非對稱粒子在不同取向下感應電荷電泳之研究

Abstract

近幾年，科學家應用設計膠體粒子的表面性質或施加電場來控制 Janus 的移動或是組裝，進而利用這些科技來完成如藥物輸送或是自組裝等目的。在均勻的低頻交流電場下，可極化表面周圍誘導出非線性的電荷，並被電場所驅動形成感應電荷電滲流，而感應電荷電滲流帶動粒子移動的行為被稱作感應電荷電泳。然而，目前並無直接的實驗數據探討非對稱粒子的感應電荷電滲流與感應電荷電泳對於電場方向的依賴性。本實驗使用粒徑為 3 微米之超順磁粒子半邊鍍有黃金形成 Janus 粒子，使其具有三種不同的物理性質，通過施加一微弱磁場操作粒子內部磁矩的轉向，實驗測量不同塗層厚度的 Janus粒子在不同取向下的極化反應行為。 在金屬塗層低於 30 奈米之薄塗層 Janus 粒子的情況下，其感應電荷電泳方向主要垂直於電場，並保持聚苯乙烯端在前方運動；接著在磁場中測量不同取向下 Janus粒子的感應電荷電泳速度和感應電荷電滲流的流動模式，實驗結果觀察到速度明顯地有取向依賴關係，垂直於電場軸線時速度有最大值，隨著運動漸漸轉為平行於電場軸線運動時，速度會慢慢降低。對於金屬塗層高於 45 奈米之厚塗層 Janus 粒子，其感應電荷電泳主要平行電場，並保持聚苯乙烯在前方運動，並且在不同取向下觀察到速度與薄塗層呈現截然相反的取向依賴性。因此我們認為 Janus 粒子的金屬塗層厚度與顆粒感應電荷電泳的取向是由感應電偶極強度所主導。另外，在高頻交流電場下，電雙層沒有足夠的時間進行充放電行為，實驗觀察到電泳運動反轉的現象，並保持金屬端在前方移動，我們認為在金屬電介質界面處會產生表面電場梯度，吸引非均勻的離子進而引起一壓力差使粒子移動。最後，本研究創新設計旋轉磁場和調整交流電場，提供了一種可編程粒子移動路徑的技術，達到在微流體中自由操作粒子的目的。

Recently, scientists reported that the flows resulting from the action of an applied electric field on its own induced charges around a polarizable structure as “induced-charged electro-osmosis” (ICEO), which is capable of driving Janus particle, whose surfaces have two or more distinct properties, under the electric fields by “induced-charge electrophoresis” (ICEP). However, the directly experimental data of particle velocity for ICEP and flow velocity for ICEO in orientation-dependency with respect to electric fields are seldom investigated by scientists. We achieve that the magnetic moment of superparamagnetic Janus particle can be manipulated by external magnetic fields, the polarization effect of Janus particles with different coating thickness experimentally measured in different orientation under the electric field. In the case of Janus particles with thin metallic coating, the ICEP motion of particle mainly moves perpendicular to the electric field without magnetic. The ICEP velocity and ICEO flow of Janus particle in different directions are measured in magnetic field and clear orientation dependent velocities are observed. However, for a thick metallic coating, the ICEP motion is dramatically parallel to the electric field without magnetic field and different orientation-dependent velocities are observed. We proposed that the strength of induced dipole of a Janus particle, depending on the thickness of the metallic coating and orientation of a particle, dominates the behavior of ICEP. Finally, this study provide a programmable method to control the particle by a rotating magnetic and frequency adjustment.