Novel Convective Instabilities in a Magnetic Fluid

Abstract

Nonlinear dynamics outside equilibrium is one of the intriguing and ubiquitous phenomena in nature. Exploring new mechanisms for driving forces and understanding relations between structures and dynamics will not only elucidate the underlying physics but also lead to new applications. In most research on instabilities, the driving forces involve gravity, surface tension, and ponderomotive force [1,2]. In this Letter, we report the observation of a novel instability in a horizontal magnetic fluid layer with a focused laser beam perpendicularly passing through the layer in the presence of a homogeneous vertical magnetic field. We found that the diffraction pattern formed by the laser beam remained stable when the field was less than a critical value, whereas it became unstable when the field was larger than the threshold. This instability is driven by a force that was rarely studied before — the magnetic Kelvin body force that originates from the interaction of the magnetic moment of the fluid with the internal field gradient induced by temperature and concentration inhomogeneities, even though the external field is uniform in space. This force can be easily adjusted, by changing the external field and the laser power, to control heat and mass transfers in the system. Studies of this novel instability should lead to not only new insights in fundamental research on nonequilibrium phenomena but also practical applications such as field-controlled heat transfer devices and liquid optical devices.