Yang Kai-Chieh, Wang Chieh, Hu Ting-Yu, Lin Hui-Ping, Cho Kuan-Hung, Chen Li-Jen
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
J Colloid Interface Sci. 2020 Nov 1;579:212-220. doi: 10.1016/j.jcis.2020.06.059. Epub 2020 Jun 16.
Self-pinning induced by the aggregation of particles at the edge of a pinned drop is a pre-requisite for the coffee ring formation. The edge (three-phase contact line) of a suspension drop on a hydrophobic surface would depin and shrink in the early stage of evaporation process. It is plausible to conjecture that the self-pinning of silica suspension drops depends on the particle size and surface property.
Two substrate materials, the alkylsilane coated surfaces and the polydimethylsiloxane surfaces, and three different sizes of silica particles are used to explore the criterion of self-pinning of silica suspension drops on these hydrophobic surfaces. The evaporation process of droplets is recorded and further analyzed.
The pinning concentration of silica suspensions of a fixed size linearly depends on the receding contact angle of the surface, irrelevant to the substrate material and initial particle concentration. The pinning concentration decreases along with an increase in particle size. In addition, the pinning concentrations of bi-dispersed silica (e.g. 400 + 1000 nm) suspensions have an excellent agreement with that of larger size (1000 nm) particle system. That implies that the larger particle dominates the system of bi-dispersed silica suspensions to initiate the self-pinning, further verified by SEM images.
钉扎液滴边缘的颗粒聚集所引发的自钉扎是形成咖啡环的一个先决条件。疏水性表面上悬浮液滴的边缘（三相接触线）在蒸发过程的早期会脱钉并收缩。可以合理推测，二氧化硅悬浮液滴的自钉扎取决于颗粒大小和表面性质。
使用两种基底材料，即烷基硅烷涂层表面和聚二甲基硅氧烷表面，以及三种不同尺寸的二氧化硅颗粒，来探究二氧化硅悬浮液滴在这些疏水性表面上的自钉扎准则。记录并进一步分析液滴的蒸发过程。
固定尺寸的二氧化硅悬浮液的钉扎浓度与表面的后退接触角呈线性关系，与基底材料和初始颗粒浓度无关。钉扎浓度随颗粒尺寸的增加而降低。此外，双分散二氧化硅（例如400 + 1000 nm）悬浮液的钉扎浓度与较大尺寸（1000 nm）颗粒体系的钉扎浓度非常吻合。这意味着较大颗粒主导双分散二氧化硅悬浮液体系以引发自钉扎，扫描电子显微镜图像进一步证实了这一点。
