Heterogeneous systems consisting of a hydrophobic nanoporous material immersed in water can be used as an unexpensive, green and efficient way to dissipate or store mechanical energy, depending on the characteristics of the material ([2-5]). In such applications, surface energy is accumulated by forcing the intrusion of water inside the pores (e.g by increasing the water pressure) and is subsequently released by decreasing pressure and triggering cavitation of vapour inside the pores. Depending on the characteristics of the material the process can display a large hysteresis and dissipate a large amount of mechanical energy, or be nearly reversible.

Molecular simulations constitute an essential tool to get insight on the physical properties allowing for the energetic applications of nanoporous hydrophobic materials: These phenomena are hardly captured by classical, capillary-based, macroscopic theories (such as Classical Nucleation Theory - CNT), as they are associated to the peculiarities of nucleation of vapour in extreme nanoscale confinement. It is also extremely hard for experiments to provide microscopic insight on the nucleation process, due to the fast dynamics and the nanometric lengthscales characterizing the phenomenon.

We employed advanced molecular dynamics techniques in combination with a rare event method known as the string method in collective variables ([6]) in order to shed light on the intrusion-extrusion process from a single hydrophobic nanopore ([1,7]). This approach allowed us to simulate, without artifacts, the microscopic mechanism of water intrusion and extrusion in the pores as thermally activated events. Simulations revealed, in accord with experimental data, the existence of important deviations from the predictions of macroscopic theory. First of all the nucleation free-energy barriers are reduced sixfold as compared to CNT predictions, allowing for spontaneous nucleation. The intrusion pressure value also deviates from the classic Kelvin-Laplace law, due to nanoscale confinement. Simulations also allowed us to confirm experimental observations ([4-5]) clearly highlighting the the suppression of the the intrusion/extrusion hysteresis for nanometer-sized pores and the possibility to use these materials as ”molecular springs” able to store and subsequently release mechanical energy. In addition to their theoretical significance, these results provide useful design criteria for the engineering of technological applications of nanoporous materials as they allow to critically discuss their behaviour as vibration dampers or molecular springs and to relate it to their physical characteristics, and in particular to their dimension.

References
1. Tinti et al. (2017), PNAS 114(48)
2. Guillemot et al. (2012), PNAS 109(48)
3. Smirnov et al. (2010), Acs Nano 4(9)
4. Eroshenko V. et al. (2001), J. Am. Chem. Soc. 123(33)
5. Grosu Y. et al. (2016), ChemPhysChem 17(21)
6. Maragliano et al. (2006), J. Chem. Phys. 125(2)
7. Tinti et al (2018) EPJ-E 41(4), 52.