"Wow. What was that?" Markus Zahn recalls saying as he watched the black drop of liquid suddenly morph into a design resembling Native American art.
The professor of electrical engineering and computer science was watching a video created by Cory Lorenz, a senior in the same department. As a project last summer, Lorenz collaborated with Zahn to explore the effects of time-varying magnetic fields on ferrofluids, materials Zahn is exploring for potential applications in nanotechnology.
Ferrofluids are mixtures of carrier liquid (such as water or oil) containing magnetic particles only 10 nanometers, or billionths of a meter, in diameter. The particles are coated with a surfactant to keep them dispersed. Lorenz applied the magnetic fields to ferrofluids in a Hele-Shaw cell, where flows are constrained to two dimensions in a small gap between two parallel glass disks.
By running a typical experiment in reverse order, he unexpectedly coaxed a ferrofluid to "undergo something analogous to a phase change," Zahn said, resulting in the unexpected pattern. "It was the first time this had ever been observed," said Zahn, the Thomas and Gerd Perkins Professor of Electrical Engineering.
No wonder, then, that the video was one of five winning entries out of 25 video submissions at last November's annual meeting of the American Physical Society's Division of Fluid Dynamics 20th Annual Gallery of Fluid Motion. As a result, it will be featured in the September 2003 issue of the Physics of Fluids.
Zahn notes that another MIT video and an MIT poster were also among the winning entries. Professor Gareth H. McKinley of mechanical engineering and colleagues at the Universite Joseph Fourier and the University of Melbourne won for their video titled "Drop Impact of Newtonian and Elastic Fluids." A team led by John W. M. Bush, an associate professor of mathematics, won for a poster titled "Water Walking."
In the first experiment shown on his video, Lorenz applied a 100 Gauss DC vertical magnetic field to a Hele-Shaw cell containing a drop of ferrofluid. This caused the drop to form a spike pattern. He then added a 20 Gauss, 25 Hertz magnetic field that rotates clockwise in the horizontal plane. The result: the ferrofluid slowly changes to a spiral pattern.
When Lorenz ran the experiment in reverse order, by first applying the rotating field, then the 100 Gauss DC vertical field, the ferrofluid drop morphed dramatically into the new protozoan-like shape.
Ferrofluids have been used commercially since the 1960s in applications such as exclusion seals to keep contaminants out of disk drives. They are also used to enhance heat transfer in high-quality audio speakers to allow higher power without overheating. Zahn's lab in the Laboratory for Electromagnetic and Electronic Systems is exploring potential ferrofluid applications in the fields of MicroElectroMechanical and NanoElectro Mechanical Systems (MEMS and NEMS). One application he is particularly interested in is magnetic-field-driven microfluidic devices.
"Most conventional MEMS are based on electric fields," he explained, "but magnetic forces have the advantages that they're generally larger than electric forces and there is no electrical breakdown mechanism to cause failure due to spark discharges."
Lorenz's summer work was sponsored by the National Science Foundation's Research Experience for Undergraduates program.
A version of this article appeared in MIT Tech Talk on February 5, 2003.
