Capillary assembly of anisotropic nanoparticles at cylindrical fluid interfaces in the immersion regime

Abstract The unique behaviour of colloids at liquid interfaces provides exciting opportunities for engineering the assembly of colloidal particles into functional materials. In particular, the deformable nature of liquid interfaces means that we can use interfacial curvature, in addition to particle properties, to direct self-assembly. In this paper, we use a finite element method to study the self-assembly of rod-shaped particles adsorbed at a curved interface formed by a sessile drop with cylindrical geometry, where the lateral width of the cylindrical drop is much greater than the length of the rods, and the height of the drop is comparable to or smaller than the radius of the rods, i.e. the system is in the so-called immersion regime. Specifically, we study the configuration of single and multiple rods as a function of drop height, particle shape (ellipsoid, cylinder, spherocylinder) and contact angle. We find that for low enough drop heights, regardless of the shape or contact angle of the particles, all rods orientate themselves parallel to the long axis of the cylindrical interface and are strongly confined laterally to be at the centreline of the cylindrical drop. The rods also experience long-range immersion capillary forces which assemble the rods tip-to-tip at larger drop heights and, in the case of ellipsoids and spherocylinders, side-to-side at smaller drop heights. We note that the capillary forces that drive particle ordering are very strong in the immersion regime, even for rods on the nanoscale, allowing us to control the configuration of nanorods using near micron-scale droplets. Our capillary assembly method therefore provides a facile method for creating functional nanoclusters. Our study also provides insights into how the structure of such clusters evolves during the drying of the droplet.