The present article discusses the physics and mechanics of evaporation of pendent, aqueous ferrofluid droplets and modulation of the same by external magnetic field. We show experimentally and by mathematical analysis that the presence of magnetic field improves the evaporation rates of ferrofluid droplets. First we tackle the question of improved evaporation of the colloidal droplets compared to water, and propose physical mechanisms to explain the same. Experiments show that the changes in evaporation rates aided by the magnetic field cannot be explained on the basis of changes in surface tension, or based on classical diffusion driven evaporation models. Probing using particle image velocimetry shows that the internal advection kinetics of such droplets plays a direct role towards the augmented evaporation rates by modulating the associated Stefan flow. Infrared thermography reveal changes in the thermal gradients within the droplet and evaluating the dynamic surface tension reveals presence of solutal gradients within the droplet, both brought about by the external field. Based on the premise, a scaling analysis of the internal magnetothermal and magnetosolutal ferroadvection behavior is presented. The model incorporates the role of the governing Hartmann number, the magnetothermal Prandtl number and the magnetosolutal Schmidt number. The analysis and stability maps reveal that the magneto-solutal ferroadvection is the more dominant mechanism, and the model is able to predict the internal advection velocities with accuracy. Further, another scaling model to predict the modified Stefan flow is proposed, and is found to accurately predict the improved evaporation rates.

中文翻译：

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磁场环境中铁磁流体液滴的铁磁平流辅助蒸发动力学

本文讨论了悬垂的铁磁流体水滴蒸发的物理和力学原理，以及外部磁场对其的调制。我们通过实验和数学分析表明，磁场的存在提高了铁磁流体液滴的蒸发率。首先，我们解决了胶体液滴与水相比改善蒸发的问题，并提出了解释这一问题的物理机制。实验表明，磁场辅助蒸发速率的变化不能基于表面张力的变化或基于经典的扩散驱动蒸发模型来解释。使用粒子图像测速仪进行探测表明，这种液滴的内部平流动力学通过调节相关的 Stefan 流对增加的蒸发率起着直接作用。红外热成像揭示了液滴内热梯度的变化，评估动态表面张力揭示了液滴内溶质梯度的存在，两者都是由外场引起的。在此前提下，给出了内部磁热和磁溶胶铁运动行为的标度分析。该模型结合了控制哈特曼数、磁热普朗特数和磁溶施密特数的作用。分析和稳定性图显示磁溶胶铁运动是更占主导地位的机制，并且该模型能够准确地预测内部平流速度。此外，还提出了另一个用于预测修正 Stefan 流的比例模型，并发现它可以准确预测改进的蒸发率。