本論文將以實驗及理論兩部份來探討微液滴在介電潤濕(Electrowetting-on-dielectric , EWOD)此操作條件下其接觸角飽和之行為。本研究中之介電潤濕晶片製作之步驟為：先將利用熱蒸鍍系統在青板玻璃(Soda-lime glass)表面鍍上鋁及鉻兩種金屬成為一電極層，再利用旋轉塗佈機塗佈高分子材料聚二甲基矽氧烷(polydimethysiloxane , PDMS)於電極層上，成為介電潤濕晶片之介電層及疏水層。本研究將探討五種介電層及疏水層厚度之介電潤濕行為，其厚度分別為17 μm、31 μm、68 μm、113 μm及234 μm。而在介電潤濕現象之理論分析方面，將包含兩部份：第一部份以最小能量之觀點(介面位能、重力位能及電能)進行分析，第二部份則以等效電路迴路分析探討微液滴的接觸角與提供電位兩者之間的關係。在本研究之實驗結果可知在不同介電層厚度其接觸角飽和之角度皆近似於 。 在介電潤濕現象下，楊-李普曼(Young-Lippmann)方程式已無法描述微液滴接觸角飽和與所施加電壓之行為，本研究推測造成接觸角飽和行為之原因為高分子材料聚二甲基矽氧烷發生介電潰堤現象。當提供的電場強度超過高分子材料聚二甲基矽氧烷之介電強度，此時高分子材料之介電性質將會改變，而造成高分子材料聚二甲基矽氧烷將產生導體性質而造成電流溢漏，故本研究之實驗結果中發現當提供之電位達到臨界電壓時，將產生電流溢漏之現象，並且定義出此時臨界電場強度為高分子材料聚二甲基矽氧烷之介電強度，介電強度也將隨著介電層厚度增加而降低，此趨勢與介電材料相關文獻中相符合。

This study presents the experimental and theoretical investigations into the issue of the micro-droplet saturated contact angle in electrowetting-on-dielectric (EWOD). The EWOD chips were fabricated by the following processes. First, the aluminum/chromium electrodes were plated on a soda-lime glass using the thermal evaporation. Then, the polydimethysiloxane (PDMS) was adopted to form the dielectric/hydrophobic layer over the electrode using the spinning coater. There are five different thicknesses of dielectric/hydrophobic layer (17 μm, 31 μm, 68 μm, 113 μm and 234 μm) were fabricated to study the EWOD. In theoretical analysis, the principle of minimum energy (e.g. interfacial potential, gravitational potential and electrical energies) and the electrical circuit analysis were both adopted to investigate the relationship between the contact angle of micro-droplet and the applied voltage. The experimental results showed that the saturated contact angles of micro-droplet for the different thicknesses of dielectric/hydrophobic layer are all approximately 57°. The Young-Lippmann equation fails to describe why the saturated contact angle of micro-droplet occurs in EWOD. We infer that the dielectric breakdown may be a possible reason to interpret this physical phenomenon. When the externally applied electric field strength is larger than the dielectric strength of the material (i.e. PDMS), the dielectric or electrical properties of the material will break down. The material becomes a partial conductor and results in the current leakage. In our experimental measurements, we find that the current leakage occurs at the critical applied voltage of the saturated contact angle. Further, we identify that the critical electric field strength as the dielectric strength of the material. We find that the dielectric strength decreases as the thickness of dielectric/hydrophobic layer is increased. This trend is consistent with the dielectric strength measurements of the dielectric materials in literatures.

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