本研究起源於高導電度緩衝溶液之生物晶片快速檢測的需求，其中一種檢測方法為施加交流電場以實現快速捕捉樣品或流動效應，來克服擴散作用在輸送上的不足。研究著重於焦耳熱產生的交流電熱流(AC Electrothermal Flow, ACET)，這是高導電度緩衝溶液中最普遍的作用。
本研究使用T字型ITO電極以多種溶液導電度來檢視ACET，我發現這種流動主要是由電極的局部焦耳熱增強而在電極尖角附近或沿著電極邊緣流動。經典ACET理論並不能完全根據誘導空間電荷理論來解釋所觀察到的流態，且測量到的流速並不總是遵循經典理論所預測的電壓四次方，因此，我提出一個基於熱致電雙層極化的全新理論來解釋我的實驗發現。
在低導電度溶液(1-10µS/cm)的研究(第四章)中，我觀察到ACET通常發生於電極表面且頻率範圍為500k-10MHz。頻率5M-10MHz時於電極尖角處形成局部匯流，在500k-1MHz時沿著電極短邊的邊緣形成小漩渦流動，後者通常會噴至電極外且有時與電極內的逆流共存。令人意外地，這些流動的特徵非常不同，前兩種流動的流速U幾乎不隨電壓V改變，而逆流則表現U∝V⁴，這意味著它們是由不同ACET機制所驅動。
於中導電度溶液( ~120µS/cm)，我觀察到於10MHz時在尖角處產生局部匯流，這是與低導電度相同的流態，但當頻率下降至5MHz時，我發現於電極短邊邊緣出現短距離的噴流，更低頻500k-1MHz時，快速的噴流沿著電極長邊邊緣流出，不同於低導電度的是，電極內沒有發生逆流。前兩種流態的流速U幾乎與電壓V無關，而噴流則遵循U∝V²。
對於高導電度溶液(2000-73000µS/cm)，這些流動特性(參閱第五章)與中低導電度溶液有很大的不同，流態基本上呈現大漩渦對，這是由於當溶液導電度提高，局部焦耳熱效應增強所致，因此在電極尖角附近的ACET流動現象擴大，我也觀察到這流速隨著導電度提高而減慢，對應的電壓關係從U∝V⁴下降至U∝V²，這些流動特性與經典ACET所預測的U∝σV⁴相抵觸。
經典ACET理論無法解釋在這些溶液中所觀察到不同的流動模式，因此，我們於第六章提出了另一種觀點來解釋這三種情況之間的差異。我推測電極尖角處的局部發散電場使尖角處產生局部熱點，導致鄰近電極尖角的水平充電比遠離尖角的垂直充電還更容易發生，而進入尖角的水平電流較熱，流出尖角的電流較冷，這兩者之間的不平衡會產生電力從而形成電極內的局部匯流或向電極外的噴流，這種機制解釋了實驗上觀察到的多種流態。
為了解釋這些偏離U∝V⁴的現象，我提出「熱致電雙層充電理論」，進一步考慮了焦耳熱對電雙層電容與溶液電阻的影響。對於後者，水平方向的充電以表面電導來表示，包含擴散層與靜止層的貢獻，這兩者分別在高和低導電度溶液下主導作用。由於電雙層電容隨溶液導電度σ變化，水平電流與電雙層競爭加熱效應會導致所得電荷密度隨施加電壓V不同次方數而變化，從而解釋流速U對V的各種冪律依賴性，這也部份解釋了為什麼實驗上U在高導電度溶液中不隨σ增長。
本研究表明了局部焦耳熱及水平充電為決定ACET表徵的重要關鍵，因此，ACET特徵可能對於電極幾何形狀非常敏感，這為生物晶片應用中如何基於ACET設計更有效的微裝置開闢一個新的途徑。

SUMMARY

This thesis originated from the detection of biosensing chip under highly conductive buffer solution, which caused by the AC electrothermal flow (ACET) driven by the alternating current field. The current theory about ACET is based on the ''induced space charge theory'' proposed by Ramos et al. But we find that the actual experiment doesn’t completely follow the behavior of the classical theory, so we conducted a series of experiments to clarify the ACET mechanism. Under different conductivity solutions, I found that the flow patterns and the dependence of flow speed U on voltage V are different from the classical theory. Research on these problems, I consider that the local electric field on the electrode corners makes the corners hotter, which forms tangential charging and jetting. The tangential charging makes the flow direction observed in the experiment different from the classical theory. Regarding the deviation from U∝V⁴ based on the classical theory, I develop the ''heated double layer charging theory'', taking into account the Joule heating on electric double-layer capacitors and bulk resistance. The heating effect of the competition between these two will cause the flow speed to vary with different powers of the voltage, which explains the various effects of the dependence of U on V. These findings not only reverse the classic ACET theory, but also provide a reference for academic research.

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