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Abstract：
In this study, the evaporation characteristics and drying patterns of various sessile ferrofluid droplets on certain substrate under horizontal magnetic fields of controlled intensities are reported. The effects of droplet concentration and magnetic field intensity on the duration of each evaporation stage and drying patterns of droplets have been systematically investigated. It turned out that a plateau appears at the initial stage of evaporation in the absence of magnetic field and it was found that the plateau value is positively correlated with the concentration of ferrofluid droplets. Under the external magnetic field, the evaporation time of droplets decreases, the stage of contact line retreat extends, the stage of late pinning mode shortens, and the deposition area of ferrofluid droplet decreases compared to that of without magnetics field. The deposition area increases gradually and becomes more uniform with the increase of magnetic field. The decrease of friction force which is due to the decrease of the number of nanoparticles at the contact line under external magnetic field is the main reason for the observed phenomena. We found that the coffee ring and the uniform deposition inside the droplet will be destroyed when the magnetic field intensity is higher than a critical value. Our work has a significant reference value for the evaporation of sessile magnetic fluid droplets under the applied magnetic field, especially when the drying pattern needs to be precisely controlled, such as in spray or biomedicine.
摘要：
在这项研究中,报道了在受控强度的水平磁场下，某些基底上各种固着铁磁流体液滴的蒸发特性和干燥模式。系统研究了液滴浓度和磁场强度对每个蒸发阶段持续时间和液滴干燥模式的影响。事实证明，在没有磁场的情况下，在蒸发的初始阶段出现了平台，并且发现平台值与铁磁流体液滴的浓度呈正相关。在外部磁场下，液滴的蒸发时间减少，接触线后退的阶段延长，后期钉扎模式的阶段缩短，与没有磁场的情况相比，铁磁流体液滴的沉积面积减小。随着磁场的增大,沉积面积逐渐增大,变得更加均匀。由于在外部磁场下接触线处纳米颗粒数量的减少，摩擦力的降低是观察到的现象的主要原因。我们发现，当磁场强度高于临界值时，咖啡环和液滴内部的均匀沉积会被破坏。我们的工作对在外加磁场下固基磁流体液滴的蒸发具有重要的参考价值,特别是当干燥模式需要精确控制时，例如在喷雾或生物医学中。
