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

H. C. Yang¹ - S. Y. Yang² - W. S. Tse² - Y. P. Chiu³ - H. E. Horng³ - Chin-Yih Hong⁴

¹ Department of Physics, National Taiwan University, Taipei 106, Taiwan
² Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan
³ Department of Physics, National Taiwan Normal University, Taipei 117, Taiwan
⁴ Department of Mechanical Engineering, Da-Yeh University, Chang-Hwa 515, Taiwan

Abstract
The shape evolution of magnetic fluid microdrops in an over-saturated magnetic fluid thin film during the application of the 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 dH/dt of the applied field but also upon the diameter a of the initially circular microdrops. For lower dH/dt and smaller a, 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 and under higher dH/dt, reached a phase in which they had a branched strucutral shape. A phase diagram with a horizontal axis, dH/dt, 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 ac-dH/dt curve, which separates the two phases in the phase diagram, follows the power law, ac ~ (dH/dt)^p, with the power p ~ -0.14. All these details will be discussed further in this paper. Figs 4, Refs 6.