本研究為了解磁流體於密封裝置之活塞-圓柱介面內流動行為，在外部磁場作用下，探討穩定磁流體對運動活塞與固定圓柱間隙內所形成的環形膜之影響。通過解析求解得出作用在活塞上的流動阻力近似解，並呈現磁流體在活塞與圓柱於不同半徑比下之結果。研究結果顯示，儘管使用磁流體可獲得不錯的密封效果，但流體內磁性奈米粒子的存在將導致流體黏度增加，而黏度的增加可進而導致流阻增加。這意指增加了摩擦損耗，從而降低了活塞的動態性能。然而，結果亦可發現，偶極粒子若越軟，亦或活塞與圓柱間的半徑差若越大，則可降低磁流體活塞於圓柱內移動所經歷的摩擦。

It is desirable to understand the fluidity of a magnetic fluid in the piston-cylinder interface gap of a magnetic fluid sealing apparatus. In this study, the effect of magnetic fluid on the action of annular film of a stable magnetic fluid existing in the long gap between a moving piston and a stationary cylinder was investigated in the presence of an applied magnetic field. The closed-form solution of the flow drag acting on the piston was derived analytically and presented for magnetic fluids under different radius ratios of the piston to the cylinder. The results revealed that, although the magnetic fluid used could seal the cylinder well, the presence of magnetic nanoparticles in the fluid results in an increase in fluid viscosity; moreover, increased viscosity leads to increased flow drag. This means that the friction loss is enhanced and the piston’s dynamic performance is thereby reduced. However, it has also been found that the softer the dipolar particles or the larger the radius difference between the piston and cylinder, the lower the friction experienced by the magnetic fluid piston.