Laplace's Equation for Surface Tension

The pressure within a spherical liquid drop or bubble is directly
proportional to the radius.
 
The Laplace law  introduces an equation that can determine the pressure
of liquid, when the liquid is in a droplet configuration.  Two equations
are derived for two different types of droplets; droplet, and bubble
shapes.  These shapes of moisture has an impact on the lining on the lungs
and, therefore has an influence on the pressure volume of the lungs.  This
influence is the attractive forces between adjacent molecules if the
liquid are much stronger than those between the liquid and gas.  This
results in the liquid surface area becoming as small as possible.
 
 Thereby, the liquid sphere will attain the smallest surface area of a
given volume, and producing a pressure that can be measured by Laplace
law.

The Following Equations apply:

 
    P = 2T/r (drop)		P = 4T/r (bubble)

Sample Problems:

1)  What is the pressure of  a drop of liquid, when surface tension is 4
    dynes and radius is 3mm ?


2)  What is the pressure of a bubble, that has surface tension of 4 dynes
    and radius of 3mm ?

Answers:

1)     P      =  2 (4 dynes ) / .003 m =  2666.6 dynes / m
        drop


2)     P      = 4 ( 4 dynes ) / .003m  =  5333.3 dynes / m
        bubble

For more information on this topic, please refer to West , page 172.

Also, check out the following links that may be helpful:

• Poultry Science 407 - Respiratory Physiology
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by Steve Kamholz, a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.