微磁性液滴薄膜之界面分裂

Abstract

磁性流體為一將強磁性超微粒子經界面活性劑的作用，安定地分散 於液體中的一種溶液，容易被磁鐵強力吸引，磁性流體包含了流動 和磁性的特性。若將磁性流體置於一平板上而形成一磁性流體層或 薄膜，當磁場作用方向與平板垂直時，此垂直磁場會造成一垂直拉 力而形成三維之突出物，而此界面不穩定現象即為所謂之 Rosensweig Instability。而其三維之突出物之最終形狀則取決於磁 力、重力與表面張力之平衡。就提案主持人所知至今為止，有關 Rosensweig Instability 之研究均僅於較大體積磁性流體層或薄膜，然 而仍無對微小量微米級薄膜之研究。此提案計畫研究方向乃基於個 人新發現於垂直磁場下，微磁性流體液滴薄膜因Rosensweig Instability 產生之界面分裂及分裂液滴之漂離現象，加以更深入探 討。 本提案計畫為全期三年之計畫，具體之研究方向與目的則為針對個 別參數對微磁性流體液滴薄膜分裂現像影響進行深入之物理機制 探討，並進而利用此簡易磁場裝制置，發展出一組可利用 Rosensweig Instability 產生更微小之次磁性流體液滴，且加以分離與 收集之裝置。 本提案計畫之第一年計劃首先在於完整實驗裝置規劃與架設並進 行實驗，同時針對控制參數(如液滴薄膜直徑，磁場大小，磁場增加 率，磁化強度大小)影響加以研究，預計研究之液滴薄膜直徑介於 O(100)~O(1000)μm 間，以深入瞭解此一微液滴薄膜分裂現像之物 理機制。於充分瞭解微液滴薄膜分裂現像之物理機制後，第二年計 劃則針對接觸面之預濕潤狀況進行研究，以瞭解接觸角與表面張力 對微液滴薄膜分裂之影響，同時亦詳加研究分裂之次磁性流體液滴 漂移現像，以有效加以收集。第三年計劃則嘗試將此一微液滴薄膜 分裂現象作一理論性定量研究，藉由完整分析主要影響之能量場(磁 力、重力與表面張力) ，導求出相觀關之無因次物理參數值。

Magnetic fluids (ferrofluids) have been synthesized for technical applications since the early 1960s. They are colloidal suspensions of nano magnetic particles coated by a layer of surfactants in carrier fluids such as water and oil. The advantages of flow controls for magnetic fluids by an external magnetic field have provided wide scopes of engineering applications, such as rotary feed in hard disc drives and speakers. In addition, practical uses in biomedicine have been achieved because of the great success in the development of stable ferrocolloids during the last decade. At the same time, the interfacial instabilities of magnetic fluids under a static perpendicular field also interest researchers from both scientific and practical points of views, such as the interfacial instability on a free surface of a horizontal magnetic layer or so called Rosensweig instability. If the magnetic strength exceeds a critical magnetic Bond number, a sudden transition from an original flat surface into a hexagonal pattern of three-dimensional liquid crests, or so-called the Rosensweig crests, is observed. Nevertheless, previous researches focus mainly on a magnetic fluid layer at a relative larger scale. With the advances in micro-technologies, it is interesting to investigate further the instability phenomena of an extremely thin film of a micro drop, which are strongly affected by the surface tension. In this proposal, experiments on an extremely thin film of a tiny magnetic drop, diameters ranging from O (102 - 103) μm, under a perpendicular field are proposed to study how instabilities evolve on the surface and their potential applications. The goal of the present proposed 3-year project is to systematically investigate the rupture instability of a thin film of magnetic droplet into numerous subdroplets. The experimental setup is planned to be established within the first year subproject. Experiments for various control parameters, such as diameters, the field strength, field sweeping rate, magnetic fluids will be analyzed as well in the first year. The subproject of second year will focus on fluids with different magnetizing responses and wetting conditions of the plates. Also being studied in the second year is the floating phenomena of the subdroplets and their separating and collecting technologies. A more quantitative and theoretical analysis, such as the influences of dimensionless parameters, will be explored in the final third year.