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MIT Electrical Engineering and Computer Science
EECS Event |
Monday, October 2, 2000
4:00 PM (refreshments 3:45)
Edgerton Hall, Room 34-101
EECS Colloquium
Abstract
A ferrofluid is a colloidal suspension of permanently magnetized particles of 10 nm diameter. Brownian motion keeps them from settling under gravity. A surfactant coating provides short range repulsion between particles and prevents agglomeration in non-uniform magnetic fields. Demonstrations and videos will show ferrofluid applications and ferrohydrodynamic instabilities.
Applications use fields from permanent magnets to make a liquid O-ring in rotary and exclusion seals, as dampers in stepper motors and shock absorbers, and to enhance heat transfer in loudspeakers. Most computer disk drives and silicon crystal-growing furnaces use ferrofluid rotary shaft seals. Biological and medical applications under investigation use water-based ferrofluids injected into the body and moved by external magnetic fields to the proper body location to deliver medically effective chemicals or remove toxins.
In a current study, infrared imaging of thermal convection in a spherical shell of heated ferrofluid in a magnetic field shows thermal convection cells in direct analogy to continental drift. The experiment speeds up geological time scales by a factor of about 1016 so the analog of continental drift on earth is visible in a short period of laboratory time.
The motion of ferrofluid in time periodic magnetic fields shows a paradoxical frequency dependent critical magnetic field strength for flow reversals. Periodic excitations also result in increases or decreases in the effective viscosity of ferrofluids, including zero and negative values. Evidence for negative effective viscosity effects will be presented and discussed.