ABSTRACT
Francis Bitter first detailed his work observing magnetic inhomogeneities, i.e., domain boundaries, using a fluid suspension of maghemite (Fe2O3) particles in 1931. Hereafter known as the “Bitter Technique,” the use of iron filings, micro- and nanoparticle ferrofluids, and other materials have long been staples of physical science demonstrations from elementary school through college. Broadly, the reason magnetic particles are attracted to these inhomogeneities is because a spatially changing magnetic field is present, i.e., a magnetic field gradient. The orientation energy of a magnetic particle in a magnetic field is given by E = − μ 0 m ⇀ ⋅ H ⇀ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315374086/dcafed40-6460-4e65-81bd-77416a7a7ddc/content/inline-math8_1.tif"/> , in SI units, with magnetic moment m in Am2, magnetic field H in A/m, and the permeability of free space μ 0 = 4 π × 10 − 7 N / A 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315374086/dcafed40-6460-4e65-81bd-77416a7a7ddc/content/inline-math8_2.tif"/> yielding E in Joules. Force is the gradient of this energy, and after applying appropriate vector identities for the magnetic field, one can write the force on a particle with magnetic moment m as F → = μ 0 ( m → ⋅ ∇ ) H ⇀ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315374086/dcafed40-6460-4e65-81bd-77416a7a7ddc/content/math8_1.tif"/> .
