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My research focuses on advancing the fundamental understanding of fluid–nanomaterial interfaces through a combination of molecular simulations and theoretical modeling. By uncovering the underlying physics at the nanoscale, my research aims to drive transformative innovations in membrane design for sustainable water and energy applications. Below, I highlight key aspects of my past and ongoing research efforts.
Fundamental Understanding of Fluid/Nanomaterial Interfaces
Fundamental Understanding of Fluid/Nanomaterial Interfaces
In systems involving polar fluids such as water and charged species like salt ions interacting with solids and nanomaterials in particular, the electric fields generated by these solutes can profoundly alter the charge distribution within the solid. These phenomena, known as electronic polarization, are inherently many-body in nature due to the vectorial nature of the electric fields. My research employs a multiscale framework that integrates quantum chemical calculations with classical molecular dynamics simulations to capture these complex interactions, particularly at electrolyte/nanomaterial interfaces. The videos below show representative results of induced charge distributions on graphene, modeled using Drude particles (DPs) and Drude cores (DCs), which are fully coupled through a dipole field tensor to reproduce both the in-plane and out-of-plane polarizabilities of the material. For clarity, only the motion of the DP/DC pair oriented toward the external species is shown. Panels (a), (b), and (c) illustrate the polarization response in the presence of a negatively charged salt ion, a water molecule, and vacuum (i.e., no electric field), respectively.
In systems involving polar fluids such as water and charged species like salt ions interacting with solids and nanomaterials in particular, the electric fields generated by these solutes can profoundly alter the charge distribution within the solid. These phenomena, known as electronic polarization, are inherently many-body in nature due to the vectorial nature of the electric fields. My research employs a multiscale framework that integrates quantum chemical calculations with classical molecular dynamics simulations to capture these complex interactions, particularly at electrolyte/nanomaterial interfaces. The videos below show representative results of induced charge distributions on graphene, modeled using Drude particles (DPs) and Drude cores (DCs), which are fully coupled through a dipole field tensor to reproduce both the in-plane and out-of-plane polarizabilities of the material. For clarity, only the motion of the DP/DC pair oriented toward the external species is shown. Panels (a), (b), and (c) illustrate the polarization response in the presence of a negatively charged salt ion, a water molecule, and vacuum (i.e., no electric field), respectively.
Additional details can be found in this publication: Langmuir 2021, 37 (2), 722–733
Wetting of Graphene and Other Emerging 2D Nanomaterials
Wetting of Graphene and Other Emerging 2D Nanomaterials
The wetting behavior of graphene and other emerging two-dimensional (2D) nanomaterials is strongly governed by the strength and nature of fluid–surface interactions. As illustrated below, a single water molecule approaching a graphene layer can exert electric fields exceeding 2 V/nm—strong enough to induce significant polarization within the material. This dynamic redistribution of charge alters local surface energies and plays a key role in determining interfacial behavior.
The wetting behavior of graphene and other emerging two-dimensional (2D) nanomaterials is strongly governed by the strength and nature of fluid–surface interactions. As illustrated below, a single water molecule approaching a graphene layer can exert electric fields exceeding 2 V/nm—strong enough to induce significant polarization within the material. This dynamic redistribution of charge alters local surface energies and plays a key role in determining interfacial behavior.
Our recent findings reveal that these polarization effects give rise to a strong coupling between the electric fields exerted by interfacial water molecules and their orientations, as the polarization energy is highly sensitive to water alignment. Notably, the water-graphene polarization interaction contributes more significantly to the interfacial entropy of water than dispersion interactions alone. When polarization is modeled explicitly using Drude oscillators, the energetic gain from polarization is substantially offset by a compensating increase in entropy—an effect not captured by traditional Lennard-Jones-based models, which underestimate water contact angles by nearly 23°.
Our recent findings reveal that these polarization effects give rise to a strong coupling between the electric fields exerted by interfacial water molecules and their orientations, as the polarization energy is highly sensitive to water alignment. Notably, the water-graphene polarization interaction contributes more significantly to the interfacial entropy of water than dispersion interactions alone. When polarization is modeled explicitly using Drude oscillators, the energetic gain from polarization is substantially offset by a compensating increase in entropy—an effect not captured by traditional Lennard-Jones-based models, which underestimate water contact angles by nearly 23°.
We further validated these findings for hexagonal boron nitride, underscoring the generality of this behavior across 2D materials. These insights highlight the importance of accounting for polarization effects when modeling nanomaterial interfaces, with direct implications for advancing selective membrane design, nanofluidic transport, and energy harvesting technologies.
We further validated these findings for hexagonal boron nitride, underscoring the generality of this behavior across 2D materials. These insights highlight the importance of accounting for polarization effects when modeling nanomaterial interfaces, with direct implications for advancing selective membrane design, nanofluidic transport, and energy harvesting technologies.
Additional details can be found in this publication: J. Phys. Chem. C 2017, 121 (50), 28166–28179
Molecular Mechanism of Salt Ion Adsorption at Planar Water/Nanomaterial Interfaces
Molecular Mechanism of Salt Ion Adsorption at Planar Water/Nanomaterial Interfaces
Salt ion interactions with water/solid interfaces are fundamental to a wide variety of natural and technological processes. Despite their ubiquity, a long-standing question remains: how do the electric fields generated by charged salt ions and interfacial water molecules influence electronic polarization within the solid, and how does this polarization, in turn, govern the ion adsorption behavior?
Salt ion interactions with water/solid interfaces are fundamental to a wide variety of natural and technological processes. Despite their ubiquity, a long-standing question remains: how do the electric fields generated by charged salt ions and interfacial water molecules influence electronic polarization within the solid, and how does this polarization, in turn, govern the ion adsorption behavior?
To address this, our research employs advanced polarizable force fields trained from quantum chemical calculations to perform fully all-atomistic molecular dynamics simulations of ion adsorption at the water/graphene interface. As illustrated in the figure below, we find that while ions can strongly polarize the graphene surface in vacuum, leading to ion binding, the presence of water drastically alters this behavior. In particular, we demonstrate that interfacial water molecules screen more than 85% of the polarization energy induced by the ion on the graphene surface which greatly diminishes the ion–solid interaction strength.
To address this, our research employs advanced polarizable force fields trained from quantum chemical calculations to perform fully all-atomistic molecular dynamics simulations of ion adsorption at the water/graphene interface. As illustrated in the figure below, we find that while ions can strongly polarize the graphene surface in vacuum, leading to ion binding, the presence of water drastically alters this behavior. In particular, we demonstrate that interfacial water molecules screen more than 85% of the polarization energy induced by the ion on the graphene surface which greatly diminishes the ion–solid interaction strength.
Our findings indicate that, under aqueous conditions, the specificity of ion adsorption is dictated largely by the interplay between the ion–water and water–water interactions, rather than the direct ion–solid effects, which is a direct consequence of the effective screening by the water molecules. We further show that this behavior is not unique to graphene but occurs universally across both superhydrophilic and superhydrophobic surfaces. These insights have important implications for engineering water/solid interfaces in applications ranging from water purification to chemical sensing and electrochemical energy storage.
Our findings indicate that, under aqueous conditions, the specificity of ion adsorption is dictated largely by the interplay between the ion–water and water–water interactions, rather than the direct ion–solid effects, which is a direct consequence of the effective screening by the water molecules. We further show that this behavior is not unique to graphene but occurs universally across both superhydrophilic and superhydrophobic surfaces. These insights have important implications for engineering water/solid interfaces in applications ranging from water purification to chemical sensing and electrochemical energy storage.
Water-mediated screening of ion-solid interactions at planar water/solid interfaces
Additional details can be found in this publication: J. Phys. Chem. C 2021, 125 (4), 2666–2679
Molecular Mechanism of Salt Ion Transport under Confinement
Molecular Mechanism of Salt Ion Transport under Confinement
Confinement introduces a powerful handle for molecularly engineering solvation environments in ways not possible in bulk solution. One striking example is the interior of carbon nanotubes (CNTs), cylindrical one-dimensional allotropes of carbon, where the nanoscale geometry enables precise control over water and ion coordination. In ultra-narrow CNTs with diameters around 0.8 nm, water molecules and salt ions are restricted to a single-file arrangement, significantly altering their interaction landscape.
Confinement introduces a powerful handle for molecularly engineering solvation environments in ways not possible in bulk solution. One striking example is the interior of carbon nanotubes (CNTs), cylindrical one-dimensional allotropes of carbon, where the nanoscale geometry enables precise control over water and ion coordination. In ultra-narrow CNTs with diameters around 0.8 nm, water molecules and salt ions are restricted to a single-file arrangement, significantly altering their interaction landscape.
Our research demonstrates that under such confinement, salt ions experience minimal dielectric screening from surrounding water molecules, which is unlike the strong screening observed at planar interfaces. As a result, ions induce substantial electronic polarization in the CNT walls, giving rise to a large ion–CNT polarization energy. This energy plays a critical compensatory role by offsetting the dehydration penalty associated with the loss of hydration shell water molecules during translocation, thereby facilitating the permeation of small ions such as potassium (K⁺) through narrow CNTs.
Our research demonstrates that under such confinement, salt ions experience minimal dielectric screening from surrounding water molecules, which is unlike the strong screening observed at planar interfaces. As a result, ions induce substantial electronic polarization in the CNT walls, giving rise to a large ion–CNT polarization energy. This energy plays a critical compensatory role by offsetting the dehydration penalty associated with the loss of hydration shell water molecules during translocation, thereby facilitating the permeation of small ions such as potassium (K⁺) through narrow CNTs.
Beyond equilibrium properties, our work reveals new transport mechanisms under extreme confinement. In collaboration with Dr. Aleksandr Noy’s group at the Lawrence Livermore National Laboratory (LLNL), we used molecular dynamics simulations, supported by experimental data, to study K⁺ ion transport through 0.8 nm diameter CNT porins (CNTPs) embedded in lipid bilayers. We found that ion diffusion in these confined environments is suppressed by nearly three orders of magnitude compared to the bulk solution. Interestingly, the simulations also reveal a disintegration of the single-file water chain upon the application of an electric field. The resulting K⁺–water clusters were then found to traverse the CNTP at high velocity. Finally, although the individual ion–water clusters were found to obey the classical Nernst–Einstein relation, the overall relation was found to breakdown due to the two distinct mechanisms of diffusion and electromigration. These findings refine our fundamental understanding of ion transport in confined environments and open new directions for membrane design, nanofluidic logic and nanoscale energy technologies.
Beyond equilibrium properties, our work reveals new transport mechanisms under extreme confinement. In collaboration with Dr. Aleksandr Noy’s group at the Lawrence Livermore National Laboratory (LLNL), we used molecular dynamics simulations, supported by experimental data, to study K⁺ ion transport through 0.8 nm diameter CNT porins (CNTPs) embedded in lipid bilayers. We found that ion diffusion in these confined environments is suppressed by nearly three orders of magnitude compared to the bulk solution. Interestingly, the simulations also reveal a disintegration of the single-file water chain upon the application of an electric field. The resulting K⁺–water clusters were then found to traverse the CNTP at high velocity. Finally, although the individual ion–water clusters were found to obey the classical Nernst–Einstein relation, the overall relation was found to breakdown due to the two distinct mechanisms of diffusion and electromigration. These findings refine our fundamental understanding of ion transport in confined environments and open new directions for membrane design, nanofluidic logic and nanoscale energy technologies.
Additional details can be found in this publication: Nature Nanotechnology 2023, 18, 177–183
Electronic Engineering of Channel Properties for Tunable Water Transport
Electronic Engineering of Channel Properties for Tunable Water Transport
Traditionally, fluid flow through pores has been viewed as primarily governed by properties of the fluid, such as its viscosity. This perspective holds for larger conduits, including pipes that transport water or oil, where the well-known "no-slip" boundary condition applies at the walls. However, this understanding changes significantly when considering single digit nanopores, which are so narrow that their dimensions approach the size of individual fluid molecules.
Traditionally, fluid flow through pores has been viewed as primarily governed by properties of the fluid, such as its viscosity. This perspective holds for larger conduits, including pipes that transport water or oil, where the well-known "no-slip" boundary condition applies at the walls. However, this understanding changes significantly when considering single digit nanopores, which are so narrow that their dimensions approach the size of individual fluid molecules.
In a multi-PI collaboration with Dr. Aleksandr Noy (LLNL) and Professor Narayana Aluru (University of Texas, Austin), we investigated how salt ions and water molecules transport through metallic and semiconducting CNTs, which are nearly identical in diameter but differ in their electronic properties. Our simulations revealed that the ion-CNT interactions are intrinsically strong even in semiconducting CNTs, such that ion transport remains relatively insensitive to the CNT band gap value. On the other hand, for water molecules, as the CNT's axial polarizability increases (i.e., in metallic CNTs), the binding energy between water and the CNT wall weakens. This weaker interaction translates to reduced friction at the wall, allowing for the higher water permeability observed in the experiments.
In a multi-PI collaboration with Dr. Aleksandr Noy (LLNL) and Professor Narayana Aluru (University of Texas, Austin), we investigated how salt ions and water molecules transport through metallic and semiconducting CNTs, which are nearly identical in diameter but differ in their electronic properties. Our simulations revealed that the ion-CNT interactions are intrinsically strong even in semiconducting CNTs, such that ion transport remains relatively insensitive to the CNT band gap value. On the other hand, for water molecules, as the CNT's axial polarizability increases (i.e., in metallic CNTs), the binding energy between water and the CNT wall weakens. This weaker interaction translates to reduced friction at the wall, allowing for the higher water permeability observed in the experiments.
These insights establish a strong mechanistic basis for developing chemical sensing technologies, where nanotubes can differentiate fluids based on their polarity. More significantly, they open the door to a new class of electronically tunable synthetic membranes, where the electronic properties such as band gap or polarizability can be engineered to control fluid–wall interactions.
These insights establish a strong mechanistic basis for developing chemical sensing technologies, where nanotubes can differentiate fluids based on their polarity. More significantly, they open the door to a new class of electronically tunable synthetic membranes, where the electronic properties such as band gap or polarizability can be engineered to control fluid–wall interactions.
Schematic highlighting the role of channel electronic properties on water transport
Additional details can be found in this publication: Nature Materials 2024, 23, 1123–1130
Current Research Interests at NUS
Current Research Interests at NUS
At the National University of Singapore (NUS), my research will focus on how fluid confinement gives rise to unique transport and reactive behaviors critical for sustainable water and chemical separations. Bridging the gap between atomic-scale interactions and macroscopic performance metrics like permeability, selectivity, and reactivity remains a key challenge. To address this, I am developing a multiscale modeling strategy that integrates molecular simulations, continuum theories, and physics-informed machine learning. My current efforts span three main thrusts: (1) enabling separations by exploiting confinement-induced changes in electrostatic interactions, rather than relying on traditional boiling-point differences in distillation, (2) building physically grounded machine-learning potentials trained on polarizable and ab initio MD data for rapid screening of next-generation materials, and (3) exploring confined nanomaterials as tunable nanoreactors to study and enhance important chemical reactions. Together, these efforts aim to advance fundamental understanding while driving practical innovations.
At the National University of Singapore (NUS), my research will focus on how fluid confinement gives rise to unique transport and reactive behaviors critical for sustainable water and chemical separations. Bridging the gap between atomic-scale interactions and macroscopic performance metrics like permeability, selectivity, and reactivity remains a key challenge. To address this, I am developing a multiscale modeling strategy that integrates molecular simulations, continuum theories, and physics-informed machine learning. My current efforts span three main thrusts: (1) enabling separations by exploiting confinement-induced changes in electrostatic interactions, rather than relying on traditional boiling-point differences in distillation, (2) building physically grounded machine-learning potentials trained on polarizable and ab initio MD data for rapid screening of next-generation materials, and (3) exploring confined nanomaterials as tunable nanoreactors to study and enhance important chemical reactions. Together, these efforts aim to advance fundamental understanding while driving practical innovations.
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