Adhesion between Liquids and Solids.
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-123
2025-11-08
34
Let us consider what may happen when a solid is brought into contact with a liquid. If a lump of sugar is dipped into water, the adhesion between the two is greater than the cohesion of the sugar, and the sugar is dissolved. If a clean glass rod is dipped into pure water, the adhesion is greater than the cohesion of the water, and the rod will be found wet when it is removed. If the glass rod is dipped into mercury, the adhesion is less than the cohesion, and none of the mercury will cling to the rod.
When the glass rod is dipped into mercury, the surface of the liquid is not broken, but extends down beside and below the rod. The surface tension, tending to decrease the area of this surface, rounds off the corners at A and B, Fig. 1, convex upward. When the rod is dipped into water (Fig. 2), the adhesion causes the water next the glass to rise above the general level, so that the surface would be as at A if it were not for the surface tension; but the surface tension decreases the area of the surface by rounding off the corners as at B, concave upward.
If two plates of glass are thrust into water with their faces parallel to each other, the liquid will rise between them, the height being greater, the nearer the plates are to each other. If the plates are held tightly together at one edge and slightly separated at the other, as in Fig. 3, this varying height will be shown in the form of a curve, highest at the angle, and lowest at the outside edge. This can be seen better if the water is slightly colored.
FIG.2.
Demonstrations. -Pour some clean water into a beaker, and thrust one end of a piece of clean glass tubing below the surface of the water. The water will rise on the inside of the tube to a considerable height above the water in the beaker. On removing the tube, it will be found to be wet. Repeat the experiment with a tube of half the diameter, and the water will rise twice as high. Pour clean mercury into a dish, and repeat the experiment,
FIG. 3
using clean glass tubes of the same sizes as before. Observe that the surface of the mercury is convex and that it is depressed in the tubes. Notice also that the glass tubes are not wet by the mercury.
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