旋转磁场中蛋黄颗粒的极相
Polar Phase of an Egg-Yolk Particle in a Rotating Magnetic Field
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中文总结 AI 辅助
本研究通过实验和扩展模型，揭示了旋转磁场中蛋黄颗粒在超过瞬时界限时出现的极相，并表明该相由内部摩擦参数κ调控，为磁性颗粒动力学提供了新见解。
中文摘要 AI 辅助
在旋转磁场作用下，粘性液体中的磁性颗粒所选择的轨道通常用瞬时Stoner-Wohlfarth极限来描述，在该极限下，磁矩始终占据各向异性能量的最小值。在此极限下，只有当约化场$B_0/B_a<1/\sqrt2$时，才存在离开磁场平面的进动，而更强的场只允许平面内的同步旋转或天平动。我们研究了一个宏观的蛋黄颗粒，即一个打印的壳体内包含两个在流体填充腔中自由旋转的永磁体，由补偿旋转磁场驱动，并通过光学方式跟踪。除了同步旋转、进动和天平动外，该颗粒还表现出一种极相：在瞬时界限之上且在高频率下，壳体轴线离开磁场平面，并锁定在围绕旋转轴的窄锥体上。我们通过磁体与壳体之间的有限粘性摩擦扩展了模型。该扩展引入了一个单一的无量纲参数，即内部与外部旋转摩擦之比$\kappa$，并在$\kappa\to0$时简化为瞬时理论。数值相图表明，增加$\kappa$会使双稳态带在高频率下向更高场倾斜，并将稳定进动扩展到$B_0/B_a=1/\sqrt2$以上，而对于大的$\kappa$，平面天平动在高频率下会回归。通过校准的各向异性场、弛豫速率以及量级为1的$\kappa$，该模型重现了所有测量记录的终态及其倾斜角，包括极锥。在模型中，极态由磁体相对于壳体的稳定旋转维持，该旋转由场和各向异性力矩驱动，并由内部摩擦耗散；因此，它仅存在于中间$\kappa$值下。
英文摘要
The orbit selected by a magnetic particle in a viscous liquid under a rotating field is commonly described in the instantaneous Stoner--Wohlfarth limit, in which the magnetic moment always occupies a minimum of the anisotropy energy. In this limit, precession out of the field plane exists only for reduced fields $B_0/B_a<1/\sqrt2$, and stronger fields admit only planar synchronous rotation or libration. We study a macroscopic egg-yolk particle, a printed shell containing two freely rotating permanent magnets in fluid-filled cavities, driven by a compensated rotating field and tracked optically. In addition to synchronous rotation, precession, and libration, the particle exhibits a polar phase: above the instantaneous bound and at high frequency, the shell axis leaves the field plane and locks onto a narrow cone about the rotation axis. We extend the model by a finite viscous friction between the magnets and the shell. The extension introduces a single dimensionless parameter, the ratio $κ$ of inner to outer rotational friction, and reduces to the instantaneous theory as $κ\to0$. Numerical phase diagrams show that increasing $κ$ tilts the bistable band to higher fields at higher frequencies and extends stable precession above $B_0/B_a=1/\sqrt2$, whereas for large $κ$ planar libration returns at high frequency. With a calibrated anisotropy field, relaxation rate, and $κ$ of order unity, the model reproduces the terminal state of all measured records and their tilt angles, including the polar cone. In the model the polar state is maintained by a steady rotation of the magnets relative to the shell, driven by the field and anisotropy torques and dissipated by the inner friction; it therefore exists only for intermediate $κ$.
发表机构
- University of Latvia（拉脱维亚大学）
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