(579a) Estimation of Drying Length and Effect of Marangoni Flow during Particle Assembly By Convective Deposition

Effect of evaporation rate and addition of surfactant on drying droplets has been well-studied. Leveraging these insights, we have analyzed the effective drying length and the effect of surfactant in convective deposition where particles are deposited due to an advancing thin film where flow is driven by evaporation. Following the work done by Y. Jung et al. we have derived the analytical expression for drying length in convective deposition for more general geometry, treating system as Darcy flow. This analysis allows prediction of coating thickness over a wider range of velocities. By digital microscopy we have experimentally validated our expression for drying length. Alternatively, the addition of surfactant can significantly alter the mode of deposition to be nearly independent of evaporation rate and deposition velocity. At lower surfactant concentrations, added surfactant had no effects on assembly as is seen in evaporating droplets. At moderate surfactant concentrations we observed an adverse effect during the particle deposition that results in the formation of streaks. Alternatively, at higher surfactant concentrations above the critical micelle concentration, Marangoni stresses due to the concentration gradient of surfactant along the thin film becomes the main driving force for flow within the thin film. These Marangoni flows, which can be much stronger than those driven by evaporation, may be tailored to produce desired particle depositions over wide range of velocity.