| 摘要: | 潤濕現象與我們日常生活息息相關，了解潤濕現象的機制原理不僅可以解釋自然界生物特有的潤濕行為，亦可以對材料的啟發與改良上有所助益。本研究著重水滴於材質表面上的潤濕行為以及接觸角遲滯的探討，內容分為四大部分: (1)實驗設計一顆小液滴結合連通材質表面上的液滴與其位置旁洞內的液滴，一般認為由於重力的因素會使得整體液滴向洞內下方流動，然而，透過Young-Laplace 方程式還有對流性的 Ostwald ripening 的理論機制得知，我們可以藉由控制此兩個液滴的曲率來決定其流動方向，由無管壁的毛細現象實驗證實，洞內下方的液滴可以被上方較小曲率滑動中的液滴吸上來。結果推論，使用超親水孔洞表面將可移除微通道系統中吸附在內壁的液滴。 (2)沙漠中的甲蟲傾斜身體迎向充滿水氣的風，藉此動作利用其背上超疏水表面相間親水區塊來收集珍貴的水滴，此過程包含水滴在此特殊表面停滯不前(pinning)以及去潤濕(dewetting)的潤濕行為。本實驗經由在低遲滯超疏水表面上設計一個特定形狀的親水區塊(如正方形、長方形與三角形)，探討液滴黏滯力(pinning force)、黏滯長度(pinning length)與親水區塊邊長長度以至於其方向的相互關係。實驗發現黏滯長度(wp)等於正方形或長方形親水區塊垂直於傾斜方向上的邊長長度(w)，對應黏滯力大小的臨界傾斜角(ac)亦與此邊長長度成正比；有別於正方形與長方形親水區塊，wp在三角形親水區塊上會隨著傾斜角增加而增加。然而，實驗證實正方形或長方形上w與sin(ac)的相對關係與三角形上wp與sin(a)的相對關係是相符合的；另一方面，模擬程式 Surface Evolver 利用自由能最小化計算方式成功模擬出與實驗結果一致。 (3)實驗利用壓版法(compression-relaxation)量測水滴於壓克力表面之接觸角遲滯。壓版法利用上板為超疏水低遲滯表面緩慢向下擠壓接著向上鬆弛於待測材質表面上的水滴，藉此觀察接觸線向外擴張時的前進角與向內收縮時的後退角。實驗發現向下擠壓的程度小或大時，水滴在向上鬆弛的過程中，可分為接觸線停滯不前(contact line pinning regime) 或接觸線撤退收(contact line withdrawal regime)兩種潤濕行為。當向下擠壓程度較小，此時接觸角隨著超疏水板向上鬆弛而角度下降；當向下擠壓程度較大，於超疏水板向上鬆弛時，接觸角會下降至後退角並維持著後退角伴隨著接觸線向內撤退，本實驗亦利用Surface Evolver 的模擬來比對實驗結果。 (4)實驗利用膠帶黏貼與撕開石墨片表面，製作出微米級粗糙度，石墨面變成疏水性，接著將石墨片放入丙酮溶液於超音波震盪，使得微米級結構上嵌入奈米級結構，以此來製作超疏水表面的石墨片。由於水潤濕入侵疏水性與超疏水性石墨表面的程度不同，接觸角遲滯在此兩表面的程度有明顯區別；另外，在超疏水石墨表面亦發現有對流性 Ostwald ripening 與接觸角遲滯程度會隨著時間改變得特殊現象。Wetting phenomena of a liquid droplet on various solid surfaces are everywhere in our daily life as well as in engineering and science. In this thesis, there are four major parts:(1)When a sessile drop encounters a pendant drop through a hole, it is generally anticipated that they will coalesce and flow downward due to gravity. However, like “wall-free” capillarity, the pendant drop may be sucked up by a sliding drop instantaneously if the radius of the curvature of the former is smaller than that of the latter. This phenomenon can be explained by Young-Laplace equation and convective Ostwald ripening. Our results indicate that superhydrophilic perforated surface can be used as an effective way for the removal of small droplets adhering to the inner walls of microchannel systems.(2)A desert beetle tilts its body forwards into the fog-laden wind to collect water by the hydrophilic patches on its superhydrophobic back. The pinning and dewetting mechanism of a tilted drop pinned by a designed patch on a superhydrophobic surface with negligible contact angle hysteresis (CAH) is explored both experimentally and theoretically. The patch is designed in different shapes including square, rectangle and triangle. For a square or rectangular patch, the uphill contact angle (CA) of the tilted drop varies with the inclined angle (a) of the plate. The drop remains pinned until the critical inclined angle (ac) is achieved. As a=ac, the uphill CA of the drop reduces to the receding angle of the patch. The magnitude of ac grows approximately linearly with the pinning length (wp), which is related to the patch size. It is found that wp equals the side-length (w) of square or rectangular patch perpendicular to the sliding direction. While wp on square patches remains essentially unchanged before sliding, wp on the triangular patch grows with increasing a. However, the relation between sin(a) and wp for the triangular patch is consistent with that between sin(ac)and w for square and rectangular patches. Surface Evolver simulations based on free energy minimization are performed to reproduce the wetting and dewetting behavior. The simulation outcomes agree quite well with the experimental results.(3) The CAH of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the CA of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different. In the contact line withdrawal regime, the contact line will withdraw with the CA remaining at the receding angle. In the contact line pinning regime, however, the contact line will be pinned at the final position and the CA will decline to a certain value higher than the receding angle. Furthermore, the advancing pinning behavior can also be realized through a successive compression-relaxation process. On the basis of the liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of the droplet during the compression-relaxation process; both contact line withdrawal and pinning regimes can also be identified. The results of the experiment and simulation agree with each other very well.(4) A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to network-like pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface. |
