Research

Thermoelectric materials convert temperature differences directly into electricity and offer a compact, solid-state route for waste-heat recovery, cooling, sensing, and energy generation. At Morgan State University, I contribute to a Department of Defense-supported program developing cost-effective and environmentally responsible thermoelectric materials. The project connects nanoscale structural characterization, materials synthesis, and computational modeling to understand how microscopic structure controls heat transport and energy-conversion performance.

My current work uses molecular dynamics and reverse non-equilibrium molecular dynamics to investigate thermal conductivity in polycrystalline solids. Periodic three-dimensional microstructures are generated by Voronoi tessellation with controlled grain number, grain size, seed distribution, and grain-boundary topology. By comparing randomly and uniformly seeded structures, the simulations separate the effects of mean grain size, grain-size distribution, boundary density, and microstructural stability on lattice-mediated heat transport.

Polycrystalline models generally conduct less heat than comparable single crystals because grain boundaries scatter atomic vibrations. The simulations further show that thermal conductivity depends not only on average grain size, but also on the organization and thermal evolution of the grain-boundary network. This atomistic framework provides design guidance for lowering lattice thermal conductivity and improving thermoelectric performance through microstructural engineering.

Selected publications

1. Xu, R. G., Chen, S., Wang, C., Shahadat, M. R. B., Chen, Y., Lan, Y., and Li, Z. Microstructural Control of Thermal Conductivity in Polycrystalline Argon: A Molecular Dynamics Study. Manuscript in preparation (2026).

Mechano-medicine connects mechanobiology with biomedicine to understand, prevent, and treat diseases whose progression is regulated by physical forces. My research focuses on the interface of molecular biomechanics, vascular biology, and computational medicine, asking how force changes receptor binding, protein conformation, cell adhesion, and mechanosignaling in platelets, endothelial cells, and cancer cells.

I combine molecular dynamics, Monte Carlo simulation, continuum blood-flow modeling, kinetic models, and microfluidic experiments to study receptor-mediated mechanosensing. Current topics include force-regulated GPIbalpha-von Willebrand factor binding, integrin conformational changes and signaling, shear-induced platelet activation, and the mechanisms that distinguish physiological hemostasis from pathological arterial thrombosis.

This multiscale framework links molecular events to vascular function and patient-level phenotypes. It also supports the development of high-throughput microfluidic platforms for thrombus profiling, disease diagnosis, drug screening, and mechanobiology-inspired therapeutic strategies for thrombosis, vascular dysfunction, and cancer metastasis.

Selected publications

1. Din, M., et al., including Xu, R. G. Multi-parametric Thrombus Profiling Microfluidics Detects Intensified Biomechanical Thrombogenesis Associated with Hypertension and Aging. Nature Communications 15, 9067 (2024).
2. O. C. Owegie, R.-G. Xu, Q. P. Kennedy, T. Penubothu, I. H. Zirena, O. Levy, Q. Q. Luu, Y. Chen, P. Davizon-Castillo, Y. Chen, and M. Yang. Molecular Mechanisms of Protein Disulfide Isomerase Antagonism by Punicalagin. Biochemical and Biophysical Research Communications, 154462 (2026).
3. Sun, S., Chen, Y., Xu, R. G., Zhang, H., Mostafa, F., and Liu, L. From Generalist to Specialist: Evolution of PS2 alpha-Integrins and Implications for Drug Targeting. bioRxiv 2026.04.29.721650 (2026).
4. Torki, M. E., Liu, F., Xu, R. G., Chen, Y., Fredberg, J., and Chen, Z. Bridging the Gap in Cancer Cell Behavior Against Matrix Stiffening: Insights from a Trizonal Model. eLife 14 (2025).
5. Sutlive, J., Liu, B. S., Kwan, S. A., Pan, J. M., Gou, K., Xu, R. G., Ali, A. B., Khalil, H. A., et al. Buckling Forces and the Wavy Folds between Pleural Epithelial Cells. BioSystems 240, 105216 (2024).

Improving the reliability and efficiency of lubrication for machines ranging from automotive engines and wind turbines to nanoscale devices requires a fundamental understanding of the phase behaviors and shear properties of complex base fluids under nanoconfinement. Industrial and environmental demands for lower fuel consumption and pollution are driving the development of multifunctional lubricants and additives with less complex formulations. From a materials perspective, hydrocarbon fluids confined between two surfaces to nanometer-scale thickness are also of fundamental interest in surface and interfacial science. Surface-force experiments have found oscillatory normal force-distance profiles within approximately seven to eight molecular diameters, while shearing nanoconfined films often produces stick-slip friction characteristic of a solid-like response.

We developed a liquid-vapor molecular dynamics (LVMD) simulation method to investigate these phenomena. Our results show that simple hydrocarbon liquids such as cyclohexane undergo a liquidlike-to-solidlike phase transition under normal compression. Under shear, slip occurs at the solid-lubricant interfaces while the solidified film structure remains stable through repeated stick-slip cycles. We also study hybrid lubricants with silicate-based core structures, including octamethylcyclotetrasiloxane (OMCTS) and its derivatives, and develop advanced molecular models and DFT-based force-field parameters for these confined organic fluids.

Selected publications

1. Xu, R. G. and Leng, Y. S. Squeezing and Stick-Slip Friction Behaviors of Lubricants in Boundary Lubrication. Proceedings of the National Academy of Sciences 115, 6560-6565 (2018).
2. Xu, R. G., Xiang, Y., Papanikolaou, S., and Leng, Y. S. On the Shear Dilation of Polycrystalline Lubricant Films in Boundary Lubricated Contacts. Journal of Chemical Physics 152, 104708 (2020).
3. Xu, R. G. and Leng, Y. S. Contact Stiffness and Damping of Liquid Films in Dynamic Atomic Force Microscope. Journal of Chemical Physics 144, 154702 (2016).
4. Xu, R. G., Xiang, Y., and Leng, Y. S. Computational Simulations of Solvation Force and Squeezing Out of Dodecane Chain Molecules in an Atomic Force Microscope. Journal of Chemical Physics 147, 054706 (2017).
5. Xu, R. G. and Leng, Y. S. Solvation Force Simulations in Atomic Force Microscopy. Journal of Chemical Physics 140, 214702 (2014).
6. Xu, R. G., Zhang, G., Liu, T., Xiang, Y., and Leng, Y. S. On the Nucleation Rate of Confinement-Induced Liquidlike-to-Solidlike Phase Transitions. Lubricants 12, 420 (2024).
7. Xu, R. G., Xiang, Y., Zhang, G., Rao, Q., and Leng, Y. S. Nucleation of Frank Dislocation during the Squeeze-Out Process in Boundary Lubrication: A Molecular Dynamics Study. Materials 15, 997 (2022).
8. Xu, R. G., Rao, Q., Xiang, Y., Bian, M., and Leng, Y. S. Computational Simulations of Nanoconfined Argon Film through Adsorption-Desorption in a Uniform Slit Pore. Coatings 11, 177 (2021).

Friction occurs at the contact between two sliding surfaces. Nanoscale and atomic-scale friction involve fundamental science at unlubricated sliding interfaces and form a multidisciplinary field spanning physics, chemistry, solid mechanics, materials science, and nanomanufacturing. Understanding friction dynamics, particularly stick-slip motion and energy dissipation at nanoscale contacts, is essential for preventing surface failure and maintaining the function of nanodevices and nanosystems. Molecular dynamics simulations can be accelerated using community software such as the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS).

We develop advanced friction models and algorithms to understand how stick-slip signals are measured in atomic force microscopy experiments. Benchmark systems include polycrystalline platinum and amorphous silica tips sliding on Au(111). Our goal is to develop cyberinfrastructure that enables researchers to study nanoscale friction across different material interfaces and optimize the design of nanoscale machines.

Selected publications

1. Zhang, G., Xu, R. G., Xiang, Y., and Leng, Y. S. Contact Stiffness and Damping in Atomic-Scale Friction: An Approximate Estimation from Molecular Dynamics Simulations. Tribology Letters 72, 24 (2024).
2. Zhou, X., Chen, P., Xu, R. G., Zhang, C., and Zhang, J. Interfacial Friction of vdW Heterostructures Affected by In-Plane Strain. Nanotechnology 34, 015708 (2023).
3. Xu, R. G., Zhang, G., Xiang, Y., and Leng, Y. S. On the Friction Behavior of SiO2 Tip Sliding on the Au(111) Surface: How Does an Amorphous SiO2 Tip Produce Regular Stick-Slip Friction and Friction Duality? Langmuir 39, 6425-6432 (2023).
4. Xu, R. G., Zhang, G., Xiang, Y., Garcia, J., and Leng, Y. S. Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick-Slip Friction? Langmuir 38, 6808-6816 (2022).
5. Xu, R. G., Xiang, Y., Rao, Q., and Leng, Y. S. On the Asymptotic Expressions of Critical Energy Barrier in the Prandtl-Tomlinson Model. International Journal of Smart and Nano Materials 10, 107-115 (2019).

Cold atmospheric plasma is a nonthermal, partially ionized gas that produces electrons, ions, photons, electric fields, and reactive oxygen and nitrogen species at near-room temperature. My research examines how these plasma-generated agents interact with molecules, liquids, soft matter, and biological interfaces. I combine reactive molecular dynamics, multiphysics modeling, and data-driven analysis to connect plasma chemistry with structural changes in biomolecules and cell-to-cell communication.

In plasma medicine, I am particularly interested in the mechanisms by which radicals modify gap-junction proteins, plasma-activated liquids create persistent chemical and interfacial effects, and plasma-liquid systems can be engineered for biomedical treatment. This broader research direction also includes acoustic cavitation-assisted plasma generation and plasma-assisted ignition and combustion, linking fundamental plasma processes to medical, agricultural, energy, and aerospace applications.

Selected publications

1. Bai, F., Yan, A., Fu, Y., Khan, I., Huang, Y., Cao, F., Martins, G. A., Xu, R. G., Lin, X., et al. Acoustic Cavitation-Assisted Plasma Generation in Liquid Media. Advanced Materials Technologies 10, e00719 (2025).
2. Li, M., Wang, Z., Xu, R. G., Zhang, X., Chen, Z., and Wang, Q. Advances in Plasma-Assisted Ignition and Combustion for Combustors of Aerospace Engines. Aerospace Science and Technology 117, 106952 (2021).
3. Chen, Z., Xu, R. G., Chen, P., and Wang, Q. Potential Agricultural and Biomedical Applications of Cold Atmospheric Plasma-Activated Liquids with Self-Organized Patterns Formed at the Interface. IEEE Transactions on Plasma Science 48, 3455-3471 (2020).
4. Xu, R. G., Chen, Z., Keidar, M., and Leng, Y. S. The Impact of Radicals in Cold Atmospheric Plasma on the Structural Modification of Gap Junction: A Reactive Molecular Dynamics Study. International Journal of Smart and Nano Materials 10, 144-155 (2019).

The mechanical behavior of aqueous films under confinement is important in materials science, biology, and engineering. Interactions between cell membranes, water transport through ion channels, protein folding, biolubrication, and membrane fouling all depend on how water interacts with hydrophilic and hydrophobic groups in complex chemical environments. Hydrated metal ions can be especially important because their structured hydration shells influence short-range forces and molecular organization under nanoconfinement.

We use liquid-vapor molecular dynamics simulations to study repulsive hydration forces in electrolyte solutions, hydrophobic interactions in water, and membrane-foulant binding. These studies show how strongly hydrated ions can support mechanical load and form ionic bridges between polyamide membranes and alginate foulants. They also examine how surface modification with polyethylene glycol and polyzwitterion coatings changes interfacial hydration, polymer conformation, and foulant adhesion, providing molecular guidance for designing water-treatment membranes with improved antifouling performance.

Selected publications

1. Xiang, Y., Lu, L., Luo, Y., R.-G. Xu, Zeng, G., and Leng, Y. S. Understanding the Termination Effect of Ti₃C₂Tₓ MXene Membrane on Water Structure and Interaction with Alginate Foulants: A Molecular Dynamics Study. Langmuir 41, 975 (2025).
2. Xiang, Y., Xu, R. G., and Leng, Y. S. How Alginate Monomers Contribute to Organic Fouling on Polyamide Membrane Surfaces? Journal of Membrane Science 643, 120078 (2022).
3. Xiang, Y., Xu, R. G., and Leng, Y. S. Molecular Simulations of Zwitterion/Cation Interactions and Ion Effect on Hydration Behavior of a Zwitterion Brush. Langmuir 36, 7648-7657 (2020).
4. Xiang, Y., R.-G. Xu, and Leng, Y. S. Molecular Dynamics Simulations of the Hydration Behaviors of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment. Langmuir 34, 2245 (2018).
5. Xiang, Y., R.-G. Xu, and Leng, Y. S. Molecular Dynamics Simulations of a Poly(ethylene glycol)-Grafted Polyamide Membrane and Its Interaction with a Calcium Alginate Gel. Langmuir 32, 4424 (2016).

Reactive Ni/Al nanolaminates store chemical energy in their numerous nanoscale interfaces and release it through rapid atomic interdiffusion and intermetallic formation. My research investigates how transport evolves from the motion of individual atoms to continuum-scale concentration fields, with particular attention to the mechanisms and rates that control interfacial mixing in metallic multilayers.

This work combines molecular dynamics, continuum diffusion modeling, and Bayesian inference to estimate temperature-dependent atomic diffusivity and quantify uncertainty in parameters derived from atomistic simulations. By linking atomic trajectories with predictive continuum descriptions, the multiscale framework helps distinguish diffusion, dissolution, and phase-formation processes and supports the design of reactive multilayers for joining, energetic materials, and controlled thermal applications.

Selected publications

1. Xu, R. G., Falk, M. L., and Weihs, T. P. Interdiffusion of Ni-Al Multilayers: A Continuum and Molecular Dynamics Study. Journal of Applied Physics 114, 163511 (2013).
2. Rizzi, F., Salloum, M., Marzouk, Y. M., Xu, R. G., Falk, M. L., Weihs, T. P., Fritz, G., and Knio, O. M. Bayesian Inference of Atomic Diffusivity in a Binary Ni/Al System Based on Molecular Dynamics. Multiscale Modeling & Simulation 9, 486-512 (2011).