Ferrofluid Field Induced Flow

Motivation

Ferrofluids consist of a carrier fluid loaded with small (nanometer sized) magnetic particles. The behavior of these fluids varies due to the carrier fluid, temperature, particle size, shape and loading, magnetic characteristics of the particles and the applied magnetic field. When exposed to a magnetic field, the magnetized particles produce a body force as opposed to rheological variations (in the case of magneto-rheological (MR) fluids). Also, unlike MR fluids that experience settling and separation of the particles, the ferrofluid particle size ensures that thermal agitation in the fluid keeps the particles in suspension. The potential exists to utilize the fundamental characteristics of ferrofluids to provide a natural hydraulic pumping methodology using only the fluid medium and external magnetic and thermal fields.

Figure 1.  Field effect on ferrofluid.

Figure 2.  Temperature effect on magnetism.

The principle of operation, shown in Figures 1 and 2, is relatively simple. The ferrofluid is attracted to the magnetic field (in this case, from a coil or permanent magnet). As the fluid enters the thermal field, the fluid temperature increases. As the fluid temperature approaches the Curie temperature of the magnetic particles in the fluid, the regions exposed to elevated temperatures will lose their attraction to the magnetic field, displayed in Figure 2. Equation (1) expresses the pressure differential, , to the temperature dependent magnetization, M(T), of the particles with representing the permeability of air.

(1)

It is clear that the closer the fluid temperature is to the Curie temperature of the particles, the higher the pressure gradient. Figure 3 shows a working prototype of this pumping mechanism. The ideal particle would have a Curie temperature close to the maximum expected thermal cycling temperature. Table 1 lists the Curie temperature of a number of ferrous materials. Most commercial grade ferrofluids are based on magnetite particles. While initially attractive from a fabrication (cost) perspective, the Curie temperature of magnetite particles is far above the expected operating temperature of many hydraulic systems. This presents focus of this research: the synthesis of ferrofluid particles with specific Curie temperatures and pyromagnetic coefficients.

Table 1. Curie Temperature and Saturation Magnetization of Ferromagnetic Solids

Substance	Curie Temp (C)
Dysprosium	-185	3.67
Gadolinium	19	2.59
Nickel	358	0.64
Magnetite	585	0.56
Iron	770	2.18
Cobalt	1120	1.82

Figure 3.  Proof-of-principle ferrofluid pump.

Ferrofluid Synthesis

There are many approaches to the synthesis of ferrous nanoparticles such as size reduction through ball milling, chemical precipitation, and thermophilic iron reducing bacteria. Ball milling was the earliest approach to the synthesis of particles for ferrofluids. Micron sized particles are submitted to a ball grinding process for approximately 1000 hours. While easy to operate, this methodology is costly and difficult to control. Synthesis by chemical precipitation is the more common approach in which the particles precipitate out of solution during chemical processes. A new approach, developed at ORNL, is based upon thermophilic bacteria that reduce amorphous iron oxyhydroxides to nanometer sized iron oxides. The thermophilic bacteria under investigation at ORNL have demonstrated an ability to reduce a number of different metal ions. As with the milling and chemical precipitation processes, it is possible to incorporate other compounds (Mn(II), Co(II), Ni(III), Cr(III)) into magnetite, Fe₃O₄, to control magnetic, electrical, and physical properties of the substituted magnetite. While the understanding of the actual process is still under investigation, ferrous nanoparticles are formed and shed on the skin of the bacteria as they move through amorphous iron oxyhydroxides plus soluble metal species, as illustrated in Figure 4. The process continues until all of the iron is reduced to the magnetite phase, at which time bacterial respiration ceases. This approach to particle synthesis is attractive from many vantage points. First, particle size and morphology are very consistent since the process occurs on the surface of the bacteria. The process is extremely scalable. Researchers at ORNL have observed little change in efficiency from 20 ml to 20 liter batches. Production rate is approximately 10,000’s mg of magnetite/gallon of culture per day. Furthermore, in a culture, the bacteria replicate approximately once every three hours.

Figure 4.  Thermophilic bacteria in magnetite.

Various ferrite compositions can be synthesized as nanoparticles using any of the above approaches. In particular, Mn_xFe_1-xFe₂O₄ ferrites have been reported to have Curie temperatures between 75°C and 325°C and to have as-synthesized particle sizes between 6 and 20 nm. These physical properties fit well with our requirements for ferrofluid particles. Specific applications of interest at ORNL are smart cooling for electromagnetic actuators and microfluidics for Lab-on-a-Chip Technologies.

For publications related to ferrofluids, see the following links:

http://www.ornl.gov/sci/ees/mssed/res/ferrofluidpump.htm

http://www.ornl.gov/sci/ees/mssed/res/smartcooling.htm

For further information, contact Dr. Lonnie J. Love.

 

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Last Updated:  June 24, 2009