Department of Physics, Center for Soft Matter and Biological Physics, and Macromolecules Innovation Institute , Virginia Polytechnic Institute and State University , Blacksburg , Virginia 24061 , United States.
Sandia National Laboratories, Albuquerque , New Mexico 87185 , United States.
Langmuir. 2018 Jun 19;34(24):7161-7170. doi: 10.1021/acs.langmuir.8b01334. Epub 2018 Jun 5.
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 ϕ ≳ c with c ∼ 1, where Pe is the Péclet number and ϕ is the volume fraction of the smaller particles. This threshold of Pe ϕ for "small-on-top" is larger by a factor of ∼α than the prediction of the model treating solvent as an implicit viscous background, where α 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 ϕ above the threshold when Pe is large but ϕ 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.
使用具有显式溶剂的双分散纳米颗粒悬浮液的大规模分子动力学模拟，研究了蒸发率和体积分数对干燥过程中纳米颗粒分布的影响。我们的结果表明，当 Peϕ≳c 且 c∼1 时，会出现“小在上”分层，其中 Pe 是 Peclet 数，ϕ 是较小颗粒的体积分数。这种“小在上”分层的 Peϕ 阈值比将溶剂视为隐式粘性背景的模型预测大一个因子 ∼α，其中 α 是大颗粒和小颗粒之间的尺寸比。我们的模拟还表明，当溶剂的蒸发速率降低时，“小在上”分层可以增强，这是现有理论所没有预测到的。这种出乎意料的行为可以用与溶剂密度正梯度相关的热泳来解释，这种正梯度是由液体/蒸汽界面处的蒸发冷却引起的。对于超快蒸发，梯度很大，会将纳米颗粒推向液体/蒸汽界面。对于较大的纳米颗粒，这种推流效应更强，因此当蒸发速率较慢时（但不会慢到导致干燥膜中纳米颗粒的均匀分布），“小在上”分层会更加明显，因为有利于较大颗粒在顶部的热泳效应会减弱。当 Pe 较大但 ϕ 较小时，类似的效应可能导致 Peϕ 超过阈值时出现“大在上”分层。我们的结果揭示了在模拟蒸发诱导颗粒分离和组织时显式包含溶剂的重要性，并指出了超快蒸发引起的密度梯度的重要作用。
