Natural Subsurface Research Articles

Factors influencing residual air saturation during consecutive imbibition processes in an air-water two-phase fine sandy medium – A laboratory-scale experimental study

The residual air saturation plays a crucial role in modeling hydrological processes of groundwater and the migration and distribution of contaminants in subsurface environments. However, the influence of factors such as media properties, displacement history, and hydrodynamic conditions on the residual air saturation is not consistent across different displacement scenarios. We conducted consecutive drainage-imbibition cycles in sand-packed columns under hydraulic conditions resembling natural subsurface environments, to investigate the impact of wetting flow rate, initial fluid state, and number of imbibition rounds (NIR) on residual air saturation. The results indicate that residual air saturation changes throughout the imbibition process, with variations separated into three distinct stages, namely, unstable residual air saturation (Sgr-u), momentary residual air saturation (Sgr-m), and stable residual air saturation (Sgr). The results also suggest that the transition from Sgr-u to Sgr is driven by changes in hydraulic pressure and gradient; the calculated values followed the following trend: Sgr > Sgr-u > Sgr-m. An increase in capillary number, which ranged from 1.46 × 10−7 to 3.07 × 10−6, increased Sgr-u and Sgr-m in some columns. The increase in Sgr ranged from 0.034 to 0.117 across all the experimental columns; this consistent increase can be explained by water film expansion at the primary wetting front along with a strengthening of the hydraulic gradient during water injection. Both the pre-covered water film on the sand grain surface and a pore-to-throat aspect ratio of up to 4.42 were identified as important factors for the increased residual air saturation observed during the imbibition process. Initial air saturation (Sai) positively influenced all three types of residual air saturation, while initial capillary pressure (Pci) exhibited a more pronounced inhibitory effect on residual air saturation, as it can partly characterized the initial connectivity of the air phase generated under different drying flow rates. Under identical wetting flow rate conditions, Sgr was higher during the second imbibition than during the first imbibition due to variations in initial fluid state, involving both fluid distribution and the concentration of dissolved air in the pore water. In contrast, NIR did not have an obvious effect on the three types of residual air saturation. This work aims to provide empirical evidences and offer further insights into the capture of non-wetting phases in groundwater environments, as well as to put forward some potential suggestion for future investigations on the retention and migration of contaminants that involves multiphase interface interactions in subsurface environments.

Containment strategy for subsurface H2 storage based on time-dependent soft solids

We propose an innovative containment strategy based on time-dependent soft solids, namely 2 wt% Laponite suspensions, to reinforce natural subsurface seals and engineer flow barriers, with an eye toward making H2 subsurface storage scalable and geographically agnostic. This suspension can be injected at its initial low viscosity and elasticity into a porous medium, allowing for easy pumping and targeted delivery. Once inside the target zone, it matures into a soft solid with much higher viscosity and elasticity, acting as a potential flow barrier. We discuss the rheological properties of the suspensions and demonstrate that hydrogen does not adversely affect their microstructure, but rather increases the suspensions’ viscosity and elasticity. Moreover, the suspensions enhance the rock samples’ compressive strength, while hydrogen exposure increases their stiffness and ductility. The ability of rock samples saturated with the suspensions to contain higher injected gas pressures is enhanced by aging at higher temperatures.

Multiscale sampling for the inverse modeling of partial differential equations

We are concerned with a novel Bayesian statistical framework for the characterization of natural subsurface formations, a very challenging task. Because of the large dimension of the stochastic space of the prior distribution in the framework, typically a dimensional reduction method, such as a Karhunen-Leove expansion (KLE), needs to be applied to the prior distribution to make the characterization computationally tractable. Due to the large variability of properties of subsurface formations (such as permeability and porosity) it may be of value to localize the sampling strategy so that it can better adapt to large local variability of rock properties.In this paper, we introduce the concept of multiscale sampling to localize the search in the stochastic space. We combine the simplicity of a preconditioned Markov Chain Monte Carlo method with a new algorithm to decompose the stochastic space into orthogonal subspaces, through a one-to-one mapping of the subspaces to subdomains of a non-overlapping domain decomposition of the region of interest. The localization of the search is performed by a multiscale blocking strategy within Gibbs sampling: we apply a KL expansion locally, at the subdomain level. Within each subdomain, blocking is applied again, for the sampling of the KLE random coefficients.The effectiveness of the proposed framework is tested in the solution of inverse problems related to elliptic partial differential equations arising in porous media flows. We use multi-chain studies in a multi-GPU cluster to show that the new algorithm clearly improves the convergence rate of the preconditioned MCMC method. Moreover, we illustrate the importance of a few conditioning points to further improve the convergence of the proposed method.

The survival strategy of direct interspecies electron transfer-capable coculture under electron donor-limited environments

Direct interspecies electron transfer (DIET) has been considered as an effective mechanism for interspecies electron exchange in microbial syntrophy. Understanding DIET-capable syntrophic associations under energy-limited environments is important because these conditions more closely approximate those found in natural subsurface environments than in the batch cultures in the laboratory. This study, investigated the metabolic dynamics and electron transfer mechanisms in DIET-capable syntrophic coculture of Geobacter metallireducens and Geobacter sulfurreducens under electron donor-limited condition. The wild-coculture and the mutant-coculture with a citrate synthase-deficient G. sulfurreducens exhibited similar rates of syntrophic metabolism under ethanol-limited and ethanol-replete conditions. Transcriptomic analyses revealed that, in the mutant-coculture in which interspecies electron exchange was the sole electron source for G. sulfurreducens, the transcription of genes associated with uptake hydrogenase in G. sulfurreducens were significantly repressed and thus DIET tended to be the preferred mode of interspecies electron exchange under electron donor-limited condition. To overcome electron donor limitation, c-type cytochromes in the coculture actively moved from outer membrane to extracellular environment, potentially via increased secretion of outer-membrane vesicles. These results suggested a preferred electron transfer mechanism for DIET-capable syntrophic communities' survival in the electron donor-limited environments, providing valuable insights into the biogeochemical processes mediated by DIET in natural and engineered environments.

Analysis of Lava Tubes’ Roughness and Radar Near-Nadir Regime Backscattering Properties

Lava tubes are terrestrial tunnel-like natural subsurface caves. Mounting evidence suggests their presence on the Moon and Mars. Planetary radar sounders are nadir-looking instruments operating in the high-frequency (HF)/very-HF (VHF) part of the spectrum with subsurface penetration capabilities. Recently, several studies either proposed future mission concepts for lava tubes’ detection or attempted to locate them on the Moon and Mars with the available radar-sounding data. Lava tubes are typically modeled as quasi-cylindrical structures but their actual geometry and their influence on the radar backscattering in near-nadir regime have never been investigated in the literature. These are crucial information for understanding the feasibility of detecting lava tubes by current and future planetary radar sounding systems. Accordingly, in this article: 1) we assess whether lava tubes are self-affine fractal surfaces at horizontal scales relevant to radio and microwave scattering and 2) we evaluate the effect of lava tube topography on the radar backscattering response in the near-nadir regime. Our experimental results, which are inferred from 3-D terrestrial laser scanning (TLS) data of planetary lava tube analogs, show that lava tubes: 1) are self-affine fractals at horizontal scales relevant to radar sounding and 2) they are electromagnetically rough surfaces, especially in the VHF band. We provide quantitative values on the lava tube fractal parameters and radar roughness losses along with a discussion on both: 1) the implication of our results on current radar sounding systems’ ability to detect lava tubes and 2) the planning of future missions devoted to lava tube detection and characterization.

Novel interventional electromagnetic thermography for subsurface defect detection

For low-emissivity metal materials, reducing the influence of non-uniform emission in detecting subsurface defects remains a challenge. This paper proposes a novel interventional electromagnetic thermography technique. In particular, a thin water film medium is involved in the detection which enhances the detectability of subsurface defects by utilizing the thermal radiation characteristic and thermophysical parameters of the participating medium. The physical mechanism of the proposed technique is analyzed in solid aspect. Static experiments have been carried out on the ferromagnetic plate with subsurface notches. The obtained results have shown that water film can eliminate the inhomogeneous emission of the detection target while the high emissivity of the water film corrects the temperature distribution. Moreover, dynamic scanning experiments have been conducted on the ferromagnetic plate containing artificial subsurface defects and the non-austenitic stainless-steel plate containing natural subsurface intergranular corrosion defect. The commonly used black paint is added to the dynamic experiment for performance comparison. The comparative experimental results have demonstrated that the proposed technique can effectively identify subsurface defects with burial depth up to 1.0 mm in the ferromagnetic plate. The results are especially more pronounced when the subsurface defects occur at buried depths ranging from 0.4 mm to 1.0 mm where higher thermal contrast and signal-to-noise ratio have been attained. The technique has shown superior detection performance for detecting natural subsurface microcrack.

Orange hydrogen is the new green

Maintaining global warming well below 2 °C, as stipulated in the Paris Agreement, will require a complete overhaul of the world energy system. Hydrogen is considered to be a key component of the decarbonization strategy for large parts of the transport system, as well as some heavy industries. Today, about 96% of current hydrogen production comes from the steam reforming of coal or natural gas (labelled black and grey hydrogen, respectively). If hydrogen is to become a solution, then black and grey hydrogen need to be replaced by a low-carbon option. One method that has received much attention is to produce so-called green hydrogen by coupling water electrolysis with renewable energies. However, green hydrogen is expensive and energy-intensive to produce. Here, we explore an alternative option and highlight the benefits of rock-based hydrogen (white and orange) compared with classic electrolysis-based technologies. We show that the exploitation of native hydrogen and its combination with carbon sequestration has the potential to fuel a large part of the energy transition without the substantial energy and raw material cost of green hydrogen. Enhancing natural subsurface hydrogen production through water injection could make a substantial contribution to achieving the low-carbon energy transition that is required to limit global warming.

Biotransformation of 8:2 Fluorotelomer Alcohol in Soil from Aqueous Film-Forming Foams (AFFFs)-Impacted Sites under Nitrate-, Sulfate-, and Iron-Reducing Conditions

The environmental fate of per- and polyfluoroalkyl substances (PFAS) in aqueous film-forming foams (AFFFs) remains largely unknown, especially under the conditions representative of natural subsurface systems. In this study, the biotransformation of 8:2 fluorotelomer alcohol (8:2 FTOH), a component of new-generation AFFF formulations and a byproduct in fluorotelomer-based AFFFs, was investigated under nitrate-, iron-, and sulfate-reducing conditions in microcosms prepared with AFFF-impacted soils. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high-resolution mass spectrometry (HRMS) were employed to identify biotransformation products. The biotransformation was much slower under sulfate- and iron-reducing conditions with >60 mol % of initial 8:2 FTOH remaining after ∼400 days compared to a half-life ranging from 12.5 to 36.5 days under nitrate-reducing conditions. Transformation products 8:2 fluorotelomer saturated and unsaturated carboxylic acids (8:2 FTCA and 8:2 FTUA) were detected under all redox conditions, while 7:2 secondary fluorotelomer alcohol (7:2 sFTOH) and perfluorooctanoic acid (PFOA) were only observed as transformation products under nitrate-reducing conditions. In addition, 1H-perfluoroheptane (F(CF2)6CF2H) and 3-F-7:3 acid (F(CF2)7CFHCH2COOH) were identified for the first time during 8:2 FTOH biotransformation. Comprehensive biotransformation pathways for 8:2 FTOH are presented, which highlight the importance of accounting for redox condition and the related microbial community in the assessment of PFAS transformations in natural environments.

Zeta Potential of a Natural Clayey Sandstone Saturated With Carbonated NaCl Solutions at Supercritical CO2 Conditions

AbstractThe zeta potential is a measure of electric potential at the mineral‐electrolyte interfaces. The zeta potential of natural sandstones depends on mineralogy, electrolyte pH, concentration, composition, amount of dissolved CO2, and temperature. We report for the first time the zeta potential measured on clayey sandstone comprising quartz, kaolinite, illite, albite and microcline saturated with NaCl solutions at supercritical CO2 conditions. Our results demonstrate that zeta potentials in clayey sandstone samples at supercritical CO2 conditions are significantly different from similar measurements conducted under ambient conditions and from those obtained with clean sandstones. Supercritical CO2 zeta potential remains negative but is influenced by clays and feldspars due to their significant presence and exposure to large pores, which yields less negative zeta potential compared to quartz, under identical conditions. Our results have significant implications to natural subsurface systems such as CO2 geo‐sequestration sites, aquifers, geothermal sources and hydrocarbon reservoirs.

Can Agents Model Hydrocarbon Migration for Petroleum System Analysis? A Fast Screening Tool to De-Risk Hydrocarbon Prospects

Understanding subsurface hydrocarbon migration is a crucial task for petroleum geoscientists. Hydrocarbons are released from deeply buried and heated source rocks, such as shales with a high organic content. They then migrate upwards through the overlying lithologies. Some hydrocarbon becomes trapped in suitable geological structures that, over a geological timescale, produce viable hydrocarbon reservoirs. This work investigates how intelligent agent models can mimic these complex natural subsurface processes and account for geological uncertainty. Physics-based approaches are commonly used in petroleum system modelling and flow simulation software to identify migration pathways from source rocks to traps. However, the problem with these simulations is that they are computationally demanding, making them infeasible for extensive uncertainty quantification. In this work, we present a novel dynamic screening tool for secondary hydrocarbon migration that relies on agent-based modelling. It is fast and is therefore suitable for uncertainty quantification, before using petroleum system modelling software for a more accurate evaluation of migration scenarios. We first illustrate how interacting but independent agents can mimic the movement of hydrocarbon molecules using a few simple rules by focusing on the main drivers of migration: buoyancy and capillary forces. Then, using a synthetic case study, we validate the usefulness of the agent modelling approach to quantify the impact of geological parameter uncertainty (e.g., fault transmissibility, source rock location, expulsion rate) on potential hydrocarbon accumulations and migrations pathways, an essential task to enable quick de-risking of a likely prospect.

Microzonation of Cisarua District Using Horizontal Vertical Spectral Ratio

The Bandung region is part of the framework of the Indonesian tectonic system, namely the tectonic plate meeting zone, where the Indo Autralia plate is infiltrated under the Eurasian plate in a convergent manner. The subduction process produces an effect in the form of an active fault geological structure in the Bandung area. One of these active faults is the Lembang Fault, which has a length of ± 29 kilometers and a shear acceleration of 3 to 5.5 millimeters per year. The microtremor measurement method is a passive geophysical method that utilizes natural subsurface vibrations so that it can provide dominant frequency data and amplification factors for soil layers. Based on the results of seismic susceptibility research using microtremor measurements using the HVSR method in the Lembang Fault zone in Cisarua Sub-District, it can be seen that the distribution of the dominant frequency values tends to be influenced by lithology and topography. In the research area, it is known to have a dominant frequency value that varies due to the different types of lithological units. In general, the dominant frequency ranges from 1-3 Hz because it is dominated by tuff sand and tuff pumice, and areas composed of volcanic breccias have a dominant frequency value between 3-6 Hz. Meanwhile, the amplification factor value will be influenced by rock deformation and weathering. The area that has a very high amplification factor value is in the southeast of the study area with an A0 value greater than 5. This indicates that the area is composed of a layer of thick and not dense tuff sand

A new pore-scale capillary pressure model for fractional wettability media using cylindrical mixed-wet tubes

The capillary-pressure/saturation relationship is a key property for modeling multiphase flow in fractional-wet (FW) porous media. However, the multiphase flow mechanism in such media has not been clearly understood yet and few capillary pressure models have been developed or verified for the range of wettability conditions that often occur in natural subsurface. This work presents a new general pore-network capillary pressure model for FW media. The pore-network is the first to adopt cylindrical mixed-wet pores and follows truncated Weibull pore size distribution. The pore level displacements of the mixed-wet pores including a new stepwise scenario are investigated by using the MS-P method and employed in the model. We simulate a primary drainage process with sequentially increased capillary pressure. Each individual pore is tested for invasion at a given capillary pressure (Pc) when ignoring the pore connectivity, then the water saturation (Sw) is calculated by integrating all pore elements to yield the Pc/Sw relation. The model can simulate capillary pressure curves for FW media in various wettability conditions. The underlying multiphase flow mechanisms for FW media have been well revealed and clarified with the model. Three sets of published experimental data with different wettability conditions are successfully predicted for the model validation. Comparisons with extant models are performed to prove the improvement and generality. This new model is an extension to the conventional uniform-wet capillary pressure theory, as it is consistent when the fraction of the oil-wet surface is either equal to zero or one.

Experimental Study on the Performance of a Sand Filtration Method and Device by Using Natural Soils and Subsurface PVC Drainage Pipes

AbstractDrip irrigation is an efficient water-saving irrigation technique. However, the clogging of emitters is a major problem for drip irrigation systems that adversely affects the uniformity of ...

Numerical modeling of groundwater flow system in the Modjo River catchment, Central Ethiopia

In recent years, groundwater pumping has increased for domestic, industrial, and irrigation use in the Modjo River catchment. Understanding changes in groundwater levels is crucial for the sustainable use and management of aquifer. This study investigates the groundwater flow system and aquifer response to increased groundwater pumping and reduced recharge using the calibrated steady-state groundwater level and budget as a baseline. The groundwater flow corresponds to the direction of the Modjo River flow, following the topographic gradient. The simulated groundwater budget indicates that recharge from precipitation and surface water (crater lakes and river) are the main inflow to the aquifer, while the outflow from the aquifer is due to groundwater pumping, natural subsurface flow to downstream area, and base flow. Analysis of the different scenarios reveals that both an increase in well pumping and a decline in recharge resulted in a decrease of the base flow to Bishoftu crater lakes and Mojo River, and to the downstream subsurface flow. In conclusion, increasing human demand for groundwater and variability in recharge will affect groundwater contribution to surface water and ultimately will be a source of concern in the future for both environmental flows and groundwater management.

Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions.

The influence of organic compounds on iodine (I2) emissions from the O3 + I– reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I2 was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I2 emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I2 emissions compared to artificial solutions with equivalent [I–], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I2 or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I2 product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O3 + I– reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.

Eco-friendly interpenetrating network hydrogels integrated with natural soil colloid as a green and sustainable modifier for slow release of agrochemicals

Natural soil colloid, an important component of natural soil, has high specific surface area and strong adsorbability. Although natural soil colloids are ubiquitous in natural subsurface environments, they have not been utilized to synthesize composite materials for any practical application. In this work, a series of eco-friendly interpenetrating polymer network hydrogels integrated with natural soil colloid were synthesized via the ionic crosslinking of alginate and covalent cross-linking of polyacrylamide (PAM) in the aqueous dispersion of natural soil colloid. The natural soil colloid was extracted from the natural soil by a simple, green and sustainable process, without any formation of pollution. The swelling characteristic experiment showed that the soil colloid in polymeric matrix accelerated water absorption by the hydrogels. Hydrogels released 34.4% of pesticide and 56.9% of fertilizer loaded during 54 and 30.5 h at the slowest release rates, respectively. The release data indicated that the hydrogels could release the fertilizer and the pesticide in a slow manner. These findings show that natural soil colloid can be used as a green and sustainable modifier to improve the slow release of hydrogels. Therefore, the synthesized hydrogels integrated with natural soil colloid show great potential as carriers of agrochemicals in modern agriculture and horticulture.

Terrain uplift due to natural hydrologic overpressure in karstic conduits

Water supply from karst sources is a worldwide natural resource and the exploitation is tied to the knowledge of the positions of the hydrologic channels. We show that surface deformation induced by flood events in karst conduits is observable, and consists in uplift and outward movement from the hydraulic channel. Precipitation events produce the natural occurrence of subsurface hydraulic overpressure up to 1 MPa. Numerical modeling shows that the stresses are so strong to uplift and dislocate the surface by several mm and induce tilts in the order of microradians. The naturally induced deformation is compatible with a transient internal pressure loading of a channel. The results can be used to find new channels with dense GNSS networks. Sea water incursion and channels accessed for tourism could be monitored. Seismicity has been shown to have a seasonal variation in some areas, which could be explained by the subsurface stresses induced by the natural subsurface overpressure. The pressure induced deformation is expected to be observed in all karstic systems worldwide.

Molecular and isotopic evaluation of the maturation history of the organic matter in an Ordovician aquiclude (Michigan Basin): Evidence for late diagenetic biodegradation

Previous characterization of a low permeability (10−16 ≤ Kh ≤ 10−12 m s−1) and high salinity (> 5 M of Cl−, > 10× seawater salinity) aquiclude, in Upper Ordovician-aged sediments situated on the eastern flank of the Michigan Basin, where a deep geological repository for low and intermediate level nuclear waste is proposed, suggested a microbial origin for CH4 and CO2 based on their stable isotope values. CH4 is believed to have been produced and trapped during the Paleozoic in a relatively discrete and high organic matter (OM) horizon at the shale/carbonate transition. To further investigate the efficiency of confinement of this aquiclude, here we present detailed isotopic and geochemical stratigraphic profiles of OM and biomarkers, with the objective of understanding the origin of CH4 and its relative timing of confinement. Classical diagnostic ratios of polycyclic aromatic hydrocarbons, relative abundances of hopanes and degraded hopanes coupled to dual compound-specific isotopic analysis of n-alkanes, as well as δ13C analysis on different fractions of OM were measured. A partitioning of the Ordovician sedimentary succession into two systems is observed with an upper system that is self-sourced and confined, hosting the microbially derived CH4 with evidence of late diagenetic OM biodegradation and a lower system that reveals later secondary oil migration which has overprinted the organic geochemical record. We propose a conceptual model that would explain the generation and preservation of this paleo-bioreactor over time using the maturation history reconstruction derived from this dataset. This work not only provides essential field empirical evidence of a relationship between methanogenesis and late diagenetic biodegradation, but also shows the power of dual-compound-specific isotope analyses in deciphering between different maturation processes affecting OM in natural subsurface settings.

Scale dependence of surface complexation capacity and rates in heterogeneous media

Reactive solutes flow, transport, and transform in natural subsurface that is ubiquitously heterogeneous. How much are the heterogeneity effects on solute reactive transport? At what spatial scale heterogeneity effects diminish? Answers to these questions are important to understand controls of and to predict solute reactive transport in natural environments. Although these questions have been explored for non-reactive tracers and solutes that go through relatively simple single-component sorption, heterogeneity effects remain poorly understood for solutes that go through surface complexation. These reactions typically include multi-component and are subject to influences of local geochemical conditions. This work aims to address these questions and to understand how and how much effective capacity (Csc,m), early (kc1) and late (kc2) rates of Cr(VI) surface complexation are influenced by heterogeneity characteristics and length scales. A two-dimensional model with Cr(VI) surface complexation were calibrated using column data. Numerical experiments were carried out in heterogeneous media characterized by different permeability variance σ2lnκ (0.2, 4.5), correlation length λL (0–6.0 cm), and domain length (0.1–1.0 m). Results show that long λL and high σ2lnκ facilitate the formation of preferential flow and elongate the diffusion length, leading to high connectivity that minimizes Csc,m and kc1. In highly connected media (>2.0), Csc,m and kc1 can be >2 orders of magnitude lower than those in corresponding homogeneous media. Connectivity however is not a good indicator for kc2 that is predominantly controlled by diffusion. The heterogeneity effects are scale dependent. As the domain length increases, Csc,m and kc1 values approach those of homogeneous media due to longer mixing lengths and continued pH increase during Cr(VI) surface complexion. The “critical length” at which heterogeneity effects disappear occurs at about 20λL and differs only slightly at different σ2lnκ values. The heterogeneity effects however linger at much longer lengths for kc2, the values of which are >2 orders of magnitude lower in heterogeneous media even at 20λ because of the longer characteristic times of diffusive transport. These results underscore the strong regulation of solute reactive transport by mineral spatial patterns, suggesting the importance of subsurface characteristics and spatial scales at which they are important in estimating natural attenuation, residence times, and natural system capacity to sorb chemicals.

Hydrophobic nanochannel self-assembled by amphipathic Janus particles confined in aqueous nano-space**Project supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB10040304), the National Natural Science Foundation of China (Grant Nos. 51490654, 11290164, and U1262109), and the Key Research Program of Chinese Academy of Sciences (Grant No. KJZD-EW-M03).

Hydrophobic nanochannel plays a significant role in many physical, biological, and geological phenomena and exhibits impressive applications due to both its ubiquitous distribution and great ability to transport hydrophobic molecules, including various oils and gases. Based on theoretical modeling, we herein reveal that the amphipathic Janus nanoparticles have a large probability to self-assemble into uninterrupted hydrophobic nanochannels inside the aqueous nano-space, although there are large portions of the Janus nanoparticles to be hydrophilic. The key to this observation is the attractions between the hydrophobic regimes on neighboring amphipathic Janus particles through hydrophobic interaction in aqueous nano-space. More surprisingly, the permeation efficiency of hydrophobic molecules through the uninterrupted hydrophobic channel in Janus particles aggregate is even higher than that in the aggregate of hydrophobic particles. We note that the proposed amphipathic Janus particles can be transported to the appropriate positions by the water since the hydrophilic regimes still remain a strong particle–water interaction. We also note that most natural subsurface rocks are not completely hydrophobic or hydrophilic but have complex surfaces with inhomogeneous wetting property. Our work therefore provides a detailed molecular level understanding of the formation of underground strata as well as the new insight for constructing the artificial hydrophobic channels for various applications, such as the design of proppants to enhance the recovery of the unconventional oil/gas.