Capstone (Math 451H) Spring 2009:
The Shape of a Soap Film in an Electric Field
Department of Mathematical Sciences
Supported by NSF grant No.
0511514
Students:
Karina Aliaga,
Daniel Bautista, Phillip Bowden, Miguel Correa, David Hamoui, Clifford Hilaire,
Tao Lin, Dawid Midura, and Jesse Slavicek
Laboratory Assistant:
Daniel Cargill
Instructor: Michael Booty
Project Description
& Background
The project combined analysis and computation
with modeling and simple experiments to see how an electrostatic field can alter
the shape of a soap film or fluid membrane.
Some appreciation that objects can experience a force when placed in an electrostatic field was
around before 1752 when Benjamin Franklin used the idea in the invention of "Franklin's Bells".
Various types of electroscope were invented soon after. A more quantitative understanding
of how an electrostatic field can deform continuous media must have waited for formulation
of the Maxwell stress tensor. Our project draws on studies by G.I. Taylor during the
1960's on liquid drops coalescing or breaking up in an electric field (cf. drops in a
rain cloud) and the "Taylor cone", which is related to experiments by Zeleny (1917) on
electrohydrodynamic "jetting". More recently, electrohydrodynamics has become a topic of
interest for the control of a fluid, or of drops and bubbles, in small-scale microfluidic
devices, and in solid-phase MEMS that have a variety of applications.
The experimental setup we used is the same as that of the University of Delaware's
MECLAB, and is seen in the two gray-scale
photos below. Two aluminum plates (0.16 mm thick) are separated (5 cm apart) by threaded
nylon bolts and nuts. The top plate has a central hole cut in it (10 cm diameter) across
which a soap film is wiped. The plates are held at a potential difference of 0-20 kV using
a standard adjustable low voltage power supply (0-20 V) which drives an Emco high voltage
power supply (model F121, 10 Watts). The potential difference across the plates is measured
by putting an Emco high voltage divider (model V1G) across them, which reduces the potential
difference by 1000, and this reduced difference is measured by a standard digital multimeter.
The multimeter can be seen in the photos, propped up behind the plates.
Far more elaborate high voltage supplies are available, but this arrangement is cheap
and fairly reliable. It has none of the safety features of more elaborate supplies, and
it is fairly easy to get an electric shock at the high voltages used, but the amount of
electric charge and current is very small. The Emco high voltage supply was often
driven above its maximum 15 V input rating for minutes and did not fail.
The main difficulty with our experiment was the setting up of a stable soap film.
It would often burst before we could get results. This was probably due to the often low
humidity of the air in the lab.
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