Content Gradient Research Articles

Variability of Vertical Total Electron Content Gradients in the Brazilian Sector

Ground Based Augmentation Systems are designed to meet demanding requirements during airport approach and landing sections of aircraft routes. It computes corrected and smoothed differential corrections and other information. These messages are transmitted to aircraft, which may use the corrections to improve their position estimation. However, residual correction errors remain, due to ionospheric spatial and temporal gradients between Ground Based Augmentation System reference receivers and approaching aircrafts. Large vertical ionospheric delay gradients detected during combinations of extreme geophysical environments indicate that there are problematic conditions for Ground Based Augmentation System operation over the Brazilian region. Motivated by the above conditions, data from Rede Brasileira de Monitoramento Contínuo, the Brazilian public network of dual-frequency Global Navigation Satellite System receivers operated by Instituto Brasileiro de Geografia e Estatística were analyzed to estimate ionospheric vertical Total Electron Content gradients. The effects from different locations (represented by the dip latitude) and geophysical conditions (represented by solar activity, season, and local time) on variations of ionospheric vertical Total Electron Content gradients have been statistically quantified. The results from the present work provide information to more detailed performance assessments of Ground Based Augmentation System procedures under equatorial and low-latitude ionospheric regions, aimed at flexibly delimiting the conditions for feasible operation in such regions.

InSAR‐Derived Horizontal Velocities in a Global Reference Frame

AbstractInterferometric Synthetic Aperture Radar is used to measure deformation rates over continents to constrain tectonic processes. The resulting velocity measurements are only relative, however, due to unknown integer ambiguities introduced during propagation of the signal through the atmosphere. These ambiguities mostly cancel when using spectral diversity to estimate along‐track motion, allowing measurements to be made with respect to a global reference frame. Here, we calculate along‐track velocities for a partial global data set of Sentinel‐1 acquisitions and find good agreement with ITRF2014 plate motion model and measurements from GPS stations. We include corrections for solid‐earth tides and gradients of ionospheric total electron content. Combining data from ascending and descending orbits we are able to estimate north and east velocities over 250 × 250 km areas and their accuracy of 4 and 23 mm/year, respectively. These “absolute” measurements can be particularly useful for global velocity and strain rate estimation where GNSS measurements are sparse.

Based on energy dissipation investigation of preparation and toughening mechanism of continuous transition coating

The plasticity and toughness of metals can absorb a large amount of energy. In this study, we focused on the preparation of the continuous transition coating of yttria-stabilized zirconia (YSZ) and CoNiCrAlY via the atmospheric plasma spraying (APS) method. The external force produced by Vickers hardness initiated the growth of cracks. Thus, the energy dissipation of the coatings was studied. The results confirmed that the continuous transition structure exhibits a hardness gradient change and excellent energy dissipation. The results indicated that the hardness curve of the continuous transition coating with slope kCTC = 5.728 compared to the double-layer coating curves with slope kDLC = 18.75 can alleviate the hardness differences between metals and ceramics. In contrast, CoNiCrAlY in the continuous transition zone dissipated energy through deformation, avoiding the failure of the coating caused by the large-scale fracture of the 8YSZ splats. Its energy dissipation increased with the gradient of the metal content in the continuous transition zone in the following order: ΔJsa=0J < ΔJsb=6.0072×10−9J<ΔJsc=7.3778×10−9J.

Characterizing Internal Flow Field in Binary Solution Droplet Combustion with Micro-Particle Image Velocimetry

Droplet internal flow participates in liquid-phase mass transfer during multicomponent solution droplet combustion. In this work, internal flow fields in the binary droplet combustion of two polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, n ≥ 1, abbreviated as PODEn), i.e., PODE2 and PODE4, are characterized using micro-particle image velocimetry (Micro-PIV). The buoyancy-driven upward vapor flow around the droplet is found to initiate two opposite radial flows in the droplet, which form two vortex cores near the surface, while the gravitational effect and Marangoni effect resulting from the content and temperature gradients in the binary droplets can induce disturbance to the two flows. The binary droplets have comparable spatially averaged flow velocities at the stable evaporation stage to those of pure droplets, which are around 3 mm/s. The velocity curves are more fluctuant and tend to slightly increase and reach the peak values at around 250 ms, and then decrease until droplet atomization. The flow velocities in the droplet interior are generally higher than those near the droplet surface, forming a parabolic velocity profile along the horizontal radial direction. The peak velocity first increases to 5–9 mm/s as the radial flow and vortex structure start to form and then decreases to around 3 mm/s until droplet atomization. The radial flow with a spatially averaged velocity of 3 mm/s can run around one lap during the stable evaporation stage, which implies that the convection-induced mass transfer is relatively weak, and consequently, the content gradient of the binary droplet is still mainly controlled by mass diffusion.

Improving the cotton simulation model, GOSSYM, for soil, photosynthesis, and transpiration processes

GOSSYM, a mechanistic, process-level cotton crop simulation model, has a two-dimensional (2D) gridded soil model called Rhizos that simulates the below-ground processes daily. Water movement is based on gradients of water content and not hydraulic heads. In GOSSYM, photosynthesis is calculated using a daily empirical light response function that requires calibration for response to elevated carbon dioxide (CO2). This report discusses improvements made to the GOSSYM model for soil, photosynthesis, and transpiration processes. GOSSYM’s predictions of below-ground processes using Rhizos are improved by replacing it with 2DSOIL, a mechanistic 2D finite element soil process model. The photosynthesis and transpiration model in GOSSYM is replaced with a Farquhar biochemical model and Ball-Berry leaf energy balance model. The newly developed model (modified GOSSYM) is evaluated using field-scale and experimental data from SPAR (soil–plant–atmosphere-research) chambers. Modified GOSSYM better predicted net photosynthesis (root mean square error (RMSE) 25.5 versus 45.2 g CO2 m−2 day−1; index of agreement (IA) 0.89 versus 0.76) and transpiration (RMSE 3.3 versus 13.7 L m−2 day−1; IA 0.92 versus 0.14) and improved the yield prediction by 6.0%. Modified GOSSYM improved the simulation of soil, photosynthesis, and transpiration processes, thereby improving the predictive ability of cotton crop growth and development.

An Analysis of Landslide Risk Using the Change in the Volumetric Water Content Gradient in the Soil Layer Per Unit Time of Effective Cumulative Rainfall

The topographical characteristics of South Korea are coupled with heavy rain and typhoons concentrated from July to September, which take up two-thirds of the country’s annual precipitation, causing substantial damage from slope failures, landslides, debris flows, and other mountain disasters. This study uses the Land-Atmosphere Modeling Package (LAMP) data to study the area, which has been affected by numerous landslides in the past. Specifically, to analyze the likelihood of slope failure, this study simulates the rainfall events in the area at the time of the landslides as well as changes in the volumetric water content in the soil layer and uses the changes in the volumetric water content per unit time, which are affected by the effective cumulative rainfall within the soil layer. To that end, this study assesses the risk of landslides based on how rainwater infiltration in the saturation depth changes the saturation depth ratio and compares the results with the landslide spots in the past. Moreover, this study analyzes the landslide risk by applying the saturation depth ratio (Ht) to an infinite slope stability analysis model. The landslide threshold proposed in this study was obtained by analyzing the rainfall events that caused the landslides in the study area using the LAMP model. It can then be concluded that, when compared with the spatial information on the previous landslides, if the change in the volumetric water content per unit time during the first and second rainfall events is 0.7, a landslide may occur. These results indicated that the landslide risk increases the volumetric water content in the soil layer, along with the congruence between past landslide risk points and spatial landslide risks.

EVOLUTION AND DISTRIBUTION OF SHAKES IN LARCH LOGS DURING AIR DRYING

This paper analyzed the length, width, and location of shakes in the air-drying process of larch log (Larix principis-rupprechtii Mayr) 1 m long and 66% initial moisture content. The development law and distribution characteristics of shakes during log drying of larch were studied and shake generation and development law were analyzed from two aspects of microstructure and growth defects. The variation of dry shrinkage deformation along the radial direction and knot influence on it was analyzed using the split-shaped stress test strips. At the cellular level, the cell wall shrinkage behavior of heartwood and sapwood and the microstructure of knots were observed. The results showed a significant relationship between eccentricity and shake distribution; the IIa region is the most prone to dry shake. The evolution of shakes is closely related to the drying rate. In the high-speed drying stage (MC ≥ 40%), shakes almost do not occur; in the decelerating drying stage (40% ≥ MC ≥ 20%), the amount, length, and width of shakes increase rapidly. In the low-speed drying stage (20% ≥MC), the area of shakes tends to stabilize or even decline. The main reasons for dry shaking are the dry shrinkage difference between tangential and radial cell walls, early and late wood, heartwood and sapwood, and moisture content gradient during the drying process.

Molecular dynamics simulation of CO2-oil miscible fluid distribution and flow within nanopores

CO2 Huff-n-Puff is a promising technology for recovering unconventional oil reservoirs nowadays. Studying the structural properties of CO2-oil inside nanoscale pores and the transport mechanism during production can help further improve crude oil recovery. In this work, molecular dynamics simulations are performed to explore the distribution and flow of CO2-decane within a 6 nm SiO2 nanopore under reservoir conditions. CO2 will substitute the decane adsorbed on the surface under the strong electrostatic interactions with the SiO2 surface. The variation of CO2 content in the adsorbed layer was quantitatively described. The flow behavior of CO2-decane on the pore surface does not obey the continuous hydrodynamics theory. The adsorbed CO2 forms an adhesion layer on the pore surface, shifting the flow boundary condition from slip to negative slip. The CO2-decane permeability model is established to investigate the influences of the negative slip on the permeability. The adhesion layer reduces the permeability of CO2-decane in pores. By contrast, the dissolved CO2 reduces the bulk phase viscosity and raises the decane mobility. The flux analysis results indicate that viscosity reduction contributes more than the flux loss due to negative slip, and CO2 improves the decane transport ability within the nanopore. Besides, sensitive factors such as oil content, pore pressure, and pressure gradient are considered. This study provides new insights into the surface flow behavior of CO2-oil in nanopores, which is essential for the accurate prediction of CO2-oil transport behavior under confined conditions.

Directed laser deposition of super duplex stainless steel: Microstructure, texture evolution, and mechanical properties

Microstructure and texture evolution of directed-laser deposited super-duplex stainless-steel, in the as-received block, were characterized using light and electron microscopies and electron backscattered diffraction. Mechanical properties in different directions were studied. Local FCC-depleted and FCC-rich zones and extensive precipitation of oxides were detected at the matrix wherein the different types of reformed austenite were surrounded by the elongated coarse ferrite. A vertical gradient of austenite content, caused by overall change in cooling rate, generated a waning hardness distribution along the building direction. The texture of austenite across the different deposition layers was not as intense as that of the ferrite. A dominant ⟨001⟩ ⫽ND fibre, embedding strong Cube {001}⟨100⟩, was calculated for ferrite on the layer away from the bottom while the Goss {011}⟨100⟩ appeared in the layer near the building substrate due to the considerable epitaxially developed grains. The less intensified multi-component texture of austenite at the layer near the substrate changed to ⟨011⟩ ⫽ND fibre adorned by Rotated-Goss and Goss components at the upper layers where an incomplete fibre {001}⟨uvw⟩ with a major Rotated-Cube was also partially inherited from the parent phase. The inter-phase boundaries obeying Kurdjumov-Sachs orientation relationship were predominantly formed at all layers. A slight increase of Σ3 coincidence site lattice interfaces was observed in austenite across the build direction. The possible mechanical anisotropy was depressed due to complex and multi-component transformation texture of the austenite. The material showed brittleness corresponding to significantly high tensile strength and low impact toughness.

Soil fertility thresholds driven by sand content indicate drylands degradation phases on the Tibetan Plateau

AbstractUnderstanding the process of dryland degradation is crucial to combating desertification and maintaining land sustainability. To identify important indicators of dryland degradation, we conducted a field survey of 46 drylands spanning an aridity gradient across the Tibetan Plateau. Our random forest model suggested that soil sand content was the most important predictor of variation in soil fertility (carbon, nitrogen, and phosphorus). Most soil fertility showed a nonlinear and abrupt decline with increasing soil sand content, and displayed three sequential thresholds. First, soil organic carbon sharply decreased beyond a soil sand content threshold of 47%. Second, exceeding a subsequent threshold of 70%, the total nitrogen and light fraction carbon declined dramatically with increasing soil sand content. Finally, soil heavy fraction carbon, inorganic carbon, and total carbon were drastically reduced when the soil sand content exceeded 80%. The three soil fertility thresholds along the gradient of the soil sand content indicated a sequence of dryland degradation phases. Our findings provide an approach for detecting the severity of dryland degradation and suggest vulnerable areas of degradation on the Tibetan Plateau.

Vertical distribution differences of the understory herbs and their driving factors on shady and sunny slopes in high altitude mountainous areas

The vertical distribution understanding of undergrowth herbaceous plants in high altitude mountain areas is of great significance to reveal the environmental gradient change law of plant diversity. However, the species composition of herbaceous plants at different altitude gradients and the relationship between diversity and environmental factors is still vague. This study takes the Sejila Mountain in southeast Tibet at an altitude of 2,000–3,300 m as the study area to analyze the vertical distribution differences of the understory herbs and their driving factors on shady and sunny slopes. The results showed that: (1) The number of families, genera and species of herbaceous plants on sunny slope is greater than that on shady slope. Asteraceae on shady and sunny slopes has the largest number of species; (2) The α diversity index of the herbs on shady slope shows a U-shaped trend. The Cody heterogeneity index (βC) is negatively correlated with the β diversity index. The herb richness index (R) on sunny slope is unimodal. The β diversity index displays the same as that of shady slope; (3) Different environmental factors have different effects on herb diversity. The correlation between environmental factors and diversity indicators indicates that there is a certain degree of clustering among samples. The environmental factors of shady slope and sunny slope are separated; and (4) The difference of water and heat conditions caused by altitude and slope aspect is an important driving factor for the diversity of herbage species, followed by soil clay content, silt content and slope gradient. This study will help to understand the species diversity and ecosystem characteristics of the region, and provide a reference for the study of plant community structure and function and rational utilization of understory plant resources in high-altitude mountain areas.

Compositional gradient affects the residual stress distribution in Si3N4/SiC functionally graded materials

Residual stress greatly affects the mechanic performance of the functionally graded materials (FGM). The residual stress distributions in different Si3N4/SiC FGMs formed in the fabricating processes were investigated with X-ray diffraction method in this work. It's found that residual stresses in FGMs with different component gradients were the gradient function of the quadratic of the component content and mainly determined by gradual changes of thermal mismatch and elastic modulus caused by the component content gradient. And the maximum tensile strength of FGM was limited by the ultimate tensile strength or shearing strength of SiC. When the maximum tensile strength exceeded the ultimate tensile strength or shearing strength of SiC and the maximum residual stress was maintained within the ultimate limit and resulted in the newly balanced stress distribution due to the same gradient function, cracks were formed. These results obviously provide ideas for regulating the performance and designing the FGM gradient for some special applications.

Effect of defoaming agent on the properties of cement mortars with hydroxyethyl methyl cellulose through adjusting air content gradient

Pore structure plays one of the most important role in the properties of cement-based materials. Hydroxyethyl methyl cellulose (HEMC) has the air entrainment effect, which may significantly change the pore structure and further affect the performance of this materials. Incorporating defoaming agent (DA) may weaken this effect, due to its defoaming effect which needs to be fully evaluated. Therefore, the working efficiency of DA and the changes of pore structure caused by stepwise adjustment of the air content were mainly studied. The results show that incorporating DA can effectively eliminate air bubbles, while the defoaming effect is weak with the air content approaching to 6.0%. There is a certain negative correlation between fresh bulk density and air content, and a positive correlation between fluidity and air content. Incorporating an appropriate dosage of DA is beneficial to control the air content of the cement mortars with HEMC, so as to ensure the cement mortars have better mechanical strengths and low drying shrinkages. With the increase of DA dosage, the large capillary pores, macro pores and total porosity all show a gradual decreasing trend due to the air content decreasing. The content of connected capillary pores in cement mortars tends to increase as the air content level decreases, but shows a small decrease at the low air content of less than 6.0%.

Nitrogen control of transpiration in grapevine.

Transpiration per unit of leaf area is the end-product of the root-to-leaf water transport within the plant, and it is regulated by a series of morpho-physiological resistances and hierarchical signals. The rate of water transpired sustains a series of processes such as nutrient absorption and leaf evaporative cooling, with stomata being the end-valves that maintain the optimal water loss under specific degrees of evaporative demand and soil moisture conditions. Previous work provided evidence of a partial modulation of water flux following nitrogen availability linking high nitrate availability with tight stomatal control of transpiration in several species. In this work, we tested the hypothesis that stomatal control of transpiration, among others signals, is partially modulated by soil nitrate ( ) availability in grapevine, with reduced availability (alkaline soil pH, reduced fertilization, and distancing source) associated with decreased water-use efficiency and higher transpiration. We observed a general trend when was limiting with plants increasing either stomatal conductance or root-shoot ratio in four independent experiments with strong associations between leaf water status, stomatal behavior, root aquaporins expression, and xylem sap pH. Carbon and oxygen isotopic signatures confirm the proximal measurements, suggesting the robustness of the signal that persists over weeks and under different gradients of availability and leaf nitrogen content. Nighttime stomatal conductance was unaffected by manipulation treatments, while application of high vapor pressure deficit conditions nullifies the differences between treatments. Genotypic variation for transpiration increase under limited availability was observed between rootstocks indicating that breeding (e.g., for high soil pH tolerance) unintentionally selected for enhanced mass flow nutrient acquisition under restrictive or nutrient-buffered conditions. We provide evidence of a series of specific traits modulated by availability and suggest that fertilization is a potential candidate for optimizing grapevine water-use efficiency and root exploration under the climate-change scenario.

Study on gas transport behavior in coal matrix based on free gas density gradient diffusion: Comparison with fick model

Understanding the mechanism of gas transport in coal matrix is crucial for coalbed methane production and gas disaster prevention. The conventional Fick's law may have certain theoretical flaws because its content gradient includes both adsorbed and free gases, ignoring the fact that free gas dominate gas transport. In this study, enclosed isothermal adsorption tests with adsorbable methane (CH4) and non-adsorbable helium (He) were done first. Then, combined with Fick's law and free gas density gradient diffusion (FGDGD), the corresponding mathematical models of gas diffusion-adsorption were numerically established and solved with finite difference method (FDM). Finally, the numerical simulation results of these two models were validated. The results show that: 1) whether CH4 or He, the simulation curves based on the FGDGD model are always consistent with the experimental data, verifying the correctness of the FGDGD model; 2) the Fick model fits the experimental data with a substantial error when simulating the CH4 diffusion and adsorption process, but fits well when modeling He, demonstrating that the model's failure can be attributed to the misconception that adsorbed gas is involved in transport; and 3) the microchannel diffusion coefficient of free gas (Dm) shows that Dm (He) is greater than Dm (CH4), which can be attributed to the pore structure and gas adsorption-desorption. Consequently, the FGDGD model is reasonable in assuming that gas migration is mainly driven by free gas density gradient, and it also offers a novel method for further research into gas flow in coal matrix.

Myosin-binding protein C stabilizes, but is not the sole determinant of SRX myosin in cardiac muscle.

The myosin super-relaxed (SRX) state is central to striated muscle metabolic and functional regulation. In skeletal muscle, SRX myosin are predominantly colocalized with myosin-binding protein C (MyBP-C) in the sarcomere C-zone. To define how cardiac MyBP-C (cMyBP-C) and its specific domains contribute to stabilizing the SRX state in cardiac muscle, we took advantage of transgenic cMyBP-C null mice and those expressing cMyBP-C with a 271-residue N-terminal truncation. Utilizing super-resolution microscopy, we determined the lifetime and subsarcomeric location of individual fluorescent-ATP turnover events within isolated cardiac myofibrils. The proportion of SRX myosin demonstrated a gradient along the half-thick filament, highest in the P- and C-zones (72 ± 9% and 71 ± 6%, respectively) and lower in the D-zone (45 ± 10%), which lies farther from the sarcomere center and lacks cMyBP-C, suggesting a possible role for cMyBP-C in stabilizing the SRX. However, myofibrils from cMyBP-C null mice demonstrated an ∼40% SRX reduction, not only within the now cMyBP-C-free C-zone (49 ± 9% SRX), but also within the D-zone (22 ± 5% SRX). These data suggest that the influence of cMyBP-C on the SRX state is not limited to the C-zone but extends along the thick filament. Interestingly, myofibrils with N-terminal truncated cMyBP-C had an SRX content and spatial gradient similar to the cMyBP-C null, indicating that the N terminus of cMyBP-C is necessary for cMyBP-C's role in enhancing the SRX gradient along the entire thick filament. Given that SRX myosin exist as a gradient along the thick filament that is highest in the C-zone, even in the absence of cMyBP-C or its N-terminus, an inherent bias must exist in the structure of the thick filament to stabilize the SRX state.

Characterizing Spray-Dried Powders through NIR Spectroscopy: Effect of Two Preparation Strategies for Calibration Samples and Comparison of Two Types of NIR Spectrometers.

Emerging portable near infrared (NIR) spectroscopic approaches coupled with data analysis and chemometric techniques provide opportunities for the rapid characterization of spray-dried products and process optimization. This study aimed to enhance the understanding of applying NIR spectroscopy in spray-dried samples by comparing two sample preparation strategies and two spectrometers. Two sets of whey protein-maltodextrin matrixes, one with a protein content gradient and one with a consistent protein content, were spray-dried, and the effect of the two preparation strategies on NIR calibration model development was studied. Secondly, a portable NIR spectrometer (PEAK) was compared with a benchtop NIR spectrometer (CARY) for the moisture analysis of prepared samples. When validating models with the samples with focused protein contents, the best PLS protein models established from the two sample sets had similar performances. When comparing two spectrometers, although CARY outperformed PEAK, PEAK still demonstrated reliable performance for moisture analysis, indicating that it is capable as an inline sensor.

Exploring Earth’s ionosphere and its effect on low radio frequency observation with the uGMRT and the SKA

The Earth's ionosphere introduces systematic effects that limit the performance of a radio interferometer at low frequencies ($\lesssim 1$\,GHz). These effects become more pronounced for severe geomagnetic activities or observations involving longer baselines of the interferometer. The uGMRT, a pathfinder for the Square Kilometre Array (SKA), is located in between the northern crest of the Equatorial Ionisation Anomaly (EIA) and the magnetic equator. Hence, this telescope is more prone to severe ionospheric conditions and is a unique radio interferometer for studying the ionosphere. Here, we present 235\,MHz observations with the GMRT, showing significant ionospheric activities over a solar minimum. In this work, we have characterised the ionospheric disturbances observed with the GMRT and compared them with ionospheric studies and observations with other telescopes like the VLA, MWA and LOFAR situated at different magnetic latitudes. We have estimated the ionospheric total electron content (TEC) gradient over the full GMRT array which shows an order of magnitude higher sensitivity compared to the Global Navigation Satellite System (GNSS). Furthermore, this article uses the ionospheric characteristics estimated from the observations with uGMRT, VLA, LOFAR and MWA to forecast the effects on the low-frequency observations with the SKA1-MID and SKA1-LOW in future.

Influence of the lay-up and curing steps in the manufacturing process of thick laminate composites on their compressive strength

The influence of the lay-up and curing steps in the manufacturing process of CFRP is investigated in terms of compressive strength. Automated Fibre Placement (AFP) and manual lay-up (HLU), from the same pre-preg material , are compared as well as the use of a porous or non-porous consumable during curing. It is shown that the latter factor may result in heterogeneous mesostructures with gradients in fibre volume content from the top of the laminate to its center. This made it necessary to propose a specific data analysis of the four-point bending test, used to evaluate the compressive strength of the most loaded unidirectional ply. Experimental results suggest a better performance of AFP vis-à-vis HLU plates, especially in terms of reproducibility. The increase in performance for plates using non porous consumables is discussed with respect to the relevance of using such a strategy on thin laminates for designing thick parts.

Mechanisms and models for water transport in reverse osmosis membranes: history, critical assessment, and recent developments.

Water scarcity is one of the greatest societal challenges facing humanity. Reverse osmosis (RO) desalination, a widely used membrane-based technology, has proven to be effective to augment water supply in water-stressed regions of our planet. However, progress in the design and development of RO membranes has been limited. To significantly enhance the performance of RO membranes, it is essential to acquire a deep understanding of the membrane separation and transport mechanisms. In this tutorial review, we cover the pivotal historical developments in RO technology, examine the chemical and physical properties of RO membrane materials, and critically review the models and mechanisms proposed for water transport in RO membranes. Based on recent experimental and computational findings, we conduct a thorough analysis of the key transport models-the solution-diffusion and pore-flow models-to assess their validity for accurately describing water transport in RO membranes. Our analysis involves examining the experimental evidence in favor of the solution-diffusion mechanism. Specifically, we explain whether the water content gradient within the membrane, cited as evidence for the key assumption in the solution-diffusion model, can drive a diffusive transport through RO membranes. Additionally, we review the recent molecular dynamics simulations which support the pore-flow mechanism for describing water transport in RO membranes. We conclude by providing future research directions aimed at addressing key knowledge gaps in water transport phenomena in RO membranes, with the goal of advancing the development of next-generation RO membranes.