Easy-Plane Alignment of Anisotropic Biofluid Crystals in a Magnetic Field: Implications for Rod Orientation

We study the orientation in a uniform magnetic field of rod-like anisotropic biofluid crystals with an easy plane that makes an oblique angle with the crystal’s c-axis. For a sufficiently strong field, these crystalline rods orient themselves such that the crystal’s easy plane is parallel to the magnetic field, the rod’s direction being defined as the direction of the crystal’s c-axis. As the rod rotates about the crystal’s hard axis there will therefore be a range of angles that the rod makes with the magnetic field. We detail this behavior by first providing illustrations of hemozoin crystals at various orientations. These illustrations clearly demonstrate that the orientation angle that the crystalline rod makes with respect to the magnetic field varies from about 30° to 150°. We also derive an analytical expression for the probability density function for the orientation angle. We find that the orientation angles are not uniformly distributed between the limits of 30° and 150°, but rather tend to cluster near these limits. This suggests experimental tests and addresses confusion about the rod orientation found in past literature. The relevance to other anisotropic biofluid crystals, such as those produced by gout, is also discussed.