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The Laplace pressure is the pressure difference between the inside and the outside of a curved surface that forms the boundary between two fluid regions. The pressure difference is caused by the surface tension of the interface between liquid and gas, or between two immiscible liquids.
The Laplace pressure is determined from the Young–Laplace equation given as
where R 1 {\displaystyle R_{1}} and R 2 {\displaystyle R_{2}} are the principal radii of curvature and γ {\displaystyle \gamma } is the surface tension. Although signs for these values vary, sign convention usually dictates positive curvature when convex and negative when concave.
The Laplace pressure is commonly used to determine the pressure difference in spherical shapes such as bubbles or droplets. In this case, R 1 {\displaystyle R_{1}} = R 2 {\displaystyle R_{2}} :