CONTACT ANGLE MEASUREMENT JUDSON L. IHRIG and DAVID Y . F. LA1 University of Hawaii, Honolulu, Hawaii
WITH the tremendous rise during the past decade in the use of surface active agents and protective coatings and film, renewed attention has been focussed upon wetting, "wettability," and allied phenomena. Interest in these effects has been manifest in such diverse fields as lithography, insecticide formulation, metal treating, waterproofing, detergent manufacture, and in the application of plastic films for all purposes. Unfortunately, no completely general index of wetting has yet been found. Perhaps the most widely used criterion for wetting is that of contact angle. The term refers to the angle a t which the phase boundaries of different substances meet. This necessitates the intersection of three phases simultaneously. The contact angle is measured a t this line of intersection. By far the most common type of phase trio encountered is that of gas-solid-liquid. The solid is assumed to have a plane surface and the gas-liquid interface meets the solid surface a t an angle to it. This angle, measured within the liquid by convention, is defined as the contact angle. The interdependence of contact angle and other surface properties has been discussed by Mankowich (I,2 ) . General treatments of the subject are readily available (5,4,5). A close relationship exists between contact angle and , angle has often been surface tension. ~ n f a c t contact defined in such terms as:
where 9 is the contact angle and n, yz, and ys are the interfacial tensions between gas-solid, liquid-solid, and gas-liquid respectively. Equilibrium conditions are assumed. I n all methods devised for contact angle measurement a t least one of these interfacial tensions must be known. Most of the methods heretofore described have very decided limitations. Quantitative procedures have almost always been time-consuming and have employed apparatus with fairly elaborate optical systems and precisely machined parts. Thus a program of contact angle studies has involved considerable preparatory work in setting up equipment. The only rapid, simple procedures, on the other hand, have described apparatus suitable only for results of the "wet" or "no-wet" type. Clearly, then, there is a need for an apparatus capable of a t least semiquantitative results and fair precision, which is rapid in operation, and composed of readily available units. It is believed that the method of contact angle determination described in this paper meets these requirements. Furthermore, it is capable of modification so as to yield any desired degree of precision from control work to research studies. THE WILHELMY METHOD
A method originally devised for surface tension determination hrts been found suitable for measuring cou196
tact angles. This is the method originally due to Wilhelmy (6). Because of the existence of a wealth of simple techniques for measuring surface tension rapidly and accurately, it has been little used although an elaborate modification has been employed by Harkins and Anderson (7) in film pressure studies. The Wilhelmy method involves the determination of the downward pull exerted by a liquid upon a thin plate of solid material which is partially immersed. The plate is suspended from a balance arm. Its apparent weight will be the sum of the actual weight (in air) and the downward force exerted by the surface tension of the liquid minus the buoyant effect of the displaced liquid. Obviously, the vertical pull depends upon the degree to which it is wet and hence upon the contact angle. The fundamental relationship between the weight of the plate partially immersed (wJ and its weight in air (w.) is given by: wi = we
2(a
+ b)r eos 8 - ahhp 9
which rearranges to cos 8 =
(wi - w, 2Aa
+ abhplg + b)
where a and b are the width and thickness of the plate, h is its depth of immersion, and g is the gravitational constant. The surface tension, y , and the density, p, of the liquid must be known or determined independently. The experimental procedure, then, consists of weighing the plate partially in and completely out of the liquid, measuring the dimensions of the plate with micrometers, and determining the depth of immersion with a cathetometer. THE APPARATUS AND PROCEDURE
Figure 1 shows the plan of the apparatus. An old chain-type balance, A, was supported on a simple, rectangular, 2- X 4- inch timber frame put together with bolts. Its gross dimensions were approximately 18- X18- X 23-inches high. Sections are seen a t H and I. One balance pan was removed and the plate, B, suspended by a thin varnished copper wire. The plate could be attached or adjusted quickly by means of an ordinary small stainless steel spring clip, C,of the type used by photographers. The experimental liquid was held in a one-liter beaker, D, which was placed inside an ordinary water bath, J, used to catch overflow from the beaker. Immersion of the sample was controlled by means of the same type of double-gear drive automotive jack, E, used so successfully in a previous investigation (8). I n order to provide a level and steady support for the beaker, the top plate of the jack was ground approximately flat and a 10- X 10- inch steel d a t e was attached. This, in turn, was covered with piece of thin plywood, F and G. The whole jack assembly was
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JOURNAL OF CHEMICAL EDUCATION
bolted to the wood frame after being leveled with shims. Such an apparatus may be constructed and assembled in less than a day. The actual operation of the equipment was extremely rapid and simple. A thin plate of solid to be tested was clamped to the balance suspension wire and weighed. It was also found helpful to attach to the top of the wire a counterpoise weight of approximately the mass of the missing balance pan. Then the beaker of liquid (water in all of the tests made) was raised so as to immerse the plate to a depth of 2-3 em. At the same time the jack was raised, approximate balance was maintained by adjusting the chain. After a final weight adjustment, this new value, wt,was recorded and the depth of the bottom edge of the plate below the water surface was read off with a cathetometer to 0.05 mm. This procedure was repeated once or twice, after which the beaker was lowered and the temperature of the liquid taken. No thermostatting was necessary due to the short time required to make readings. As just described, the results obtained give the socalled "advancing" contact angle. Because of the well-known hysteresis of contact angle, it was desirable to obtain values of the "receding" angle. This was done in the same manner, except that the plate was immersed by raising the jack and then balance was obtained after a slight lowering of the jack. The effect was then one of slightly withdrawing the plate. As might be expected, considerable pains to prevent surface contamination of both water and plate were necessary to obtain reproducibility. The various plates were handled only with clean surgical gloves or forceps. All glassware was soaked in concentrated nitric acid and thoroughly rinsed with distilled water. Immediately before the plate was immersed, enough fresh distilled water was added to the beaker to cause a slight overffowwhich created a new surface. The plates used in this work were of glass, polymethyl methacrylate ("Plexiglas"), paraffin-coated glass, brass, aluminum, and pressed amorphous carbon. The glass plates were standard microscope slides and were cleaned with nitric acid. The plastic plates were made from sheet Plexiglas, cut to 1- X 3- inch dimensions with their edges polished with emery paper and finally with flannel and levigated alumina. They were cleaned with synthetic detergent and thoroughly rinsed with water. Paraffin surfaces were obtained by dipping glass slides into molten paraffin. The paraffin was kept a t a temperature just above its melting point, and the slides were kept in the molten bath for a few minutes in order to heat them all the way through. In this way, thin, fairly uniform films were deposited on the slide. Shorter immersion times produced thick films which later wrinkled, and coating the slides with a benzene solution of paraffin left irregular patches of the solid deposited after evaporation of the solvent. The metal plates were prepared by first grinding all surfaces until the dimensions desired were nearly obtained. They were then polished to mirror finish with emery paper and alumina on flannel. The surfaces were kept planar by backing the polishing media with plate glass. The carbon plate used was porous and demanded special treatment. The absorption and subsequent evaporation of water by the plate gave drifting results. VOLUME 34, NO. 4, APRIL. 1957
Consequently, the plate was first prepared in the same manner as were the metal plates. Then the carbon was submerged in distilled water and reduced pressure applied. The plate was then attached to the suspension wire and allowed to dry until the free surface water had evaporated. (It was assumed that evaporation of absorbed capillary water was a much slower process.) From previous work it was found that the lo& of capillary water by evaporation was a linear function of time. I n practice, then, the weight of the plate suspended in air was taken a t three intervals of time and a plot made, so that the weight a t the exact moment of immersion could he determined by extrapolation. RESULTS
The results of numerous determinations are summariaed in the table. Contact Angles of Water Against Various Materials Plale material
Adoancing Sld. angle dm. (degrees) (degrees)
Receding angle (degrees)
Std. dm. (degrees)
It will be noted immediately that the precision of measurement is least in the case of glass. Here the solid is usually presumed to be completely wetted and hence should have a contact angle of zero degrees, or
nearly so. This is because the change in an angle per unit change in its cosine is very large near zero but falls off rapidly as the angle increases. Thus it appears that the method is most suitable for measuring contact angles appreciably greater than zero. Deviations for the other materials tested are well within the range of those found using other, more cumbersome, methods. Other investigators have reported a range of values for paraffin from 96' to 115" with deviations of the order of lo-7' within each set of data (9,10,11,18,1S). "Best" values appear to be in the range of 106°-1150 for the advancing angle. As appears to be usual with most workers, we have found greater reproducibility with the advancing angle compared to the receding one. A direct comparison of the values obtained for Plexiglas, brass, aluminum, and carbon is not possible. Contact angles for the metals and the plastic do not seem to have been recorded in the literature. The previous history of the pressed carbon block used was unknown. Its angle was found to be about half that recorded for cleaved Ceylon graphite (IS), a much different form of the element, of course. I n any case, the principal object of all these determinations was to check the method rather than to provide results of archival value. ADVANTAGES AND LIMITATIONS
The principal advantages of this method for contact angle determinations are those of simplicity and speed. The apparatus is composed of standard components to be found available in most laboratories and may he put together or dissembled quickly. The measurements themselves may be performed quickly, thus commending the method as a tool for coating or film screening programs involving many repetitive determinations. Tentatively a t least, it may be stated that the results are comparable in accuracy and precision to most other procedures which have been described. Furthermore, it appears that the method can be elaborated to any desired degree by the use of amore sensitive balance or length-measuring devices, even though in its present
form the method appears to he sufficiently refined for most purposes. A final advantage may be cited which suggests the possible use of contact angle determination by this procedure as an experiment in the physical or applied chemistry laboratory. The effect of surface irregularity upon contact angle and, hence, upon solid wetting, may be observed readily by noting any waviness or distortion in the line of solid-liquid-gas contact. Examination of this intersection with a hand lens or even with the naked eye shows up such irregularities very clearly. There are two major limitations. First, is the fact that experimental errors are most significant when the contact angles are small. For applications with solids of low wettability, however, the precision appears to be satisfactory. Inherent in the method itself is the limitation that the solids employed must be in the form of thin laminae. This, of course, is no real restriction in the testing of materials for the coating and protection of solid surfaces. LITERATURE CITED (1) M A N K ~ ~ IA.~ M., H , Ind. Eng. Chem., 44,1151 (1952). (2) M~NKowrcn,A. M., Ind. Eng. Chem., 45,2759 (1953). ( 3 ) ADAM,N. L., "The Physics and Chemistry of Surfaces," 3rd ed., Oxford University Press, London, 1941. (4) BIKERMAN, J. J., "Surface Chemistry for Industrial Research," Academic Press Inc., New York, 1948. (5) SCHWARTZ, A. M., AND J. W. PERRY,''Surface Active
Agents, Their Chemistry and Technology," Interscience Publishers, Ine., New York, 1949. (6) WILHELMY, L., Ann. Physik, 119, 177 (1863). ( 7 ) HARKINS, W. D. AND T . F. ANDERSON, J. Am. Chem. Soc.. 59, 2189 (1937). (8) IERIG,J. L., AND R. G. CALDWELL, J. CHEM.EDUC.,32,320 (19.55).
(9) A E ~ E T T :R., Phil. Mag., 46, 244 (1923). (10) ADAM,N . K., AND G. JESSOP,J. Chern. Soe., 127, 1863 (1925). (11) BOSANQUET, C. H., AND H. HARTLEY, Phil. Mag., 42, 456 !,09,, ,A
