| 摘要: | 自泳流體是指含有自泳粒子的溶液，例如自然界中的微生物（大腸桿菌）利用其肢體運動來達到自泳的目的。近年來，各式各樣的自泳動膠體粒子亦透過人工方式製造，例如半面覆蓋白金的球型氧化矽粒子。自泳粒子的運動皆靠消耗自身能量來實現，由於過程中會製造熵，所以自泳流體是一個非平衡系統。自泳流體展現許多獨特的性質，具有發展新世代裝置的潛力，因此針對自泳流體，有許多的實驗觀察、理論推導、及分子模擬的研究。在熱力平衡狀態，系統內的壓力是到處均一，與器壁特性無關，所以壓力為狀態函數。然而，自泳流體為非平衡系統，故系統壓力是否為狀態函數仍有爭議。近年來的研究指出：雖然球型自泳粒子的壓力是狀態函數，非球型自泳粒子的壓力會隨其與器壁的作用而改變。但目前為止，仍無研究探討『流體力學作用』是否會使系統壓力不為狀態函數。此外，當自泳粒子濃度高於某一定值時，自泳粒子會產生高密度與低密度的兩相。若系統壓力不再是狀態函數，過去用來描述相分離的條件可能不適用於預測自泳流體的相行為。在為期三年的計畫中，我們將利用『耗散粒子模擬』方法，探討自泳流體之基本性質。於第一至第二年，我們將計算自泳流體於無邊界和有邊界系統中之溶液壓力及表面壓力；此外，我們亦會探討相分離的形成條件。於第二至第三年，我們將持續研究在自泳流體相分離狀態下，系統的壓力分佈和兩相間的界面行為。最終目標是要發展出可用來預測自泳流體發生相分離的理論模型。 除理論模擬外，我們將研究以毛細作用驅動之微液滴自泳行為。在民生及工業應用中，如何精準地製造微液滴並控制其運動一直是個挑戰。液滴在一般表面時，由於『接觸角遲滯』特性而呈現類似摩擦阻力，導致液滴抗拒運動。通常會透過施加外力於液滴，使其克服表面的阻滯而產生運動。若能將材質表面改質成低(無)遲滯的表面，摩擦阻力消失可使微液滴易於運動。若含非揮發性溶質液滴的表面發生不均勻蒸發，可誘發毛細作用力（馬倫哥尼效應），導致液滴在低(無)遲滯的表面產生自我泳動。本計畫將製造低（無）遲滯的表面，探討微液滴的自泳行為。目前有兩種簡易方法可製備低（無）遲滯表面：超親水（油）表面和液體浸潤的多孔表面。這些表面必須具有長時間的穩定性，並可抵抗溶劑的溶解。實驗的首要目標是能產生極性和非極性微液滴在這些表面上的自泳現象。為可應用於微型裝置，液滴的大小將從毫米延伸至微米。透過各式實驗設計，我們將詳細探討微液滴自泳的機制。 ;An active fluid is a dispersion of active particles in fluid media. Active particles like E. coli perform self-propelled motions by converting energy to work. The artificial active particles have also been synthesized and their motions are achieved by spatially-asymmetric catalytic reactions. Active fluids have been demonstrated out of equilibrium and exhibited unique behaviors. Because active fluids are essential for the development of new generation devices, they have been studied extensively in recent years. When a system is at thermodynamic equilibrium, the pressure is uniform everywhere and independent of the properties of confining walls. Consequently, the pressure is a state function. However, because the active fluid is a nonequilibrium system, whether the equation of state exists or depends on the wall-particle interactions is still unclear. It has been reported recently that the pressure for active fluids made of non-spherical active particles varied with wall-particle interactions, although the pressure is a state function for active spheres. Additionally, whether the hydrodynamic interactions can destroy the state properties of active fluids is an open question. On the other hand, when the concentration of active particles is high, the active fluid was found to phase separated into a dense phase and a dilute phase. If the pressure of active fluids isn’t a state function, the thermodynamic criteria for phase separation cannot be applicable to active fluids. Thus, in this three-year project, fundamental properties of active fluids will be explored by “dissipative particle dynamics simulations.” In the 1st ~2nd years, both bulk and surface pressures will be calculated in unbounded and confined systems. The criteria associated with the onset of phase separation will be explored. In the 2nd~3rd years, as the two phases coexist in the system, the interfacial behaviors between the two phases and the criteria of phase coexistence will be investigated. Our goal is to develop a theoretical model that can explain the occurrence of phase separation. In addition to theoretical simulations, the capillary-driven self-propelled motion will be studied. The precise generation and control of drop motion are essential for a variety of applications such as microfluidic systems. The drop motion driven by external forces is usually hindered by contact line pinning caused by contact angle hysteresis (CAH). In order to overcome this resistance, the surface has to be modified to have low/negligible CAH. On such surfaces, the self-propulsion of a liquid droplet can be driven by the Maragoni effect as non-volatile solutes are contained in the drop. This project aims at fabrication of surfaces with low/negligible hysteresis and to achieve the self-propulsion of liquid drops. Two typical approaches to acquire CAH-free surfaces are total wetting surfaces and liquid-infused porous surfaces. The resultant surface must have long-term stability and withstand against various solvents. For the applications in droplet-based microfluidics, both mini-sized and micro-sized drops will be considered. On the basis of those experiments, the detailed mechanism responsible for self-propelled aqueous/non-aqueous liquid drops (soluto-capillary flow) will be proposed. |
