频率依赖的磁调制对沉积形貌的影响
Frequency-Dependent Magnetic modulation of deposition morphology
AI总结：
研究通过频率依赖的磁场调制影响铁流体蒸发过程中的沉积形貌，发现驱动频率影响环的数量及咖啡环效应的抑制。
摘要：
本文提出了一种新的方法，用于在蒸发的铁流体液滴中通过磁调制来调节沉积形貌。磁场强度和铁流体浓度保持不变，而驱动频率则从0.016 Hz到5 Hz变化。在没有磁场的情况下，观察到咖啡环形形成，并与之前的研究所一致\cite{deegan1997capillary,deegan2000contact,saroj2019drying}。应用一个随时间变化的磁场显著改变了沉积形貌。周期性磁场诱导了蒸发过程中多个同心环的形成。环的数量最初随着电磁铁的驱动频率增加而增加。然而，在临界驱动频率（$f_c = 0.2\,\text{Hz}$）以上，环的数量减少。在更高的驱动频率下，磁性颗粒优先沉积在液滴的中心区域，导致咖啡环效应的抑制。此外，随着驱动频率的增加，内环的厚度和环间距在临界驱动频率之前减少。从多环形形成到咖啡环抑制的转变由磁力、毛细流动和粒子扩散之间的竞争决定。通过缩放论证评估了周期性磁场下液滴动态和沉积形貌的内在物理机制。结果表明，扩散粒子运输在确定沉积模式中起主导作用。引入了一个基于磁扰动时间尺度的无量纲磁开关数作为控制参数，以表征频率依赖的沉积行为。
英文摘要：
This paper presents a novel approach for magnetic modulation of deposition morphology in an evaporating ferrofluid droplet. The magnetic field strength and ferrofluid concentration are kept unchanged, while the actuation frequencies are varied from 0.016 Hz to 5 Hz. In the absence of a magnetic field, a coffee-ring formation is observed and consistent with previous studies\cite{deegan1997capillary,deegan2000contact,saroj2019drying}. The application of a time-dependent magnetic field significantly modifies the deposition morphology. The periodic magnetic field induces the formation of multiple concentric rings during evaporation. The number of rings initially increases with increasing actuation frequency of the electromagnet. However, beyond a critical actuation frequency ($f_c = 0.2\,\text{Hz}$), the number of rings decreases. At higher actuation frequencies, magnetic particles preferentially deposit in the central region of the droplet, resulting in suppression of the coffee-ring effect. Additionally, the thickness of the inner rings and the ring spacing decrease with increasing actuation frequency up to critical actuation frequency. The transition from multi-ring formation to coffee-ring suppression is governed by the competition among magnetic forcing, capillary flow, and particle diffusion. The underlying physical mechanisms responsible for droplet dynamics and deposition morphology under periodic magnetic fields are evaluated using scaling arguments. The results demonstrate that diffusive particle transport plays a dominant role in determining the deposition pattern. A non-dimensional magnetic switching number, based on the magnetic perturbation timescale, is introduced as a control parameter to characterize the frequency-dependent deposition behavior.
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