(70d) Size Fractionation of Nanoparticles by Magnetophoresis Using Structured Wire Arrays

Annavarapu, V. N. R., Massachusetts Institute of Technology
Hatton, T. A., Massachusetts Institute of Technology
Smith, K. A., Massachusetts Institute of Technology

In the fine particles industry, precise control of size of nanometer and sub-micrometer particles is critical for numerous applications. This control is difficult during synthesis. Hence, there is a genuine need for a generic industrial scale separation technology in the nanometer size range.

We have developed such a generic macroscale separation system for submicron and nano-sized particles based on the principle of magnetophoresis and also demonstrated the separation capability of our technique experimentally. When any non-magnetic particle (viz. polymer, metal, metal oxide etc.) is immersed in a magnetic fluid it behaves like a magnetic hole. A magnetic hole is subjected to size dependent magnetophoretic forces (proportional to volume) in the presence of an inhomogeneous magnetic field (IMF). We exploit the size dependence of these magnetophoretic forces (MF) for separating particles of different sizes. Our separation system consists of a structured array of iron wires in a flow cell which generates an IMF and preferentially retains larger nanoparticles while allowing smaller particles to flow through.

We performed particle tracking simulations to understand the variation of non-magnetic particle motion with size in the presence of a wire array. By a dominant force balance, the motion of the particle was found to depend mainly on the balance of fluid drag vs. MF. Particles of different sizes behaved differently with larger particles getting captured on wires whereas smaller particles flowed through the system. This behavior was quantified with the help of a dimensionless number FR which was the force ratio of MF to fluid drag. This quantification helped in the separation system design and geometry optimization of the wire array.

The proposed separation concept was verified experimentally. A lab scale prototype of the separation device was fabricated using the optimized wire geometry. Capture experiments were performed at different FRs (ratios of MF to fluid drag) in the device. The particle behavior was observed using fluorescence microscopy. The results were consistent with simulations and indicated that at larger FR particles get captured on the wires whereas at smaller FR they flow through. Larger particles were captured preferentially using this device and feasibility of separation was shown experimentally.