Brian J. S. Pirie and David W. Gregory Department of Bacteriology University of Aberdeen Aberdeen A69 220, United Kingdom
The Measurement of Wetta bility A simple method for contact angle determinations
There are many practical situations where the assessment of wettability is important e.g., cleaning of glassware, lubrication, crop spraying, treating fabrics, photographic processing, coating surfaces, etc. One of the easiest ways to measure wettability for a given situation is to determine the contact angle between the liquid and the solid involved. In 1805 Young ( I ) proposed that there was an appropriate contact angle for each combination of liquid and solid. The contact angle is defined as the angle between the surfaces of the liquid and the solid substrate a t the line of contact, as measured through the liquid (i.e., 0 in Figure 1). Although stated only in words, Young's theory described the contact angle as resulting from the equilibrium of forces acting upon a liquid drop resting on a solid surface. As shown in Figure 1, these forces are the surface tension of the liquid PL, the surface tension of the solid Ps, and the interfacial tension of the solid and liquid PSL where 0 is the contact angle. This theory is true whether the forces are transient, metastable, or in equilibrium, whether the surface is clean or dirty, flat or rough, homogeneous or heterogeneous (2). The liquid surface tension PL acting tangentially to the drop at the line of contact has a vector P~.cosO in the plane of the solid surface. Thus the drop is subject to the action of PSLand the force vector P~.cosOwhich oppose spreading, and Ps which favors spreading, such that (see ref. (2)) P,
=
P,,.
+ P, .cos0
It follows that as the tendency for liquids to spread or wet increases. the magnitude of the contact anele decreases. and vicelversa. fierefore determination of the contact anele nrovides a auantitative measurement of wettahilitv. '?heirnodynamic studies have resulted in an equation similar to that given above Y , ' Ys,. + YI.C0S0
where ? is free surface enerw. -. This ~ r e d l c t sthat in an idealizeh situation where a pure liquib is spreading on a perfectly clean, uniform, plane solid in a vapor-saturated atmosphere there should be only one contact angle at equilibrium. However, in practice it is difficult to attain thermodynamic equili'hrium between the solid, liquid, and vapor phases because of surface imperfections, so advancing and retracting angles are usually measured. The former angle is the larger, and is measured when advancing the liquid over the dry surface of the solid, whereas the latter is determined when the liquid is receding from a previously wetted solid surface. The ability of a liquid drop to exhibit several stable contact angles is called "hysteresis." Contact angle hysteresis may he caused by surface roughness (3), adsorption (4),.penetration (5), friction (6), or other factors, e.g., see ref. (7). The existence of hysteresis is not easily reconciled with thermodynamic theory, and has resulted in much controversy (8, 9). This is because thermodynamics, based on the concept of free 682
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Journal of Chemical Education
Figure 1. Diagram to illustrate the meaning a1 contact angle.
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surface enerev., is descrihine an idealized situation. whereas hysteresis is caused by departures from ideality. However. as Grav (2) indicated, these difficulties do not invalidatk ~ o u n 2 theory, s based on forces, as an adequate description of the non-idealized, practical situation. Adam and Elliott (10) indicated that the mean of the advancing and receding angles is more constant than either extreme, and suggested that it represents the theoretical equilihrium contact angle because it is in good agreement with measurements of single equilihrium angles. However Phillips and Riddiford (11) did not agree and suggested that the advancing angle may be closer than the receding angle to the theoretical equilihrium angle for smooth surfaces when penetration is the only likely cause of hysteresis. Techniques for Contact Angle Measurement Johnson and Dettre (12) stated that the accuracy of contact angle measurements is not generally limited by the measuring technique, but rather by the unreproducihility of the surfaces heing studied. Instruments that measure to an accuracv of 1" are nsuallv more than adequate. The commonly-used techniques for measuring contact angles have been reviewed by Johnson and Dettre (12). Most technioues have involved o ~ t i c a observation l of the line of contact, the contact angle'then heing measured either directlv or indirectlv. Direct methods have included t h e reflection of light (13), or use of a microscope or telescope fitted with a goniometer eyepiece (14). Indirect methods have involved constructing a tangent to the liquid surface at the line of contact either on a projected image (15) or on a photograph of the drop profile (16). In the latter method the image was focussed on a ground glass screen, photographed with a camera and the contact angle was measured on an enlarged print. This procedure may he simplified using the apparatus described below. Apparatus The technique used here is a modification of the sessile drop method described by Poynting and Thomson (17). The advantages of this method are that i t requires only a small volume of liquid and a small area of the solid surface under study. The apparatus is illustrated in Figures 2 and 3 and must he housed in a darkroom equipped for processing photographic prints.
Figure 2. Photograph of apparatus used.
Figure 4. Examples of drop profiles obtaned using the apparatus deSCrbed in this paper. The solid surface, the tangent to the liquid surface at the line of contact, and the contact angle have been drawn in at one side of each profile. (a) 0.01% (v/u) Teepol detergent advancing on dental wax. (b) 0.005% (v/v) Teepoi detergent advancing on dental wax. ( c ) 0.002% (v/Y) Teepol detergent advancing on dental wax. (d) 0.002% (v/v) Teepoi detergent retracting on dental wax. (el 0.002% (v/v) Teep01 detergent advancing on glass. (1) Mercury advancing on glass.
Figure 3. Diagram of apparatus used
It consists of a suitable slide projector directed a t a small screen (e.g., a piece of white card) upon which a piece of photographic printing paper can he fixed. The type of projector used must be one where the slide holder can be removed and replaced by a support for the surfaces to be examined. A drop of liquid is formed on the solid surface and a n image of the drop is focussed on the screen. The size of the image is adjusted to fall within the size of printing paper being used (e.g., 8 X 10 in.) by selecting the appropriate distance between projector and screen. The solid surface used for investigations should he as clean as possible. It must be supported horizontally on the axis of the light beam. The liquid to he studied is applied to the solid surface from a hypodermic syringe so that the drop can be advanced or retracted as required. The syringe must he thoroughly cleaned (e.g., by using an ultrasonic cleaning bath), and the needle has a squared-off tip so that it can always he kept immersed in the drop of liquid. The syringe is clamped with the needle tip just above the solid surface because it is necessary to avoid vibration or distortion of the drop by removing the needle before measurement. It is desirable to prevent as much stray light as possible from leaving the projector by masking off, e.g., with aluminium foil. It is useful to have a neutral density filter (e.g., Kodak Wratten filter No. 96, N.D. 2.0) in the light beam to reduce its intensity so that the exposure time can he more easily controlled. Procedure With the projector switched on, a drop of liquid is applied to the solid surface from the hypodermic syringe and advanced or retracted as required. The magnified projected image of the drop is focussed on the small screen. The projector lamp is switched off and under the appropriate safelight conditions a sheet of photographic paper is fixed in place an the screen. The lamp is switched on just long enough to give the correct exposure and then switched off again. (If a neutral density filter is not available
the paper can be exposed by flashing the lamp momentarily). The photographic paper is removed from the screen and processed according to the manufacturer's recommendations. The time spent on this step can be shortened by using an automatic print processor (e.g., Kodak Ektamatic or Ilford Ilfoprint) which produces a dry, light-stable print immediately. As soon as the print is ready, tangents t o the liquid surface a t the line of contact with the solid may be drawn at each end of the drop profile. Thus two determinations are obtained from each print, the contact angle (0 in Figure 1) being measured with a protractor. The tangent technique is reported to have an accuracy of 1" or 2" (12). The uncertainty is somewhat higher for very small (
