| 摘要: | 本篇論文著重於學習利用PMMA模具製造出PDMS微模型，分析蒸發後的微模型進行對混合物的影響。因此使用複製模塑法將PMMA模具成功地製作出PDMS微模型，模具是在PMMA板材上使用CNC銑削及重複使用製作出PDMS微模型，模具包含模型1以及模型2，分別為顆粒間距差異，各別尺寸為120µm及90µm。
皆進行了蒸發及混合的實驗，微模型在低濃度的染色乙醇(舊液體)中充分地佈滿內部，而微模型中須在穩定的排水情況下，使用舊液體將內部空氣排空。改變液體的注入率直到為0.2 µl/min而停止，當舊液體到達飽和程度(即為So)並使其在內部蒸發後，在微模型混合通過注射高濃度的染色甲醇(新液體)得到相同的注入率(Q)。此論文使用兩種實驗，第二種實驗是沒有經過排水過程。
液體注入率(Q)根據毛細數(Ca)為3x10-6，3x10-5和3x10-4，對應到流速(Q)3組數值為模型1與模型2分別為1.25µl/min與1.55µl/min;12.5µl/min與15.5µL/min;125µl/min與155µL/min;而預期得到So分別是So=10%，So=20%以及So=30%。
液體蒸發So=10%由於有更多的水相蒸發，相較於So=20%以及So=30%有最長的蒸發時間與最高的濃度。模型2的蒸發率比模型1來得快，因為小液體群在緩和的狀態下，透過縫隙傳遞且加速蒸發。
在模型1中，因混合分散作用形成了新液體的手指現象；在模型2中，液體蒸發So=30%，混合主要由擴散造成的，而So=10%及So=20%則在平流引起的。增加毛細數Ca在新的液體注入期間限制擴散並且推進加速混合的過程。當平流在So=10%和So=20%時，由於有重大濃度差，控制更高的So導致更大的空間擴散。另外，在模型1中，當新液體注入毛細數Ca為3x10-5和3x10-4後，空氣氣泡完全被取代。在Ca為3x10-6時，因為有殘餘的氣泡在微模型的中間，導致新液體通過模型的邊緣；在模型2中，空氣氣泡被發現在一些區域並扮演了防止新液體經過與混合的角色，然而，當毛細數Ca數值的減小相對也減少氣泡的產生。
總而言之，在混合的過程中，由於模型2中的液體通過無序通道導致混合機會增加，而高於模型1的混合能力；Ca的增加也提高混合能力，因為它改善了平流過程；較高的So也具有更多的混合能力，這是由於更顯著的濃度梯度影響在擴散期間有更高的質量傳遞。
模型2(無排水)比模型2(有排水)需要更長的蒸發時間。蒸發後，模型2(有排水)因為有排水過程導致有許多未連接的液體團在內部傳播。而模型2(無排水)因為沒有排水過程，所以僅有少許未連接的液體團，且能均勻的散佈內部。因此，排水過程影響了混合能力，模型2(無排水)中的空氣面積比模型2(有排水)來得少，這表示模型2(無排水)有較大的傳遞面積，提高混合能力。
;The focuses are to study PMMA mold fabrication for preparing PDMS micromodel, drying in micromodel, and drying impacts on mixing behaviors in micromodel. PDMS micromodels were successfully prepared by replica molding using PMMA molds built by CNC milling. The molds included Model 1 and Model 2 with the average throat sizes are ~120 µm and ~90 µm, respectively.
The micromodel was fully imbibed by a low concentration of dyed ethanol (the old liquid). Drainage was done by invading gas air and stopped at steady condition. Drying was conducted by changing the injection rate of gas air to be 0.2 µl/min and stopped when the dried old liquid reached certain saturations (So). Then, mixing was conducted by injecting a high concentration of dyed ethanol (the new liquid) into the micromodel with the same injection rate (Q) as in the imbibition and the drainage. There were two kinds of experiments, the second was conducted without drainage.
The liquid injection rates (Q) were varied based on capillary numbers (Ca) of 3x10-6, 3x10-5, 3x10-4 which were responsible Q of 1.25µl/min and 1.55µl/min (Model 1 and Model 2); 12.5µl/min and 15.5µL/min; 125µl/min and 155µL/min; respectively. The desired So were varied around So=10%, So=20%, and So=30%.
The dried liquid of So=10% has the longest drying times and the highest concentration than So=20% and So=30% due to the more aqueous phase is evaporated. Model 2 has faster drying rates than Model 1 because much small liquid cluster spreads in the pore that eases and accelerates evaporation.
In Model 1, finger of the new liquid is formed and mixing happened by dispersion. In Model 2, at So=30%, mixing is dominated by diffusion while advection dominates in So=10% and So=20%. The increase of Ca during the new liquid injection limits the diffusion and advances the advection. The higher So induces longer diffusion.
In Model 1, the air bubble is completely displaced during the new liquid injection with Ca=3x10-5 and Ca=3x10-4. At Ca=3x10-6, it remains in the middle of micromodel. In Model 2, the air bubbles trapped in some areas prevent mixing by blocking the new liquid to pass. The decrease of Ca reduces the air bubble trapping in Model 2.
The mixing areas in Model 2 are higher than those are in Model 1 due to disorder liquid channel in Model 2 that upsurge the chance of mixing; the increase of Ca rises the mixing areas because it improves the advection process; the higher So has the more mixing areas due to the more significant concentration gradient.
Model 2 (no drainage) needed much longer drying times than Model 2 (with drainage). After drying, In Model 2 (with drainage) has many disconnect liquid clusters spreading in micromodel. In Model 2 (no drainage), it has fewer disconnect liquid clusters and several big bulks of liquid. It influences mixing behaviors in Model 2. The air area in Model 2 (no drainage) is lower than it is in Model 2 (with drainage), thus in Model 2 (no drainage) has higher mixing area. |
