超親水表面之潤濕行為與超疏水表面的氣泡運動;Wetting Behavior of Superhydrophilic Surfaces and Bubble Motion on Superhydrophobic Surfaces

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Title:	超親水表面之潤濕行為與超疏水表面的氣泡運動;Wetting Behavior of Superhydrophilic Surfaces and Bubble Motion on Superhydrophobic Surfaces
Authors:	吳權璋;Wu, Chuan-Chang
Contributors:	化學工程與材料工程學系
Keywords:	表面潤濕性;超親水;超疏水;氣泡運動;surface wettability;superhydrophilic;superhydrophobic;bubble motion
Date:	2017-03-08
Issue Date:	2017-05-05 17:10:37 (UTC+8)
Publisher:	國立中央大學
Abstract:	隨著各類科學與工業技術的蓬勃發展，許多應用皆涉及液體與基材表面的相互作用。因此，各類具有不同潤濕特性的材料表面被開發，以因應各式各樣的產品需求。在經過適當的表面改質後，可以賦予材料表面獨特的潤濕特性並提高材料的應用價值。舉例而言，超親水表面可以利用在抗霧、抗反射、及抗生物沾黏等領域；相對地，超疏水表面則具備自清潔的能力，因而有潛力可以作為抗汙材料或減少原物料輸送時的能量損耗。合成並研究這些具有極端潤濕特性的表面(如：超親水或超疏水表面)將有助於表面改質工藝的發展。氣泡廣泛的存在於日常生活中，其沿基材表面的運動涉及氣/液/固三相的相互作用，並且在流體輸送(如微流道系統)中扮演重要的角色。研究氣泡的運動行為將可解決一些輸送或工業程序的難題，如氣泡於微流道中的柱塞現象。在本研究中，將結合實驗與理論，探討超親水表面之潤濕現象與超疏水表面的氣泡運動。將磺基甜菜鹼(SBSi)與羧基甜菜鹼(CBSi)等兩類矽烷分別接枝至玻璃表面，即可完成雙離子性表面的製備。並利用水、極性有機溶劑、與十六烷觀察其表面潤溼行為。於CBSi表面上，液滴呈現部分潤濕，水與十六烷的接觸角皆小於10度，顯示其超雙親的特性。於SBSi表面上，所有測試液體皆自發性地不斷向外擴散(無平衡接觸角)，顯示其完全潤濕的特性。根據液滴種類的不同，其潤濕面積隨時間的變化可以區分為三種類型：對水的「擴張-收縮」、對極性有機液體的「擴張-停止」、及對十六烷的「連續擴張」。此於SBSi表面上的自發性擴張行為是由固體表面的高表面能所驅動且其運動行為可以冪次法則來表徵。即使雙離子性表面在一般大氣環境中看似親水亦親油，其仍具有偏好性。將雙離子性表面浸入水中並置上十六烷油滴，十六烷在CBSi表面上的接觸角為120度，但在SBSi表面上變為180度，即可發雙離子性表面仍是親水更勝親油。將CBSi與SBSi兩種表面分別浸入各類測試液中並釋放氣泡至其表面上，可以發現氣泡會沾黏並停滯在CBSi表面上。反之，在SBSi表面上的氣泡卻會自由的滑動。實驗結果清楚的彰顯了CBSi與SBSi表面之潤濕性質的不同，前者為超親水表面而後者則為完全潤濕表面。蒸餾是混合水溶液溶質分離的常用方式，其過程主要由水的蒸發所主導。而水滴的蒸發速率則會隨著其液氣界面的面積增加而上升。水滴在完全潤濕表面上的自發性擴張行為可大幅增加每單位體積的液氣接觸面積，因而具有快速揮發溶劑達到溶質分離的應用潛力。然而，當水滴中含有非揮發性的溶質(如小分子或高分子)，原先的自發性擴張行為將會被抑制而造成自釘扎的現象。在本研究中，將利用簡單的重力方式，克服這類由溶質所誘發的自釘扎行為，且產生的非等向性擴張，其速率更顯著的超越水平表面上的等向擴張。對於一個10微升的液滴而言，將其置於雙離子型SBSi表面上並利用重力進行非等向性的擴張，將可大幅提升其蒸發速率。與靜置在壓克力板上相比，其蒸發速率增加了近25倍。此利用面積擴張急遽增強蒸發作用的方法，可由液膜上方瞬間同步生成的霧來證實，於溶質分離與濃縮等領域有相當的應用潛力。在非等向性擴張的過程中，溶質於30秒內即會快速析出，顯示其快速的溶質分離效果。此外，將含有染劑的水溶液重複進行6次擴張實驗，濃度即可上升一倍，其濃縮效率高達100%。此由重力驅動於完全潤濕表面上的非等向性擴張方法可在常溫下進行，並具備簡單大規模化與節能的優點，因此在溶質分離與濃縮等應用領域有優秀的應用潛力。由於接觸角遲滯的影響，微小氣泡很容易黏著在基材表面上。當基材表面的接觸角遲滯很小時，由於浮力能夠克服毛系阻力，因此小氣泡將可緩慢的在傾斜的表面上滑動。在本研究的實驗中，很驚訝地觀察到3~15微升的小氣泡(直徑約1.79~3.06毫米)於傾斜超疏水表面下的移動速度大幅超越了同體積氣泡的自由浮升速度(雷諾數約O(102))。隨著傾斜角增加，滑動氣泡的阻力係數基本上與自由浮升氣泡(球狀)的值相近，然而滑動氣泡(扁平)的前方投影面積卻呈現單調上升的情形。僅管如此，滑動氣泡的前方投影面積仍是小於自由浮升氣泡。結果顯示，超疏水表面下的滑動氣泡因為其扁平的形狀而具有非常小的前方投影面積，因而大幅減少運動過程中的拖曳阻力，使其滑動速度可以超越自由浮升氣泡。;Owing to numerous scientific and industrial applications involving fluid/substrate interfaces, the material surfaces with various types of wettability are needed. After suitable surface modifications, substrates can show specific wetting behavior and possess high-value applications. For instance, superhydrophilic surfaces can be used for anti-fog, anti-reflection, and anti-biofouling fields; in contrast, superhydrophobic surfaces possess the self-cleaning ability and have the potential to be utilized for anti-smudge materials and reducing energy consumption during mass transport. Synthesis and investigation of the surfaces with extreme wettability (superhydrophobicity or superhydrophilicity) can shed light on the development of surface modification. Gas bubbles in aerated drinks are often present in our daily life and nature. Their motion along the material surfaces involve the interactions among three phases (gas/liquid/solid) and play an important role in fluid transport such as the dynamic microfluidic. The difficulties of industrial processes and the products encountered in the transport phenomena, such as the bubble plunger in micro-fluidic systems and syringe devices, can be resolved by studying the motion of bubbles. In this work, the wetting behavior of superhydrophilic surfaces and bubble motion on superhydrophobic surfaces have been investigated both experimentally and theoretically. Zwitterionic surfaces are fabricated by grafting sulfobetaine silane (SBSi) and carboxybetaine silane (CBSi) on glass slides. Their wetting behaviors are investigated using water, polar organic liquids, and hexadecane as test liquids. For the CBSi surface, partial wetting is observed, and contact angles of water and hexadecane are lower than 10o, revealing super-amphiphilicity. For the SBSi surface, all test liquids spread spontaneously and contact angles are absent, corresponding to total wetting. The time evolution of the wetting area of a liquid drop can be divided into three types: spread-withdrawal for water, spread-pin for polar organic liquids, and continuous spread for hexadecane. The spontaneous spreading on SBSi surfaces is driven by the high solid-gas interfacial tension and can be characterized by the power law. Although zwitterionic surfaces like both water and hexadecane in ambient air, their preference for water over hexadecane is typically demonstrated by a hexadecane drop in a water environment. Nonetheless, the contact angle of the hexadecane drop is 120o on the CBSi surface, but becomes 180o on the SBSi surface. As the zwitterionic surfaces are immersed in all test liquids, bubbles generally adhere to the CBSi surface but freely move beneath the SBSi surface. Our experimental results clearly show the wettability difference between the CBSi and SBSi surfaces. The former is superhydrophilic, while the latter is total wetting. Solute separation of aqueous mixtures via distillation is mainly dominated by water vaporization. The evaporation rate of an aqueous drop grows with increasing the liquid-gas interfacial area. The spontaneous spreading behavior of a water droplet on a total wetting surface provides huge interfacial area per unit volume but is halted by the self-pinning phenomenon with the addition of nonvolatile solutes including small molecules and polymers. In this work, it is shown that the solute-induced self-pinning can be overcome by gravity, leading to anisotropic spreading much faster than isotropic spreading. The evaporation rate of anisotropic spreading on a zwitterionic sulfobetaine surface is 25 times as large as that of a drop of 10 l on a poly(methyl methacrylate) surface. The dramatic enhancement of evaporation by the area expansion is demonstrated by the simultaneous formation of fog atop the liquid film and it is very useful for solute separation and concentration. During anisotropic spreading, the solutes are quickly precipitated out within 30 sec, showing the rapid solute-water separation. After repeating 6 times of the spreading process dropwisely for the dye-containing solution, the mean concentration of the collection is doubled, revealing the concentration efficiency as high as 100 %. Gravity-enhanced spreading on total wetting surfaces at room temperature is easy to scale-up and much less energy consumption and thus it has great potentials for the applications of solute separation and concentration. Tiny bubbles readily stick onto substrates due to contact angle hysteresis (CAH). Nevertheless, tiny bubbles can slide slowly on a tilted surface with ultralow CAH since capillarity is overcome by buoyancy. It is surprising to observe experimentally that bubbles of 3~15 l (diameter 1.79~3.06 mm) slide beneath a tilted superhydrophobic surface at a vertical ascent rate faster than freely rising ones of high Reynold numbers ~O(102). As the tilted angle increases, the drag coefficient remains essentially the same as that of a freely rising bubble but the frontal area of the flat bubble rises monotonically. Nonetheless, the frontal area of the sliding bubble always stays much smaller than that of a freely rising bubble. Consequently, the small drag force associated with sliding bubbles is attributed to their substantially small frontal areas on superhydrophobic surfaces.
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