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→Capillary pressure concepts
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg|300px|thumb|{{figure_number|2}}The wetting phase rises above the original or free surface in the capillary tube experiment until adhesive and gravitational forces balance. Capillary pressure (P<sub>c</sub>) is the difference in pressure measured across the interface in the capillary (''P''<sub>c</sub> = ''P''<sub>nw</sub> - ''P''<sub>w</sub>). This pressure results from the contrast in pressure gradients caused by the different densities of the nonwetting (''ρ''<sub>nw</sub>) and wetting (''ρ''<sub>w</sub>) phases (right).]]
[[File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg|300px|thumb|{{figure_number|2}}The wetting phase rises above the original or free surface in the capillary tube experiment until adhesive and gravitational forces balance. Capillary pressure (P<sub>c</sub>) is the difference in pressure measured across the interface in the capillary (''P''<sub>c</sub> = ''P''<sub>nw</sub> - ''P''<sub>w</sub>). This pressure results from the contrast in pressure gradients caused by the different densities of the nonwetting (''ρ''<sub>nw</sub>) and wetting (''ρ''<sub>w</sub>) phases (right).]]
If the end of a narrow capillary tube is placed in a wetting fluid, net adhesive forces draw the fluid into the tube ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg||Figure 2]]). The wetting phase rises in the capillary above the original interface or ''free surface'' until adhesive and gravitational forces are balanced. Because the wetting and nonwetting fluids have different densities, they also have different pressure gradients ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg|Figure 2]]). ''Capillary pressure'' (''P''<sub>c</sub>) is defined as the difference in pressure across the meniscus in the capillary tube. Put another way, capillary pressure is the amount of extra pressure required to force the nonwetting phase to displace the wetting phase in the capillary. Capillary pressure can be calculated as follows:
If the end of a narrow capillary tube is placed in a wetting fluid, net adhesive forces draw the fluid into the tube ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg|Figure 2]]). The wetting phase rises in the capillary above the original interface or ''free surface'' until adhesive and gravitational forces are balanced. Because the wetting and nonwetting fluids have different densities, they also have different pressure gradients ([[:File:charles-l-vavra-john-g-kaldi-robert-m-sneider_capillary-pressure_2.jpg|Figure 2]]). ''Capillary pressure'' (''P''<sub>c</sub>) is defined as the difference in pressure across the meniscus in the capillary tube. Put another way, capillary pressure is the amount of extra pressure required to force the nonwetting phase to displace the wetting phase in the capillary. Capillary pressure can be calculated as follows:
:<math>P_\mathrm{c} = (\rho_\mathrm{w} - \rho_\mathrm{nw}) g h</math>, or
:<math>P_\mathrm{c} = (\rho_\mathrm{w} - \rho_\mathrm{nw}) g h</math>, or