Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada.
Langmuir. 2011 Jun 7;27(11):7163-7. doi: 10.1021/la200476n. Epub 2011 May 10.
Evaporation of aqueous droplets of carbon nanotubes (CNTs) coated with a physisorbed layer of humic acid (HA) on a partially hydrophilic substrate induces the formation of a film of CNTs. Here, we investigate the role that the global geometry of the substrate surfaces has on the structure of the CNT film. On a flat mica or silica surface, the evaporation of a convex droplet of the CNT dispersion induces the well-known "coffee ring", while evaporation of a concave droplet (capillary meniscus) of the CNT dispersion in a wedge of two planar mica sheets or between two crossed-cylinder sheets induces a large area (>mm(2)) of textured or patterned films characterized by different short- and long-range orientational and positional ordering of the CNTs. The resulting patterns appear to be determined by two competing or cooperative sedimentation mechanisms: (1) capillary forces between CNTs giving micrometer-sized filaments parallel to the boundary line of the evaporating droplet and (2) fingering instability at the boundary line of the evaporating droplet and subsequent pinning of CNTs on the surface giving micrometer-sized filaments of CNTs perpendicular to this boundary line. The interplay between substrate surface geometry and sedimentation mechanisms gives an extra control parameter for manipulating patterns of self-assembling nanoparticles at substrate surfaces.
在部分亲水基底上，被物理吸附层腐殖酸（HA）覆盖的碳纳米管（CNTs）液滴蒸发会导致 CNTs 形成薄膜。在这里，我们研究了基底表面整体几何形状对 CNT 薄膜结构的影响。在云母或二氧化硅的平整表面上，凸面 CNT 分散液滴的蒸发会引起众所周知的“咖啡环”效应，而 CNT 分散液的凹面（毛细弯月面）在两片平行云母片之间或在两个交叉圆柱片之间的楔形物中蒸发会导致大面积（>mm(2))的纹理或图案化薄膜，其特点是 CNTs 的短程和长程取向和位置有序性不同。所得图案似乎由两种竞争或协作的沉降机制决定：(1) CNT 之间的毛细力导致与蒸发液滴边界线平行的微米级长丝，和 (2) 蒸发液滴边界线处的指状不稳定性以及随后 CNT 在表面上的固定导致垂直于该边界线的微米级 CNT 长丝。基底表面几何形状和沉降机制之间的相互作用为在基底表面操纵自组装纳米粒子的图案提供了额外的控制参数。
