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A series of novel donor-acceptor (D-A) type viologens with through-space conjugation (TSC) are synthesized. The synergistic effect of D-A conjugation and TSC endow these compounds with a narrower energy gap, excellent redox properties, strong absorption in the visible range (up to 450 nm), and high efficiency of intramolecular charge transfer (ICT). These compounds also exhibit a long-lived charge separation state and stable radical formation. Considering their impressive photophysical and redox properties, TSC viologens with D-A conjugation are explored for visible-light-driven photocatalysis. As a catalyst in oxidative coupling reactions under visible light, compound 9 can achieve a yield of 94%, attributed to its strong ICT effect. Additionally, hybridized into graphitic carbon nitride (g-C₃N₄), compound 9 enhances hydrogen production performance under visible light, achieving an H 2 generation rate of 4102 µmol h -1 g -1 . This study highlights the potential applications of TSC viologens in photocatalytic systems., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Hydrogels are an extraordinary soft matter system that serve as a laboratory for a rich plethora of multiphysical phenomena and find applications that range from biocompatible sensors to soft robots. Here, we report a peculiar experimental observation suggesting concurrent stiffening and softening in hydrogels during the dehydration process. Theories based on Flory's work fail to capture the scaling of mechanical behavior with water content, observed in our experiments. We perform coarse-grained molecular dynamics simulations to elucidate the mechanisms underpinning the odd softening-stiffening behavior during dehydration and propose a theoretical model to correctly represent the underlying physics and the divergence from Flory-based theories.

It has been shown that the nanoplastic particles present in graphene membranes have a high tendency to cause fouling in them due to the high affinity between graphene and nanoplastic molecules. This poses a significant challenge for the use of graphene membranes for desalination. In this paper, we introduce a double-layer graphene slit membrane as a viable solution to significantly reduce fouling caused by the presence of nanoplastic particles in graphene membranes. The molecular dynamics (MD) simulations performed in this work show that when fouling occurs in a single-layer membrane, the presence of nanoplastics reduces the average permeability by close to 40%, from 1877 LMBH to 1148 LMBH, with a large standard deviation of 26% between runs. With the addition of the secondary membrane, the average permeability increases by 17%, with a significantly reduced standard deviation of 7%. These suggest that the secondary layer acts as a sacrificial shield, attracting the nanoplastic contaminants and preventing them from coming into close proximity with the primary membrane, thus preventing fouling at the primary rejection layer. Furthermore, due to the affinity of the nanoplastic particles with the secondary graphene membrane, this membrane design points toward an effective and efficient way of extracting nanoplastic particles for further analysis or processing.

In this paper, we perform molecular dynamics simulations to investigate the performance of multilayer graphene slit membranes. Graphene slit membranes at a critical slit size have been found to be promising desalination membranes. In this contribution, it is shown that multilayer slit membranes have the potential to provide significantly better permeability while retaining outstanding salt rejection. Improved permeability of the membrane is achieved by using slits of widths larger than the critical slit size required to reject salt through size exclusion, and desalination of sea water is performed by increased resistance to salt passage through the multilayering. To facilitate the design process of future multilayer membranes, we analyze the flow resistance of the membrane as a combination of electrical resistors in series and show that this analogy works for membranes where the layers possess the same slit size, as well as membranes with layers of different slit sizes. Comparing with single layer graphene membranes, it was shown that it is possible to obtain 55% improvement in permeability without loss in salt rejection capabilities through multilayering. This opens up possibilities for membrane designers to be free from the restrictions of using a single layer graphene slit membrane with a fixed slit width.

Structural vibration induced by low frequency elastic waves presents a great threat to infrastructure such as buildings, bridges, and nuclear structures. In order to reduce the damage of low frequency structural vibration, researchers proposed the structure of seismic metamaterial, which can be used to block the propagation of low frequency elastic wave by adjusting the frequency range of elastic wave propagation. In this study, based on the concept of phononic crystal, a ternary seismic metamaterial is proposed to attenuate low frequency vibration by generating band gaps. The proposed metamaterial structure is periodically arranged by cube units, which consist of rubber coating, steel scatter, and soft matrix (like soil). The finite element analysis shows that the proposed metamaterial structure has a low frequency band gap with 8.5 Hz bandwidth in the range of 0-20 Hz, which demonstrates that the metamaterial can block the elastic waves propagation in a fairly wide frequency range within 0-20 Hz. The frequency response analysis demonstrates that the proposed metamaterial can effectively attenuate the low frequency vibration. A simplified equivalent mass-spring model is further proposed to analyze the band gap range which agrees well with the finite element results. This model provides a more convenient method to calculate the band gap range. Combining the proposed equivalent mass-spring model with finite element analysis, the effect of material parameters and geometric parameters on the band gap characteristic is investigated. This study can provide new insights for low frequency vibration attenuation.

Magnetic hydrogels have found a myriad of applications in bioengineering and soft robotics. As the function of magnetic hydrogels is affected by the distribution of magnetic nanoparticles, it is imperative to propose a strategy for fabricating patterned magnetic hydrogels. However, previous strategies can only achieve very simple distribution by using external magnetic fields to guide the chain-like assembly of nanoparticles. It remains challenging to realize the complex distribution of magnetic nanoparticles in a hydrogel. Here we propose an ion transfer printing strategy to prepare patterned magnetic hydrogels, taking advantage of the ion permeation and nanoparticle precipitation in the hydrogel. The polyacrylamide (PAAm) hydrogel is loaded with Fe 2+ /Fe 3+ ions and covered with a patterned filter paper with OH - ions to generate Fe 3 O 4 nanoparticles locally. The effect of the ion concentration and covering time on the generation of nanoparticles is investigated by using a reaction-diffusion model. Furthermore, the magnetothermal response of the patterned magnetic hydrogels has been characterized to reveal the distribution and thermogenesis of magnetic nanoparticles. We hope that the fabricated magnetic hydrogels with complex patterns can open up new opportunities for applications.

Shape memory polymers (SMPs) have gained much attention in biomedical fields due to their good biocompatibility and biodegradability. Researches have validated the feasibility of shape memory polymer stent in treatment of vascular blockage. Nevertheless, the actual application of SMP stents is still in infancy. To improve the mechanical performance of SMP stent, a new geometric model based on metamaterial is proposed in this study. To verify the feasibility and mechanical behavior of this type of stent, buckling analysis, and in vivo expansion performance of SMP stent are simulated. Numerical results exhibit that stent of a smaller radius behaves a higher critical buckling load and smaller buckling displacement. Besides, a smaller contact area with vessel and smaller implanted stress are observed compared with traditional stents. This suggests that this SMP stent attributes to a reduced vascular restenosis. To characterize the radial strength of SMP stent, an analytical solution is derived by the assumption that the deformation of stent is mainly composed of bending and stretch. The radial strength of SMP stent is assessed in form of radial force. Analytical results reveal that radial strength is depended on the radius of stent and periodic numbers of unit cell in circumferential direction.

Tensile-compressive asymmetry and the ratcheting effect are two significant characteristics of shape memory alloys (SMAs) during uniaxial cyclic tests, thus having received substantial attention in research. In this study, by redefining the internal variables in SMAs by considering the cyclic accumulation of residual martensite, we propose a constitutive model for SMAs to simultaneously reflect tensile-compressive asymmetry and the cyclic ratcheting effect under multiple cyclic tests. This constitutive model is temperature dependent and can be used to reasonably capture the typical features of SMAs during tensile-compressive cyclic tests, which include the pseudo-elasticity at higher temperatures as well as the shape-memory effect at lower temperatures. Moreover, the proposed model can predict the cyclic mechanical behavior of SMAs subjected to applied stresses with different peak and valley values under tension and compression. Agreement between the predictions obtained from the proposed model and the published experimental data is observed, which confirms that the proposed novel constitutive model of SMAs is feasible., Competing Interests: The authors declare no conflict of interest.

In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order.

Meso-scale models for hydrogels are crucial to bridge the conformation change of polymer chains in micro-scale to the bulk deformation of hydrogel in macro-scale. In this study, we construct coarse-grain bead-spring models for polyacrylamide (PAAm) hydrogel and investigate the large deformation and fracture behavior by using Dissipative Particle Dynamics (DPD) to simulate the crosslinking process. The crosslinking simulations show that sufficiently large diffusion length of polymer beads is necessary for the formation of effective polymer. The constructed models show the reproducible realistic structure of PAAm hydrogel network, predict the reasonable crosslinking limit of water content and prove to be sufficiently large for statistical averaging. Incompressible uniaxial tension tests are performed in three different loading rates. From the nominal stress-stretch curves, it demonstrated that both the hyperelasticity and the viscoelasticity in our PAAm hydrogel models are reflected. The scattered large deformation behaviors of three PAAm hydrogel models with the same water content indicate that the mesoscale conformation of polymer network dominates the mechanical behavior in large stretch. This is because the effective chains with different initial length ratio stretch to straight at different time. We further propose a stretch criterion to measure the fracture stretch of PAAm hydrogel using the fracture stretch of C-C bonds. Using the stretch criterion, specific upper and lower limits of the fracture stretch are given for each PAAm hydrogel model. These ranges of fracture stretch agree quite well with experimental results. The study shows that our coarse-grain PAAm hydrogel models can be applied to numerous single network hydrogel systems., (Copyright © 2020 Lei, Xu, Li and Liu.)

Existing theories cannot predict the mechanical property changes of polyacrylamide hydrogels with different water content because of the absence of side chains. In this study, polyacrylamide hydrogels are prepared and tested to investigate the side chain effect on their mechanical properties. First, the comparison between the effective chain density and total chain density provides proof of the large amount of side chains in the polymer network of PAAm hydrogel. We propose a practical chain density fraction to measure the side chain fraction. Then, the abnormal Young's moduli-polymer volume fraction relationship reveals that side chains affect the mechanical properties of hydrogel through the insufficient lubrication of water. Water confined in narrow space within a molecular-level size can bear shear force to provide extra deformation resistance. A constitutive mode considering the effect of the insufficient lubrication of water is proposed. Combining this constitutive model with experimental results, we find that this insufficient lubrication of water exists even in equilibrium PAAm hydrogel. Molecular dynamics simulations reveal that this insufficient lubrication of water comes from the constraint of polymer chains. It also demonstrates that when there is insufficient lubrication, the rearrangement of water molecules leads to the persistent energy dissipation in the Mullins effect of PAAm hydrogel., Competing Interests: The authors declare no conflict of interest.

A series of chalcogen-containing carborane derivatives were synthesized through a one-pot reaction. The nonconjugated six-membered ring of carborane-fused disulfide ( 1 ) can be electrochemically reduced to a dithiolate derivative. The five-membered ring of carborane-fused chalcogenophenes ( 2 - 4 ) showed aromaticity and considerable σ-π conjugation. The dicarborane chalcogenides ( 5 - 7 ) showed intramolecular charge-transfer-induced emission. These chalcogen-containing carborane derivatives provided a useful platform to study the electron interaction systematically and shed some light on the design of carborane-based optoelectronic materials.

A polymer network can imbibe copious amounts of water and swell, and the resulting state is known as a hydrogel. In many potential applications of hydrogels, such as stretchable conductors, ionic cables, and neuroprostheses, the thermal conductivities of hydrogels should be understood clearly. In the present work, we build molecular dynamics (MD) models of random cross-linked polyacrylamide hydrogels with different water volume fractions through a reaction method. On the basis of these models, thermal conductivities of hydrogels at the nanoscale are investigated by a none-equilibrium MD method. This work reveals that when the water fraction of hydrogels is under 85%, the thermal conductivity increases with the water fraction, and can be even higher than the thermal conductivities of both pure polymer networks and pure water because of the influence of the interface between polymer networks and water. However, when the water fraction in hydrogels is bigger than 85%, its thermal conductivity will decrease and get close to the water's conductivity. Accordingly, to explain this abnormal phenomenon, a 2-order-3-phase theoretical model is proposed by considering hydrogel as a 3-phase composite. It can be found that the proposed theory can predict results which agree quite well with our simulated results.

As an excellent model material for fundamental studies on temperature-sensitive hydrogels, poly( N -isopropylacrylamide) (NIPA) hydrogel has been applied in drug delivery, tissue engineering, and soft robotics. However, the lack of study on fracture and fatigue hinders further development of hydrogels for applications where cyclic loading-unloading is unavoidable. In this study, the fracture and fatigue damage of the NIPA hydrogel were studied for the first time by pure shear tests at different temperatures. Fracture behaviors were investigated under monotonic load from 31 °C to 39 °C. It is found that the fracture energy increases with the increase in temperature. The fracture energy is approximately 20 J m -2 near the volume phase transition temperature. Temperature also significantly influences the fatigue life. By fitting the experimental data, the fatigue limit λ f is determined. The results obtained from the experimental tests would be important for the engineering applications of the NIPA hydrogels., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We investigate the effect of varying carbon nanotube (CNT) size on the desalination performance through slit confinements formed by horizontally aligned CNTs stacked on top of one another. By increasing the CNT size, the results obtained from this study indicate a corresponding increase in the water flow rate, accompanied by a slight reduction in salt rejection performance. However, due to the increase in the membrane area with CNT size, the permeability performance is observed to reduce as the CNT size increases. Nevertheless, a comparison with nanoporous 2D membranes shows that the permeability of an outer-wall CNT slit membrane remains significantly higher for all CNT sizes considered. This indicates that precise dimensions of the CNTs are not highly crucial for achieving ultra-high permeability performance in such membranes, as long as the critical slit size is maintained. In-depth analytical studies were further conducted to correlate the influence of curvature effects due to increasing CNT size on the flow characteristcis of the outer-wall CNT membrane. These include the analysis of the measured velocity profiles, oxygen density mapping, potential of mean force profile and friction profile. The present numerical results demonstrate the superb desalination performance of the outer-wall CNT slit membrane, regardless of the size of CNTs used. In addition, an extensive analysis conducted provides detailed characterization of how the curvature affects flow across outer-wall CNTs, and can be used to guide future design and fabrication for experimental testing.

The inverse Stone-Wales defect is a typical defect in graphene, which causes local bumps and local deformation in graphene sheets. Our molecular dynamics simulations show that the spontaneous rolling up of graphene sheets can be induced by orderly distributed inverse Stone-Wales defect bumps, when defective graphene is cut into small strips. This spontaneous process is mainly dominated by the defect density and tailored graphene size. When the tailored length is longer than the upper threshold length, graphene sews up as a curly one-dimensional structure: heart-shaped nanotube. For medium length graphene (the length is in between the lower threshold value and upper threshold value), the results reveal that graphene finally curls into a completely or incompletely stitched nanotube similar to a cylindrical shell. This spontaneous process is produced by a high-frequency damped vibration accompanied by elastic and viscoelastic deformation in defective graphene. Thus, the properties of vibration are further investigated for graphene that has the tailored length shorter than the lower threshold length. This kind of graphene gradually forms a curved nanoribbon rather than a nanotube. It is also found that the bending rigidity of defective graphene is larger than that of pristine graphene.

Due to its excellent temperature sensitivity, the Poly( N -isopropylacrylamide) (NIPA) hydrogel has attracted great interest for a wide variety of applications in tissue engineering and regenerative medicine. NIPA hydrogel undergoes an abrupt volume phase transition at a lower critical solution temperature (LCST) of 30⁻35 °C. However, the mechanical behaviors of NIPA hydrogel induced by phase transition are still not well understood. In this study, phase transition effects on mechanical properties of NIPA hydrogel are quantitatively studied from experimental studies. The mechanical properties of NIPA hydrogel with the LSCT around 35 °C are systemically studied with varying temperatures (31⁻39 °C) under a tensile test. We find that the mechanical properties of NIPA hydrogel are greatly influenced by phase transition during the tension process. The maximum nominal stress and maximum stretch above the LCST are larger than those of below the LCST. The Young's modulus of NIPA hydrogel is around 13 kPa at 31 °C and approximately 28 kPa at 39 °C. A dramatic increase of Young's modulus values is observed as the temperature increases through the phase transition. The samples at a temperature around the LCST are easy to rupture, because of phase coexistent. Additionally, NIPA hydrogel displays toughening behavior under a cyclic load. Furthermore, the toughening characteristic is different between the swollen state and the collapsed state. This might originate from the internal fracture process and redistribution of polymer chains during the tension process., Competing Interests: The authors declare no conflict of interest.

It is well known that elastic instabilities induce pattern transformations when a soft cellular structure is compressed beyond critical limits. The nonlinear phenomena of pattern transformations make them a prime candidate for controlling macroscopic or microscopic deformation and auxetic properties of the material. In this present work, the novel mechanical properties of soft cellular structures and related hydrogel⁻elastomer composites are examined through experimental investigation and numerical simulations. We provide two reliable approaches for fabricating hydrogel⁻elastomer composites with rationally designed properties and transformed patterns, and demonstrate that different geometries of the repeat unit voids of the periodic pattern can be used to influence the global characteristics of the soft composite material. The experimental and numerical results indicate that the transformation event is dependent on the boundary conditions and material properties of matrix material for soft cellular structures; meanwhile, the deformation-triggered pattern of matrix material affects the pattern switching and mechanical properties of the hydrogel⁻elastomer material, thus providing future perspectives for optimal design, or serving as a fabrication suggestion of the new hydrogel⁻elastomer composite material., Competing Interests: The authors declare no conflict of interest.

Mixed results were reported on the anticorrosion of graphene-coated metal surfaces-while graphene serves as an effective short-term barrier against corrosion and oxidation due to its low permeability to gases, the galvanic cell between graphene and the metal substrate facilitates extensive corrosion in the long run. Defects in the graphene layer provide pathways for the permeation of oxidizing species. We study the role of defects in graphene in the anticorrosion using first-principles theoretical modeling. Experiments in the highly reactive environment indicate that the oxidized products primarily distribute along the grain boundaries of graphene. We analyze the thermodynamics of the absorption of S and O on the grain boundaries of graphene on the basis of density functional theory. The insertion of S and O at the vacancy sites is energetically favorable. The interstitial impurities facilitate structural transformation of graphene and significantly decrease the mechanical strength of the graphene layer. Furthermore, the presence of the interstitial S and O reduces the chemical stability of graphene by enhancing the formation of vacancies and promoting dispersive growth of corrosive reactants along the grain boundaries.

Cell adhesion plays a crucial role in many biological processes of cells, e.g., immune responses, tissue morphogenesis, and stem cell differentiation. An essential problem in the molecular mechanism of cell adhesion is to characterize the binding affinity of membrane-anchored receptors and ligands under different physiological conditions. In this paper, a theoretical model is presented to study the binding affinity between a large number of anchored receptors and ligands under both tensile and compressive stresses, and corroborated by demonstrating excellent agreement with Monte Carlo simulations. It is shown that the binding affinity becomes lower as the magnitude of the applied stress increases, and drops to zero at a critical tensile or compressive stress. Interestingly, the critical compressive stress is found to be substantially smaller than the critical tensile stress for relatively long and flexible receptor-ligand complexes. This counterintuitive finding is explained by using the Euler instability theory of slender columns under compression. The tension-compression asymmetry in the binding affinity of anchored receptors and ligands depends subtly on the competition between the breaking and instability of their complexes. This study helps in understanding the role of mechanical forces in cell adhesion mediated by specific binding molecules.

How to maintain sustained deformation in one-dimensional nanostructures without localized failure is an important question for many applications of nanotechnology. Here we report a phenomenon of torsional detwinning domino that leads to giant rotational deformation without localized failure in nanotwinned one-dimensional metallic nanostructures. This mechanism is demonstrated in nanotwinned Cu nanorods via molecular dynamics simulations, where coherent twin boundaries are transformed into twist boundaries and then dissolved one by one, resulting in practically unlimited rotational deformation. This finding represents a fundamental advance in our understanding of deformation mechanisms in one-dimensional metallic nanostructures.

This paper studies the nonlinear behavior of a nano-optomechanical actuator, consisting of a free-standing arc in a ring resonator that is coupled to a bus waveguide through evanescent waves. The arc deflects when a control light of a fixed wavelength and optical power is pumped into the bus waveguide, while the amount of deflection is monitored by measuring the transmission spectrum of a broadband probe light. This nanoactuator achieves a maximal deflection of 43.1 nm, with a resolution of 0.28 nm. The optical force is a nonlinear function of the deflection of the arc, leading to pull-back instability when the control light is red-tuned. This instability is studied by a combination of experiment and modeling. Potential applications of the nanoactuator include bio-nanomotor, optical switches, and optomechanical memories.

Classical molecular dynamics with the AIREBO potential is used to investigate and compare the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing various densities of Stone-Thrower-Wales (STW) and double vacancy defects, within a temperature range of 100-600 K. Our results indicate that the presence of both kinds of defects can decrease the thermal conductivity by more than 80% as defect densities are increased to 10% coverage, with the decrease at high defect densities being significantly higher in zigzag compared with armchair nanoribbons. Variations of thermal conductivity in armchair nanoribbons were similar for both kinds of defects, whereas double vacancies in the zigzag nanoribbons led to more significant decreases in thermal conductivity than STW defects. The same trends are observed across the entire temperature range tested.

The deformation behavior of a pH-sensitive hydrogel micro-fluidic valve system is investigated using inhomogeneous gel deformation theory, in which the fluid-structure interaction (FSI) of the gel solid and fluid flow in the pipe is considered. We use a finite element method with a well adopted hydrogel constitutive equation, which is coded in commercial software, ABAQUS, to simulate the hydrogel valve swelling deformation, while FLUENT is adopted to model the fluid flow in the pipe of the hydrogel valve system. The study demonstrates that FSI significantly affects the gel swelling deformed shapes, fluid flow pressure and velocity patterns. FSI has to be considered in the study on fluid flow regulated by hydrogel microfluidic valve. The study provides a more accurate and adoptable model for future design of new pH-sensitive hydrogel valves, and also gives a useful guideline for further studies on hydrogel fluidic applications.