Size effect on shape evolution of microdrops in magnetic fluid thin film under perpendicular magnetic fields

The shape evolution of magnetic fluid microdrops in an oversaturated magnetic fluid thin film during the application of a magnetic field h perpendicular to the film was investigated in this study. It was found that the shape evolution depends upon not only the sweep rate dhldt of the applied field but also upon the diameter a of the initially circular microdrops. For lower dhldt and smaller a's, the circular shape of the microdrops evolved as they reached a dumbbell phase as the magnetic field was increased. On the other hand, the microdrops with larger a's and under higher dhldt reached a phase in which they had a branched structural shape. A phase diagram with a horizontal axis, dhldt, and a vertical axis, a, was then constructed for the pattern evolution of the microdrops in the magnetic fluid thin film. Further analysis indicated that the a_c-dh/dl curve, which separates the two phases in the phase diagram, follows the power law, a_c ∞ (dh/dt)^p, with the power p ~ -0.14. All these details will be discussed further in this paper.