The present work reports the dynamics of an isolated ferrofluid slug of predefined length driven by air inside a dry capillary under the influence of constant as well as time dependent magnetic fields. It is shown that the pressure characteristic (pressure drop across the slug versus time) of slug, due to the supportive (slug at upstream of magnet) and resistive nature (slug at downstream of magnet) of the magnetic force field, exhibits a drop and rise pattern. We also noted that the peak pressure becomes constant beyond a threshold slug length $$\left( {L/D = 10} \right)$$ for a given strength of applied field. Also, we observed that the pressure characteristics of the slug are qualitatively similar for both types of magnetic perturbation, though, differ quantitatively due to the development of complex force field in the magnetically influenced zone. Substantial draining of liquid film has been observed from the receding meniscus of the slug, under constant magnetic field at two different regions viz., as the slug enters into the “Magnetically Assisted (MA)” zone and when it leaves from the “Magnetically Opposed (MO)” zone (From here onwards, we will be using “MA” and “MO” zones unambiguously.). Finally, the effect of the alternating magnetic field modulated by the frequencies $$\left( f \right)$$ gives rise to an impulsive force on the slug in the magnetically influenced zone, which in turn leads to accelerated–decelerated motion of the slug. This accelerated–decelerated motion yields substantial amount of film deposition on the wall for $$f = 3$$ Hz. A theoretical model is developed and subsequently compared with the experimental results, for estimation of the maximum pressure drop exhibited by the ferrofluid slug in presence of magnetic field. The inferences drawn from the study may have far reaching consequences in designing microscale thermal management systems/devices.

中文翻译：

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存在外加磁场时微毛细管内单个隔离铁磁流体塞的动力学

目前的工作报告了在恒定磁场和时间相关磁场的影响下，由干燥毛细管内的空气驱动的预定长度的隔离铁磁流体段塞的动力学。结果表明，由于磁力场的支持性（磁铁上游的段塞）和阻力性质（磁铁下游的段塞），段塞的压力特性（段塞上的压降随时间变化）表现出下降和上升模式。我们还注意到，对于给定的外加场强度，峰值压力超过阈值段塞长度 $$\left( {L/D = 10} \right)$$ 时变得恒定。此外，我们观察到，对于两种类型的磁扰，段塞的压力特性在性质上是相似的，但是，由于磁影响区中复杂力场的发展，在数量上有所不同。在两个不同区域的恒定磁场下，从段塞的后退弯月面观察到液膜大量排出，即，当段塞进入“磁辅助（MA）”区域时，以及当它离开“磁反对”区域时(MO)”区域（从这里开始，我们将明确使用“MA”和“MO”区域。）。最后，由频率 $$\left(f\right)$$ 调制的交变磁场的影响在磁影响区中的段塞上产生了一个脉冲力，这反过来导致了加速-减速运动蛞蝓。这种加速-减速运动会在 $$f = 3$$Hz 的情况下在墙上产生大量的薄膜沉积。开发了一个理论模型并随后与实验结果进行了比较，以估计存在磁场时铁磁流体段塞所表现出的最大压降。从研究中得出的推论可能对设计微型热管理系统/设备产生深远的影响。