| Summary: | 碩士 === 國立臺灣師範大學 === 物理研究所 === 87 === Abstract
Magnetic fluids are fluids consisting of particles with magnetic dipoles. Under the influence of externally applied magnetic fields, the magnetic particles in the fluid start to agglomerate and align with the direction of the field. When a magnetic fluid thin film is subjected to a magnetic field parallel to the plane of thin film (parallel magnetic field), the magnetic particles, inside magnetic fluid thin films, agglomerate and form a one-dimensional periodic long chain structure. Contrarily, under the action of magnetic fields perpendicular to the plane of thin film (perpendicular field), the particles in the film agglomerate and form particle columns. With the increasing H at a lower sweep rate, the columns evolved from a disordered column phase to a first-level hexagonal structure pattern and finally reached a second-level hexagonal structure pattern through a phase transition. During the transition phase, each column was observed to split into two columns. The split of a column was attributed to the dipolar repulsive force in each column. However, there was only one level of hexagonal structure pattern for a high sweep rate within the range of the magnetic field used here.
The effect of the temperature T to the hexagonal structure of the film revealed that the increase of the distance d between particle columns of hexagonal structures was caused by the thermal agitation of the particles in the film as T was increased. The magnetic fluid in this work became over-saturated at T 26oC, and a plateau was found for the d-T curve when T was within the range from 19.4 oC to 28.5 oC. As T 28.5oC, the d was increased rapidly as T increased.
The pattern forming system of magnetic fluid drops have also been investigated. The original cylindrical drops, which were formed by the condensation of magnetic particles in an over-saturated magnetic fluid film, evolved to a dumbbell phase or a branched structure under the influence of perpendicular magnetic fields. The cause of the pattern formation is believed to be mainly due to the competition between surface tension and the magnetic dipolar interactions between particles in each column. As the field strength increase, the pattern changes to a labyrinthine structure and finally to an equilibrium hexagonal structure. In this work, a critical diameter of the original cylindrical drops, which separated these two types of evolution processes, exists in the pattern formation for a given sweep rate. A dumbbell phase occurred when the diameter is below this value; otherwise, a branched structure formed.
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