Comment on "Patterns in Drying Drops Dictated by Curvature-Driven Particle Transport".

In a recent article, Mondal et al. (Mondal, R.; Semwal, S.; Kumar, P. L.; Thampi, S. P.; Basavaraj, M. G. , , 11473-11483) demonstrated different patterns (coffee-rings and coffee-eyes) in dry deposits from solutions of concentrated well-stabilized nanofluids. Coffee rings created from dried sessile droplets result mainly from internal radial flow as proposed by Deegan et al. (Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. , , 827-829). To generate coffee-eyes from pendent droplets, Mondal et al. have proposed a new particle transport route involving particle adsorption at the interface and its consequent curvature-driven settling along the interface due to gravity acting on the droplet. In this comment, we demonstrate that coffee-eyes can also be formed from pendent droplets by increasing the nanoparticle size to destabilize the colloidal liquid, causing nanoparticle accumulation at the water droplet apex without particle adsorption at the interface.