Coalescence characteristics of bulk nanobubbles in water: A molecular dynamics study coupled with theoretical analysis
Coalescence characteristics of bulk nanobubbles in water: A molecular dynamics study coupled with theoretical analysis
复制标题
DOI：
10.1103/physrevfluids.6.093604
复制
发表时间：
2021-09
影响因子：
2.7
通讯作者：
E. Bird;Eric Smith;Zhi Liang
中科院分区：
文献类型：
--
作者：
E. Bird;Eric Smith;Zhi Liang
两个纳米气泡（NB）在水中的聚结对于许多工业应用是非常重要的过程。在这项工作中，我们使用分子动力学（MD）模拟和基于连续介质的理论分析研究了两个相同大小的氮NB在水中的聚结。我们改变的NB直径从30到50 nm和研究的合并特性，包括两个合并NB之间的毛细管桥的膨胀速度，NB合并的动态制度，完全合并的NB的直径，以及在合并过程中的温度变化的NB。对于所有的情况下，我们表明MD模拟结果可以很好地理解在这项工作中开发的理论模型。由于模型NB中的大的拉普拉斯压力，完全合并的NB与其子NB的直径比为，这解释了最近的实验结果，该实验结果显示水中NB的尺寸以均匀的增量因子离散地分布[Maet al.，J. Phys. Chem. B 124，5067（2020）10.1021/acs.气体在聚结NB内部的膨胀以及气体NB与周围液体之间的热传递导致聚结期间气体温度的波动。从理论分析中，我们发现的NBs的聚结动力学是在交叉制度，在周围的液体中的粘性应力和惯性应力都不占主导地位，即使当粘性应力是超过十倍以上的惯性应力。在Ohnesorge数为0.33 ~ 0.82的范围内，NB聚合后期毛细桥半径随时间的标度指数约为0.75 ± 0.05，明显高于粘性主导区的0.5。
The coalescence of two nanobubbles (NBs) in water is a process of great importance to many industrial applications. In this work, we study the coalescence of two equal-sized nitrogen NBs in water using molecular dynamics (MD) simulations and continuum-based theoretical analysis. We vary the NB diameter from 30 to 50 nm and study the coalescence characteristics including the expansion speed of the capillary bridge between two coalescing NBs, the dynamic regime of NB coalescence, the diameter of fully merged NBs, and the temperature variation of NBs during the coalescence process. For all cases, we show the MD simulation results can be well understood by the theoretical models developed in this work. Due to the large Laplace pressure in the model NBs, the diameter ratio of fully merged NBs to their daughter NBs is, which explains the recent experimental result showing that the size of NBs in water is distributed discretely with a uniform increment factor of[Maet al., J. Phys. Chem. B 124, 5067 (2020)10.1021/acs.jpcb.0c02279]. The expansion of gas inside the coalescing NBs and the heat transfer between the gas NB and surrounding liquid leads to fluctuations of gas temperature during coalescence. From the theoretical analysis, we find the coalescence dynamics of NBs is in the crossover regime where neither viscous stress nor inertial stress in the surrounding liquid dominates even when the viscous stress is more than ten times higher than inertial stress. In the range of Ohnesorge number from 0.33 to 0.82, we show the scaling exponent for the capillary bridge radius vs time at late times of NB coalescence is around 0.75 ± 0.05, which is considerably higher than 0.5 in the viscous-dominated regime.