Edges facilitate water evaporation through nanoporous graphene

Solar-driven abnormal evaporation of nanoconfined water

Intrinsic water evaporation demands a high energy input, which limits the efficacy of conventional interfacial solar evaporators. Here, we propose a nanoconfinement strategy altering inherent properties of water for solar-driven water evaporation using a highly uniform composite of vertically aligned Janus carbon nanotubes (CNTs). The water evaporation from the CNT shows the unexpected diameter-dependent evaporation rate, increasing abnormally with decreasing nanochannel diameter. The evaporation rate of CNT10@AAO evaporator thermodynamically exceeds the theoretical limit (1.47 kg m-2 hour-1 under one sun). A hybrid experimental, theoretical, and molecular simulation approach provided fundamental evidence of different nanoconfined water properties. The decreased number of H-bonds and lower interaction energy barrier of water molecules within CNT and formed water clusters may be one of the reasons for the less evaporative energy activating rapid nanoconfined water vaporization.

Graphene Nanopores Enhance Water Evaporation from Salt Solutions: Exploring the Effects of Ions and Concentration

With increased water stress, the development of clean water technologies is an active area of research. Evaporation-based solutions offer the advantage of low energy consumption, and recently a 10-30 fold enhancement in water evaporation flux has been observed through Å-scale graphene nanopores (Lee, W.-C., et al., ACS Nano 2022, 16(9), 15382). Herein, using molecular dynamics simulations, we examine the suitability of Å-scale graphene nanopores in enhancing water evaporation from salt solutions (LiCl, NaCl, and KCl). Cation-π interactions between ions and the surface of nanoporous graphene are found to significantly influence ion populations in the nanopore vicinity, leading to varied water evaporation fluxes from different salt solutions. The highest water evaporation flux was observed for KCl solutions, followed by NaCl and LiCl solutions, with the differences reducing at lower concentrations. Relative to the bare liquid-vapor interface, 4.54 Å nanopores exhibit the highest evaporation flux enhancements ranging from 7 to 11, with an enhancement of 10.8 obtained for 0.6 M NaCl solution, which closely resembles seawater compositions. Functionalized nanopores induce short-lived water-water hydrogen bonds and reduce surface tension at the liquid-vapor interface, thereby lowering the free energy barrier for water evaporation with a negligible effect on the ion hydration dynamics. These findings can aid in developing green technologies for desalination and separation processes with low thermal energy input.

Enhanced Water Evaporation from Å-Scale Graphene Nanopores

Enhancing the kinetics of liquid-vapor transition from nanoscale confinements is an attractive strategy for developing evaporation and separation applications. The ultimate limit of confinement for evaporation is an atom thick interface hosting angstrom-scale nanopores. Herein, using a combined experimental/computational approach, we report highly enhanced water evaporation rates when angstrom sized oxygen-functionalized graphene nanopores are placed at the liquid-vapor interface. The evaporation flux increases for the smaller nanopores with an enhancement up to 35-fold with respect to the bare liquid-vapor interface. Molecular dynamics simulations reveal that oxygen-functionalized nanopores render rapid rotational and translational dynamics to the water molecules due to a reduced and short-lived water-water hydrogen bonding. The potential of mean force (PMF) reveals that the free energy barrier for water evaporation decreases in the presence of nanopores at the atomically thin interface, which further explains the enhancement in evaporation flux. These findings can enable the development of energy-efficient technologies relying on water evaporation.

Hydrophilicity gradient in covalent organic frameworks for membrane distillation

Desalination can help to alleviate the fresh-water crisis facing the world. Thermally driven membrane distillation is a promising way to purify water from a variety of saline and polluted sources by utilizing low-grade heat. However, membrane distillation membranes suffer from limited permeance and wetting owing to the lack of precise structural control. Here, we report a strategy to fabricate membrane distillation membranes composed of vertically aligned channels with a hydrophilicity gradient by engineering defects in covalent organic framework films by the removal of imine bonds. Such functional variation in individual channels enables a selective water transport pathway and a precise liquid-vapour phase change interface. In addition to having anti-fouling and anti-wetting capability, the covalent organic framework membrane on a supporting layer shows a flux of 600 l m^-2 h^-1 with 85 °C feed at 16 kPa absolute pressure, which is nearly triple that of the state-of-the-art membrane distillation membrane for desalination. Our results may promote the development of gradient membranes for molecular sieving.

Mechanistic Investigation of Electrostatic Field-Enhanced Water Evaporation

Investigations on external electrostatic field (EEF)-enhanced liquid water evaporation have been reported decades ago, which suggest that molecular alignment and polarization tuned by EEF accelerating the phase change process could be responsible for EEF-enhanced water evaporation. However, a detailed study revealing the role of EEF in altering the intermolecular and intramolecular water structure is lacking. Herein, an EEF is proved to tune water state by accelerating the thermal movement of water molecules, lowering the molecular escaping energy, and loosening the hydrogen bond structure. The detailed mechanisms and field interactions (heat and electrostatic) are investigated by in situ Raman characterizations and molecular dynamic simulations, which reveal that an EEF can effectively reduce the free energy barrier of water evaporation and then increase the evaporated water molecule flux. As a proof of concept, an EEF is integrated into an interfacial two-dimentional solar steam generator, enhancing the efficiency by up to 15.6%. Similar to a catalyst lowing activation energy and enhancing kinetics of a chemical reaction, the EEF enhances water state tuning, lowers evaporation enthalpy, and then boosts steam generation rate with negligible additional energy consumption, which can serve as a generic method for water evaporation enhancement in water harvesting, purification, and beyond.

Evaporation in nano/molecular materials

Evaporation is a physical phenomenon with fundamental significance to both nature and technology ranging from plant transpiration to DNA engineering. Various analytical and empirical relationships have been proposed to characterize evaporation kinetics at macroscopic scales. However, theoretical models to describe the kinetics of evaporation from nano and sub-nanometer (molecular) confinements are absent. On the other hand, the fast advancements in technology concentrated on development of nano/molecular-scale devices demand appropriate models that can accurately predict physics of phase-change in these systems. A thorough understanding of the physics of evaporation in nano/molecular materials is, thus, of critical importance to develop the required models. This understanding is also crucial to explain the intriguing evaporation-related phenomena that only take place when the characteristic length of the system drops to several nanometers. Here, we comprehensively review the underlying physics of evaporation phenomenon and discuss the effects of nano/molecular confinement on evaporation. The role of liquid-wall interface-related phenomena including the effects of disjoining pressure and flow slippage on evaporation from nano/molecular confinements are discussed. Different driving forces that can induce evaporation in small confinements, such as heat transfer, pressure drop, cavitation and density fluctuations are elaborated. Hydrophobic confinement induced evaporation and its potential application for synthetic ion channels are discussed in detail. Evaporation of water as molecular clusters rather than isolated molecules is discussed. Despite the lack of experimental investigations on evaporation at nanoscale, there exist an extensive body of literature that have applied different simulation techniques to predict the phase change behavior of liquids in nanoconfinements. We infer that exploring the effect of electrostatic interactions and flow slippage to enhance evaporation from nanoconduits is an interesting topic for future endeavors. Further future studies could be devoted to developing nano/molecular channels with evaporation-based gating mechanism and utilization of 2D materials to tune energy barrier for evaporation leading to enhanced evaporation.

Analysis of Mg2+/Li+ separation mechanism by charged nanofiltration membranes: visual simulation

The mechanism of the nanofiltration (NF) membrane separation of Mg²⁺ and Li⁺ needs to be further investigated, but some commonly used model theories are abstract, which makes them difficult to understand. More importantly, the relationship between the membrane charge and separation performance of Mg²⁺ and Li⁺ cannot be quantitatively analyzed. It is worth studying these challenges and providing a performance boost for Mg²⁺/Li⁺ filtration applications of NF membranes. Here, various NF membranes, with the membrane volumetric charge density increasing from -4.69 to 7.02 mol · m^-3, were fabricated via interfacial polymerization. For these membranes, the separation factor S _Mg,Li was decreased from 0.41 to 0.20. Importantly, the visual simulation results were consistent with the experimental results as a whole. The separation factor S _Mg,Li decreased with the increase of volumetric charge density, and the minimum separation factor S _Mg,Li of the NF membranes was 0.20 (experiment) and 0.17 (simulation), respectively. This meant that the performance of the positively charged NF membrane was not fully developed. Furthermore, we analyzed the relationship between the membrane charge and separation performance, and visualized the simulation of the NF membrane filtration and separation.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.