频率与重力对在均匀外加交流电场下平移的活性金属-介电 Janus 颗粒取向的影响
The influence of frequency and gravity on the orientation of active metallo-dielecric Janus particles translating under a uniform applied alternating-current electric field
AI总结：
针对交流电场中金属-介电 Janus 颗粒取向偏离问题，本文通过实验和转矩平衡分析揭示频率、电压、重力及颗粒-壁面作用的共同影响，并证明取向偏离会降低迁移率。
摘要：
活性 Janus 颗粒的理论和数值模型通常假设金属-介电界面平行于驱动所施加的电场。然而，我们的实验观察表明，电动驱动的 Janus 颗粒的平衡取向角随所施加电场的频率和电压而变化。在此，我们量化了取向相对于电场的变化，并证明平衡位置体现了重力、静电和电流体动力学转矩之间的相互作用。后两类转矩是所施加场（频率、电压）以及颗粒在基底上方高度的函数。在具有感应电荷电泳特征的低频条件下，以及重力主导静电和电流体动力学转矩的低电压条件下，会出现与电场对齐的最大偏离。通过比较仅受电驱动力的自由悬浮 Janus 颗粒与利用磁转矩使界面与电场对齐的磁化 Janus 颗粒，研究表明界面偏离与电场对齐会降低颗粒迁移率。考虑重力转矩和颗粒-壁面相互作用的作用，可以解释活性物质系统中理论、数值与实验之间的某些差异。
英文摘要：
Theoretical and numerical models of active Janus particles commonly assume that the metallo-dielectric interface is parallel to the driving applied electric field. However, our experimental observations indicate that the equilibrium angle of orientation of electrokinetically driven Janus particles varies as a function of the frequency and voltage of the applied electric field. Here, we quantify the variation of the orientation with respect to the electric field and demonstrate that the equilibrium position represents the interplay between gravitational, electrostatic and electrohydrodynamic torques. The latter two categories are functions of the applied field (frequency, voltage) as well as the height of the particle above the substrate. Maximum departure from the alignment with the electric field occurs at low frequencies characteristic of induced-charge electrophoresis and at low voltages where gravity dominates the electrostatic and electrohydrodynamic torques. The departure of the interface from alignment with the electric field is shown to decrease particle mobility through comparison of freely suspended Janus particles subject only to electrical forcing and magnetized Janus particles in which magnetic torque is used to align the interface with the electric field. Consideration of the role of gravitational torque and particle-wall interactions could account for some discrepancies between theory, numerics and experiment in active matter systems.
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