Exact experimental methods. Surface-pressure measurements

Exact experimental methods. Surface-pressure measurements. Nearly all the methods in use at present are modifications of Langmuir s method, of directly measuring the outward force exerted on a light floating barrier which divides the film-covered surface from a clean surface and for the manipulation of the films, and adjustment of the area available for them, modifications of Pockels s barriers are almost always used. Ths essential points requiring attention for accuracy are as follows  

One satisfactory type of apparatus is illustrated diagrammatically in Mg. 7. There is a trough on three levelling screws (not shown), filled to the brim with water an instrument with two torsion wires for measuring 

The tops of the sides of the trough should be several millimetres wide at least, and ground fiat, so that the barriers fit them. Both the tops of the sides of the trough, and if desired the whole surface of the trough, must be covered with hard paraffin wax. This is easily painted on from benzene solution, and adheres best if the trough is heated sufficiently for the wax to be molten. The function of the wax is to provide a non-wetting layer, over which the water 

1 Opaque silica troughs may be obtained from the Thermal Syndicate, Ltd. 

Under favourable conditions troughs require cleaning only every few days, the residual material from each experiment being collected behind barriers out of the way, at the end of the trough. 

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In surface tension measurements using the maximum bubble pressure method several sources of error may occur. As mentioned above, the exact machining of the capillary orifice is very important. A deviation from a circular orifice may cause an error of 0.3%. The determination of the immersion depth with an accuracy of 0.01 mm introduces an error of 0.3%. The accuracy of 1 Pa in the pressure measurement causes an additional error of 0.4%. The sum of all these errors gives an estimated total error of approximately 1%. Using the above-described apparatus, the standard deviations of the experimental data based on the least-squares statistical analysis were in the range 0.5% < sd > 1%. 

Eq. (2.18) is the exact definition of the experimental relationship for the determination of surface tension by measuring the corresponding pressure differences and radii of curvature. This relationship is the basis of many experimental surface and interfacial tension methods measuring for example the volume of detaching drops (Section 5.2), the pressure inside bubbles (Section 5.3) or drops (Section 5.5), and the shape of sessile or pendent drops (Section 5.4). 

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