Fluid Dynamics Videos
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.
Click here for a movie (only 89
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
downward gravitational force is cancelled by the upward d.c.
magnetic force. We then apply a small
(sinusoidal) current to the magnet at a frequency of 1 Hz (1
per second); this a.c. current is on top of (i.e.,
the d.c. current. When the total magnetic force (d.c. plus the
part) is larger than the earth's gravitational force, the liquid is
upward; when the total magnetic force is less than gravity,
pulls the liquid downward. After several seconds we
the amplitude of the time-varying magnetic force, and the amplitude of
bridge oscillations increases accordingly. When the amplitude of
magnetic force is sufficiently large, the bridge is no longer stable
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.
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