A new method for determining the surface tension of liquids has been derived. This involves the consideration of the advancing and receding contact angles of a liquid drop on a tilted solid surface. The theory has been tested by an improved optical projection technique for a variety of liquid/ solid systems and the results obtained are in agreement with the accepted values. It is shown that the advancing and receding contact angles are characteristic constants of liquid/solid system s and the calculated and measured values of the minimum receding angles are in agreement. The prevailing views of ‘hysteresis’ effects or ‘stationary’ contact angles which have arisen to account for the data available are incorrect and the discordant experimental results reported are due to inadequate technique. The difference between the adhesions corresponding to the advancing and receding angles is ascribed to the work done in removing an adsorbed unimolecular layer. The work done in gcal./mol. in forming this adsorbed layer is in reasonable agreement with that expected from studies in gas/solid systems and the forces involved are van der Waals’. Further, different solids that might be expected to show similar surface structures yield similar values for the work done. The variation in the value of the advancing angle in some liquid/solid systems and its constancy in others is reconciled with the polar character of the solid surface, i.e. it is suggested that short-range forces are involved. It has been found that monolayers of ferric stearate on glass are orientated with their hydrocarbon tails away from the interface in agreement with electron diffraction measurements. It is suggested that the methods may be useful for investigating the structure of monofilms and built-up layers of monofilms.

Frequently we experience the existance of adhesive forces between small particles. It seems natural to ascribe this adhesion for a large part to London-v.d. Waals forces. To obtain general information concerning their order of magnitude the London-v. d. Waals interaction between two spherical particles is computed as a function of the diameters and the distance separating them. A table is calculated which enables numerical application of the formulae derived. Besides approximations are added, which may be used when the distance between the particles is small. In a separate section it is investigated how the results must be modified, when both particles are immersed in a liquid. Here we are led to the important conclusion that even in that case London-v. d. Waals forces generally cause an attraction.

It has been suggested in a previous communication (3) that widely variant surface energies may exist at closely adjoining points on a surface. Well-substantiated theory as to the surface structure of solid catalytic materials is in accord with thisview (7). The “active patches” on the catalytic surfaces are an extreme example of irregularity in the surface energy, but it seems reasonable to suppose that such irregularities may exist to a lesser degree in nearly all ordinary surfaces. Photographic evidence in support of this proposition appears in the work of Wark and Cox (9), who found that the same air bubble under a mineral surface wet with water might have an angle of contact on the right side different from that on the left.

Instead of measuring the contact angle directly, it may be calculated from the dimensions of the drop. The angle so obtained may be regarded as the integral of the sum of all the various contact angles existing along the circumference of the drop. Thus each determination yields an average result not unduly influenced by irregularities at a given point on the surface. For precise determinations the method should have an especial advantage over the usual procedure of direct measurement, because the error in personal judgment involved in drawing the tangent to the curved drop surface at the point of contact is eliminated. This error becomes increasingly important as the contact angle approaches 180, while the dimensions of the drop may be measured with the same degree of accuracy as before.

In the waterproofing of light-weight I woven or knitted fabrics, it is generally essential to preserve the airporosity of the material. The waterproofness that can be effected is therefore definitely limited by the size of the openings, because water will readily pass through if the pressure behind it is sufficient to break the surface film across the openings. Water will penetrate, however, at a much lower pressure or even against pressure, if it can spread over the surface of the threads from one face of the cloth to the other. The waterproofing of open fabrics, therefore, presents the problem of preventing this spreading of water over the thread surfaces. The desired effect is attained by depositing on the fabric some chemical substance that has of itself this ability to resist wetting.

For practical reasons, preparations intended for use in waterproofing open fabrics commonly consist of emulsions. In these preparations the active water-repellent agent is combined with other ingredients whose presence is required to ensure the desired fluidity and stability in the emulsion, to provide proper pH control, to increase the permanence of the proofing effect, and to modify the appearance and feel imparted to the finished fabric. These auxiliary constituents may impair, or they may enhance, the effectiveness of the proofing treatments. The complexity of the problem thus presented makes it desirable to study carefully the wetting characteristics of materials selected for this use.

The present methods of measuring contact angles all require that the solid material be obtainable in some special shape, such as a flat plate or capillary tube. Many surfaces, for example, those of plant materials, occur in irregular forms and must be dealt with in situ, because of the inhomogeneity of the body. The chief value of the method herein described is that its applicability is largely independent of the form of the solid surface. Some of the earliest determinations of contact angles were made from measurements of the dimensions of bubbles and drops. The work has been confined to large drops, but the use of very small drops may be shown to possess several advantages . . .

Although many thousands of measurements have been made to determine the pull necessary to detach a ring from the surface of a liquid, it is a surprising fact that until three years ago there was no “ring method” for the measurement of surface tension. Thus in “International Critical Tables,” nine experimental methods for surface tension are listed but a ring method is not included, since the procedure which had been designated by this term did not supply even one single measured value of surface tension of these tables.

The failure of the ring procedure was due to the fact that the theory had not been sufficiently developed to permit its use as a method of measurement, although an incomplete theory had been developed by Cantor,¹ Lohnstein,² Lenard,³ Tichanowsky⁴ and MacDougall.⁵

In 1926 Harkins, Young and Cheng,⁶ on the basis of the well-justified assumption that the capillary height method, properly applied, gives correct values for the surface tensions of suitable liquids, showed how the ring procedure could be used as a moderately accurate method for such measurements. In the present paper the method is given a still higher degree of accuracy (about 0.25%).

Penetration of Liquids into Cylindrical Capillaries.—The rate of penetration into a small capillary of radius r is shown to be: dl dt= P (r 2+ 4 ε r) 8 η l, where P is the driving pressure, ε the coefficient of slip and η the viscosity. By integrating this expression, the distance penetrated by a liquid flowing under capillary pressure alone into a horizontal capillary or one with small internal surface is found to be the square root of (γ rt· cos θ 2 η), where γ is the surface tension and θ the angle of contact. The quantity (γ cos θ 2 η) is called the coefficient of penetrance or the penetrativity of the liquid.

Surface tension is probably one of the most difficult phenomena to measure. Although a great deal of ingenuity has been spent for almost a century in devising accurate techniques, the figures obtained deviate more from each other for the same substance, according to different authors, than any other constant characterizing the substance. It is well ,known that the two classes of methods of measurement, the static and the dynamic give entirely different results when applied to the same liquid.

Dr. Young’s principal objects in this paper are to reduce the phenomena of the capillary action of fluids to the general law of an equable tension of their surfaces; to investigate the properties of the curves resulting from this law; to determine the magnitude of the apparent adhesion of solids to fluids, and the cohesion of moistened solids; and to show in what manner the law itself is probably derived from the fundamental properties of matter. Dr. Young observes, that a fluid which is not capable of wetting a given solid, forms with it an angle of contact which is constant in all circumstances; that the curvature of the surface of a fluid, or the sum of the curvatures, where the curvature is double, is always proportional to the elevation or depression with respect to the general surface, and that the curve itself admits, in all cases, an approximate delineation by mechanical means, but is not always capable of being easily subjected to calculation. When, however, the curvature is simple, the direction of the surface, at any given height, admits a correct determination. Hence the elevation of a fluid in contact with a given surface, whether vertical, horizontal, or inclined, is deduced from its ascent between plates, or in a tube, of the same substance; and the result is shown to agree with experiments. Thus, taking 1/25-th of an inch for the diameter of a tube, in which water rises to the height of an inch, it is inferred that the apparent adhesion of water, to a square inch of any horizontal surface capable of being wetted by it, must be 50½ grains, which is only half a grain more than the result of Taylor’s experiments. The adhesion of alcohol, and of sulphuric acid, as measured by Achard, are also found to agree with the ascent of those fluids in capillary tubes. Lord Charles Cavendish’s table of the depression of mercury in barometer tubes, is compared with the same principles by means of diagrams constructed for each particular case; and the apparent adhesion of the surface of mercury to glass, as well as the depth of a portion of mercury spread on a plate of glass, is deduced from these measures, and is shown to agree with experiments. The observations of Morveau, on the attraction of the different metals to mercury, are also discussed; and some remarks are made on the magnitude of drops of various substances.