本論文的主要探討微液珠於微流體系統中的運動及應用。我們藉由微流道設計以及流量的調控來操控微珠大小及型態以配合不同的實驗需求。
本論文有四部份。第一部份，我們觀察油珠在具不同親疏水性質的Y型分岔微流道中之運動行為，發現油珠因為兩分岔管的濕潤性質不同而偏向親水端移動，且油珠的運動模式可隨流量不同而改變。我們同時發現存在一組最佳流量使油珠往親水端支管偏移的傾向為最大。我們藉由尺度分析來評估相關效應的相對大小，同時發現不同油珠之偏移模式主要是由於表面改質效應與油珠間流體力學效應競爭下的結果。此系統對於不同親疏水性的混合微粒系統之分離、流體式邏輯閘以及非接觸式分子探針可提供有效指引。
論文的第二部份，我們嘗試利用電場來驅動微珠運動。我們觀察發現微珠會朝電場相反方向移動。我們推測造成此現象的原因是因為微珠與管壁間的薄膜層會有電滲流發生。然而，因為微珠界面與管壁的表面電荷不同，電滲流速度的差異會造成一與電滲流(電場)方向相反的壓力迴流，因而帶動微珠移動。
論文的最後兩部份則致力於開發拉伸DNA分子的新方法。我們利用微流道系統產生一扁長狀微珠，並於微珠與管壁間形成一厚度相當細薄的次微米薄膜層。而此次微米薄膜層具有以下二特性：第一、此微米薄膜層具有能量障礙上的限制，以至於DNA分子必須改變其結構來降低自由能方可進入薄膜層。第二、由於次微米薄膜層入口端厚度急遽的縮減，此可大幅提高薄膜層之剪應力或電場，我們則利用此特性來達到有效拉伸DNA分子的目的。於論文的第三及第四部份中，我們分別利用壓力及電力方式使DNA分子於薄膜層內產生動態拉伸。我們不僅討論DNA分子於不觀測區域或外力作用下型態上如何變化，同時探討DNA分子於提濃及分離應用上的可能性。

In this thesis, we study the motion of microdroplets and explore their applications to microfluidic systems. Through control of flow rates in concert with appropriate channel designs, we are able to generate microdroplets in different sizes and shapes for desired processes.
There are four parts in this thesis. In Part I, we examine the motion of microdroplets in a bifurcating microchannel with hydrophilic and hydrophobic branches. We find that the drops tend to be re-directed towards the hydrophilic end due to the wettablility difference between the two branches . Such asymmetric motion, however, can exhibit a variety of behaviors, depending on applied flow rates. In addition, there exists an optimal flow rate at which the tendency toward the hydrophilic end is maximal. With the aid of a scaling analysis, we identify that the observed phenomena are attributed to the competition between the surface-directed effects and inter-droplet hydrodynamic interactions. This system has potential applications to separation of a suspension of fluid particles of different affinities to the channel surface. It can also serve a fluidic gate or non-contact intermolecular probe.
In Part II, we make an ab initio attempt in driving microdroplets with electric fields. We observe that the drops move against the applied field. The motion could be attributed to backflow pressures induced by the electro-osmotic flow in the thin film region that exists different surface charges between the fluid interface and the channel wall.
The last two parts of this thesis are devoted to developing new microfluidic strategies for dynamic stretch of single-molecule DNA. Here we invoke long, closely-fitted fluid slugs to create submicron films in microchannels. The effects of the submicron films are twofold. On the one hand, they create energy barriers or confinement effects, so that a DNA must change its conformation for minimizing the free energy as entering the film. On the other hand, due to dramatic reduction of dimensions, thin films set up large shears or fields and hence provide a more robust means to stretch DNA therein. In Part III and IV, we employ hydrodynamic pressures and electric forces, respectively, to examine the corresponding dynamic behaviors of stretched DNA within the films. We not only demonstrate how the conformation of DNA changes in response to various constraints or applied forces, but also discuss possible applications for concentration and separation of DNA molecules.

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