Magnetic-field-induced optical transmittance in colloidal suspensions

Through simulation and experiment we demonstrate that when a magnetic field is applied to a suspension of magnetic particles, the optical attenuation length along the direction of the field increases dramatically, due to the formation of chainlike structures that allow the transmission of light between the strongly absorbing particles. This phenomenon is interesting for two reasons; first, there might be practical applications for this effect, such as optical-fiber-based magnetic field sensors, and second, measuring the time evolution of the optical attenuation length enables us to determine the kinetics of structure formation, which can be compared to the predictions of simulation and theory. In agreement with both simulation and theory, the optical attenuation length increases as a power of time, but much less light is actually transmitted than expected, especially at higher particle concentrations. We conclude that particle roughness, which is not included in either theory or simulation, plays a significant role in structural development, by pinning structures into local minima. copyright 1999 The American Physical Society