Tunable coffee-ring formation of halloysite nanotubes by evaporating sessile drops.

Halloysite nanotubes (HNTs) are one-dimensional clay nanomaterials with a length of 200-1000 nm and a diameter of ∼50 nm. Understanding the self-assembly behavior of such unique nanoparticles is important to develop their applications in functional devices. In this study, the "coffee-ring" patterns of HNTs are investigated which are formed by evaporation of the sessile droplets of HNT aqueous dispersion on different substrates. The coffee-ring pattern with various dimensions was characterized using a polarizing microscope (POM), a scanning electron microscope (SEM), and a 3D optical profilometer. The diameter, height, and area of the coffee-ring patterns depend on the concentration of HNT dispersion, the droplet volume, and surface wettability. POM and SEM results suggested that the nanotubes were highly ordered in the edge and the middle of the coffee-ring. The coffee-ring effect of HNTs could be suppressed by increasing the evaporation temperature of substrates or adding polymer additives. In addition, multiple-ring patterns consistent with protein rings surrounding HNT rings were formed, which can be utilized to detect the presence of proteins in biological samples. This work illustrated the relationship between the formation of coffee-ring patterns and the experimental conditions, which provided an additional research chance and allowed application development for HNTs using the liquid droplet self-assembly.