The Laplace pressure is the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

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 Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...

of the interface between liquid and gas, or between two immiscible liquids. The Laplace pressure is determined from the

Young–Laplace equation In physics, the Young–Laplace equation () is an algebraic equation that describes the capillary pressure difference sustained across the interface between two static fluids, such as water and air, due to the phenomenon of surface tension or ...

given as :

\Delta P \equiv P_\text - P_\text = \gamma\left(\frac+\frac\right),

where

R_1

and

R_2

are the principal radii of curvature and

\gamma

(also denoted as

\sigma

) 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

R_2

: :

\Delta P = \gamma\frac

For a gas bubble within a liquid, there is only one surface. For a gas bubble with a liquid wall, beyond which is again gas, there are two surfaces, each contributing to the total pressure difference. If the bubble is spherical and the outer radius differs from the inner radius by a small distance,

R_o=R_i+d

, we find :

\Delta P=\Delta P_i+\Delta P_o=2\gamma\left(\frac+\frac\right)=\frac\left(1+\frac\right)=\frac\left(1 + \frac-\frac\right) = \frac\left(1-\frac\frac\right)\approx\frac+\mathcal(d).

Examples

A common example of use is finding the pressure inside an air bubble in pure water, where

\gamma

= 72 mN/m at 25 °C (298 K). The extra pressure inside the bubble is given here for three bubble sizes: A 1 mm bubble has negligible extra pressure. Yet when the diameter is ~3 μm, the bubble has an extra atmosphere inside than outside. When the bubble is only several hundred nanometers, the pressure inside can be several atmospheres. One should bear in mind that the surface tension in the numerator can be much smaller in the presence of surfactants or contaminants. The same calculation can be done for small oil droplets in water, where even in the presence of surfactants and a fairly low interfacial tension

\gamma

= 5–10 mN/m, the pressure inside 100 nm diameter droplets can reach several atmospheres.

References

{{Reflist Pressure Fluid dynamics Bubbles (physics) Articles containing video clips

Examples

See also

References