Pore Aspect Ratio Research Articles

Fabrication of model ultrafiltration membranes with uniform, high aspect ratio pores

In this manuscript, we report the facile fabrication of large-area model membranes with highly uniform and high aspect ratio pores with diameters <20 nm. These membranes are useful for fundamental investigations of separation by size exclusion in the ultrafiltration regime, where species to be separated from solution have dimensions of 1–100 nm. Such investigations require membranes with narrow pores and high aspect ratios such that the Hagen–Poiseuille equation is followed, enabling well-known models such as the hindered transport model to be evaluated and other affecting factors to be ignored. We demonstrate that the sub-20 nm pores in the membrane are of sufficiently high aspect ratio such that water flux through the membrane is consistent with the Hagen–Poiseuille equation. The fabrication relies on self-assembling block copolymers to form uniform, densely packed patterns with sub-20 nm resolution, sequential infiltration synthesis to convert the block copolymer in situ into a mask with adequate contrast to etch pores with an aspect ratio >5, and low-resolution photolithography to transfer the pattern over a large area into a silicon nitride membrane. Model membranes with narrow pore-size distribution fabricated in this way provide the means to investigate parameters that impact size-selective ultrafiltration separations such as the relationships between solute or particle size and pore size, their distributions, and rejection profiles, and, therefore, test the validity or limits of separation models.

Modeling the effects of pore aspect ratio, porosity, and seismic anisotropy on wave velocity dispersion and attenuation patterns in oil- and brine-saturated carbonates using a dynamic self-consistent anisotropic approach

Modeling the effects of pore aspect ratio, porosity, and seismic anisotropy on wave velocity dispersion and attenuation patterns in oil- and brine-saturated carbonates using a dynamic self-consistent anisotropic approach

Direct Writing of graphene/graphitic foam through picosecond pulsed laser-induced transformation of soluble polyimide suspension

We report the direct writing of graphene/graphitic foam with high electrical conductivity using laser-induced-transformation of polyimide (PI) resin films on glass surfaces. Raman spectroscopy of the treated surfaces indicated that average laser power irradiation between 900 and 1500 kW/cm2 transformed the PI film into a few layered graphene-dominated film, and the increase in irradiation power above 1500 kW/cm2 led to the formation of graphitic (multilayered graphene) material. The electrical conductivity of the transformed film was between 5800±750 S m-1 (lower power irradiation) and 1250±300 S m-1 (higher laser power irradiation). SEM imaging showed that the transformed material has a closed cell foam morphology enclosed between the smooth top and bottom layers. The results indicate that heat treatment of the polyimide suspension films, and subsequent ultra-short, pulsed laser irradiation resulted in a closed-cell graphene/graphitic foam with high electrical conductivity. The pore aspect ratio, density, and film conductivity are used to estimate the conductivity of the solid phases in the laser-treated films at different powers. Laser-induced transformation of the PI suspension into graphene/graphitic foam is conducive to additive manufacturing and may enable the direct printing of graphitic foam-based three-dimensional components.

Tensile deformation and failure of AlSi10Mg random cellular metamaterials

Cellular metamaterials are an emerging class of porous materials, in which the topology of the solid is precisely-engineered to achieve attractive combinations of properties. This work specifically examines the tensile response of a novel type of cellular metamaterials, in which pores of arbitrary elliptical shape are randomly-dispersed into an aluminum alloy matrix. Their porous mesostructure is generated numerically via a random sequential absorption algorithm, and is fabricated by laser powder bed fusion from AlSi10Mg powders. The results – obtained by means of digital image correlation combined with X-ray tomography – highlight the advantages offered by a random cellular topology. They are notably a low sensitivity to geometric imperfections (which inevitably result from manufacturing), coupled with the ability to delay long-wavelength strain localization, which is in turn responsible for failure. Structural disorder also leads to highly heterogeneous deformation patterns, which result from the interaction between geometric pores and promote void growth and coalescence during plastic straining. The presence of manufacturing defects exacerbates void interaction and promotes early plastic flow localization. Interestingly, experiments reveal that this class of porous solids effectively displays features of the ductile fracture of metals at the mesoscale. For example, void-sheeting is observed with increasing pore aspect ratio and is accompanied by large geometric distortions of the voids upon coalescence. Measured data for the pore strains are highly scattered and show a departure from McClintock’s model predictions for the voids within the fracture band. Collectively, this study highlights the potential offered by random porous metamaterials, which can be harnessed to revisit the complex mechanisms of ductile fracture at the mesoscale.

The effect of pore morphology on thermal insulation properties of thermal barrier coatings

The inhomogeneous-geometry microstructures caused by pores make thermal barrier coatings (TBCs) to have excellent thermal insulation properties, thus ensuring that the superalloy matrix is stable in its service at higher temperatures. Consequently, significant attention has been paid to understand the relationship between pore morphology and thermal conductivity. However, unclear the influence mechanisms of pore morphology-thermal insulation properties leads to a poor guide to the structural tailoring for better thermal insulation for TBC. In this study, a finite element method was used to simulate the thermal transport behavior of porous structures, and the influence mechanism of pore morphology on the thermal transport behavior of coatings was clarified. It indicated that thermal conductivity has a significant dependence on the pore aspect ratio, orientation angle, and porosity. The thermal insulation properties would be enhanced when large aspect ratio prolate spheroids are inserted with a pillar at the center. Further, the results shed light on the heat transport process across highly porous structures and provide a guide to design the coating with high thermal insulation properties.

Bayesian linearized inversion for petrophysical and pore-connectivity parameters with seismic elastic data of carbonate reservoirs

Abstract Carbonate reservoirs are important targets for promoting the oil and gas reserve exploration and production in China. However, such reservoirs usually contain the developed complex pore structures, which heavily affect the precision in seismic prediction of petrophysical parameters. As one of the most important parameters to characterize reservoir rock, pore-related parameters can not only describe the pore structure, but also be used to evaluate the oil/gas bearing capabilities of potential reservoirs. The conventional rock-physics models (e.g. Gassmann's model) are formulated assuming fully-connected pores, which is unable to accurately capture the geometrical complexity in real rocks. In order to characterize the influences of multiple pores on the elastic properties, this work presents a rock-physics modelling method for carbonates, wherein the percentage composition of connected pores is equivalently quantified as the pore-connectivity factor. The method treats the pore-connectivity factor as an objective variable to characterize the spatial variations of pore structure. Specifically, the method combines the differential equivalent medium theory and Gassmann's model, and derives a linearized forward operator to quantitatively link porosity, water saturation, and pore-connectivity factor to seismic elastic parameters. According to the Bayesian linear inverse theory, the simultaneous estimation of petrophysical and pore-connectivity parameters is achieved. To characterize the statistical variations with multiple lithofacies, the Gaussian mixture model is employed to quantify the prior distribution of the objective variables. The posterior distribution of the objective variables is analytically expressed with the linearized forward operator. Numerical experiments show that the accuracy of the proposed method in predicting elastic parameters is improved. Compared with the conventional Xu-White model and the varying pore aspect ratio method, the accuracy of predicted P-wave velocity increases by 10.29% and 1.33%, respectively, and the predicted S-wave velocity increases by 6.44% and 0.03%, in terms of correlation coefficient. The application to the field data validates the effectiveness of the method, wherein the porosity and water saturation results help indicating the spatial distribution of potential reservoirs.

Fabrication of low aspect ratio solid-state pores from sub-micron to microscales utilizing crossing blades

This article introduces a novel method for fabricating low aspect ratio pores in various sizes, from sub-micron to microscales. This technique utilizes the intersection of two collided blades as a soft lithography mold. The theoretical details of the fabrication method, including the pore’s shape and geometrical parameters, were inspected, modeled, and discussed. Multiple mechanical and electrochemical measurements were performed to characterize the fabricated pores. Pore fabrication results showed that a wide range of pore sizes, from 350 nanometers to more than 30 micrometers, can be constructed by employing this method. Electrochemical impedance spectroscopy (EIS) implied that the blades-intrusion pores have the same electrochemical behavior as the ones on 2D materials. Meanwhile, the durability of the fabricated pores against the pressure of two bars ensures great mechanical stability of these pores. Fabricated pores with sizes of 2.3 µm and 20 µm were successfully employed as sensing gates in resistive pulse sensing (RPS) for detecting polystyrene beads of 600 nm, 1 µm, and 2 µm in diameter and downsized yeast cells, respectively. A distinct scatter of pulses of different particle sizes in the pulse amplitude-width plane verified the blades-intrusion pore’s capability as a sensing gate.

Shear-wave velocity prediction of tight reservoirs based on poroelasticity theory: A comparative study of deep neural network and rock physics model

The absence of shear-wave (S-wave) velocity in well log data limits the effective seismic characterization of lithology, petrophysical properties, and fluid distribution of tight reservoirs in the Chang 7 shale oil formations of Ordos Basin, west China. However, conventional rock physics models (RPMs), e.g., the Xu-White model, struggle to accurately obtain the key parameters such as pore aspect ratio in the process of S-wave velocity prediction. This work performs a comparative study of S-wave velocity prediction based on deep neural network (DNN) and RPM for such reservoirs. By employing the poroelasticity equations which are capable of describing elastic wave propagation in the complex reservoirs saturated with viscous fluid, the relation between elastic and petrophysical properties is established with a DNN-based method. This approach quantifies the connection between P-/S-wave velocities and rock properties. On the other hand, we reformulate the traditional Xu-White model by incorporating the decoupled Kuster-Toksöz (K-T) model and the poroelasticity equations with viscoelastic fluid in the modeling process. The variation of pore aspect ratio with depth is also considered to characterize the complex pore structures, which is appropriate for improving the accuracy of prediction. Log data from the three wells are considered for verification, of which results show that the S-wave velocity prediction methods help improve the final result. Aided by the DNN-based method, the prediction exhibits a better performance if the training data is sufficient. In particular, provided a small amount of measured S-wave data (even ten data points), the trained DNN model can also provide reasonable predictions.

Preliminary study on the mechanical behavior of clay in one-dimensional compression with DEM simulations

The mechanical behavior and physical properties of clay are closely associated with its microstructure. Current research on the macroscopic mechanics and physical properties of clay is comprehensive and systematic. However, the microstructural variations underlying these characteristics have been predominantly examined under mercury intrusion porosimetry and scanning electron microscopy, while quantitative and systematic studies are notably limited. This research employs the discrete element method (DEM) simulation to investigate the microscopic responses of clay, simulating one-dimensional compression tests on two clay types: card-house and book-house structures. The analysis of pore size and morphology was conducted using virtual mercury intrusion porosimetry and the pore segmentation method. The clay microstructure was quantitatively characterized by parameters such as pore aspect ratio, orientation distribution, and cumulative pore volume curve. The findings demonstrate that DEM effectively replicates the fundamental mechanical behavior of clay, revealing and explaining the evolution pattern of clay pore structure during one-dimensional compression in terms of platelet alignment adjustment. This macroscopic compression is characterized by variations in pore size distribution, orientation, and aspect ratio.

Uniaxial Magnetization and Electrocatalytic Performance for Hydrogen Evolution on Electrodeposited Ni Nanowire Array Electrodes with Ultra-High Aspect Ratio.

Ni nanowire array electrodes with an extremely large surface area were made through an electrochemical reduction process utilizing an anodized alumina template with a pore length of 320 µm, pore diameter of 100 nm, and pore aspect ratio of 3200. The electrodeposited Ni nanowire arrays were preferentially oriented in the (111) plane regardless of the deposition potential and exhibited uniaxial magnetic anisotropy with easy magnetization in the axial direction. With respect to the magnetic properties, the squareness and coercivity of the electrodeposited Ni nanowire arrays improved up to 0.8 and 550 Oe, respectively. It was also confirmed that the magnetization reversal was suppressed by increasing the aspect ratio and the hard magnetic performance was improved. The electrocatalytic performance for hydrogen evolution on the electrodeposited Ni nanowire arrays was also investigated and the hydrogen overvoltage was reduced down to ~0.1 V, which was almost 0.2 V lower than that on the electrodeposited Ni films. Additionally, the current density for hydrogen evolution at -1.0 V and -1.5 V vs. Ag/AgCl increased up to approximately -580 A/m2 and -891 A/m2, respectively, due to the extremely large surface area of the electrodeposited Ni nanowire arrays.

Revealing the impact of Hot Isostatic Pressing temperature on the microstructure and mechanical characteristics of Selective Laser Melted CuAlNiMn shape memory alloy

The present research examines the impact of temperature during Hot Isostatic Pressing (HIP) on the mechanical, microstructure, and density characteristics of Cu-based shape memory alloys (SMAs) made by Selective Laser Melting (SLM). HIP improved the density of the built components by 11.5 %, with a reduction in density with an increase in HIP temperature. Pore aspect ratio analysis revealed non-spherical pores at higher temperatures, while spherical and small pores were observed at 950 °C. SEM analysis demonstrated the disappearance of shrinkage porosity and an increase in grain size with higher HIP temperatures. XRD analysis confirmed phase transformation and variation in crystallite size with an increase in HIP temperature. After HIP treatment, microhardness and tensile strength significantly improved, with the 950 °C sample showing the highest values.

The effects of pore structure on wave dispersion and attenuation due to squirt flow: A dynamic stress-strain simulation on a simple digital pore-crack model

Squirt flow, a phenomenon typically observed in porous cracked rocks, occurs due to the contrasting compressibility between the pores and cracks, leading to the pore pressure diffusion and dissipation of wave energy. Understanding the influence of pore structure on wave dispersion and attenuation signatures due to squirt flow is essential for interpreting seismic and sonic logging data in various fields of earth and energy sciences, such as hydrocarbon exploration, geothermal energy exploitation, and CO2 sequestration. In this study, we construct a simple digital pore-crack model with varying pore structures and use a dynamic stress-strain simulation approach to characterize wave dispersion and attenuation signatures due to squirt flow. Numerical simulation suggests that, in addition to the commonly considered parameters such as porosity, crack density, and crack aspect ratio, additional pore structure parameters, such as pore size, pore aspect ratio, crack orientation, crack length, and crack width, significantly affect the wave dispersion and attenuation signatures. Increasing the pore size leads to the pronounced enhancement of attenuation magnitude and a decrease in characteristic frequency. We demonstrate that variations in crack length have a pronounced impact on attenuation magnitude, whereas crack width is decisive in controlling the characteristic frequency. Furthermore, it is found that the saturation paths (the gas filling the pore first or gas filling the crack first) considerably affect the velocity-saturation and attenuation-saturation relationship, suggesting that the coupling effects of pore structure and fluid distribution complicate the fluid pressure diffusion and wave attenuation behaviors. The presented results offer insights for deciphering pore structure parameters using attenuation- or dissipation-related seismic attributes.

A novel rapid generation algorithm for Representative Volume Element (RVE) in composite materials considering pore geometrical parameters

Pore defects are inevitable in the manufacturing process and significantly affect the mechanical properties of composite materials. In this work, a novel algorithm capable of concurrently generating both ellipsoidal and gourd-shaped pores in composite materials is proposed and the interference detection procedure is optimized for efficient interference detection between the generated geometrical bodies. Besides, the algorithm is integrated with RVE-based finite element (FE) simulation to explore the effects of various pore types, volumes, aspect ratios, and porosity on the elastic properties of unidirectional C/C composites. The present work provides a valuable reference for generating general pore defects and investigating their resulting effects on mechanical properties of related composite structures.

Facilely tuning the morphologies of tertiary amine-functionalized SBA-15 for optimizing selective SO2 capture

The amine-based materials pose high potential for the adsorption of sulfur dioxide (SO2). This work is aimed at modulating the morphologies of SBA-15 adsorbents by controlling the precursor self-assembly processes, for the sake of optimizing the utilization efficiency of amine species in the polytertiary amine (PTA)-functionalized SBA-15. It results that worm-like SBA-15 (WLS) material shows a better particle aspect ratio and well-defined pore structure, which helps to optimize the adsorption sites invoked by the preferable dispersion and exposure of PTA, thus amplifying the chemisorption of SO2 on the adsorbent. Consequently, the functionalized WLS-PTA adsorbent exhibits remarkable SO2 adsorption performance (15.43 mmol/g), exceptionally high selectivities of SO2/N2 (27522) and SO2/CO2 (652). Surprisingly, the desulfurization capacity can be further enhanced by the positive effect of the water vapor co-fed. It is unveiled that the whole SO2 adsorption process mainly follows the dual site Langmuir-Freundlich isotherm model and Bangham model. This work sheds light on the design of polymer-solid composite adsorbents for selective capture of SO2.

Effect of pore on the deformation behaviors of NiTi shape memory alloys: A crystal-plasticity-based phase field modeling

Porous NiTi shape memory alloys (SMA) have been developed into important biomedical and damping materials. However, the effect of pore on the martensitic microstructure evolution, functional properties, and plastic deformation has not been fully clear yet. In this work, to reveal the functional properties including superelasticity (SE), elastocaloric effect (eCE), one-way shape memory effect (OWSME), and stress-assisted two-way shape memory effect (SATWSME), as well as plastic deformation of porous NiTi SMAs, the polycrystalline SMA system is considered as a composite containing the grain-interior (GI) and grain-boundary (GB) phases, a thermo-mechanically coupled, grain-size-dependent and crystal-plasticity-based phase field model is proposed for the GI phase, and an elasto-viscoplastic constitutive model is established for the GB phase. The effects of porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution, as well as the interaction between pore and grain size, are comprehensively revealed and discussed. The simulations demonstrate that the pores cause a heterogeneous stress field, and the interaction among pores and that between pores and grain boundaries can enhance such heterogeneity, which can promote the initiation of martensitic transformation and reorientation. Moreover, the geometrical constraints caused by the pores can significantly affect the morphology and evolution of martensite microstructures. However, due to the diverse microstructure evolutions during SE, OWSME, and SATWSME, their interactions with the pores are quite different, resulting in various dependencies of each functional property and plastic deformation on the pore, and such dependencies are strongly influenced by porosity, pore diameter, pore aspect ratio, pore orientation, and pore distribution. The simulated results in this work are helpful to comprehensively and deeply understand the pore effect and its microscopic mechanism.

Deep Dive into the factors influencing acoustic velocity in the Dalan-Kangan formations, the central Persian Gulf

The propagation of acoustic waves in rocks is directly influenced by the rocks’ elastic properties. To evaluate petrophysical properties from acoustic data, it is essential to understand the factors that influence the variations in acoustic velocity of sedimentary rocks. Carbonate reservoirs are heterogeneous, and their evaluation is complicated. A closer look at the factors affecting the elastic properties of these rocks is essential because they reveal important information about the reservoir properties and their formation evaluation. Rock physics modeling relies on laboratory-measured acoustic data. This study, however, employs acoustic logs to elucidate the key factors influencing sonic velocity, offering a more comprehensive and field-applicable perspective. Samples were prepared from the Permian–Triassic Upper Dalan and Kangan formations. Porosity, permeability, sedimentary textures, mineralogy, and pore types were investigated. Furthermore, compressional, shear, and Stoneley well-logs data were recorded and converted to acoustic velocity in the sampled interval. Porosity-velocity modeling based on the differential effective medium (DEM) theory was performed for different values of equivalent pore aspect ratio (EPAR). The results show that there is a strong inverse relationship between acoustic velocities and porosity. Acoustic velocities also exhibit an inverse correlation with permeability, but the strength of these relationships is relatively weak. Acoustic velocities are not affected by carbonate textures. A specific texture may exhibit different velocities due to the influence of diagenetic processes. Mineralogy has only a minor influence on acoustic velocities. Dolostones have a slightly higher velocity than limestones, but the difference is insignificant. Stoneley log values are slightly different in limestone and dolostone, because this wave propagates along the mud-borehole interface. The velocities were affected by the variations in pore types within the studied formations. Therefore, in carbonate rocks, porosity, and the pore types play a significant role in controlling acoustic velocities. The value of EPAR = 0.15 can be considered as the boundary between stiff pores (moldic and vuggy) and soft pores (interparticle, intercrystalline, micro-intercrystalline, and microporosity). The lower EPAR value of interparticle pores in limestones compared to dolostones can be attributed to diagenetic processes such as dolomitization and compaction. The findings of this study can help to better understand what factors affect acoustic velocity in carbonate rocks. This study also shows that acoustic logs data can be effectively used in rock physics modeling and pore type identification. By identifying the pore types, it is possible to recognize diagenetic processes and determine the rock types to control the heterogeneity and estimate reservoir quality.

Laboratory experiments and theoretical study of pressure and fluid influences on acoustic response in tight rocks with pore microstructure

AbstractWave‐induced fluid flow is considered to be a major source of seismic attenuation and dispersion in porous rocks. From the physical description of partially saturated reservoirs, numerous analytical solutions based on upscaling homogenization theories have been employed to calculate equivalent frequency‐dependent poroelastic media. Nevertheless, dispersion and attenuation predictions are often not reasonably consistent with laboratory and field measurements in a broad frequency range, particularly due to influences of biphasic fluids and their distribution, presence of heterogeneities on various length scales, and pore microstructure. We investigate the role of pore microstructure on pressure and fluid saturation dependence of elastic velocities in tight sandstones. Previous work points out that differentiating the impacts of heterogeneities at various scales on dispersion within seismic exploration and sonic frequencies can be very difficult. In practice, this is because fluid‐related dispersion mechanisms are impossible to be independent. Thus, it is important for a theoretical and more quantitative analysis of the relative contribution of interrelated energy dissipation processes through a better understanding of combined influences due to the presence of microscopic and mesoscopic heterogeneities. Based on microscopic squirt flow and mesoscopic flow in a partially saturated medium, we develop a poroelastic model that allows evaluating the overall frequency‐dependent dispersion via considering a random distribution of fluid heterogeneities as well as the broadly distributed aspect ratio of compliant pores. Experimental validation of the model is accomplished via a comprehensive comparison of predictions with measurements of partially saturated velocities versus pressure and fluid for sandstones with specific pore microstructures.

Alkadiyne-Pyrene Conjugated Frameworks with Surface Exclusion Effect for Ultrafast Seawater Desalination.

Billions of populations are suffering from the supply-demand imbalance of clean water, resulting in a global sustainability crisis. Membrane desalination is a promising method to produce fresh water from saline waters. However, conventional membranes often encounter challenges related to low water permeation, negatively impacting energy efficiency and water productivity. Herein, we achieve ultrafast desalination over the newly developed alkadiyne-pyrene conjugated frameworks membrane supported on a porous copper hollow fiber. With membrane distillation, the membrane exhibits nearly complete NaCl rejection (>99.9%) and ultrahigh fluxes (∼500 L m-2 h-1) from the seawater salinity-level NaCl solutions, which surpass the commercial polymeric membranes with at least 1 order of magnitude higher permeability. Experimental and theoretical investigations suggest that the large aspect ratio of membrane pores and the high evaporation area contribute to the high flux, and the graphene-like hydrophobic surface of conjugated frameworks exhibits complete salt exclusion. The simulations also confirm that the intraplanar pores of frameworks are impermeable for water and ions.

Modeling the ionic diffusion coefficient of unsaturated hardened cement paste: A micromechanical approach

Most of the concrete structures in service are unsaturated, estimating the ionic diffusion coefficient of unsaturated cement-based material is of paramount significance to assessment of corrosion of reinforced concrete. Taking account of the contributions from different water configurations at multi-scale (water films, interlayer water, large gel pore (LGP) water and capillary pore water) and the corresponding diffusion mechanisms of each type of water, a micromechanical model for estimating the ionic diffusion coefficient of unsaturated hardened cement paste (UHCP), is developed in this study. Model results show that the interconnectivity of the spheroidal capillary pore water is closely related to the aspect ratio of spheroidal capillary pore, the slender the prolate pore, the higher the interconnectivity of the capillary pore water; when the capillary pore water is totally drained, water films and interlayer water within C-S-H governs the ionic diffusion, the higher the water to cement ratio (w/c), the less the contribution of interlayer water, and the greater the contribution of water film, to ionic diffusion within UHCP. Application on saturated hardened cement paste and UHCP shows the capability of the model to accurately reproduce the experimental results.

Microstructure Imaging and Characterization of Rocks Subjected to Liquid Nitrogen Cooling

Liquid nitrogen (LN2) fracturing is a potential stimulation method in unconventional hydrocarbon recovery, showing its merits in being water free, creating low formation damage and being environmentally friendly. The microstructure evolution of rocks subjected to LN2 cooling is a fundamental concern for the engineering application of LN2 fracturing. In this paper, pore-scale imaging and characterization were performed on two rocks, i.e., tight sandstone and coal specimens subjected to LN2 cooling using computed tomography scanning. The digital core technique was employed to reconstruct the microstructures of rocks and give a quantitative analysis of the pore structure evolution of both dry and water-saturated rocks. The results indicate that LN2 cooling has a great effect on the pores’ morphology and their spatial distribution, leading to a great improvement in pore diameter and aspect ratio. When compared to the sandstone, coal is more sensitive to LN2 cooling and thermal stresses, having a more noticeable growth in pore–throat size. The porosity growth of coal is 291% higher than that of sandstone. There is a growing trend in the irregularity and complexity of pore structures. After LN2 cooling, the fractal dimensions of the pores of sandstone and coal grow by 11.7% and 0.87%, respectively, and the proportion of pores with a shape factor > 100 increases. More bundle-like and strip-shape pores with multiple branches are generated, which causes a significant growth in the throat size and the proportion of connected pores with a coordination number ≥ 1, enhancing the complexity and connectivity of pore structures dramatically. Additionally, pore water plays an important role in aggravating rock damage during LN2 cooling, enhancing the pore space and connectivity. The porosities of the saturated sandstone and coal samples grow by 22.6% and 490.4%, respectively, after LN2 cooling, which are 5.6% and 186.6% higher than dry samples. The generation of macropores ≥ 70 μm is the primary contributor to porosity growth during LN2 cooling, although such pores account for only a small proportion of the total. These findings contribute to our understanding of the microscopic mechanism of LN2 cooling on rock damage and may provide some guidance for the engineering application of LN2 fracturing.