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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/66757


    Title: 潤濕性質對咖啡圈效應之影響與石墨潤濕性質之探討;Wetting Properties on Graphite and Patterns of Evaporation Stain Formation
    Authors: 李岳峰;Li,Yueh-Feng
    Contributors: 化學工程與材料工程學系
    Keywords: 潤濕現象;接觸角;石墨;咖啡圈效應;wetting phenomena;contact angle;graphite;coffee-ring effect
    Date: 2014-12-12
    Issue Date: 2015-03-16 15:10:19 (UTC+8)
    Publisher: 國立中央大學
    Abstract: 隨著奈米科技的發展,眾多的奈米材料被應用來增強材料的機械強度、熱導度,以及電導度等性質。近年來,石墨奈米材料憑著優越的物理特性,包括高電子遷移率、高透光性,以及高機械強度與熱導性質,在電子元件、生醫工程、複合材料、光電材料與儲能技術等應用上掀起一股熱潮。於上述應用中,印刷噴墨成為一種重要的技術,而此技術與含有特定溶質之液滴於蒸發過程中的沉積行為息息相關。本研究中,由電化學剝落法所製備石墨膠體粒子之界面性質與液滴乾燥圖案之形成行為將被探討。
    疏水的石墨電極,透過電化學剝落法,竟可得到超親水石墨表面以及可於水中均勻分散的石墨膠體粒子。此種與直覺相反的特徵,透過檢驗電解反應後的石墨電極與石墨膠體粒子,發現成因乃部分氧化所造成。然而,置於陽極的石墨電極的潤濕程度可透過電解電流與時間來改變。經過一段有效電解時間後,陽極石墨成為一超親水表面,而此超親水石墨表面可藉由膠帶剝離法,使其回復原有之疏水表面。此結果顯示,電解水反應中於陽極產生之氧氣使陽極石墨產生了氧化作用,而此氧化作用僅發生於石墨表層。由此推論,由電化學剝落法所得之石墨膠體粒子為殼層結構,同時具有氧化石墨的外殼以及石墨的內部。除此之外,電解液之酸鹼度也將影響電解後石墨粒子之氧化層厚度。
    當一具有膠體粒子溶質液滴於材質上,因為液滴接觸線固定以及液滴內的外向液流,在完全蒸發後,通常會在材質上殘留環狀的乾燥圖案。在此研究中,溶質採用小尺寸溶質及高分子,材質採用各式不同的親水材質,研究過程中我們觀察到不同的乾燥圖案,包括點狀圖案、環狀圖案,及點狀環狀結合圖案,其形成機制則取決於溶質的表面活性以及材質的接觸角遲滯程度。對於小分子且不具有表面活性的溶質而言,點狀沉積圖案將出現在弱接觸角遲滯的材質上(例如硫酸銅水溶液於玻璃基材上)而環狀沉積圖案將出現在強接觸角遲滯的材質上(例如硫酸銅水溶液於石墨基材上)。倘若溶質具有表面活性,其將改變液滴於材質上的潤濕性質,而此結果將導致環狀沉積圖案的出現(例如Brij-35 於聚碳酸酯基材上)。然而,若混合具表面活性與不具表面活性溶質於溶液中,則可觀察到點狀與環狀的結合圖案。上敘論述同樣可套用至高分子溶質,點狀沉積圖案出現於弱接觸角遲滯(例如聚苯乙烯磺酸鈉)而環狀沉積圖案則沉積出現於強接觸角遲滯(聚乙烯吡咯烷酮)。由上述結果顯示,液滴蒸發後殘留圖案的形成,取決於液滴接觸線內縮以及溶質飽和析出兩者之競爭。換句話說,透過控制接觸角遲滯的程度,便可抑制咖啡圈效應。
    此外,溶質的表面活性與系統接觸角遲滯的程度將受溶質濃度影響,進而改變液滴蒸發後的沉積圖案。在濃度效應的部分,我們分類了四種不同的行為,其系統為含高分子溶質液滴至於聚碳酸酯基材上。針對不具表面活性的高分子溶質(例如聚葡萄糖),其潤濕性質與接觸角遲滯不受溶質濃度影響,故液滴於蒸發後將出現點狀沉積圖案。針對具有弱表面活性的高分子溶質(例如聚乙二醇),其前進與後退接觸角隨著添加聚乙二醇而降低,點狀沉積圖案隨著濃度而改變並且出現類似山狀的沉積。針對具有中等程度表面活性的高分子溶質(例如聚苯乙烯磺酸鈉),後退接觸角隨著濃度增加而明顯下降,增加了接觸角遲滯的程度,其環狀沉積圖案也隨濃度增加而改變。針對具有強表面活性的高分子溶質(例如聚乙烯吡咯烷酮),在添加少量溶質後,前進接觸角維持不變但後退接觸角卻大幅降低,造成大程度的接觸角遲滯。由於大程度的接觸角遲滯造成接觸線固定,於液滴蒸發後將出現環狀沉積圖案。
    ;With the development of nanotechnology, lots of nanomaterials are being used to enhance mechanical strength, as well as thermal and electric conductivity. In recent years, graphitic nanomaterials have roused a great deal of interest in the fields of electronics, biological engineering, composite materials, photovoltaics, and energy storage due to their high carrier mobility, excellent optical transparency, and extraordinary mechanical and thermal properties. For such applications, inkjet printing has also become an important technology, as it is related to pattern formation in a drying drop containing specific solutes. This study investigates the surface properties of graphite colloids by electrochemical exfoliation and the behavior of pattern formation.
    Superhydrophilic graphite surfaces and water-dispersible graphite colloids are obtained by electrochemical exfoliation with hydrophobic graphite electrodes. Such counterintuitive characteristics are caused by partial oxidation and studied by examining both the graphite electrodes and the exfoliated particles after electrolysis. The degree of wettability of the graphite anode can be altered by the electrolytic current and time. After a sufficient time, the graphite anode becomes superhydrophilic and its hydrophobicity can be recovered by peeling with adhesive tape. This result reveals that anodic graphite is oxidized by oxygen bubbles but that the oxidation occurs only in the outer layers of the graphite sheet. The structure of this partially oxidized graphite may consist of a graphite core covered with an oxidized shell. The properties of the exfoliated colloids are also influenced by the pH of the electrolytic solution. As the pH increases, the extent of oxidation and the thickness of the oxidized shell decrease. These results reveal that the degree of oxidation of exfoliated nanoparticles can be manipulated by controlling pH.
    A ring-shaped stain is frequently left on a substrate for a drying drop containing colloids due to contact line pinning and outward flow. However, different patterns are observed for drying drops that contain small-sized solutes or polymers on various hydrophilic substrates. Depending on the surface activity of the solutes and the contact angle hysteresis (CAH) of substrates, the pattern of evaporation stain will vary and may include concentrated stains, ring-like deposits, or combined structures. For small-sized surface-inactive solutes, the concentrated stain is formed on substrates with weak CAH, such as a copper sulfate solution on silica glass. On the contrary, a ring-like deposit is developed on substrates with strong CAH, such as a copper sulfate solution on graphite. For surface-active solutes, the wetting property can be significantly altered and the ring-like stain always appears, such as Brij-35 solution on acrylic glass. For a mixture of surface-active and surface-inactive solutes, a combined pattern of ring-like and concentrated stains can appear. Similar results are observed for various polymer solutions on a polycarbonate. Concentrated stains are formed for weak CAH, such as sodium polysulfonate (NaPSS), while ring-shaped patterns are developed for strong CAH, such as polyvinyl pyrrolidone (PVP). The stain pattern is determined by the competition between the time scales associated with contact line retreat and solute precipitation. The suppression of the coffee-ring effect can thus be acquired by controlling CAH.
    Furthermore, depending on the surface-activity of the solutes, the extent of contact angle hysteresis (CAH) can vary with their concentration, accordingly altering the pattern of the evaporation stain. Four types of concentration-dependent CAH and evaporation stains have been identified for a water drop containing polymeric additives on polycarbonate. For polymers with surface-inactivity such as dextran, advancing and receding contact angles (a and r) are independent of solute concentrations and a concentrated stain is observed in the vicinity of the drop center after complete evaporation. For polymers with weak surface-activity such as polyethylene glycol (PEG), both a and r are decreased by the solute addition and the stain pattern varies with increasing PEG concentration, including a concentrated stain and a mountain-like island. For polymers with intermediate surface-activity such as NaPSS, a decreases slightly while r decreases significantly after a substantial addition of NaPSS and a ring-like stain pattern is observed. Moreover, the size of the ring stain can be controlled by the NaPSS concentration. For polymers with strong surface-activity such as PVP, a remains essentially constant but r is significantly lowered after a small addition of PVP and the typical ring-like stain is seen. For a mixture of surface-active and surface-inactive solutes such as PVP and CuSO4, the surface-inactive solutes can be deposited along the perimeter of the ring pattern due to contact line pinning induced by surface-active solute.
    Appears in Collections:[化學工程與材料工程研究所] 博碩士論文

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