225 CAPILLARITY
Capillarity arises from a fluid property known as surface tension, which occurs at the interface
between different materials. For soils and rocks, it occurs between surfaces of water, mineral grains,
and air. Fundamentally, surface tension results from differences in forces of attraction between the
molecules of the materials at the interface.
The phenomenon of capillarity may be demonstrated in many ways. Placing the end of a dry
towel in a tub of water will eventually result in a saturated towel. To illustrate the effects of capillarity in porous materials such as soils and rocks, we can use the analogy of small-diameter glass tubes to represent the voids between the mineral grains. Capillary tubes demonstrate that the adhesion forces between the glass walls and water cause the water to rise in the tubes and form a meniscus‘ between the glass and the tube walls. The height of rise is inversely proportional to the diameter of the tubes; the smaller their inside diameter, the greater the height of capillary rise. The meniscus formed is concave upward with the water “hanging,” so to speak, on the walls of the glass tube [Fig. 6.1(a)]. With some
225 CAPILLARITYCapillarity arises from a fluid property known as surface tension, which occurs at the interfacebetween different materials. For soils and rocks, it occurs between surfaces of water, mineral grains,and air. Fundamentally, surface tension results from differences in forces of attraction between themolecules of the materials at the interface.The phenomenon of capillarity may be demonstrated in many ways. Placing the end of a drytowel in a tub of water will eventually result in a saturated towel. To illustrate the effects of capillarity in porous materials such as soils and rocks, we can use the analogy of small-diameter glass tubes to represent the voids between the mineral grains. Capillary tubes demonstrate that the adhesion forces between the glass walls and water cause the water to rise in the tubes and form a meniscus‘ between the glass and the tube walls. The height of rise is inversely proportional to the diameter of the tubes; the smaller their inside diameter, the greater the height of capillary rise. The meniscus formed is concave upward with the water “hanging,” so to speak, on the walls of the glass tube [Fig. 6.1(a)]. With some
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