Surface tension and capillary rise
Interface forces can support pressure jumps and capillary columns.
Key knowledge
Surface tension γ is force per unit effective contact-line length for a liquid interface. A thin soap film has two surfaces, so a movable wire of length ℓ feels 2γℓ. In a narrow circular vertical tube, static capillary height relative to a reservoir is approximately h=2γcosθ/(ρgr): wetting θ<π/2 raises the liquid, while θ>π/2 depresses it.
Values and equations
γ=F/L_effective; F_film=2γℓ; h=2γcosθ/(ρgr)
- γ
- Surface tension N m⁻¹
- F,F_film
- Interface force and force on the two-surface film wire N
- L_effective,ℓ,r,h
- Effective contact length, wire length, tube radius and signed rise m
- ρ
- Liquid density when gas density is neglected kg m⁻³
- g
- Gravitational acceleration magnitude m s⁻²
- θ
- Contact angle measured through liquid rad
Conditions and scope
- Uniform γ, static equilibrium, circular tube radius r>0, density contrast approximated by liquid density ρ>0 and uniform g>0. Contact angle is measured through the liquid.
- The capillary formula neglects the small meniscus-volume contribution and gas density. Contamination, contact-angle hysteresis, variable γ and dynamic wetting require more detailed models.
Sources · 1
References
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Evidence and review
Read College Physics2e §11.8 surface-tension definition11.47, two-surface film Figure11.26 and static capillary-height formula11.51/contact-angle convention. Medical examples, the adjoining rubber-balloon inverse-radius generalization, dated tree-water uncertainty prose and universal molecular inward-force explanation are outside this record.
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