以往的文獻對於溶液滴在固體材質表面上所進行的乾燥研究，大多是在親水材質上用粒子溶液來探討，因此一般對乾燥圖形的認知，是會形成"Ring-like"-因 Coffee ring effect 所造成外深內淺的環狀殘留圖形。因此我們選用了其它種類型的溶液，如高分子和小分子鹽類等尺寸遠小於粒子的溶質，並搭配親疏水性不同的材質表面來進行有別於以往文獻的乾燥實驗探討。經我們實驗後發現乾燥後生成的圖樣主要取決於兩個因素: Contact angle hysteresis 和 Adhesion－當材質的接觸角遲滯足以讓液珠在材質上 Pinning 至溶液邊緣的濃度達析出或結晶濃度，便可在乾燥的最後看到環狀殘留圖樣；反之，要是材質的接觸角遲滯無法讓液珠 Pinning 蒸發至足夠濃度，便會看到液珠往內縮，使得在最後無法形成環狀的乾燥殘留。而溶質與材質間的黏附若是夠強，便可發現溶質佈滿在與原始液珠覆蓋範圍相同面積大小的材質表面，反之則可看到溶質只殘留在明顯小於原始液珠大小的局部表面。而溶質尺寸較大的粒子型溶液與高分子和小分子鹽類溶液相比，因粒子溶液可藉沉降作用而較容易附著在材質表面，因此想要形成Ring-like 圖樣需要的材質遲滯較小；尺寸較小的高分子或小分子鹽類是靠濃度達飽和來進行析出或結晶，因此需要的材質遲滯較大；然而，若在溶液中加入了 Surfactant 以增強溶液和材質表面的黏附效果，並大幅的降低表面能使液珠後退角改變至近乎零度，如此一來即便是小分子鹽類也可很容易的形成 Ring-like 乾燥圖樣。因此，是否會生成環狀的乾燥殘留主要取決於接觸角遲滯，而黏附的強弱則可決定溶質在材質上的殘留範圍。並可把乾燥圖型的生成看作是溶劑蒸發內縮的速度與溶質的沉降、析出或結晶間的相互競爭。依循此原則，只要遲滯夠大即使在疏水表面上或者是小分子鹽類溶液也可形成環狀的乾燥圖樣；反之要是遲滯太小即使是親水材質也無法看到環狀的蒸發殘留。When a drop of liquid dries on a substrate, its nonvolatile solute is deposited in ring-like fashion and this phenomenon is known as coffee-ring effect. During the drying process, drop edges become pinned to the substrate. Meanwhile capillary flow is induced by differential evaporation rates across the droplet surface outward from the center of the drop. The outflow brings suspended particles to the edge as evaporation proceeds. In general, there are three necessary conditions to form coffee ring including nonzero contact angle, contact line is pinned to its initial position as solvent evaporates. However, the mechanism of drying process on different substrates is seldom mentioned. In our study, the drying process and drying patterns are observed on the various substrates with different contact angle hysteresis such as glass, polypropylene, polycarbonate, graphite and PMMA. In addition, different solutes in solution are also discussed.From the observation of our experiments, we generalize some conclusions in the following: (1)The type of drying pattern depends on the contact angle hysteresis of substrates and adhesion.(2)There are two determined factors for forming a ring like stain: one is time needed for reaching the concentration of phase separation and the other is reduction of contact line.(3)The existence of outflow is not responsible for the contact line pinning.
