the original authorship of the method of measuring angles of contact with the rotable solid surface. English ( I ) ascribes it to Stellwaag ( 2 ) ; Stellwaag credited it to Bosanquet and Hartley (13)or to Sulman ( 3 ) . Bosanquet and Hartley are silent on this point, and Sulman states that it was devised by Dr. H . -4.Wilson and was communicated by A. K. Huntington in the Faraday Society, 190j,under the title, “The Concentration of Metalliferous Sulfides”. Up to the present I have not been able t o read this article. But from the quotations it is evident that it led to disagreements with the more exact methods of the physicists and physical chemists. Instead of taking up the vagaries of the results on biological material like leaves and twigs, which introduce more complicating factors, it may be sufficient to point out that all users of the method find that it gives erroneous results with water and glass. Hence it can not be used in its present form for other liquids and other solids, where the results by this procedure can not be verified by recourse to other methods or previous work.
Discussion I t is self-evident that the finished spray must be capable of completely wetting the object sprayed, leaving no dry areas. S o w wetting is said to occur when an object, on being dipped into a liquid, can not be completely separated from the liquid by the simple process of emergence, for a film of the liquid adheres to the solid. More rigorous definitions are difficult to set up. TTetting occurs, says Freundlich in “Colloid and Capillary Chemistry,” (1922) when the surface tension of the solid is greater than the sum of the surface tension of the liquid and the tension of the liquid-solid interface. Returning to the diagram of Quincke (14),Fig. I, the cosine of the angle of contact assumes its maximum value, one; the angle becomes zero, and yet the surface tension of the solid is greater than the sum of the tensions that oppose it. I t is reasonable to expect, in general, that this indicates a value of the inter-
927
OBSERVATIOKS O S WETTISG POKER
facial tension that is small in comparison with the surface tension of the solid. llillikanz ( 7 ) , twenty years earlier, said that if wetting occurred the angle of contact was zero. The fact that Freundlich used a zero angle of contact t o define wetting and Millikan used the idea of wetting as a foundation for his approach to a zero contact angle shows that an intimate connection was a familiar idea. At any rate, the tendency of the liquid to spread over the solid has brought the angle of contact to an irreducible minimum, and its cosine to an ultinlate maximum, but the tendency itself may not have reached a limit. The angle of contact is a bit of evidence over only a limited portion of the possible range of this tendency; which, according to Quincke’s notation as adopted by Freundlich is from Tz - TIZ= -TI to TP- TI2 = TI. N o w from figures available today TP (the surface tension of the solid) may be a hundred times as great as TI (the surface tension of the liquid). When one of these terms is so large the total probable range must be very much larger than from minus TI to plus TI.
+
The entomologist will probably never be content with wetting that remains within this range. In fact both physical chemists and physicists say, in effect, that where a finite contact angle is observed wetting does not occur. Therefore, nearly all of the spraying operations of today use wetting phenomena not characterized by cont’act angles. From this discussion, wetting power or wetability may be defined as t’he degree in which the surface tension of the solid exceeds the tension of the liquid solid interface. Following this concept there may be positive, negative, slight, or great wetting. Returning to the needs of the entomologist, it will not suffice to supply him with a spray liquid that will eventually spread over the objects to which he seeks to apply it. He works in many cases out in the open orchard; if his liquid is slow in spreading it d 1 drain off the tree and finally evaporate before it has attained complete coverage. He must have as much speed as possible without producing an excessively thin film of liquid; for then he will not be able to place upon the surface enough of the insecticide. Here, possibly, the mobility of the liquid plays a part. I t is difficult to imagine good spraying being done with a viscous liquid. Yet it isslso easy to imagine excessively low viscosity causing a troublesome run-off. The surface tension, also, from an inspection of Quincke’s formula, must not, be too high. However, since water is to used be for the dispersing medium, it is perhaps idle to speculate upon viscosity and surface tension, because from considerations of expedience and expense they cannot be greatly altered. The remarks previously made about the magnitudes of Quincke’s quantities TI,Tz,and TI, are of importance in the case where the surface is contaminated with some other liquid or gas, as with air or grease. The surface and interfacial tensions of these materials are of the same low order as those of water, hence wetting is retarded unless the spray liquid displaces them rapidly. The force with which the spray is applied may be sufficient to take care of the air film. Its action resembles rubbing the liquid into the solid with the fingers.
928
E. L. GREEN
But in the case of certain hairy insects and plant organs the difficulty has not been successfully met. In fact the factors operating to cause the difficult wetting have not been completely explained. The velocity of wetting might be proposed as a definition of “spreading” as the word is used by practical sprayers.
FIG.4 (Taken from Millikan)
There is still a word to be said about the diagram of Quincke. It has been pointed out that surface tension is not in every sense a force, but may also be regarded as potential energy. It is true that a surface acts like a stretched flexible cord. Nevertheless, according to the physicists, all its manifestations are due to forces a t right angles to the surfaces, and can be explained upon that basis. This aspect is neglected in the treatment by Quincke. Also there is the matter of that resolution of the surface tension TI; letting it be considered a force for the moment. What has become of TI sin 0? Why does it not curl up the edge of the drop? It should; for it would appear that it is unopposed except by gravity, which is vanishingly small for the mass a t 0. Perhaps it is taken care of by an attraction of the solid. Then there should be a similar pull upon the adjacent liquid where the surface tension has no opposing effect. It would appear then that the drop is not in equilibrium and will spread over the solid, making the contact angle always smaller.
OBSERVATIOSS ON WETTING POWER
929
h rational consideration of the situation is possible when no forces are considered as capable of causing motion except those at right angles to the surfaces. For the fundamental theory, the following is taken from Millikan (7 P. 187): “It only remains to show why a liquid in a capillary tube assumes a curved surface-a task of no difficulty when it is remembered that a liquid surface can be in equilibrium only when it is perpendicular to the resultant force acting upon its molecules. This fact follows simply from the fact of the mobility of the particles. For, if the force acting upon the surface molecules had any component parallel to the surface, the molecules would move over
FIG.5
the surface in obedience to this component, i.e. equilibrium would not exist. If, then,’O l S (Fig. 4) represents the line of junction of a liquid with a solid wall; f l the resultant of all the forces exerted upon the molecules a t o by such portion of the liquid as lies within the molecular range when the liquid surface is assumed horizontal and f 2 the resultant of forces exerted upon the same by the molecules of the wall which lie either above or below the horizontal line passing through 0, then three cases may be distinguished: “I. That in which f l = 2 f,. I n this case, as appears from Fig. 4 the cohesion of the liquid is exactly equal t o twice the adhesion of the solid and liquid and the final resultant R is parallel with the wall. Hence, equilibrium exists in the condition assumed, i.e., the angle of contact is 90’. “2. That in which f l > z f ? ; the resultant R then falls to the right of 0 1-- - - - - - - - - - - - -
93 0
E. L. GREEK
“ 3 . That in which f l < 2 f 2 . The resultant R then falls to the left of 0 K’. (See Fig. 5 ) . Hence equilibrium can not exist until the surface near 0
has become concave and the angle of contact obtuse.” This is the case of greatest interest in this discussion and it seemed worth while to continue the examination of the resultants of the forces. The angle which Millikan calls alpha is the supplement of the angle theta, and this is called the angle of contact by many writers, especially the physical chemists. It is within the liquid and approaches zero for liquids that wet well and 180’ for liquids that wet poorly. The case of a liquid and a solid whose
FIG.6
angle of contact 0 is less than 90’ is illustrated in Fig. j. For this case, Millikan has shown in the above citation that fl < z f p . This relation between the forces occurs a t the instant when the solid has just been dipped into the liquid, the line of contact is beginning to rise with respect to the free liquid surface, and the angle 0, changing t o attain its equilibrium value, passes through 90’ or A , 2. I t seems reasonable to suppose that, for the same liquid and solid, no matter what value 0 may have a t a given instant, the force exerted upon the ultimate particle of liquid a t 0 by the solid is constant. This assumption is made in what follows and this force is called Fs. I t is the resultant of the two fz’s of Fig. 4, and is equal to l/;f2. The characteristic of the case under discussion is that zf? is quantitatively greater than fl, the
OBGERYATIONS O S WETTISG POWER
931
force cxerted upon the ultimate particle of liquid at 0 by all the other molecules of the liquid that are near enough to affect it at the instant when 8 passes through 90'. Let this difference be A. Then z f Z = f l X = d/;Fs SOTV as theta varies the result,ant of the forces exerted upon the ultimate particle of liquid at 0 by all the other molecules of liquid that are near enough t o affect it may not be constant. The range beyond which these forces may be npglected is represented in the figures by the circle. It has been reported to be between a ten-thousandth and a millionth of a millimeter ( j ) . If this variable force is called FL,then only as 8 passes through and is momentarily equal to 90'; FL = f l . Yew let 8, the angle of contact, diminish by the climbing of the liquid, according to the conclusion for this case reached by Xllikan. Fig. 6 represents a certain instant when it is less than 90' and greater than zero. The liquid within the molecular range of 0 is contained in the segment of the circular cylinder whose base is the segment of the circle cut out by the angle 8 and whose altitude is the unknown, but constant length of the line of contact. Therefore this volume is linearly proportional to 8 expressed in radians. And if the forces summed up in FL are in proportion to the number of molecules exerting them, then FL is likewise proportional to 8. That is
+
FL = C (an unknown constant)
e
= 8,!2,
F~ = f,
=
X
8 and when
cx
As Millikan has shown, FLlies along the bisect,or of 8. It is so represented in Fig. 6. S o w let FL,which is assumed to be constant, be resolved into OS parallel to OW, the line of action of FL,and OR perpendicular to it. O W represents FLin magnitude and direct'ion. From W lay off WT equal to OS. Then the resultant OP of OR and OT represents the resultant in magnitude and direction of those forces acting upon the ultimate particle of liquid at 0 that have been taken into account in this discussion. It makes an angle ROP with OR, whose magnitude is to be determined. By various elementary propositions in geometry the angles LIOW, W O T , SQO and QOR are all equal to 8 ; ' z . Sow let it be assumed that the liquid has attained equilibrium and has ceased climbing, and attempt t o derive an expression for 8 in terms of the constants A and C. As has been quoted above from Millikan, who takes it originally from Laplace, at equilibrium the final resultant O P must be perpendicular to 031, the surface of the liquid; that is PO31 must be a right angle. KOQ and QOR have been drawn to be right angles. Then a t equilibrium the angle ROP is equal to 8 ' 2 . Also O T = OS = TW 2
os
=
OK
E. L. GhEElV
932
And OS =
ow xe - =c -
Substituting and simplifying
If the assumptions made may be justified, and if no other forces need to be taken into consideration, it may be stated that, for given values of A and C, equilibrium will not exist except for some value of the angle 8 that satisfies the equation :
e
has the value r a t 8 = r (180'); 2 . 2 2 0 a t 8 = 7r/z sin e l 2 (90") ; and from this value it decreases slowly but steadily for decreasing values of 8 until, for such small angles that sin e / 2 may be set equal to e/2, it becomes exactly 2 . The right hand side of the equation has the value 2 . 2 2 0 2 X zA/C. Thus, if A were zero, 8 equal to ~ / would 2 satisfy the equation, which is Rlillikan's case I quoted above, under Fig. 4. But if A and C are real, positive quantities, as has been assumed, no value of 8 between 0 and 7r/2 will satisfy it. Thus the tendency of the liquid to climb will not be limited by reaching an equilibrium angle. The inference may be drawn that, as Millikan ( 7 ) shows for "perfect wetting", the liquid will continue to climb up the solid until it becomes a sheet so thin that FLmay be said to vanish. Then the resultant force upon the upper margin of this film of liquid is simply Fs, which is perpendicular to the surface of the solid and thus also to the surface of a thin sheet of liquid adhering to the solid. Then equilibrium, as far as these forces affect the orientation of the surface of the liquid will be established. The liquid may climb still higher than would be required to arrive a t this condition, as may be observed with water upon glass. The solid, in such cases, seems to press the liquid against itself and causes a thin film to rise above the level of the main body of the liquid until it is halted by a combination of gravity and surface tension. Kow the term-
OBSERVATIOYS ON WETTING POWER
933
I t is to be emphasized that these conclusions have been reached by considering only the magnitudes and presumable directions of the cohesive forces of the liquid and of the adhesive forces displayed between the liquid and solid. The employment of the terms “wetting” and “perfect wetting” has been successfully avoided. This is desirable because they have not been clearly defined and are not easy to define, and because different writers have not the same conception of them.
Conclusion The data are not submitted for the reasons given above and further because it has been possible to show that the values obtained depend more on the characteristics of the apparatus than on the properties of the experimental materials. The phrase “if no other forces enter,” and their equivalents deserve some emphasis. I t is known that with exact technique some angles less than 90’ and greater than zero can be found; compare, for instance, Bosanquet and Hartley (13). The possibility of other forces must not be neglected, although for the purposes of this paper it is impossible to evaluate them. The profound modifications of surfaces by small amounts of contamination are well known (4, I O , 12) and for this reason the consideration of the wetting of solid surfaces by liquids such as mixtures, solutions, dispersions and emulsions must be approached with the utmost caution. One of the components may be selectively adsorbed, and thus phenomena observed and referred to one set of causes when an entirely different set are responsible. Finally it must be considered that there must be another fluid already in contact with and perhaps very strongly adsorbed upon the surface of the solid. That fluid is the gases of the atmosphere, and the fact that it is gaseous and not liquid brings about quantitative rather than qualitative differences in its behavior toward surfaces. Gtellwaag ( 2 ) gave considerable thought to \That appears t o be a resistance of the air film to displacement. Thus he says “The bare spots upon leaves which wet can be made to disappear if the water is spread uniformly over the leaf or if it is applied to the surface with great pressure. It then penetrates into the interspaces between the outgroFvths (of the leaf) and after that is carried along by capillarity.” Kow if the angle is changing, such as when the water is “carried along by capillarity” it can scarcely be accurately measured. If a steady state has not been reached when the angle of contact is measured the value obtained will be dependent upon many other factors. I t would appear, upon reflection, that a steady state, or equilibrium, must be awaited, no matter ho\v long that takes. I t would seem that emphasis upon the necessity of a plane surface as a foundation for angle of ccntact measurements is superfluous. Severtheless, according to their own statements, several of the investigators have proceeded to determine the angle of contact of liquids with surfaces such that even remote accuracy in the orientation of the tangent that is to represent the solid plane surface is out of the question. Such are segmented and other-
934
E. L. GREEN
mise convoluted larvae, more mature insects, leaves with ribs, and stems with nodes and scars. Furthermore, they have not allowed for the effects of hairs and other outgrowths that still further modify these surfaces. It was intended a t the start to endeavor to relate these measurements to glass so that glass might be used as a reference surface if it could be shown that parallel variations occurred on tree tissue and glass. This has had to be abandoned for the reasons as summarized below: I. The set-up as described permits the operator to deceive himself into the belief that the angle of contact is much larger than it is unless the observation is confined to an ultimate film of the liquid, no wider than the range of action of the molecular forces and above which is perfect dryness. 2. The question of the establishment of equilibrium enters too largely into the problem. If equilibrium is established when the measurement is taken no information is obtained regarding the ease of replacing the air film and other factors tending to retard spreading. The rapidity of spreading is of as great if not greater importance in spraying than the angle of contact where this is less than 90 degrees. With this method the measurement can not be taken accurately until equilibrium is established. 3. The materials that are to be wetted are not suitable to measurements of the angle of contact. Acknowledgment is gratefully accorded to Dr. R. 0. Hutchinson for suggestions and criticisms in the theoretical considerations.
References E y l i s h , L. L. Some Properties of Oil Emulsions influencing Insecticidal Efficiency”. Illinois State Katural History Survey Bulletin XVII Article V (1928). 2 . Stellwaag, F. “The Wetting Power of Liquid Material used to control Pests on Plants and the direct hleasurement thereof by a S e w Procedure. (Trans. title). 2. angew. Entomologie, 10, 163-176 (1924). 3 . Su,lman, H . Livingstone A Contribution to the Study of Flotation.” Transactions of the Institution of Mining and Metallurgy, 29, 44-204 (1919). 4 . W!!lows, R. S., and Hatschek Surface Tension and Surface Energy” (1923). 5 . Be,‘;‘by Surface Structure of Colloids.” J. Soc. Chem. Ind., 22, 1167 (1903). 6 . Huntingdon, A. K. ‘Concentration of hletalliferous Sulfides.” Faraday Society 1905. Mentioned in Sulman (1919) as containing the first reference to the rotated solid plane method of measuring a contact angle. He credits it to Dr. H . A. Wilson. 7 . Millikan, R. A. “Mechanics, Molecular Physics and Heat,” (1903). 8 . Poynting, J. H., and Thomson, J. J. “Properties of hlatter” (1902). 9 . Richards, T. TV., and Carver, E. K. “A Critical Study of the Capillary Rise Method of Determining Surface Tension,” J. Am. Chem. Soc:, 43, 827 (1921). IO. Harkins, W.D., and Brown, F. E. ‘The Determination of Surface Tension (Free Surface Energy) and the Weight of Falling Drops: The Surface Tension of Jf-ater and Benzene by the Capillary Height Method.” J. Am. Chem. Soc., 4 1 , 4 9 9 ( 1 9 1 9 ) . 1.
OBSERVATIONS ON WETTING POWER
93 5
Frzundlich, H., and Hatfield, H. S. Colloid and Capillary Chemistry” (1922). 12. Harkins, W.D., Davies, E. C. H., and Clark, G. L. “The Orientation of Molecules in the Surface of Liquids, Etc.” J. Am. Chem. Soc., 39, 541 (1927). 1 3 . Bosanquet, C. H., and Hartley, Harold. “Sotes on the Angle of Contact,” Phil. Mag., 42, 456 (1921). 14 Quincke (Measurement of Angle of Contact on Drops resting on Surfaces) (Citation by Richards and Carver) Ivied. Ann., 2, 1 4 j (1877);27,219 (1886); 52, I (1894);61, 2j; (1897);64,618 (1898). II.
‘?%ation by Richards and Carver) Wed. Ann., 25, 432 (188j); Phil. Mag., 26, 162 (1888). 16 Ramsay and Shields Z. physik. Chem., 12, 452 (1893). The Right Hon. John William Strutt, Third Baron Rayleigh I; Proc. Roy. SOC., 9zA, 184 (1915).
