Measuring how well body wash rinses away?

[Soap Girl]

I need help with designing my experiment. I'm going to determine which body wash rinses away better. Any ideas on how to measure soap residue?

[Louise]

I need help with designing my experiment. I'm going to determine which body wash rinses away better. Any ideas on how to measure soap residue?

Interesting question. Do you want to quantify exactly how much residue each soap deposits or do you just want a yes/no answer? I'm guessing most soaps don't leave all that much, unless you don't rinse well or it has something designed to stay like a moisturizer.

You would need to do these tests on some object, like a glass or plastic disk. You could treat them with soap in the same way, same rinsing conditions. Then you need to find some properity that is different if there is residue. The mass of the residue might not be enough to weigh. What about a chemical property, like pH? Could you test a few of your body washes and see if they are more basic than plain water? If this is the case, then a poorly rinsed object might have a different pH. Or you could look at materials properties, like how water beads on the object. Residue might cause the water to either bead up more or less than the plain glass disk. This is actually used by researchers to characterize a surface- they measure the "contact angle" of the drop with the surface:
http://www.ksvinc.com/contact_angle.htm
http://quest.nasa.gov/space/teachers/mg ... angle.html

I think you will need to do some initial testing to see what methods might work best. Can you think of any other properties of an object that might change if you had residue (that you could measure)?

Louise

[deleted-71576]

One approach that comes to mind is measuring the electrical resistance of the resulting residue (might have to add a constant amount of water to the surface (glass, plastic, etc.)

This paper looks like someone was looking into this approach over a century ago.

"On the Thickness and Electrical Resistance of Thin Liquid Films. [Abstract]
A. W. Reinold, A. W. Rucker
Proceedings of the Royal Society of London, Vol. 53, 1893 (1893), pp. 394-398"

I'm sure there are many more recent references. Try doing a google search on Thin Film Electrical Resistance. Most of the references seem to be of thin films of metals/metallic compounds, but I would think you would find some relating to thin films of soaps.

[johnsteele]

Another approach is not to measure what is left behind, but what is washed away. Maybe measure the specific gravity of the raise water, or its pH, or conductivity or something like that could do the trick. As has been said before, if the amounts left behind are really small, which one would hope they are, this could be difficult to measure.