Role of pH and substrate on drying patterns of Laponite® droplet. (1st November 2021)

• Record Type: Journal Article
• Authors: Haque, Samiul; Dutta, Tapati
• Is Part Of: Physica scripta. Volume 96:Number 12(2021)
• Publication Date: 2021-11-01
• Subjects: droplet -- desiccation cracking -- clay -- pH -- hydrophobicity; Physics -- Periodicals; 530.05
• DOI: 10.1088/1402-4896/ac3151
• Languages: English
• ISSNs: 0031-8949

Abstract

Abstract: Disc shaped Laponite ® particles in aqueous solution have a net negative charge on the faces and a positive charge along the rim. The charge on the rim of these particles changes to negative around a pH of 12. The pH of the aqueous Laponite ® solution thus determines whether the Laponite ® particles arrange themselves in a repulsive glass phase or form a cage-like gel structure as they gel during drying. When droplets of Laponite ® solution having different pH are allowed to evaporate, the arrangement of the particles during drying is further perturbed by currents of mass transfer due to uneven evaporation flux from the droplet surface. In this work we report the interesting patterns left by drying droplets of aqueous Laponite ® with different pH, dried on substrates of different hydrophobicity. On hydrophobic substrate, in highly acidic pH, a prominent coffee-ring was observed, while in a strong basic pH, salt crystals on Laponite ® particles gathered at the centre of the dried droplet. Similar studies were done on hydrophilic substrates and quite different dried patterns obtained. Further in the case of very acidic pH of the aqueous Laponite ® droplet drying on a hydrophobic substrate, the dried pattern revealed collapsed cavities whose walls showed rings of salt crystallites. We offer an explanation of the different patterns on the basis of particle-substrate electrostatic interactions in tandem with the role of pH, in the determination of the final pattern.