Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08854, USA.
Langmuir. 2010 Jun 15;26(12):10147-57. doi: 10.1021/la100268q.
We studied cavitation in metastable fluids drawing on the example of liquid nitrogen confined to spheroidal pores of specially prepared well-characterized mesoporous silica materials with mean pore diameters ranging from approximately 6 to approximately 35 nm. Cavitation was monitored in the process of evaporation/desorption from fully saturated samples with gradually decreasing vapor pressure at the isothermal conditions. The onset of cavitation was displayed by a sharp step on the desorption isotherm. We found that the vapor pressure at the onset of cavitation depended on the pore size for the samples with pores smaller than approximately 11 nm and remained practically unchanged for the samples with larger pores. We suggest that the observed independence of the cavitation pressure on the size of confinement indicates that the conditions of bubble nucleation in pores larger than approximately 11 nm approach the nucleation conditions in the bulk metastable liquid. To test this hypothesis and to evaluate the nucleation barriers, we performed grand canonical and gauge cell Monte Carlo simulations of nitrogen adsorption and desorption in spherical silica pores ranging from 5.5 to 10 nm in diameter. Simulated and experimental adsorption isotherms were in good agreement. Exploiting the correlation between the experimental cavitation pressure and the simulated nucleation barrier, we found that the nucleation barrier increased almost linearly from approximately 40 to approximately 70 k(B)T in the range of pores from approximately 6 to approximately 11 nm, and varied in diapason of 70-75 k(B)T in larger pores, up to 35 nm. We constructed the dependence of the nucleation barrier on the vapor pressure, which asymptotically approaches the predictions of the classical nucleation theory for the metastable bulk liquid at larger relative pressures (>0.6). Our findings suggest that there is a limit to the influence of the confinement on the onset of cavitation, and thus, cavitation of nanoconfined fluids may be employed to explore cavitation in macroscopic systems.
我们以受限在具有特殊制备的、具有从大约 6 纳米到大约 35 纳米的平均孔径的、具有很好特征的中孔二氧化硅材料的各向异性的、球形孔中的液氮为例研究了亚稳流体中的空化现象。在等温条件下，随着蒸气压力逐渐降低，对完全饱和的样品进行蒸发/解吸的过程中监测了空化现象。在解吸等压线的急剧上升处显示出空化的起始。我们发现，对于小于大约 11 纳米的孔的样品，空化起始时的蒸气压力取决于孔径；而对于较大孔径的样品，蒸气压力实际上保持不变。我们认为，所观察到的空化压力对受限尺寸的独立性表明，在大于大约 11 纳米的孔中气泡成核的条件接近亚稳液体的体核化条件。为了检验这个假设并评估成核势垒，我们在直径从 5.5 纳米到 10 纳米的球形二氧化硅孔中进行了氮吸附和解吸的巨正则和规范单元蒙特卡罗模拟。模拟和实验吸附等压线吻合良好。利用实验空化压力与模拟成核势垒之间的相关性，我们发现，在大约 6 纳米到大约 11 纳米的孔范围内，成核势垒从大约 40 k(B)T 增加到大约 70 k(B)T，几乎呈线性增加，而在更大的孔中（直径从大约 6 纳米到大约 35 纳米），势垒在大约 70-75 k(B)T 的范围内变化。我们构建了成核势垒与蒸气压力的关系，该关系在较大的相对压力（>0.6）下，渐近地接近亚稳体相液体的经典成核理论的预测。我们的发现表明，受限对空化起始的影响存在一个极限，因此，纳米受限流体中的空化可以用于探索宏观体系中的空化现象。
