Thermal transport in a defective pillared graphene network: insights from equilibrium molecular dynamics simulation.

Graphene-based hybrid nanostructures have great potential to be ideal candidates for developing tailored thermal transport materials. In this study, we perform equilibrium molecular dynamics simulations employing the Green-Kubo method to investigate the influence of topological defects in three-dimensional pillared graphene networks. Similar to single-layer graphene and carbon nanotubes, the thermal conductivity (k) of pillared graphene systems exhibits a strong correlation with the system size (L), following a power-law relation k ∼ Lα, where α ranges from 0.12 to 0.15. Our results indicate that the vacancy defects significantly reduce thermal conductivity in pillared graphene systems compared to Stone-Wales defects. We observe that, beyond defect concentration, the location of the defects also plays a crucial role in determining thermal conductivity. We further analyze the phonon vibrational spectrum and the phonon participation ratio to obtain more insight into the thermal transport in the defective pillared graphene network. In most scenarios, longitudinal and flexural acoustic phonons experience significant localization within the 15-45 THz frequency range in the defective pillared graphene system.

Temperature-dependent differential capacitance of an ionic liquid-graphene-based supercapacitor.

One of the critical factors affecting the performance of supercapacitors is thermal management. The design of supercapacitors that operate across a broad temperature range and at high charge/discharge rates necessitates understanding the correlation of the molecular characteristics of the device (such as interfacial structure and inter-ionic and ion-electrode interactions) with its macroscopic properties. In this study, we use molecular dynamics (MD) simulations to investigate the influence of Joule heating on the structure and dynamics of the ionic liquid (IL)/graphite-based supercapacitors. The temperature-dependent electrical double layer (EDL) and differential capacitance-potential (CD-V) curves of two different ([Bmim][BF4] and [Bmim][PF6]) IL-graphene pairs were studied under various thermal gradients. For the [Bmim][BF4] system, the differential capacitance curves transition from 'U' to bell shape under an applied thermal gradient (∇T) in the range from 3.3 K nm-1 to 16.7 K nm-1. Whereas in [Bmim][PF6], we find a positive dependence of differential capacitance with ∇T with a U-shaped CD-V curve. We examine changes in the EDL structure and screening potential (ϕ(z)) as a function of ∇T and correlate them with the trends observed in the CD-V curve. The identified correlation between the interfacial charge density and differential capacitance with thermal gradient would be helpful for the molecular design of the IL-electrode interface in supercapacitors or other chemical engineering applications.

Use of Multiple Fluid Biomarkers for Predicting the Co-occurrence of Diabetes and Hypertension Using Machine Learning Approaches.

The co-existence of diabetes and hypertension can complicate and affect the management of these diseases. The early detection of these comorbidities can help in developing personalized preventive treatments and thereby, reduce the healthcare burden. The inclusion of readily available fluid biomarkers from different body fluids can be used as diagnostic tools and can facilitate in the designing of treatment strategies. In this work, an attempt has been made using multiple fluid biomarkers to differentiate diabetic from diabetic and hypertensive comorbid (DHC) condition. The fluid biomarkers are obtained from a publicly available dataset for diabetic (N=105) and DHC (N=57) conditions. The features, such as systolic blood pressure, fasting blood glucose, diastolic blood pressure, and total cholesterol are extracted and statistically analyzed. Data balancing technique namely synthetic minority oversampling technique is applied on the minority class to balance the dataset. Machine learning techniques namely, linear discriminant analysis, random forest, K-nearest neighbor, and linear support vector machine are used to perform the classification between the two groups. The results show that systolic blood pressure, diastolic blood pressure, and total cholesterol are elevated in the comorbid condition. These features also exhibit a statistical significance (p<0.001) between the two groups. This study also addresses the data imbalance issue, which is resolved by using an oversampling technique to mitigate the bias resulting from imbalanced data. The LDA classifier achieves a maximum accuracy of 61.2% in distinguishing between the two conditions. Machine learning based approaches may help in the prediction of comorbid conditions. This can act as a guideline for future studies on the progression of diseases and the identification of fluid biomarkers.

Effects of interfacial molecular mobility on thermal boundary conductance at solid-liquid interface.

The effects of interfacial molecular mobility on the thermal boundary conductance (TBC) across graphene-water and graphene-perfluorohexane interfaces were investigated using non-equilibrium molecular dynamics simulations. The molecular mobility was varied by equilibrating nanoconfined water and perfluorohexane at different temperatures. The long-chain molecules of perfluorohexane exhibited a prominent layered structure, indicating a low molecular mobility, over a wide temperature range between 200 and 450 K. Alternatively, water increased its mobility at high temperatures, resulting in an enhanced molecular diffusion that significantly contributed to the interfacial thermal transport, in addition to the increasing vibrational carrier population at high temperatures. Furthermore, the TBC across the graphene-water interface exhibited a quadratic relationship with the rise in temperature, whereas for the graphene-perfluorohexane interface, a linear relationship was observed. The high rate of diffusion in interfacial water facilitated additional low-frequency modes, and a spectral decomposition of the TBC also indicated an enhancement in the same frequency range. Thus, the enhanced spectral transmission and higher molecular mobility of water with respect to perfluorohexane explained the difference in the thermal transport across the interfaces considered herein.

Modeling the effect of chirality on thermal transport in a pillared-graphene structure.

The anisotropic heat transport in graphene-CNT based materials provoked the development of three-dimensional pillared-graphene (PG) systems. In this study, we performed non-equilibrium molecular dynamics simulations to analyze PG thermal conductivity and thermal boundary conductance. For the first time, we have considered the influence of pillar chirality and the temperature effect on PG heat transport. We analyzed the influence of pillar chirality and pillar length on the in- and out-of-plane transport properties. For the temperature-dependent analysis, the chosen temperatures were in the range of 100 K to 500 K. To elucidate the mechanism underlying the heat transport, we investigated the phonon density of states (DOS) in the different regions of PG systems. The overlap factor was calculated to quantify the mismatch in the phonon DOS profiles. Across the pillar region, the overlap factor correlates directly with the thermal boundary conductance. When heat is transported in an out-of-plane direction, the zig-zag PG system performs better than the armchair PG system. The atomic arrangement at the graphene-CNT interface plays an inevitable role in limiting heat transport in PG systems. The calculated phonon energy in the zig-zag PG interface is higher than that in the armchair PG interface.

Dynamic response of red blood cells in health and disease.

The viscoelastic response of the red blood cells (RBCs) affected by hematological disorders become severely impaired by the altered biophysical and morphological properties. These include traits like reduced deformability, increased membrane viscosity, and change in cell shape, causing substantial changes in the overall hemodynamics. RBCs, by virtue of their highly elastic membrane and low bending rigidity, exhibit complex dynamics when exposed to cyclic, transient forces in the microcirculation. Here, we employ mesoscopic numerical simulations based on the dissipative particle dynamics (DPD) framework to explore the dynamics of healthy, schizont stage malaria-infected and type 2 diabetes mellitus affected RBCs subjected to external time-dependent loads. The paper focuses on the imposition and cessation of external forcing on the cells of two different typologies, saw-tooth cyclic wave loading and sudden loads in the form of creep and relaxation phenomena. The effects of varying the rate of stress and the applied stress magnitude were investigated. Our simulations disclosed unique shape transitions of the hysteresis curves at varied loading rates. A careful analysis reveals a critical threshold of half cycle time of the from wherein the deformation of all cells observed, healthy or otherwise, falls under the nearly reversible deformation regime displaying minimal energy dissipation. Finally, we also examined the individual effects of the different constitutive and geometric characteristics attributed to the pathological cells and observed interesting recovery dynamics of spherocytes and cells having high shear moduli. The distinguished deformation behaviour of healthy and diseased cells could establish external force as a valuable initial biomarker.

A data driven approach to model thermal boundary resistance from molecular dynamics simulations.

A new method is proposed to model the thermal boundary resistance (TBR) at the nanoscale, solid-liquid interface from macroscopic observables that characterize a nanoscale interface. We correlated the TBR with thermodynamic state variables, material properties, and geometric parameters to derive a generalized relationship with the help of data-driven heuristic algorithms. The results show that TBR can be expressed in terms of physical observables of the systems and material-specific parameters. We investigated the mutual independence of descriptor variables and quantified the weightage for each observable parameter in the TBR models. The interfacial liquid layering has a robust correlation with TBR. However, for systems with phonon size effects and under extreme thermodynamic conditions, the work of adhesion and system geometry also affects the variation in TBR. The data-driven approach followed in this study helps us gain better insight into the mechanism of TBR at nanoscale solid-liquid interfaces and shows significant improvement in our knowledge about interfacial thermal transport.

Nanoscale gas accumulation at solid-liquid interfaces: a molecular dynamics study.

The development of the interfacial gas enrichment layer at the solid-liquid interface is coupled with the stability of surface nanobubbles. Depending upon the concentration of gas molecules, solid-liquid-gas interaction strengths, and other thermodynamic parameters, gas molecules can take several different forms such as dense gas layer, bulk and surface nanobubble, and other gaseous domains. Using molecular dynamics simulations we study the characteristics of gas accumulation into a dense gas layer, surface nanobubble and local gas aggregation at the graphene-water interface with no pinning sites. We find that gas molecules can migrate over the solid surface and can collect together to take the morphological form of a surface nanobubble. The developed nanobubble is mobile and can move over the homogeneous hydrophobic solid surface without losing its shape. We find that the gas adsorption on surfaces in the presence of a solvent is strongly affected by the wetting characteristics of the solid. In the absence of a solvent, gas adsorption is found to be universal for all surface types. Individual gas adsorption is found to be prominent and occurs in a short period, and is essential for the stability of the formed gaseous domains. Simulation results show gas adsorption density on surfaces to have a strong dependence on the solid-liquid interaction parameter than on solid-gas interaction strength.

Effects of Electrostatic Interactions on Kapitza Resistance in Hexagonal Boron Nitride-Water Interfaces.

Electrostatic interactions in nanoscale systems can influence the heat transfer mechanism and interfacial properties. This study uses molecular dynamics simulations to investigate the impact of various electrostatic interactions on the Kapitza resistance (Rk) on a hexagonal boron nitride-water system. The Kapitza resistance at hexagonal boron nitride nanotube (hBNNT)-water interface reduces with an increase in diameter of the nanotube due to more aggregation of water molecules per unit surface area. An increase in the partial charges on boron and nitride caused the reduction in Rk. With the increase in partial charge, a better hydrogen bonding between hBNNT and water was observed, whereas the structure and order of the water molecules remain the same. Nevertheless, the addition of NaCl salt into water does not have any influence on interfacial thermal transport. Rk remains unchanged with electrolyte concentration because the cumulative Coulombic interaction between the ions and the hBNNT is significantly less when compared with water molecules. Furthermore, the effect of electric field strength on interfacial heat transfer is also investigated by providing uniform positive and negative surface charges on the outermost hBN layers. Rk is nearly independent of the practical range of applied electric fields and decreases with an increasing electric field for extreme field strengths until the electrofreezing phenomenon occurs. The ordering of water molecules toward the charged surface leads to an increase in the layering effect, causing the reduction in Rk in the presence of an electric field.

Hydrodynamic slip of alkali chloride solutions in uncharged graphene nanochannels.

Using non-equilibrium molecular dynamics simulations, we demonstrate the effect of concentration and alkali cation types (K+, Na+, and Li+) on the hydrodynamic slip of aqueous alkali chloride solutions in an uncharged graphene nanochannel. We modeled the graphene-electrolyte interactions using the potential of Williams et al. [J. Phys. Chem. Lett. 8, 703 (2017)], which uses optimized graphene-ion Lennard-Jones interaction parameters to effectively account for surface and solvent polarizability effects on the adsorption of ions in an aqueous solution to a graphene surface. In our study, the hydrodynamic slip exhibits a decreasing trend for alkali chloride solutions with increasing salt concentration. The NaCl solution shows the highest reduction in the slip length followed by KCl and LiCl solutions, and the reduction in the slip length is very much dependent on the salt type. We also compared the slip length with that calculated using a standard unoptimized interatomic potential obtained from the Lorentz-Berthelot mixing rule for the ion-carbon interactions, which is not adjusted to account for the surface and solvent polarizability at the graphene surface. In contrast to the optimized model, the slip length of alkali chloride solutions in the unoptimized model shows only a nominal change with salt concentration and is also independent of the nature of salts. Our study shows that adoption of the computationally inexpensive optimized potential of Williams et al. for the graphene-ion interactions has a significant influence on the calculation of slip lengths for electrolyte solutions in graphene-based nanofluidic devices.

The disparate effect of strain on thermal conductivity of 2-D materials.

Thermal transport in 2-D (dimensional) structures is highly susceptible to external perturbations such as strain, owing to their high surface-to-volume ratio. In this study, we investigate the influence of strain on the thermal conductivity of flat (graphene and hexagonal boron nitride), buckled and puckered (molybdenum disulfide and black phosphorous) 2-D materials. Unlike bulk materials where the thermal conductivity reduces with strain, the thermal conductivity of 2-D materials under strain is observed to be unique and dependent on the material considered. To understand such diverse strain-dependent thermal conductivity in 2-D materials, the phonon mode properties are calculated. It was observed that the strain softens the longitudinal mode (LA), whereas the out-of-plane acoustic mode (ZA) undergoes stiffening albeit various extents. In flat 2-D materials, the dispersion of ZA mode is linearized under strain while it tends to linearize in buckled and puckered structures. The variation in the phonon group velocity of ZA mode coupled with the anomalous behavior of the phonon lifetime of acoustic modes results in a diverse strain dependence of the thermal conductivity of 2-D materials. Our findings offer insight into the influence of strain of 2-D materials and will be helpful in tailoring the thermal properties of these materials for various applications such as nanoelectronics and thermoelectric devices.

Rotational dynamics of proteins in nanochannels: role of solvent's local viscosity.

Viscosity variation of solvent in local regions near a solid surface, be it a biological surface of a protein or an engineered surface of a nanoconfinement, is a direct consequence of intermolecular interactions between the solid body and the solvent. The current coarse-grained molecular dynamics study takes advantage of this phenomenon to investigate the anomaly in a solvated protein's rotational dynamics confined using a representative solid matrix. The concept of persistence time, the characteristic time of structural reordering in liquids, is used to compute the solvent's local viscosity. With an increase in the degree of confinement, the confining matrix significantly influences the solvent molecule's local viscosity present in the protein hydration layer through intermolecular interactions. This effect contributes to the enhanced drag force on protein motion, causing a reduction in the rotational diffusion coefficient. Simulation results suggest that the direct matrix-protein non-bonded interaction is responsible for the occasional jump and discontinuity in orientational motion when the protein is in very tight confinement.

Nanoconfinement Effects on the Kapitza Resistance at Water-CNT Interfaces.

The Kapitza resistance (Rk) at the water-carbon nanotube (CNT) interface, with water on the inside of the nanotube, was investigated using molecular dynamics simulations. We propose a new equilibrium molecular dynamics (EMD) method, also valid in the weak flow regime, to determine the Kapitza resistance in a cylindrical nanoconfinement system where nonequilibrium molecular dynamics (NEMD) methods are not suitable. The proposed method is independent of the correlation time compared to Green-Kubo-based methods, which only work in short correlation time intervals. Rk between the CNT and the confined water strongly depends on the diameter of the nanotube and is found to decrease with an increase in the CNT diameter, the opposite to what is reported in the literature when water is on the outside of the nanotube. Rk is furthermore found to converge to the planar graphene surface value as the number of water molecules per unit surface area approaches the value in the graphene surface and a higher overlap of the vibrational spectrum. A slight increase in Rk with the addition of the number of CNT walls was observed, whereas the chirality and flow do not have any impact.

Variation of momentum accommodation coefficients with pressure drop in a nanochannel.

Accommodation coefficients (ACs) are the phenomenological parameters used to evaluate gas-wall interactions. The gas transport through a finite length nanochannel will confront the variation of properties along the length of the channel. A three-dimensional molecular dynamics simulation has been carried out to examine this streamwise inhomogeneity of flow characteristics in a nanochannel. The rarefaction of the flow to the downstream direction is a crucial behavior in a pressure-driven nanochannel flow. This is manifested as the variation in velocity and temperature along the length of the channel. Subsequently, the interactions between the gas and wall particles will get reduced considerably. Moreover, the characteristics near the wall are examined in detail. A nonhomogeneous behavior in density and velocity profile near the wall is reported. Further, the momentum accommodation coefficient (MAC) in both the tangential and normal directions is examined along the lengthwise sections of the channel. The results show a significant variation of tangential and normal MACs along the length. Further, three channels with different length-to-characteristic dimension (L/H) ratios are considered to investigate the effect of L/H ratio. All three channels are subjected to the same pressure drop along the length. It is observed that the MACs and slip length show distinct behavior for different (L/H) ratios. The work establishes that the variation of MAC along the length of the channel has to be considered in modeling the nano- and microtransport systems.

Kapitza resistance at water-graphene interfaces.

Heat transfer across fluid-solid interfaces in nanoconfinement has received significant attention due to its relevance in nanoscale systems. In this study, we investigate the Kapitza resistance at the water-graphene interface with the help of classical molecular dynamics simulation techniques in conjunction with our recently proposed equilibrium molecular dynamics (EMD) method [S. Alosious et al., J. Chem. Phys. 151, 194502 (2019)]. The size effect of the Kapitza resistance on different factors such as the number of graphene layers, the cross-sectional area, and the width of the water block was studied. The Kapitza resistance decreases slightly with an increase in the number of layers, while the influence of the cross-sectional area and the width of the water block is negligible. The variation in the Kapitza resistance as a function of the number of graphene layers is attributed to the large phonon mean free path along the graphene cross-plane. An optimum water-graphene system, which is independent of size effects, was selected, and the same was used to determine the Kapitza resistance using the predicted EMD method. The values obtained from both the EMD and the non-equilibrium molecular dynamics (NEMD) methods were compared for different potentials and water models, and the results are shown to be in good agreement. Our method allows us to compute the Kapitza resistance using EMD simulations, which obviates the need to create a large temperature gradient required for the NEMD method.

The effect of temperature on water desalination through two-dimensional nanopores.

Two-dimensional (2D) materials such as graphene, molybdenum sulfide, and hexagonal boron nitride are widely studied for separation applications such as water desalination. Desalination across such 2D nanoporous membranes is largely influenced by the bulk transport properties of water, which are, in turn, sensitive to the operating temperature. However, there have been no studies on the effect of temperature on desalination through 2D nanopores. We investigated water desalination through hydrogen functionalized graphene nanopores of varying pore areas at temperatures 275.0 K, 300.0 K, 325.0 K, and 350.0 K. The water flux showed a direct relation with the diffusion coefficient and an inverse relation with the hydrogen-bond lifetime. As a direct consequence, the water flux was found to be related to the temperature as per the Arrhenius equation, similar to an activated process. The results from the present study improve the understanding on water and ion permeation across nanoporous 2D materials at different temperatures. Furthermore, the present investigation suggests a kinetic model, which can predict the water and ion permeation based on the characteristics of the nanopore.

Non-isothermal flow of an electrolyte in a charged nanochannel.

Electrokinetic flows are generally analyzed, assuming isothermal conditions even though such situations are hard to be achieved in practice. In this paper, the flow of a symmetric electrolyte in a charged nanochannel subjected to an axial temperature gradient is investigated using molecular dynamics simulations. We analyze the relative contribution of the Soret effect, the thermoelectric effect, and the double layer potential in the electrical double layer for various surface charges and temperature gradients. We find the flow driven by thermal gradient is analogous to electroosmotic flow. The thermophoretic motion of the electrolyte is significant for negative surface charge than the positive surface charge. The vibrational spectrum of graphene is calculated to delineate the effect of the surface charge polarity on the observed thermophoretic motion of the electrolyte. A unique structure of interfacial water layer is observed for the positive and negative surface charges. We attribute the presence of these structures to the differences in water-carbon interactions existing for various surface charge polarity. For an applied thermal gradient in the range 2.6 K nm-1 to 8 K nm-1, we observe a continuous net flow with average velocities reaching up to 9.4 m s-1 inside the channel for a negative surface charge of -0.101 C m-2. The results indicate that in a charged graphene-based nanochannel, temperature gradients can be employed to induce streaming current, depending on the relative influence of the Soret effect and the double layer potential.

Thermal conductivity of graphene under biaxial strain: an analysis of spectral phonon properties.

Thermal transport in graphene is strongly influenced by strain. We investigate the influence of biaxial tensile strain on the thermal conductivity of zigzag and armchair graphene (AG and ZG) using non-equilibrium molecular dynamics simulations (NEMD). We observe that the thermal conductivity is significantly reduced under strain with a maximum reduction obtained at equi-biaxial strain. It is interesting to note that the high lateral to longitudinal strain ratios reduce the negative impact of strain on the thermal conductivity of AG and ZG. The in-plane acoustic modes are found to be the major heat carriers in unstrained graphene but are severely softened due to strain, and hence, their contribution to the conductivity drops down significantly. Strain alleviates the out-of-plane fluctuations in graphene and the group velocity of the out-of-plane acoustic mode (ZA) increases due to the linearisation of its dispersion relation. These factors result in the dominance of ZA mode in the thermal transport of strained graphene. Significant increase in the size dependence of the thermal conductivity of strained graphene is observed, which is attributed to the long-wavelength ZA phonons. The discrepancies between the results of BTE studies and NEMD are also discussed. This study suggests that biaxial strain can be an effective method to tune the thermal transport in graphene. Our findings can lead to better phonon engineering of graphene for various nanoscale applications.

Phonon coupling induced thermophoresis of water confined in a carbon nanotube.

The controlled transport of water through nanoscale devices is an important requirement in the design and development of various nanofluidic systems. Molecular dynamics simulations are performed to investigate the phonon coupling induced thermophoretic transport of water through a carbon nanotube (CNT). Phonon coupling is believed to have a significant role in the transport of heat at the liquid-solid interface. The thermally induced vibrational modes of water-filled and empty CNTs are examined at various thermal gradients. The spatial asymmetry along the length of a CNT due to the imposed thermal gradient contributes to the diffusion enhancement of water confined in the CNT, but does not have a strong correlation with the applied thermal gradient. Analysis shows that the vibrational modes present in the center-of-mass oscillations of CNTs do not play any significant role in the development of the thermophoretic force on water. The low-frequency phonon vibrational modes of CNTs are suppressed due to the phonon coupling between water and the CNT. Also, we observed that the spectral heat current across the water-CNT interface dominates at frequencies below 5 THz, which is the same frequency range as radial breathing modes observed in the vibrational spectrum of CNT. This observation leads us to the conclusion that the coupling of radial breathing phonon modes contributes significantly to thermophoresis. This study substantiates the existence of phonon coupling at the water-CNT interface and quantifies the accumulated heat transfer across the interface.