An alternative approach to mixing materials

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2012

Two new magnetic techniques offer a unique way of mixing.

KEY CONCEPTS
• Two new techniques that utilize magnetism provide reliable mixing.
• Vortex field mixing involves the application of a magnetic field that disperses and assembles micron-sized iron particles into countless microscopic stir bars.
• Isothermal Magnetic Advection subjects thin, 20-micron magnetic platelets in a fluid to a biaxial magnetic field. This method can be used at scales ranging from nanoliters up to large commercial quantities.

AS A CHEMIST I have spent much time in my career dealing with mixing, whether it is reacting components at an elevated temperature or preparing a lubricant mixture at room temperature. There is a variety of mixing tools available, ranging from stirring bars to mechanical stirrers. On a larger commercial scale, companies use mixing vessels having various types of agitators. These are sometimes even equipped with baffles to further improve the mixing. Still there is a continuing interest in improving mixing methods.

In the development of new lubricants, more efficient mixing is desirable, both on the lab and the plant scale to improve productivity. In a previous TLT article, an automated blending unit was discussed that can prepare 160 blends in a 24-hour period (1). Using robotic arms, the automated unit can also handle viscous raw materials and disperse powders into liquids.

The application of magnetic fields to magnetic materials has been used as an alternative for some common applications. For example, the concept of magnetic refrigeration was examined previously in TLT (2). Conventional refrigeration uses gas compression to extract heat from a specific environment. This technique has some drawbacks due to the use of certain refrigerants that have negative impacts on the environment and difficulty in improving efficiency. Magnetocaloric metal alloys have been developed that can expel heat in the presence of a magnetic field. Once the field is turned off, the magnetocaloric material cools off and is ready to absorb more heat.

Jim Martin, member of the technical staff at Sandia National Laboratories in Albuquerque, N.M., says, “Conventional mixing techniques can be problematic, especially in congested or complex volumes, or in those cases where the need for a controlled atmospheric mitigates against the use of extraneous seals for rotating shaft. For example, the use of a magnetic stir bar eliminates the need for an additional seal, but one often finds that at high stirring speeds the stir bar goes into fibrillation. Stir bars are also not a good approach in complex fluid volumes such as those penetrated by cooling pipes.”

A second weakness is that conventional stirring techniques are very ineffective in mixing tiny volumes such as those in micrometer-sized channels. Martin says, “When fluid is pushed down a large pipe, eddies are generated to create mixing. But mixing does not occur in a very small pipe unless the fluid is subjected to very high pressure.”

New techniques are needed to more efficiently facilitate mixing such as two techniques described below that employ magnetism.

VORTEX FIELD MIXING
Martin and his associates have developed a new approach to mixing by suspending microscopic, magnetic particles in a fluid subjected to a dynamic magnetic field. The technique is known as vortex field mixing.

Martin says, “We first introduce a small volume fraction of 4- to 7-micron diameter iron or nickel particles into the fluid. Application of a vortex magnetic field then disperses these particles and assembles them into countless microscopic stir bars. The vortex field has a precession-like motion, something like a slowly spinning top before it collapses, and the particle stir bars mimic this motion throughout the entire fluid volume, leaving no stagnation regions. Because the particle stir bars constantly fragment and reform in response to their environment, the mixing instabilities one associates with a conventional stir bar do not occur. Fluid mixing is completely constant and reliable.”

Figure 3 shows a lab setup for vortex mixing in which the vessel containing the fluid is literally surrounded by a multiaxial magnet (This magnet is a research tool and is unnecessarily large for mixing). Fluid mixing using conventional methods is usually improved by increasing the rotational speed of a stir bar or a propeller of some sort, but increasing the frequency of the vortex magnetic field does not improve mixing at all. The mixing frequency can instead be selected for convenience. An energy-efficient vortex field mixer can be created by running the coils in series resonance with a capacitor chosen to produce resonance at the line frequency of 60 Hz, which mitigates the need for an amplifier.


Figure 3. Vortex field mixing is a new technique where iron particles dispersed in a fluid can be turned into microscopic stir bars when a magnetic field is applied. This results in a very effective way to prepare uniform mixtures. (Courtesy of Sandia National Laboratories)

In contrast to the conventional stir plate, increasing the magnetic field will lead to stronger mixing. Martin adds, “We found that doubling the strength of the magnetic field increases the strength of mixing by a factor of four.” This effect is due to the agglomeration of particles into longer stir bars. Of course, if the field is too small, mixing will not occur at all. For low-viscosity fluids, the required field is quite small, but as the liquid viscosity increases the required field increases proportionately.

Vortex field mixing is scalable to arbitrary fluid volumes, with the only significant limitation being the particle size selected. This makes microfluidic applications a real possibility. Martin adds, “With nanoscale magnetic particles, we believe this technique can be used on liquid volumes in the nanoliter range.”

ISOTHERMAL MAGNETIC ADVECTION
While vortex field mixing is effective, it does require a magnetic field comprised of three components. Such an assembly can get cumbersome, particularly at larger production scales.

Martin and his associates have discovered a way to stimulate highly organized fluid flows they call advection lattices by using magnetic platelets instead of spheres. This phenomenon, called Isothermal Magnetic Advection, occurs when these suspensions are subjected to biaxial fields comprised of only two components.

Martin says, “These flow patterns are created by suspending thin, 20-micron magnetic platelets in a fluid. Subjecting this suspension to a biaxial fluid comprised of two orthogonal components, each with a strength of 75-150 Gauss rms, creates a vigorous fluid flow. By using this process, we can create mixing flows that are useful for mixing or heat transfer with very little wall plug power.”

The magnetic fields can be generated with copper coils because the field strength required is low. Resonant copper coils are scalable devices whose power dissipation is proportional to their accessible interior volume. As a result, mixing can be potentially done at any scale from nanoliter up to large commercial quantities with the same energy efficiency. In fact, Martin has used this technique to disperse 10-nanometer iron particles that were so magnetically agglomerated they could not be dispersed by conventional means.

In the future, do not be surprised if you come across a reactor or large blend tank that is covered in copper coils. Additional information can be found in two recent articles (3, 4) or by contacting Stephanie Holinka of Sandia National Laboratories at slholin@sandia.gov.

REFERENCES
1. Canter, N. (2008), “Automated Lab Blending,” TLT, 64 (10), pp. 16-17.
2. Canter, N. (2009), “Magnetic Refrigeration: Another Way to Cool,” TLT, 65 (5), pp. 12-13.
3. Martin, J., Rohwer, L. and Solis, K. (2009), “Strong Intrinsic Mixing in Vortex Magnetic Fields,” Physical Review E, 80 (1), DOI: 10.1103/PhysRevE.80.016312.
4. Solis, K. and Martin, J. (2010), “Isothermal Magnetic Advection: Creating Functional Fluid Flows for Heat and Mass Transfer,” Applied Physics Letters, 97 (3), DOI: 10.1063/1.3462310.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.