碰撞铁磁流体液滴中的磁聚并现象

中文摘要 AI 辅助

我们研究了在水平磁场存在下，自由下落的铁磁流体液滴与同种液体的座滴碰撞的流体动力学；该构型将液滴间聚并与伴随的场控润湿和铺展耦合在一起。利用高速成像，我们追踪了从冠状形成、径向铺展到边缘脱离（在特定条件下）的事件过程，涵盖了三种铁磁流体浓度、两种不同润湿性的基底（玻璃和PET）以及一系列撞击速度和磁场强度。最大冠状高度被注意到遵循标度关系 $H_{c,\max}/D_t\sim Fr^{0.5}$（$Fr$：弗劳德数），远低于弹道上限 $H_{c,\max}/D_t\sim Fr$。在零场条件下，当用合并后的撞击速度和合并液滴尺寸表示时，最大铺展因子符合边界层标度 $\beta_{0,\max}\sim (We_0/Oh)^{1/6}$（$We_0$：韦伯数，$Oh$：奥内佐格数）。施加磁场后，体耗散能量平衡预测 $\beta_{\max}\sim Z^{1/5}$，其中 $Z$ 结合了磁驱动项和惯性-毛细-粘性项，但观测结果反而遵循明显更弱的 $\sim Z^{1/11}$ 标度，这一偏差归因于磁粘效应引起的增强耗散，并通过有效奥内佐格数体现。最后，边缘脱离发生在依赖于场强和高度的阈值之上，由与尺寸无关的判据 $Fr^2Bo\approx 3550$（$Bo$：邦德数）描述，超过该阈值后边缘可能碎裂成子液滴。这些标度关系为磁辅助打印、按需液滴系统和涂覆工艺提供了预测工具，在这些应用中，重复的液滴碰撞发生在残余液滴或液层上。

英文摘要

We explore the collision hydrodynamics of a ferrofluid droplet falling freely onto a sessile droplet of the same liquid, in the presence of a horizontal magnetic field; a configuration that couples droplet-on-droplet coalescence with concomitant field-governed wetting and spreading. Using high-speed imaging, we track the events through crown formation, radial spreading, and rim detachment (under specific conditions), across three ferrofluid concentrations, two substrates of different wettability (glass and PET), and a range of impact velocities and magnetic field strengths. The maximum crown height is noted to scale as $H_{c,\max}/D_t\sim Fr^{0.5}$ ($Fr:$ Froude number); well below the ballistic upper bound of $H_{c,\max}/D_t\sim Fr$. At zero-field, the maximum spreading collapses onto the boundary-layer scaling $β_{0,\max}\sim (We_0/Oh)^{1/6}$ ($We_0$: Weber number, $Oh:$ Ohnesorge number) when expressed in terms of the merged impact velocity, and coalesced-drop size . With the field applied, a bulk-dissipation energy balance predicts $β_{\max}\sim Z^{1/5}$, where $Z$ combines the magnetic-driving, and inertial-capillary-viscous terms, but the observations instead follow a markedly weaker $\sim Z^{1/11}$, a deficit traced to enhanced dissipation from the magnetoviscous effects, and manifested via an effective Ohnesorge number. Finally, rim detachment occurs beyond a field- and height-dependent threshold, described by a size-independent criterion $Fr^2Bo\approx 3550$ ($Bo:$ Bond number), above which the rim may fragment into daughter droplets. These scalings provide predictive tools for magnetically assisted printing, droplet-on-demand systems, and coating processes, where repeated droplet collisions occur on the residual liquid droplet or layer.

发表机构

• Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）
• Indian Institute of Technology Ropar（印度理工学院罗帕尔分校）

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