Evaporation-Induced Branched Structures from Sessile Nanofluid Droplets

We investigate the formation of branched nanoparticle aggregates resulting from the evaporation of sessile nanofluid droplets of the copper water-based nanofluids experimentally. Both symmetric and asymmetric drying patterns were found as the sessile droplet evaporated. A kinetic Monte Carlo (KMC) approach is developed to explain the drying process in a circular domain, representing the top view of a drying sessile droplet. It is found that the lattice-gas-based Monte Carlo model can describe the nanoparticle self-assembly into a solid highly branched aggregate. While the chemical potential function is coupled to the nondimensional spherical droplet size during evaporation, the results reveal that the fingering contact line instabilities can emerge under a given condition and force the formation of a branched nanoparticle structure. The pattern comparison shows that the simulation results have a qualitative agreement with the experiments. The parameter study shows that the model parameters, such as domain diameter, chemical potential distribution, particle interaction energy, and so on, have significant influence on the resulting patterns.