Intrusion and extrusion of water in hydrophobic nanopores

Abstract

SignificanceMolecular springs, constituted by nanoporous materials immersed in a nonwetting liquid, are compact, economical, and efficient means of storing energy, owing to their enormous surface area. Surface energy is accumulated during liquid intrusion inside the pores and released by decreasing liquid pressure and thus triggering confined cavitation. State-of-the-art atomistic simulations shed light on the intrusion and extrusion of water in hydrophobic nanopores, revealing conspicuous deviations from macroscopic theories, which include accelerated cavitation, increased intrusion pressure, and reversible intrusion and extrusion processes. Understanding these nanoscale phenomena is the key to a better design of molecular springs as it allows relating the characteristics of the materials to the overall properties of the devices, e.g., their operational pressure and efficiency.

Cavitation of Water Confined in Hydrophobic Nanoporous Materials

“…This trend of hysteresis with the pore size is consistent with the experimental observations on zeolites and metal-organic frameworks available in the literature [1,3,[18][19][20][21]. For a more quantitative discussion of these nanoscale effects, along with an in-depth comparison with individual experimental results, we refer the interested reader to [16]. Fig.…”

“…A second important nanoscale effect is related to the intrusion pressures. Using a procedure explained in detail elsewhere [16], it is possible to add a pressure term, linear in the volume, to the atomistic free energy, in order to render the cavitation free-energy profile at different thermodynamical conditions (This term mimics the ∆PVv contribution in the classical expression Ω = ∆PVv + γlv (Alv + cosθY Asv ) for the free energy of the system).…”

“…The string method uses a coarse grained density field as the collective variable. For further details on the simulations we refer to [16] and for the string method in collective variables to the original article [12]. A snapshot of the atomistic system is presented in Fig.…”

Recovering superhydrophobicity in nanoscale and macroscale surface textures

“…Thus, for the smallest pores, around 2 nm wide or below, drying was observed even at pressures as high as tens of megapascals. [35][36][37] This phenomenon is related to the thermodynamic destabilization of the liquid phase due to nanoconfinement, which may occur even under ''normal'' conditions; this fact could also be exploited for designing materials endowed with perpetual superhydrophobicity. Importantly, the drying pressure is controlled by the wetting and the geometric characteristics of the pores, a topic which is rationalized in the present study and which is crucial for designing surfaces featuring perpetual superhydrophobicity.…”

Phase Field/Fluctuating Hydrodynamics Approach for Bubble Nucleation

“…Presumably, nucleation -the precursor of the phase change in metastable systems -is however the most common fluctuation-dominated phenomenology found in applications. Examples are cavitation [10], freezing rain [11], or the intrusion of liquids in microporous materials [12], to cite a few. Since the energy barriers for phase transition is overcome by thermal fluctuations [13][14][15][16][17], the nucleation time may be long and the phenomenon is labeled as a ''rare event''.…”