Message-Id: <v01540b24b0279e2936e8@[192.174.2.173]>
Date: Mon, 25 Aug 1997 12:40:49 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: experiments with bubbles
X-Sender: jlahr@icefog.polarnet.com
Date: Sun, 24 Aug 1997 00:17:16 -0800
To: webhead-l@eskimo.com
From: "J. Lahr" <jlahr@polarnet.com>
Subject: Re: antibubbles
Cc: thuynhdi@laedu.lalc.k12.ca.us, pauld@exploratorium.edu, donrath@aol.com
Mime-Version: 1.0
At 12:05 PM 8/23/97 -0700, you wrote:
>
>Speaking of bubbles, anyone encounter the old Amateur Scientist column in
>SciAm, on antibubbles? I just put together a little article on how to
>make them. http://www.eskimo.com/~billb/amateur/antibub/antibub1.html
>
>.....................uuuu / oo \ uuuu........,.............................
>William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623
William,
Thanks for the antibubble web page! At the Exploratorium Teacher Institute
this summer I asked Paul Doherty why, if a glass of water was filled brim
full, so that the surface was convex as seen from above, something floating
in the water would move to the center of the glass, where the water was the
highest. When I go rafting, the raft always slides to the bottom of "holes"
created by flow around rocks.
At least for small objects floating in a glass (no flow involved, unlike
the raft), Paul reasoned that the object would float to the high point
if the surface tension was holding down and toward the low point if the
surface tension was holding it up. I did some quick tests and found this
to be true, and to make sense. The force of the surface tension would be
normal to the surface of the water, so that the direction of the horizontal
component of the surface tension force always pointed in the direction of
motion of the object.
By this reasoning, I'm sure that anti bubbles (globules) will move to the
lowest point of a curved water surface and that bubbles will move to the
highest point. I had the most fun with small particles of ground coffee.
If dropped on a slanted water surface (a "tall" drop of water placed
on a coin works well for this - a suggestion of one of the teachers there)
the coffee ground would at first remain dry and slide down to the lower edge
of the drop because it was being pushed outward by the water tension. After
a second or so, the coffee ground would become wetted, so that the water
tension was pulling it in. This caused the ground to slide up the bubble to
its highest point. It was fun to see some of the coffee grounds first slide
down the bubble, pause briefly, and then zoom up to the top. The hard part
was getting them to land on the steep part of the bubble, as otherwise they
would get wet too quickly would not move down the bubble.
Some particles acted more ambiguously, as they seemed to have a meniscus that
was concave upwards in some places and concave downward elsewhere. I
suppose if you could create a floating object that was wetted (meniscus
concave upward) on one side and dry (meniscus convex up) on the other
side, then it would rotate so that its dry side was downhill. It would
either move uphill or downhill depending on which side was the largest.
Hope this isn't too obscure without pictures. Someday (I bet soon) there
will be a universal way to easily include a sketch with an Email message.
Cheers,
John
-- * John C. and Jan H. Lahr * * jlahr@polarnet.com * * P.O. Box 83245, Fairbanks, AK 99708 * * (907) 474-7997 * * http://www2.polarnet.com/~jlahr * * http://giseis.alaska.edu/Input/lahr *