Tinti Antonio, Giacomello Alberto, Casciola Carlo Massimo
Max-Planck Institut für Intelligente Systeme, 70569, Stuttgart, Germany.
Sapienza Università di Roma, 00184, Rome, Italy.
Eur Phys J E Soft Matter. 2018 Apr 19;41(4):52. doi: 10.1140/epje/i2018-11658-y.
In recent years, technologies revolving around the use of lyophobic nanopores gained considerable attention in both fundamental and applied research. Owing to the enormous internal surface area, heterogeneous lyophobic systems (HLS), constituted by a nanoporous lyophobic material and a non-wetting liquid, are promising candidates for the efficient storage or dissipation of mechanical energy. These diverse applications both rely on the forced intrusion and extrusion of the non-wetting liquid inside the pores; the behavior of HLS for storage or dissipation depends on the hysteresis between these two processes, which, in turn, are determined by the microscopic details of the system. It is easy to understand that molecular simulations provide an unmatched tool for understanding phenomena at these scales. In this contribution we use advanced atomistic simulation techniques in order to study the nucleation of vapor bubbles inside lyophobic mesopores. The use of the string method in collective variables allows us to overcome the computational challenges associated with the activated nature of the phenomenon, rendering a detailed picture of nucleation in confinement. In particular, this rare event method efficiently searches for the most probable nucleation path(s) in otherwise intractable, high-dimensional free-energy landscapes. Results reveal the existence of several independent nucleation paths associated with different free-energy barriers. In particular, there is a family of asymmetric transition paths, in which a bubble forms at one of the walls; the other family involves the formation of axisymmetric bubbles with an annulus shape. The computed free-energy profiles reveal that the asymmetric path is significantly more probable than the symmetric one, while the exact position where the asymmetric bubble forms is less relevant for the free energetics of the process. A comparison of the atomistic results with continuum models is also presented, showing how, for simple liquids in mesoporous materials of characteristic size of ca. 4nm, the nanoscale effects reported for smaller pores have a minor role. The atomistic estimates for the nucleation free-energy barrier are in qualitative accord with those that can be obtained using a macroscopic, capillary-based nucleation theory.
近年来，围绕疏液性纳米孔应用的技术在基础研究和应用研究中都备受关注。由于具有巨大的内表面积，由纳米多孔疏液性材料和非润湿性液体构成的非均相疏液体系（HLS）有望成为高效存储或耗散机械能的候选材料。这些不同的应用都依赖于非润湿性液体在孔内的强制侵入和挤出；HLS用于存储或耗散的行为取决于这两个过程之间的滞后现象，而这又由系统的微观细节决定。不难理解，分子模拟为理解这些尺度下的现象提供了无与伦比的工具。在本论文中，我们使用先进的原子模拟技术来研究疏液性中孔内蒸汽泡的成核过程。在集体变量中使用弦方法使我们能够克服与该现象的活化性质相关的计算挑战，从而呈现出受限环境中成核的详细图景。特别是，这种罕见事件方法能够在原本难以处理的高维自由能景观中有效地搜索最可能的成核路径。结果揭示了存在几条与不同自由能垒相关的独立成核路径。具体而言，存在一族不对称转变路径，其中气泡在其中一个壁上形成；另一族涉及形成具有环形形状的轴对称气泡。计算得到的自由能分布表明，不对称路径比对称路径的可能性要大得多，而不对称气泡形成的确切位置对该过程的自由能影响较小。我们还将原子模拟结果与连续介质模型进行了比较，结果表明，对于特征尺寸约为4nm的中孔材料中的简单液体，较小孔所报道的纳米尺度效应作用较小。原子模拟对成核自由能垒的估计与使用基于宏观毛细管的成核理论所得到的结果在定性上是一致的。
