Three-dimensional simulation of dual-scale deposition structures from evaporative self-assembly of nanofluid films
Authors: Gao Chao , Yuan Jun-Jie , Cao Jin-Jun , Yang Hui-Nan , Shan Yan-Guang ,   Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, School of Energy and Power Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China
Published: December 2020

Self-assembly of nanomaterials from the drying of nanofluid films has aroused great interest due to its applications in micro/nano fabrication, ink-jet printing, and thin film coatings. Numerical models are developed to investigate the single-scale deposition structures from the drying of nanofluid films, including network structures, continuous labyrinthine, branched structures and micro-sized rings. In the case of the actual drying of nanofluid films, dual-scale cellular networks and nano-rings are also discovered. In order to study the formation mechanism of dual-scale deposition structures, a three-dimensional kinetic Monte Carlo model is developed based on two-dimensional lattice gas model, and the dynamic chemical potential which couples solvent evaporation rate is implemented. Different dynamic chemical potentials are defined for each layer of the thin-film in the model to mimic the real evaporation situation. Considering the Brownian motion and the interaction between particles, the formation of dual-scale cellular networks and nano-rings coexisting with small scale patternis achieved via coupling the chemical potential to the solvent evaporation rate. The simulation results accord well with the results from many experimentally studied de-wetting systems. The effects of the chemical potential sharpness and critical evaporation rate of fluids on the dual-scale deposition structures are discussed. It can be found that the evaporation mode of thin-film is dominated by nucleation and growth at the initial stage. If the spinodal point is passed, the residual solvent will evaporate suddenly, and the nanoparticles do not accumulate further but directly deposit into small-scale structures, thus forming a dual-scale deposition structures at the final stage of the evaporation. The simulation results also show that the chemical potential sharpness will affect the deposition structure after the mutation in a certain range. When the chemical potential sharpness equals zero, the sedimentary structure is the same as the single-scale sedimentary structure when the constant chemical potential is applied. When the chemical potential sharpness is small, the large-scale network structure interacts closely with the small-scale network structure. With the increase of chemical potential sharpness, the large-scale deposition structure remains unchanged, while the dense small-scale network structure becomes small-scale point structure. When the chemical potential sharpness exceeds a certain large value, the effect of chemical potential sharpness on the deposition structure will gradually decrease, and finally the dual-scale deposition structure will remain unchanged. The critical evaporation rate of fluids determines the area ratio of the two kind of structures in the dual-scale deposition. With the increase of the critical evaporation rate of fluids, the area ratio of small-scale structures decreases while that of the large-scale structure increases. When critical evaporation rate increases to a certain value, the final deposition structure will evolve into a single-scale deposition structure.