| Summary: | 碩士 === 國立雲林科技大學 === 機械工程系碩士班 === 94 === Abstract
Experiments are carried to investigate the interfacial morphologies of a circular magnetic thin film under a perpendicular field. Four patterns of striking film rupturing instabilities are recorded .Formations of different modes of instability are found to depend strongly on the initial diameters of the circular film. For an instant field strength =584 gauss, three modes of well-ordered breakup instability patterns are identified for drop sizes ranging from d ≈ 400~1200μm. The pattern structures can be categorized into three main parts: (1) a central droplet, (2) the outer fluids in forms of either an outer residual annulus/ring or an outer array of secondary droplets with/without derivative droplets, and (3) the middle region that might evolve into a single middle array of derivative droplets. The mode I instability appears only a central droplet is fully evolved. Numerous incompletely developed droplets, or waves, are observed at the outer fluid annulus. These waves are further destabilized and fully evolve into an outer circular array of secondary droplets, referred to as the mode II instability. A more vigorous Mode III instability is observed, that an additional circular array of derivative droplets appears in the middle region. In addition, smaller derivative Hext droplets are formed between the secondary droplets at the outer array. A very complex and disorderly mode IV instability is recorded for large diameter of d ≧ 1,500μm , that numerous derivative droplets with multiple arrays in the middle region and between the secondary droplets at the outer array are formed. Owing to the expelling interactions between the magnetized fluids or possible non-uniformity of the field, the secondary droplets at the outer array are drifting outward very significantly. Besides the strong dependence on the initial diameters, the present study also confirm the common expectations that a more unstable mode of instability occurs in a field condition with either a stronger magnetic field strength or a higher field increasing rate.
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