Formation of bubbles in a microfluidic flow-focusing device

本研究利用玻璃及聚二甲基矽氧烷(Polydimethylsiloxane，簡稱PDMS)製成寬度為100 μm之矩型疏水性(Hydrophobic)流道，將空氣作為分離相之唯一流體，連續相流體則為純水及不同濃度之甘油、乙醇及界面活性劑-十二烷基硫酸鈉(sodium dodecyl sulfate，簡稱SDS)之水溶液，利用十字聚焦之氣泡產生機制並藉由液氣二相流率比的控制，觀察在僅改變單一參數下(黏度、表面張力及界面分子效應)氣泡大小及成形之拉伸變化。利用毛細數(capillary number, Ca)及歐氏數(Ohnesorge number, Oh)來區分氣泡於疏水性流道之流態，當Ca < 0.0045時為非濕潤性氣泡；0.0045 < Ca < 0.007為過渡區域；Ca > 0.0070時為濕潤性氣泡。拉伸變化於非濕潤區域時將隨著流率比之增加而出現第一峰值，利用毛細數與流率比之乘積可得其值約為0.0025，當流率比增加至過渡區域當下將產生第二峰值，且兩峰值將隨著歐氏數線性增加並於Oh = 0.0370而重合，而進入濕潤性區域後之氣泡大小及拉伸長度將逐漸穩定，最後將不再隨流率比而改變，而拉伸長度主要由黏滯性所主導，與表面張力較無關係。氣泡大小於非濕潤區域時將隨著流率比之增加而大幅減小，進入濕潤區域後將逐漸平緩，最後氣泡大小將維持定值，且不同連續相液體將產生不同氣泡大小之變化，即氣泡大小將隨著黏度增加而減小且同樣隨著表面張力降低而減小。 除了實驗外，本研究利用模擬軟體-Fluent在同樣控制連續相液體參數的條件下進行分析，但由於氣泡與壁面之接觸角為動態接觸角，而動態接觸角將隨氣泡傳輸速度而改變，因此僅能定性分析氣泡流動之流態。 最後，利用毛細數將疏水性流道之流態進行分界，以利於低流速也可達到濕潤之效果，減少氣泡與壁面接觸所產生之不穩定，可有效的應用於電子晶片散熱及燃料電池之效率提升。In this paper, liquid/gas flows were investigated in hydrophobic microchannels with square cross-section of 100 x 100 μm made of glass and PDMS. Liquid and gas were mixed in flow-focusing device in way to generate monodisperse gaseous bubbles. Air bubbles were produced in glycerol-water, ethanol-water and water with concentrations of surfactant sodium dodecyl sulfate (SDS). Flow morphologies were obtained by using Ca-Oh diagram. This diagram shown that the non-wetting region occurred at about Ca < 0.0045, the transition region occurred at about 0.0045 < Ca < 0.0070, and the wetting region occurred at about Ca > 0.0070. The experimental data of the bubbles size and elongated length were correlated as a function of the width of the microchannels, the ratio of the liquid/gas flow rates and the capillary number. The bubbles elongated length appeared first peak in non-wetting region could be predicted by the flow rates ratio of liquid and gas phases and the capillary number. Bubbles elongated length and size finally stable in wetting region occurred at about Ca > 0.0075. The bubbles size decreased rapidly in non-wetting region and decreased smoothly in wetting region. The bubbles size decreased with the increased of the viscosity and decreased with the decrease of the surface tension of liquid phase. Additionally, we also used a commercial software, Fluent, to analysis bubble formation. In Fluent, equilibrium contact angle is used as a boundary condition, however, the contact angle in the present experiment may be changed by the velocity of bubble, thus we can only analyzed the bubble formation qualitatively. Finally, the experimental results shown that the wetting bubbles would exist at low flow rates ratio which decreased the instability between the wall and the bubbles, and could be applied in fuel cells and heat dissipation of electronic equipment