微/奈米粒子粒徑與材質在液滴中對於不同親疏水表面咖啡環形成之自附著現象影響探討

Effect of Mirco-/Nano-particle Size and Material on Self-Pinning Behavior for Coffee Ring Formation on Different Hydrophobic Surfaces

楊凱傑(Kai-Chieh Yang)

指導教授 ： 陳立仁

國立臺灣大學/工學院/化學工程學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201702448

摘要

當含有非揮發性微/奈米粒子的膠體懸浮液在均勻固體表面乾燥後會殘留環形的微/奈米粒子堆積在液滴的周圍，此種現象被稱為咖啡環效應(coffee ring effect)，粒子的聚集堆積而導致的自附著現象是造成咖啡環效應的必要條件之一，隨著溶劑(水)逐漸揮發的過程微/奈米粒子溶液濃度會慢慢上升，直到其最終達到附著濃度使得自附著現象開始發生並在表面上形成咖啡環，在本研究當中，我們探討在固定的環境溫度(30 °C)與濕度(50%)之下關於粒子粒徑大小、粒子材料兩者與有機矽烷塗佈表面親疏水性之間的關聯性並透過實驗計算其附著濃度，對於二氧化矽微/奈米粒子而言其附著濃度會隨著粒徑的上升而下降並且不隨著液滴的起始粒子濃度而改變，其中粒徑400奈米的二氧化矽粒子其附著濃度與表面親疏水性(以水在表面的後退角作為依據)會呈線性關係，隨著粒徑的上升，二氧化矽粒子附著濃度與表面水滴後退角的線性關係依然維持但會些許的向下平移，並與粒徑400奈米的二氧化矽粒子系統幾乎保持平行，此結果顯示對於粒徑較大的二氧化矽微/奈米粒子而言，其三相接觸線的附著能力比起小粒子來的更為優勢，然而對於聚苯乙烯的系統(粒徑100、500奈米及1.0微米)在有機矽烷表面上的揮發情形而言，雖然線性關係仍然存在，但粒子大小對於附著濃度趨勢造成影響卻完全相反，粒徑較小的聚苯乙烯粒子比起較大的粒子具有更強的三相接觸線附著能力。
除此之外，我們也對不同粒徑的奈米粒子混和溶液的系統進行探討，透過將兩者不同粒徑的粒子混和於同一溶液當中(粒徑400 + 1000奈米與760 + 1000奈米之二氧化矽粒子及粒徑 100 + 1000奈米之聚苯乙烯系統)進行揮發實驗，附著濃度與最終的咖啡環沉積在混和粒徑的系統當中會和兩粒子當中具有較低附著濃度的粒徑的結果較為相近，這也顯示了自附著現象會被混和粒子當中具有較強三相接觸線附著能力的粒子所主導。

關鍵字

後退角 ； 自附著現象 ； 親疏水性 ； 奈米溶液 ； 咖啡環

並列摘要

When a colloidal droplet containing nonvolatile micro-/nano-particles dries out on homogeneous solid surfaces, a ring-like deposit of micro-/nano-particles develops at the edge of the droplet, known as the coffee ring effect. Self-pinning induced by the aggregation of nanoparticles at the drop edge is a pre-requisite for the coffee ring effect. The micro-/nano-particle concentration in the colloidal droplet increases during the evaporation process due to the solvent (water) evaporation and eventually reaches the pinning concentration of particles to initiate the self-pinning for the coffee ring formation on the alkylsilane coated surfaces. The effect of particle sizes, particle material and surface hydrophobicity on the pinning concentration was experimentally explored and examined at a fixed surrounding temperature (30 °C) and humidity (50 %). The pinning concentration of silica nanoparticles decreases along with an increase in the size of nanoparticles and is independent of the initial concentration of nanoparticles. The pinning concentration of silica nanoparticles of 400 nm in diameter linearly depends on the surface hydrophobicity, characterized by the receding contact angle of water on the surface. The linear regression curve between the pinning concentration of silica nanoparticles of 760 nm (and 1000 nm) in diameter and the receding contact angle of water on the alkylsilane coated surface remains intact but shifts downward and almost parallel to that of 400 nm in diameter, implying that the ability of self-pinning the three phase contact of lager silica nanoparticles is stronger than smaller ones. However, for the polystyrene nanoparticles (100, 500 and 1000 nm in diameter) on alkysilane coated surface, the tendency of pinning concentration affected by particle size totally opposite to that of silica particle system, although the linear dependence on the receding contact angle of the surface remains the same as that of silica particle system.
In addition, the evaporation of colloidal droplet containing a mixture of bi-dispersed particles of two different sizes (400 + 1000 nm, 760 + 1000 nm in diameter silica particle and 100 + 1000 nm in diameter polystyrene particle) was also examined. The pinning concentration and the size of residual deposit of this bi-dispersed particle system are consistent with that of particles with lower pinning concentration (larger particle for silica particles and smaller particle for polystyrene particles), implying that the self-pinning is dominated by the particles with strong pinning effect to the three phase contact line.

並列關鍵字

receding contact angle ； self-pinning ； hydrophobicity ； nanofluid ； coffee-ring

參考文獻

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