Abstract
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 (L/D = 10) 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 (f) 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.
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