# Surface Tension(fluid mechanics)

1. Oct 2, 2009

### Godwin Kessy

May any one help me out on this model!

On surface tension the model used is that it acts as a skin on top of liquids, So when a body floats on top of liquid due to surface tension does it stay on top of the fluid molecules or between the surface molecules! And how does the molecules prevent the body from sinking!

2. Oct 5, 2009

### tiny-tim

Hi Godwin!

From http://en.wikipedia.org/wiki/Surface_tension#Cause"
So the body stays outside the liquid, and it's for roughly the same reason that it would stay outside a solid … liquids and solids both "stay together"!

Last edited by a moderator: Apr 24, 2017
3. Oct 5, 2009

### clem

The surface is pulled down slightly at the edges of the suspended object.
This gives an upward component to the surface tension at the object.

4. Oct 5, 2009

### Godwin Kessy

Thank you! I got you well!
There comes else, May you help me out on the derivation of excess pressure
How comes pressure due to surface tension is simply
Pressure T = 2(pie)rT/A

Where 2(pie)r=Circumference
T=Surface Tension
A=Surface Area of bubble

While the surface Tension force considered in per length and not on the whole surface Area as Pressure is concerned!

5. Oct 5, 2009

### Andy Resnick

This is an unfortunate model to use- an effect of demanding that everything reduce to some sort of atomic model.

First, objects do not 'float' due to surface tension- they float (or sink) due to bouyancy- i.e. gravity.

Surface tension affects *wetting*. Insects that can stay on top of a water surface do so because they do not get wet- the interfacial energy creates an energy barrier, preventing their skin (or whatever passes for insect skin) from getting wet. If the water had a thin film of oil, which decreases that wetting barrier, the insects would most likely fall right in.

The Laplace formula for the pressure jump at a curved interface is simply due to this energy barrier- the energy cost associated with creating a surface. The idea that surfaces have an energy independent of the bulk material is due to Gibbs. So, interfacial tension has units of energy/area. Note there is also line tension, which occurs at a three-phase line (e.g. water-air-solid).