帶電高分子噴流之電液動力穩定性分析

Abstract

電紡是目前唯一可製造連續性奈米絲的技術。雖然已有利用電紡技術製造濾器的案例，還有在藥物傳遞以及觸媒工業上也有相關的應用，不過，受制於電紡過程中，帶電高分子噴流的各種不穩定擾動模態之影響，使其會產生串珠狀的噴流或是發生鞭甩的現象，導致無法有效的控制噴流路徑以及未能獲得均勻的成絲形態，因此，此項技術還停留在發展的階段。 本論文研究旨在以時間的線性穩定性理論，分析帶電高分子噴流之電液動力行為的不穩定性，同時剖析各種物理機制對噴流不穩定性的影響，以期能定性上的預測噴流實驗中可能觀察到的現象以及趨勢。 由研究結果可知，黏滯力為使能量消散的機制，會抑制各種模態的擾動波；彈性效應則與黏滯力相反，利用交纏力所產生的彈性儲存外部所施加的能量，延遲應力響應的時間，進而增長各種擾動波的能量，使噴流更加不穩定。此兩種機制皆不會激發更多的不穩定模態；庫倫力則會激發非軸對稱的擾動波，而表面張力抑制非軸對稱的擾動效應降低時，空氣壓力也會誘發非軸對稱擾動波，使噴流產生不穩定的擾動。由頻散關係可知，表面張力勝過庫倫力的效應時，會主導噴流產生串珠狀的噴流；當庫倫力機制較強時，則會增長各種擾動模態的不穩定性，而電荷之間相互的斥力會提供噴流一個徑向的合力，使其產生鞭甩不穩定。 本文最後以參數地圖定性上的預測噴流實驗中，可能觀察到的噴流動力行為現象和趨勢，發現增加噴流的彈性機制可能會產生噴流分岔的不穩定現象，同時規範出各種模態主導噴流不穩定性的區域並對應指出實驗上可能觀察到的現象。

Electrospinning is currently the only technique to produce continuous nanofibers with submicron-scale diameters. Although numerous successful applications have found in filtration, medicine, and catalysis industries, the uncontrolled random motion caused by inherent instability during jetting process often leads to the formation of undesirable patterns such as beads-on-string or nonwoven mats, which severely retards the technique toward wider applications. Therefore, a further understanding of the flow instability is necessary to improve the controlling of the movement of electrospinning nanofibers. This paper aims to carry out the electrohydrodynamic instability of a charged polymer jet to explore the instability mechanisms. The effects of physical parameters on the onset of jet instability are investigated by employing temporal linear stability theory, so as to give a qualitative explanation of observed phenomena. Results show that the liquid viscosity is a mechanism responsible for energy dissipation, decreasing the growth of disturbances and postponing the breakup of the jet. The entanglement effect of polymer in terms of elasticity can enhance the instability by storing the strain energy and releasing the stresses out of phase. Both of the two mechanisms do not trigger more modes to grow. On the contrary, the electrical Coulomb force and the aerodynamic drag arising at the jet surface will be able to induce the growths of more asymmetric modes. The surface tension has a stabilizing effect on all asymmetric modes while it always destabilizes the axisymmetric one. Furthermore, the formation of beads-on-string is an outcome of the predominance of the axisymmetric mode, and the bending instability due to the presence of the electrically repulsive force must be responsible for the chaotic whipping motion. In particular, we also find that, under certain conditions, an increase in the elasticity may lead to the occurrence of branching. The detailed parametric diagrams provide a basic guideline for applications of electrospinning nanofibers technology in future.