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What do Penn and Teller and Charles Rosenblatt's group in the physics department at Case Western Reserve University have in common? They can make objects seem to float in air.
The difference is that Rosenblatt's objects really do float. In fact, Rosenblatt has recently gone from simply levitating liquids and crystals to varying the effective gravitational force on these objects over time.
His latest scientific wizardry is to study what happens to liquid bridges in a simulated zero-gravity environment when the Earth's gravity -- or the moon's, or Saturn's, or Pluto's -- is suddenly switched on. Rosenblatt's ability to simulate a spectrum of gravitational conditions in his laboratory is important when planning missions to the moon and planets.
The professor of physics suspends a fluid such as water or glycerol, mixed with highly paramagnetic magnesium chloride tetrahydrate, between two specially designed pole pieces of an electromagnet. By revving up the current to the magnet, he is able to generate an upward magnetic force on the fluid that completely compensates the downward force due to gravity. The result is a floating fluid.
In order to prevent the fluid from drifting off, as astronauts tend to do during their space walks, the fluid is tethered at two points, but is otherwise a freely floating "bridge" suspended between the two surfaces.
When he reduces the magnetic force, he finds that the bridge sags, and when he increases the magnetic force, the bridge arches upward. When he wiggles the electric current in the magnet up and down with time, the bridge dances in a pattern that is determined by gravity and the properties of the fluid. From the motion of the fluid bridge he is able to determine many of its physical properties.
Suspense in Rosenblatt's laboratory hangs in the air as computer-recorded images detail the changes in the shape of the bridge as his assistants -- graduate students Milind Mahajan and Shiyong Zhang -- control the magnet's power supply and computer-assisted imaging system. If the magnetic force is changed too much, the bridge collapses.
Images of collapsing bridges have revealed new physical phenomena that he and his colleagues -- Philip Taylor, Perkins Professor of Physics; J. Iwan D. Alexander, associate professor of mechanical and aerospace engineering; and graduate student Mesfin Tsige -- have been examining theoretically. The group reported its findings in the Physical Review Letters article, "Collapse Dynamics of Liquid Bridges Investigated by Time-Varying Magnetic Levitation."
Like Houdini, who practiced to perfect his magic act, the physicists repeat their experiments with differing bridge lengths and magnetic forces. These experiments provide important material information about the fluid, and have revealed a surprising new scaling relationship that shows that the collapse time of the bridge does not directly depend on its length.
Currently, Rosenblatt and his students explore how bridges resonate when subjected to a gravitational field that oscillates as often as 30 times per second. In particular, they are attempting to determine how the resonances would differ in different gravitational environments, such as Earth as compared to Mars.
While several methods are known to simulate low gravitational environments, until now there were no "good ways to simulate changes with time in the effective gravitational environment, and to follow the progression of these changes in the liquid," Rosenblatt says.
This technique allows researchers to simulate sudden or periodic accelerations without physically moving the fluid. It thus has important implications for understanding how materials act under a variety of gravitational conditions.
For example, this technique may be used to simulate the vibrational effects of earthquakes, the motion of fluids that build up in the lungs with heart failure, and even what may happen aboard a spacecraft during a planetary or lunar landing, says Rosenblatt.
While others have done similar work using techniques involving motors or by suspending a liquid within another liquid bath, countering gravity against a magnetic field has a multitude of advantages. He points out that with this technique scientists can study phenomena on incredibly fast time scales, down to milliseconds. Additionally, the magnetic approach allows the liquid or solid to float freely, unencumbered by the physical constraints of mechanically driven motion or the effects of a surrounding liquid bath.
"Perhaps most importantly, the magnetic approach facilitates the study of dynamic changes in any gravitational environment, ranging from one-g experienced on Earth, to about one-sixth of a g on the moon, to zero-g experienced by astronauts on a space shuttle," he adds.
"This is the first time anyone has been able to so completely control the effective gravitational force on an object," explains Rosenblatt. He adds that this magnetic technique opens possibilities to study liquid dynamics that cannot be studied any other way.
In addition to their liquid crystal research support from the National Science Foundation, Rosenblatt and Taylor share a four-year, $280,000 grant from NASA, with which they previously studied free-floating crystals before beginning their work on the dynamics of levitated liquids.
Rosenblatt and his students do the experimental work, while Taylor, a theoretical physicist, and his group translate the findings into equations that can be used to predict behavior of other materials and under other conditions.
The researchers also are studying liquid bridge resonance, which involves quickly varying the gravitational force and observing the liquid's movement for clues on stability and surface tension. "Professor Taylor and I intend to use this technique to study how the surface tension of pulmonary fluids (liquids that coat the surfaces of the alveoli in the lung) varies with time during the breathing process," Rosenblatt said.
The two physicists say that the climate for microgravity research at CWRU has been greatly enhanced by the establishment of the National Center for Microgravity Research under the leadership of Simon Ostrach, the Wilbert J. Austin Professor of Engineering, and by the recent addition of Alexander to the CWRU faculty.
Journal
Physical Review Letters