| 摘要: | 小從我們日常生活的枝微末端，大至科學及工程的實際應用，在各種材料表面的『潤濕』現象一直都扮演著舉足輕重的角色。在我的博士論文中，我將針對有關潤濕現象的研究成果，簡單分成五個部份敍述如下： 首先，就改變一個疏水表面的潤濕性質而言，最普遍的方法便是使用界面活性劑溶液。超疏水表面的潤溼行為可簡單概分為二類，隨著其界面活性劑濃度而改變。當界面活性劑濃度超過臨界微胞濃度時，典型的線性界面活性劑溶液在超疏水表面的接觸角主要受到表面張力主導，仍維持在疏水的區域。而具有分枝結構的界面活性劑如sodium bisethylhexylsulfosuccinate 及 didodecyldimethylammonium bromide則可利用降低固液界面能的方式，將超疏水表面調整至超親水的區域。 其次，對溶液中的微小氣泡而言，其傾向於粗糙、疏水的表面析出，因此在許多疏水表面具有氣泡殘留不易去除的問題。我們利用多孔性的超疏水表面可以達到將小氣泡收集集中的目的，配合浮力即可以有效解決此問題。而這個機制即是所謂的對流性Ostwald ripening，因為超疏水特性使得網絡狀的孔洞可以避免水的潤溼，進而扮演溝通微小氣泡間橋樑的角色，使得氣體可以在氣泡間流動。因此，大氣泡可以利用與小氣泡間的毛細管壓差達到吸收小氣泡的目的。在這個章節中，我們的實驗結果清楚地展示了此一現象可以有效應用於微流體的氣泡去除。 再者，對一典型的超疏水表面而言，其本身僅具極低的吸附力，也就是所謂的低接觸角遲滯，最典型的例子便是『蓮花效應』。然而，某些植物的葉面，像葱和蒜的葉面具有超過150度的前進角卻又伴隨極大的接觸角遲滯。雖然表面的粗糙度及披覆的石蠟可以解釋其高前進角的性質，卻無法解釋其遲滯。為此，我們的研究指出，位於葱蒜葉面的疏水性缺陷(diallyl disulfide)在接觸線的釘附上扮演了重要角色。原本低遲滯高接觸角的聚四氟乙烯薄膜，在塗抹了diallyl disulfide後即變成高遲滯的超疏水表面。 一般而言，我們相信對生活在水面的水生昆蟲而言，具有極佳疏水性質的四肢是必須的。但實際上，經由實驗的證明配合理論的推導，我們成功地展示了，利用邊緣效應(edge effect)可以將一個親水性表面的物質漂浮在水面上。在理論推導上，我們借由最低自由能的概念，將物體的漂浮概分為二個區間。當浮力主導時，液面的接觸角會與其本質接觸角相同。然而，當表面張力主導時，浮體則會藉由調整液面接觸角的方式達到力平衡的狀態。 最後，我們將探討普遍存在於稜角邊緣的潤溼現象。例如在杯緣行將溢出的液體以及稜角邊緣所懸掛的液滴，便是在日常生活常見的例子。在這個章節中，我們利用錐盤形的圓柱體以及楔形的立方體，同時針對邊界效應的理論以及實驗部份進行計算及觀察。在最低自由能的前提下，我們可以定義出三個區域來描述這個概念。當三相接觸線未碰觸到邊界前，所得到的接觸角是遵守楊氏方程式。一但接觸線到達邊界，其接觸線便會因為自由能產生邊界最低值而釘附不動。換句話說，隨著液滴體積的增加，此時所得到的接觸角將會超過其本質接觸角。當接觸角到達一臨界值(臨界接觸角取決於固體邊界的角度)後，液體便會跨越此一邊界呈現出新接觸面的本質接觸角。相似的行為模式不論在潤溼或去潤溼中都可以得到印證，並且實驗結果亦與理論預測吻合。 Wetting phenomena of liquid on various substrates are of crucial concern in our daily life as well as in engineering and science. In this paper, there are five main topics about the wetting phenomena, described as follow: First, the wettability of hydrophobic surfaces is generally improved by surfactant solutions. The wetting behavior of superhydrophobic surfaces can be classified into two types, in terms of the variation of contact angle with surfactant concentration (cs). Contact angle is controlled by surface tension for common linear surfactants and becomes independent of cs as cs > cmc (critical micelle concentration). Consequently, superhydrophobic surfaces remain in hydrophobic range, as reported. However, for branch-tailed surfactants such as sodium bisethylhexylsulfosuccinate and didodecyldimethylammonium bromide, superhydrophobic surfaces can turn superhydrophilic by increasing cs owing to continuous reduction of solid-liquid interfacial tension. The superhydrophobicity is recoverable simply by water rinsing. Secondly, tiny bubbles are easily formed on the rough, hydrophobic surface results in difficulties in bubble detachment and removal. We show that bubbles captured by porous superhydrophobic surfaces merge into larger ones, which can detach by buoyancy. The responsible mechanism is convective Ostwald ripening because networklike pores in the superhydrophobic film remain nonwetted and provide passage for gas flow between adhered bubbles. A large bubble grows spontaneously by absorbing all small adhered bubbles due to capillary pressure differences. Our results demonstrate that porous hydrophobic film can be an efficient, passive way of bubble removal in microfluidic systems. Thirdly, the typical superhydrophobic surface is essentially nonadhesive and exhibits very low water contact angle CA hysteresis, so-called Lotus effect. However, leaves of some plants such as scallion and garlic with an advancing angle exceeding 150° show very serious contact angle hysteresis. Although surface roughness and epicuticular wax can explain the very high advancing contact angle, our analysis indicates that the unusual hydrophobic defect, diallyl disulfide, is the key element responsible for contact line pinning on allium leaves. After smearing diallyl disulfide on an extended polytetrafluoroethylene (PTFE) film, which is originally absent of contact angle hysteresis, the surface remains superhydrophobic but becomes highly adhesive. Fourthly, it is generally believed that a water-repellent surface is necessary for small insects to stand on water. Through a combined experimental and theoretical study, we demonstrate that an object with hydrophilic surface can float with apparent contact angle greater than 90o due to edge effect. The apparent contact angle rises with increasing loading even to a value typically displayed only by superhydrophobic surfaces. On the basis of free energy minimization, two regimes are identified. When buoyancy controls, the meniscus meets the object with the intrinsic contact angle. As surface tension dominates, however, contact angle is regulated by total force balance. Finally, the wetting phenomenon in the vicinity of a corner boundary is ubiquitous. The daily life examples include the meniscus of water near the mouth of a container and the halt of the movement of a sliding droplet by the edge. In this study the wetting behavior near the edge is investigated both theoretically and experimentally by considering the volume growth of a droplet atop a conical frustum and the gradual immersion of a wedge. On the basis of free energy minimization, three different regimes are identified. When the contact line is away from the edge and Young’s equation is followed. Once the contact line reaches the edge, the contact line is pinning at the edge due to the boundary minimum of the free energy. Consequently, the apparent contact angle exceeds the intrinsic contact angle and grows with increasing droplet volume or water level. As the apparent contact angle reaches the critical angle, which depends on the solid edge angle, liquid extends over the edge and the contact line advances along new surface at its intrinsic contact angle. Similar behavior can be observed for wetting retreat but in a reverse order. The theoretical prediction has been experimentally confirmed. |
