title: Orientation of charged clay nanotubes in evaporating droplet meniscus.
abstract: During drying, an aqueous suspension of strongly charged halloysite clay nanotubes concentrates at the edge of the droplet ("coffee-ring" effect) which provides alignment of the tubes along the liquid-substrate contact line. First, the surface charge of the nanotubes was enhanced by polyanion adsorption inside of the lumen to compensate for the internal positive charges. This increased the magnitude of the ξ-potential of the tubes from -36 to -81 mV and stabilized the colloids. Then, colloidal halloysite was dropped onto the substrate, dried at 65 °C and after a concentration of ∼0.05 mg mL(-1) was reached, the alignment of nanotubes occurred starting from the droplet edges. The process was described with Onsager's theory, in which longer nanorods, which have higher surface charge, give better ordering after a critical concentration is reached. This study indicates a new application of halloysite clay nanotubes in polymeric composites with anisotropic properties, microchannel orientation, and production of coatings with aligned nanotubes.
authors.affiliation: Institute for Micromanufacturing, Louisiana Tech University, Ruston, USA.
authors.forename: Yafei
authors.initials: Y
authors.lastname: Zhao
authors.affiliation: Department of Physics and Chemistry, University of Palermo, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Cavallaro
authors.affiliation: Institute for Micromanufacturing, Louisiana Tech University, Ruston, USA; Kazan Federal University, Tatarstan, Russian Federation. Electronic address: ylvov@latech.edu.
authors.forename: Yuri
authors.initials: Y
authors.lastname: Lvov
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D000077215:Clay; D003460:Crystallization; D006863:Hydrogen-Ion Concentration; D008855:Microscopy, Electron, Scanning; D043942:Nanotubes / Q000737:chemistry*; D009994:Osmolar Concentration
chemical_list: D000538:Aluminum Silicates; D000077215:Clay
keywords: Alignment; Coffee ring; Droplet evaporation; Halloysite nanotubes; Liquid crystal