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Principles of Colloid and Surface Chemistry, Third Edition, Revised and Expanded, 3rd Edition by Raj Rajagopalan, Paul C. Hiemenz

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6.4b  Laplace Equation and Capillary Rise

As noted above, it is possible that a different pair of radii of curvature applies at different locations on a surface. In this case the Laplace equation shows that Δp also varies with location. This is the reason for the variation of the pressure with z in the meniscus shown in the capillary in Figure 6.3b. As is often true of pressures, it is convenient to define pressure variations relative to some reference plane.

With this idea in mind, the horizontal surface in Figure 6.3b can be taken as a reference level at which Δp = 0. Just under the meniscus in the capillary the pressure is less than it would be on the other side of the surface owing to the curvature of the surface. The fact that the pressure ...

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