"Lewis Perin" > wrote in message
news
> "Alex Chaihorsky" > writes:

>
> - the pot drips if a drop can form where tea liquor and spout meet;
>
> - the surface tension needed for drop formation is increased by a
> relatively fluffy spout lining like a glaze, while a dense material
> like unglazed ceramics attracts the liquid, preventing drop
> formation.

Approximately

>
> In the naive and sloppy part of my mind where I've accounted for this
> kind of thing until now, I assumed that vitreous surfaces repelled
> water because they were dense.

Let me put it this way - a drop of liquid has to "decide" if it rather be
attracted to contact material (and then it wets it) or to itself (and then
it forms some sort of spherical surface. The density of the immediate thin
film on the contact surface is critical for such decision.If the desnity of
such is higher than the liquid's it is likely to wet it and form the
negatively curved surface. If it is lower - it will likely to form
positively curved surface. A good example is ater and mercury in a glass
tube.
The more sperical is that positively curved surface, the taller is the drop,
the more distance between the surface and the center of the drop's gravity
and the more likely it will lose contact with the surface and actually drip.
the bettr it wets the surface the thinner is the liquid film and the less
likely it would lose contact with the surface and drip.

>But really, at the level where this
> stuff takes place, I suppose it's a long way between particles, so
> there had to be something else in play, right?
>
> /Lew
> ---

Not that I am aware of.
I have not looked at any math models recently, but if I remember right
gravitational pull plays major role in this. In some special cases like with
oils ionic forces are also at play.
One could have suspected also some additional molecular forces at play,
especially in water, bust since all liquids are subject to this type of
effect disregarding the chemistry its unlikely that such forces play any
significant role in common cases.

Sasha.