| 摘要: | 以往的科學家在進行咖啡漬圈環效應的研究時，多選用微米等級的微粒作為溶質，本論文選用不同尺寸的溶質，研究對咖啡漬圈環表現圖形的影響。溶質方面選擇奈米粒子、小分子(金屬鹽類溶液與界面活性劑)以及高分子。
經過實驗後發現，無論是奈米粒子、小分子或是高分子溶質，溶液形成類環狀圖形與否的重要因素皆為接觸角遲滯的大小，若接觸角遲滯小，接觸線不斷內縮，溶質將隨著接觸線撤退最後形成點狀圖形；反之，若接觸角遲滯大，溶質就能被outflow帶往液滴邊緣，最終形成類環狀圖形。我們可以藉由選擇接觸角遲滯大的基材，或是使用具有界面活性的溶質來控制類環狀圖形的出現，反之則可消除類環狀圖形。
在高分子與奈米粒子溶液中，有部分的溶質會在不同濃度時表現出不同的蒸發殘留圖形，這種特殊的現象稱為濃度效應。濃度效應有兩種成因：(1)NaPSS型：溶液之後退角隨著溶質濃度的上升而大幅度地下降，在溶液於低濃度時，蒸發時接觸角角度下降速率高於後退角下降的速率，使接觸角角度能夠到達後退角，接觸線depinning，最終形成點狀圖形；反之形成類環狀圖形。(2) PEG、銳鈦礦奈米二氧化鈦溶液型：液滴蒸發過程中溶質濃度到達飽和析出。故越高濃度的液滴越有可能在接觸線來不及內縮時到達飽和，形成看似接觸線不會退縮的類環狀圖形。
值得注意的是，如NaPSS一般會因為濃度改變而其活性亦隨之改變的溶液，不同接觸角的量測方法所得到的結果將有所差異。如果使用蒸發法，因為隨著蒸發，溶液的濃度不斷地上升，溶液的後退角隨之改變，蒸發法將不能量出一開始欲量測濃度的後退角。但是微量針頭法、平板法與傾斜板法因為量測時所耗用的時間短暫，對於溶液濃度的影響不大，仍可使用。
Drying of a droplet containing non-volatile solutes, initially dispersed over the entire drop, is an everyday phenomenon and it commonly leaves a ring of solute deposit on a surface, rather than a uniform spot. Although droplet drying is often observed, it gives rise to surprisingly rich morphologies, which depend on the contact line geometry, solute size and chemistry, and substrate-solvent interaction.
The formation of the well-known coffee-ring pattern after the liquid drop evaporation has been successfully explained by Deegan et al. The radial outward flow is thus induced by the differential evaporation rates across the drop. Contact line pinning of the drying drop on the surface leads to liquid flow from the interior to replenish the liquid evaporating from the edge. The resulting edgeward flow can carry nearly all the dispersed solutes toward the edge and deposit them in the vicinity of the contact line to form a ring-like stain. Evidently, the appearance of the coffee-ring effect involves three ingredients for a drying droplet: non-volatile solute, outward flow, and contact line pinning.
Contact angle hysteresis dominates the formation of ring-like stain and there are two factors to influence the contact angle hysteresis: substrate and solutes. In our study, the drying process and drying patterns are observed on the substrate of low contact angle hysteresis with different sizes solutes.
From the observation of our experiments, we generalize some conclusions in the following:
(1) In order to acquire a ring-like stain on a hydrophilic substrate with weak CAH, the wetting property of the liquid drop on the substrate must be modified and it can be achieved by surface-active solute. The separation of a mixture can be achieved on evaporation stain based on their difference of surface-activity. The pattern of evaporation stain can be determined by the competition between contact line receding and small-sized solute precipitation.
(2) There are three types of solutes :
(i) No surface-activity. The addition of some solutes, such as Dextran and PDDC, has no influence on the surface tension of solution. Therefore, the dot pattern is observed.
(ii) Weak surface-activity. The surface tension will decrease slowly with increasing the concentration of some solutes such as NaPSS. Hence, the formation of dot pattern is found at low concentration while the ring-like pattern takes place at high concentration.
(iii) Strong surface-activity. The presence of a small amount of some solutes, such as PVP and PVA may result in a rapid decrement of surface tension of solution. As a result, the ring-like pattern is formed.
(3)By using nanoparticle solution, stick-slip pattern is observed and particle size may affect the evaporation stain. |
