流體中微磁珠串操控技術之研究

Abstract

本研究主要探討微磁性粒子在擺動磁場下之運動模式，並透過實驗結果分析其操控機制。實驗係利用微米尺寸之超順磁性粒子，先施以單一方向之外加磁場將磁性粒子串接，再施加擺動磁場進行操控粒子串之實驗，因粒子具超順磁性，將磁場關閉後，粒子會失去磁性，而回復原始狀態，利用此特性，可形成一簡易且可逆式之微機構。由實驗結果發現，藉由增加粒子串長度或擺動磁場強度，可將粒子串之運動區分為剛體擺動、扭曲及斷裂等模式。有關粒子串長度對於擺動軌跡之影響，其較短之粒子串，由於所受阻力較小，可與外加磁場較同步地擺動，亦即其擺動軌跡與外加磁場較貼近；相反地，長度較長之粒子串，其擺動軌跡則較偏離外加磁場；而磁場強度大小對於粒子串擺動軌跡之影響，則與前述不同。事實上，在較大的擺動磁場下，粒子串擺動軌跡與磁場較同步，但與磁場間的相位角差，在特定時間點存在較大值。由本研究之實驗結果證實，粒子串斷裂時之時機可由N‧Mn1/2之值來判斷，其中N 表示粒子串所含之粒子數，Mn為Mason number，為無因次參數，其定義係流體阻力與外加磁力之比值。 此外，本研究亦發現微磁性粒子串在擺動磁場下會出現之一有趣現象-「軌跡轉換」，當粒子串與外加磁場間之相位角差超過90度時，此現象即會發生，此時，粒子串之擺動軌跡不再依原軸線擺動，而將轉換成沿著垂直原軸線的方向擺動，此現象即為「軌跡轉換」。本研究將有系統地呈現粒子串在不同實驗條件下的軌跡轉換情形，並說明如何將軌跡轉換的原理應用於提升微機電系統之操控機制，如改善微游泳器之操控等。在微游泳器之操控方面，我們利用了磁粒子串穩定擺動及軌跡轉換之機制，將大小不同的磁性粒子串接，形成不對稱之粒子串，在擺動磁場下產生淨推力，成功地操控其向前游動及轉向，而形成可操控方向之微游泳器，後續研究將著重於精進操控方法及增進其應用性。

We investigate experimentally the dynamics of microchains containing superparamagnetic particles and the further application in an oscillating field. The chains are first formed by a static directional field, and then manipulated by an additional dynamical perpendicular field. The present methodology represents a simple reversible chaining process, whose particles can be re-dispersed after removal of the field. The motion of superparamagnetic chains is dominated by magnetic torque and induced hydrodynamic drag. The effects of key parameters, such as field strengths and the lengths of particle chains, are thoroughly analyzed. Distinct behaviors, from rigid body oscillations and bending distortions to rupture failures, are observed by increasing the amplitudes of oscillating fields or chains’ lengths. Because of lower induced drag, a shorter chain follows the field trajectory closely and oscillates more synchronically with the external field. On the other hand, the influences of field strengths are not consistent. Even the overall oscillating phase trajectory in a stronger external field deviates less significantly from the corresponding field trajectory, a stronger dynamical component of the external field results in larger phase angle lags at certain points. The experimental results confirm the criterion of ruptures can be effectively determined by the value of (N*Mn1/2), where Mn is the Mason number defined as the ratio of induced drag to dipolar attraction, and N represents the number of particles contained in a chain. In addition, we report an interesting phenomenon of “trajectory shift” of magnetic chains in an oscillating field as well. When the phase angle lags of chains to the external field exceeding 90 degrees, the phenomenon shifts the chain’s oscillating trajectory along a new axis, which is perpendicular to its original axis. Applicability of the phenomenon to a stable chain in various conditions is experimented systematically. The trajectory shift provides an effective manipulating mechanism in a MEMS system, such as steering of micro-swimmers. By utilizing the mechanism of the stable oscillating chain and trajectory shift, we successfully develop a micro-swimmer by connecting particles of different sizes to break the symmetry of the chains to generate thrust to move forward and reverse. Further research is required to refine the manipulating methodology and expand the applicability of the micro-swimmers.