擺動磁場下磁性粒子串之不穩定現象實驗研究

Abstract

近年來關於擺動磁場下磁性粒子串的不穩定現象已被提出作廣泛的探討，相關問題透過系統性的研究及實驗已能獲得有效的解決，磁性粒子串的斷裂模式可區分為兩種不同範圍，分別為微變形斷裂模式及強烈變形斷裂模式，微變形斷裂模式的特徵為其斷裂形態較具剛性，經常發生在外加動態磁場強度極大，造成磁性粒子串擺動振幅十分明顯的時候，其斷裂發生位置通常接近於粒子串的兩側，除此之外，還可以觀察到反轉斷裂模式這樣一個有趣的現象，其斷裂型態為中心斷裂段的擺動方向與外加磁場方向相反，並且會導致磁性粒子串結構產生永久的破壞。另外一方面，強烈變形斷裂模式的特色是磁性粒子串斷裂前會有明顯的彎曲變形，此模式容易發生於黏滯度較高的溶劑流體中，粒子串的斷裂範圍接近中間區塊，粒子串斷裂模式的不穩定程度可透過降低串接磁場強度或增加粒子串的組成顆數來作提升，這些實驗觀察結果在定性上與用正向力分佈判定粒子串斷裂不穩定的標準是一致的。從這些不同操控條件的實驗中，對於判斷粒子串斷裂不穩定性的無因次參數值 得到了更進一步的確認及驗證。另外針對擺動磁場頻率對微磁性粒子串運動模式的影響，透過一系列的實驗來觀察擺動粒子串在不同擺動磁場頻率下的運動情形，藉由增加擺動磁場頻率的過程，一樣可以觀察到粒子串有剛體擺動、扭曲及斷裂等運動行為，擺動磁場頻率較小時，由於粒子串有足夠時間跟上磁場擺動軌跡，其擺動振幅會變大與外加磁場較接近，也有明顯的形變，與外加磁場更同步地擺動；相反地，擺動磁場頻率較大時，粒子串在尚未跟上磁場擺動軌跡時，磁場擺動軌跡即已反轉，擺動振幅會明顯降低，沒有明顯的形變，擺動軌跡與磁場更不同步，利用此特性可造成粒子串與磁場瞬間相位差大於90度，形成軌跡轉移的現象。

Issues concerning the structural instability of an oscillating micro-bead chain are addressed based on systematic experiments. The patterns of rupture are categorized into two distinct regimes, referred to as a weak ductile fracture and a strong ductile fracture. A weak ductile fracture describes a more rigid rupture, which often occurs in a pronounced oscillation driven by strong field strengths. The position of the rupture usually favors toward the two sides of the chain. An interesting phenomenon of a reversed rupture, wherein the ruptured segments oscillate in opposite directions, is observed when there is excessive field strength. An important consequence of the reversed rupture is to cause permanent failure of the chaining structure. On the other hand, a strong ductile fracture, featuring significant deformation before rupture, is favored in a more viscous solvent fluid. The positions of the breakages in this regime favor the central region of the chain. The prominence of rupture instability is enhanced by a weaker directional field or by a longer chain, which is in agreement with quantitative assessments by the normal forces acting between the interfaces of beads. In addition, results of the present experiments provide further validations of the global criterion for rupture instability given byN×√Mn=1.7~2 ; where Mn and N, respectively, represent the dimensionless Mason number and the number of beads in the chain. Distinct behaviors, from rigid body oscillations and bending distortions to rupture failures, are observed by increasing the frequency of oscillating fields. In addition, we report an interesting phenomenon of “trajectory shift” of magnetic chains in an oscillating field as well. We increase the frequency of oscillating fields to make the phase angle lags of chains to the external field exceeding 90 degrees and the phenomenon shifts the chain’s oscillating trajectory along a new axis, which is perpendicular to its original axis. The trajectory shift provides an effective manipulating mechanism in a MEMS system, such as steering of micro-swimmers.