Help surface tension

1. Aug 4, 2007

shamstar

help!!! surface tension

i am doing a physics experiment on the relationship between a liquids temperature and resulting surface tension. i am going to record the various surface tension of various liquids at different temperatures. the only problem is, is that im not sure how to accurately measure the surface tension. ive looked on the internet but the methods seem a bit complicated. i would like something simple that hopefully wont take to long to do. if anyone has any ideas or experience in this area please let me know. it would be much appreciated. thanks
also, i know that as temperature increases surface tension decreases, but does anyone know what sort of relationship would exist between the surface tension of a liquid and its temperature. is is linear, exponential,etc?
thankyou

2. Aug 5, 2007

chaoseverlasting

One thing you could do is turn a test tube upside down in a beaker of liquid at a certain temperature and record the height of liquid column. It should form a U shape near the top of the liquid column, if you can accurately calculate that, you can find the surface tension. Here, the weight of the liquid column is balanced by the surface tension of the liquid. You could even ignore the U part if you want. Try to derive the equations.

HINT: F=S*L where F is the force exerted by the liquid surface, S is the surface tension, and L is the length (in this case the circumference) of the liquid exposed.

3. Aug 5, 2007

chaoseverlasting

There is a second definition of surface tension based on the surface energy of the liquid. Using this, you could derive an expression between the temperature and surface tension.

4. Aug 6, 2007

shamstar

thanks, that helps alot. just one thing. when you've measured the height of the liquid column, how do you find the force? do you use mass x gravity? how does the u shape effect the surface tension measurment? thanks

5. Aug 6, 2007

shamstar

i found an equation on the internet that seems easy enough to use. it uses the idea of putting a test tube into a beaker of water and measuring the height difference. it is :
y = (2 gammalg cos theta) / (dgr)
where
• y is the vertical displacement (up if positive, down if negative)
• gammalg is the liquid-gas surface tension
• theta is the contact angle
• d is the density of the liquid density=mass/volume
• g is the acceleration of gravity
• r is the radius of the capillary

the only problem is, i think it may be hard to measure the angle of contact. you mentioned that you could do this experiment without measuring the angle. is that with another equation? please let me know. thanks

6. Aug 6, 2007

chaoseverlasting

You can ignore the angle of contact. Here, since the weight of the liquid column is balanced by the force exerted by the surface tension, $$\rho \pi r^2 lg=S*2\pi r$$ where l is the height of the liquid column, rho is the density, g is the acceleration due to gravity and S is the surface tension.

Here, the approximation is that the volume of the liquid forming the U shape is so small that it can be neglected. Simplifying the above equation, you get:

$$S=\frac{\rho rlg}{2}$$

7. Aug 7, 2007

shamstar

thankyou so much!!! you're a legend! this helps me so much. thanks again.

8. Aug 7, 2007

andrevdh

The classic way of measuring surface tension is to pull a sharp edged cylinder out of the liquid with a sensitive spring scale and note the maximum force just before it is released by the surface. The surface tension is broken over a distance of twice the circumference of the cylinder in such a case (it clings to both sides of the cylinder). The edge of the cylinder need to be clean in order for the liquid to adhere to it.

9. Aug 7, 2007

chaoseverlasting

You're welcome. Its what PF is for. Anyone would do it.

10. Sep 1, 2007

shamstar

one more question about the formula you gave me. when i get an answer, is the units for surface tension in dyns/cm or ergs/cm^2? thankyou