Re: Lycopodium Powder

[Jamil Ahmad <ahmadj@MOPIPI.UB.BW>]

The explanation of such phenomena  is given in "Physical Chemistry of
Surfaces" by A.W. Adamson (Wiley Interscience).  Briefly, think of the
powder on the surface of water as a bundle of countless capillaries,
coated with water-repelling material, and dipping perpendicularly into
the liquid.  Wetting of the powder is now akin to rise of the water into
the capillaries.

If the inside of a capillary tube is coated with a hydrophobic material,
water will not rise into it.  The driving force in capillary rise is the
pressure difference across the curved surface of the miniscus.  The
surface of the miniscus in such a case is convex.  Another way of saying
the same thing is that the contact angle between the liquid and the
solid is greater than 90.  The value of contact angle is determined by
the net Gibbs free energy, which in turn depends on the relative
magnitudes of the interfacial tensions between the liquid and the solid,
between solid and the vapor and between liquid and the vapor.

A bit of an appropriate detergent added to the water may cause the
wetting to take place, since this addition may change the balance of
these interfacial tension values.

Jamil Ahmad

cliff parker wrote:

> A couple of years ago I posted a question asking for explanations
> of the mechanism involved in a commonly performed Lycopodium
> Powder demonstration.  The one where a beaker filled with water
> has a small amount of powder sprinkled on the surface.  A finger
> submerged into the water will stay dry and have a very thin
> coating of lycopodium powder covering its surface.  I did not
> receive any explanations that satisfied and have continued, off
> and on, to wonder about this and seek a satisfactory
> explanation.  This evening I found something that I believe is
> moving me in the right direction.  I am finishing a newly
> installed hardwood floor.  Sanding between coats of polyurethane
> leaves a very fine dust that reminded me of lycopodium powder.
> It turns out that this powder behaves in very much the same way.
> Any ideas why?  I assume that it has something to do with the
> size of the particles, but why should very small particles cause
> this strange phenomenon?
>
> --
> Cliff Parker
>
> Never express yourself more clearly than you can think.  --
> Niels Bohr