Evaporative drying of a particle-laden droplet is an everyday phenomenon and is also widespread in industrial and technological processes. It can produce a plethora of residual surface patterns, including multiple rings, regular stripes, multilayer deposit and, most commonly, coffee rings. Recognised as a simple and versatile method for the organization of materials of interest (polymers, nanoparticles and biomaterials) into sophisticated structures, the subject of controlled evaporative self-assembly (CESA) continues to be topical and academically and technologically important.
Here we report unexpected and unprecedented residual surface patterns from a CESA process – very different from those previously reported. When a tiny droplet of a nanofluid containing ZnO nanorods dries on a glass slide at room temperature, we find that, within tens of seconds, a uniform thin film with a 3D porous network structure comprising centimetre-long Zn(OH)2 nanofibres can spontaneously form. The observation of this rapid chemical (ZnO à Zn(OH)2) and morphological (nanorods à cm-long fibres) transformation of a drying nanofluid is surprising and striking in itself, but more importantly, reveals a new mechanism which is very different from that associated with the coffee rings, as we elucidate in this presentation. Our results point to a facile route for generating self-assembled 3D structures with ultralong nanofibres, important to a range of modern technologies.
wuge.briscoe@bris.ac.uk
