Optical Properties and Applications of Ferrofluids—A Review

In recent years, ferrofluid has emerged as a new class of 'smart nanofluid,' whose properties are controllable by an external magnetic field. It is well known for its unique property of having both liquid and magnetic properties. Ferrofluids have been a topic of intense research during the last few decades due to their potential applications in mechanical, biomedical and optics industries. Some of the exciting applications of this material includes leak free seals, smart cooling, micro robot, micromechanical sensor, nondestructive defect sensors, tunable optical filters, actuators, optical grating, optical limiter, optical fiber modulator, magneto optical waveguide, magneto optical wavelength filter, optical switches, biosensors, hyperthermia treatment and tumor treatment, cell separation, magnetic resonance imaging based drug targeting, quantification of biomolecule agglutination etc. Besides several interesting optical effects have been observed in magnetic nanofluids under magnetic field. For example, on increasing the applied field, the transmitted intensity diminishes drastically, followed by the appearance of a 'ring' like structure in the scattered pattern, which finds applications in light limiters and switches. The scattering anisotropy factor and dipolar resonances within the scatterers are postulated as the origin for observed extinction of light. The formation of cylindrical rod-like structures, along the direction of light propagation, is manifested as a circular ring pattern in the scattered pattern. The observation of several critical fields at regular intervals in the transmitted intensity is attributed to the zippering transitions of the chains when they are in off registry. In this article, we present a critical review on various optical effects observed in ferrofluids under external magnetic field, the underlying physics and potential applications. It must be emphasized that the main focus is on scattering studies under magnetic field parallel to the incident light beam. The scattering and diffraction studies in ferrofluids (Section 4) have been elaborated for specialists.