Stratification in Drying Films Containing Bidisperse Mixtures of Nanoparticles

YF Tang and GS Grest and SF Cheng, LANGMUIR, 34, 7161-7170 (2018).

DOI: 10.1021/acs.langmuir.8b01334

Large scale molecular dynamics simulations for bidisperse nanoparticle suspensions with an explicit solvent are used to investigate the effects of evaporation rates and volume fractions on the nanoparticle distribution during drying. Our results show that "small-on-top" stratification can occur when Pe(s)phi(s) greater than or similar to c with c similar to 1, where Pe(s) is the Peclet number and phi(s) is the volume fraction of the smaller particles. This threshold of Pe(s)phi(s) for "small-on-top" is larger by a factor of similar to alpha(2) than the prediction of the model treating solvent as an implicit viscous background, where alpha is the size ratio between the large and small particles. Our simulations further show that when the evaporation rate of the solvent is reduced, the "small-on-top" stratification can be enhanced, which is not predicted by existing theories. This unexpected behavior is explained with thermophoresis associated with a positive gradient of solvent density caused by evaporative cooling at the liquid/vapor interface. For ultrafast evaporation the gradient is large and drives the nanoparticles toward the liquid/vapor interface. This phoretic effect is stronger for larger nanoparticles, and consequently the "small-on-top" stratification becomes more distinct when the evaporation rate is slower (but not too slow such that a uniform distribution of nanoparticles in the drying film is produced), as thermophoresis that favors larger particles on the top is mitigated. A similar effect can lead to "large-on-top" stratification for Pe(s)phi(s), above the threshold when Pe(s) is large but phi(s) is small. Our results reveal the importance of including the solvent explicitly when modeling evaporation-induced particle separation and organization and point to the important role of density gradients brought about by ultrafast evaporation.

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