Ruiz Pestana Luis, Head-Gordon Teresa
Department of Civil and Architectural Engineering, University of Miami, Coral Gables, Florida 33146, United States.
Department of Chemistry, Bioengineering, and Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
ACS Nano. 2022 Mar 22;16(3):3563-3572. doi: 10.1021/acsnano.1c10244. Epub 2022 Feb 2.
While experiments and continuum models have provided a relatively good understanding of the evaporation of macroscopic water droplets, elucidating how sessile nanodroplets evaporate is an open question critical for advancing nanotechnological applications where nanodroplets can play an essential role. Here, using molecular dynamics simulations, we find that evaporating nanodroplets, in contrast to their macroscopic counterparts, are not always in thermal equilibrium with the substrate and that the vapor concentration on the nanodroplet surface does not reach a steady state. As a result, the evaporative behavior of nanodroplets is significantly different. Regardless of hydrophobicity, nanodroplets do not follow conventional evaporation modes but instead exhibit dynamic wetting behavior characterized by huge, non-equilibrium, isovolumetric fluctuations in the contact angle and contact radius. For hydrophilic nanodroplets, the evaporation rate, controlled by the vapor concentration, decays exponentially over time. Hydrophobic nanodroplets follow stretched exponential kinetics arising from the slower thermalization with the substrate. The evaporative half-lifetime of the nanodroplets is directly related to the thermalization time scale and therefore increases monotonically with the hydrophobicity of the substrate. Finally, the evaporative flux profile along the nanodroplet surface is highly nonuniform but does not diverge at the contact line as the macroscopic continuum models predict.
虽然实验和连续介质模型已经让我们对宏观水滴的蒸发有了相对较好的理解，但阐明固着纳米液滴如何蒸发仍是一个悬而未决的问题，这对于推进纳米技术应用至关重要，因为在这些应用中纳米液滴可以发挥重要作用。在此，我们通过分子动力学模拟发现，与宏观液滴不同，正在蒸发的纳米液滴并不总是与基底处于热平衡状态，并且纳米液滴表面的蒸汽浓度也不会达到稳态。因此，纳米液滴的蒸发行为有显著差异。无论疏水性如何，纳米液滴都不遵循传统的蒸发模式，而是表现出动态润湿行为，其特征是接触角和接触半径存在巨大的、非平衡的、等体积波动。对于亲水性纳米液滴，由蒸汽浓度控制的蒸发速率随时间呈指数衰减。疏水性纳米液滴遵循拉伸指数动力学，这是由于与基底的热化过程较慢所致。纳米液滴的蒸发半衰期与热化时间尺度直接相关，因此随基底的疏水性单调增加。最后，沿纳米液滴表面的蒸发通量分布极不均匀，但不像宏观连续介质模型预测的那样在接触线处发散。
