Fluid Dynamics Videos |
Liquid Bridges
Liquid bridges are regions of
liquid
that span the gap between two or more solid supports. This
includes
water suspended between particles of sand, liquid in a fabric, and
water
that may enter the lungs. The classic "liquid bridge" is a column
of liquid that spans the gap between the ends of two cylindrical
rods.
In zero gravity, Lord Rayleigh showed theoretically that one can
maintain
a perfect cylindrical bridge, supported only at the ends, as long as
the
ratio of the length to diameter of the liquid cylinder is less than pi
(3.14159...); the bridge ruptures when it becomes too long.
We
have investigated
some dynamics issues of bridges in simulated low gravity conditions by
magnetic levitation. Two movies in Microsoft Media format (.wmv)
may be seen below. Financial support from NASA is acknowledged.
Bridge Collapse: Click here for a movie (only 91 kB) showing the collapse of a vertical bridge composed of glycerol and manganese chloride tetrahydrate. The manganese chloride tetrahydrate is dissolved in the glycerol, and makes the mixture "paramagnetic," which means that it is attracted toward the strongest region of magnetic field (near the pole pieces). When the movie begins, the bridge is initially in a "zero gravity environment," where the downward gravitational force is cancelled by the upward magnetic force on the mixture. [The two pole pieces of the magnet are seen as solid objects and located to the left and to the right of the bridge]. The magnetic field is then turned off suddenly, leaving only gravity; this causes the bridge to sag and eventually collapse. Because of the high viscosity of the glycerol in the bridge -- it has the consistency of honey -- the bridge collapses very slowly. Notice the thin thread connecting the top and bottom regions just before separation.
Bridge
Oscillations:
Click here for a movie (only 89
kB) of
a vertical bridge subjected to an oscillating force. This bridge is
composed of manganese chloride tetrahydrate and water, which
means that the liquid is much less viscous than the glycerol bridge,
and therefore flows much more easily.
Initially the bridge is in a simulated zero gravity environment, where
the
downward gravitational force is cancelled by the upward d.c.
(steady)
magnetic force. We then apply a small
time-varying
(sinusoidal) current to the magnet at a frequency of 1 Hz (1
cycle
per second); this a.c. current is on top of (i.e.,
added to)
the d.c. current. When the total magnetic force (d.c. plus the
time-varying
part) is larger than the earth's gravitational force, the liquid is
pushed
upward; when the total magnetic force is less than gravity,
gravity
pulls the liquid downward. After several seconds we
increase
the amplitude of the time-varying magnetic force, and the amplitude of
the
bridge oscillations increases accordingly. When the amplitude of
the
magnetic force is sufficiently large, the bridge is no longer stable
and
ruptures.
Rayleigh-Taylor
Instability
Click here for
movie (1.38 MB)
showing a Rayleigh-Taylor instability. When the
movie begins, two immiscible liquids are housed between two glass
plates. The heavier chloroform contains a red dye, and the
lighter aqueous mixture of water and paramagnetic manganese
chloride tetrahydrate is pulled downward by the magnetic force.
Thus, we have created a metastable condition in which the heavier fluid
rests on top of the lighter fluid. When the magnetic field is
turned off, the instability sets in. At first one sees the
exponential growth of the most rapidly growing mode, which gives the
interface a sinusoidal shape. Eventually the rising aquesous
mixture begins to form mushroom shapes, which pinch off into individual
bubbles. The use of a magnetic field to compensate for gravity
allows us to look at very long times (> 30 initial inverse growth
rates) and to examine the nonlinear behavior of the instability.
Having problems? Contact rosenblatt@case.edu