How this calculator works
The Young-Laplace relation connects excess pressure with the two principal curvatures of an interface. A spherical droplet has two equal curvatures; a cylindrical jet has only one. A thin soap bubble has two interfaces, doubling the spherical pressure jump. The flat mode uses total active contact-line length, which must include both sides when a film has two pulling surfaces. Contact angles and hydrostatic pressure gradients are omitted.
Formula
Flat: F = γL. Hemispherical balancing force: F = πDγ for a droplet, 2πDγ for a thin soap bubble. Jet transverse force over length L: F = 2γL. Droplet: Δp = 4γ/D. Thin soap bubble: Δp = 8γ/D. Cylindrical jet: Δp = 2γ/D.
Worked example
A water-like droplet with γ = 0.072 N/m and diameter 1 mm has excess pressure 288 Pa. A soap bubble with γ = 0.025 N/m and diameter 4 mm has excess pressure 50 Pa.
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Frequently asked questions
Why is a soap bubble different from a droplet?
A thin soap film has an inner and an outer interface, while a liquid droplet has one.
Is pressure absolute or gauge?
Enter the pressure inside minus the pressure outside the interface.
Can I use a non-spherical drop?
The simple diameter formula assumes a sphere. A general shape needs both local principal curvatures.