@article{shi_smith_santiso_gubbins_2023, title={A perspective on the microscopic pressure (stress) tensor: History, current understanding, and future challenges}, volume={158}, ISSN={["1089-7690"]}, DOI={10.1063/5.0132487}, abstractNote={The pressure tensor (equivalent to the negative stress tensor) at both microscopic and macroscopic levels is fundamental to many aspects of engineering and science, including fluid dynamics, solid mechanics, biophysics, and thermodynamics. In this perspective paper, we review methods to calculate the microscopic pressure tensor. Connections between different pressure forms for equilibrium and non-equilibrium systems are established. We also point out several challenges in the field, including the historical controversies over the definition of the microscopic pressure tensor; the difficulties with many-body and long-range potentials; the insufficiency of software and computational tools; and the lack of experimental routes to probe the pressure tensor at the nanoscale. Possible future directions are suggested.}, number={4}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Shi, Kaihang and Smith, Edward R. and Santiso, Erik E. and Gubbins, Keith E.}, year={2023}, month={Jan} }
@article{shi_santiso_gubbins_2021, title={Can we define a unique microscopic pressure in inhomogeneous fluids?}, volume={154}, ISSN={["1089-7690"]}, DOI={10.1063/5.0044487}, abstractNote={The estimation of a microscopic pressure tensor in an adsorbed thin film on a planar surface remains a challenge in both experiment and theory. While the normal pressure is well-defined for a planar surface, the tangential pressure at a point is not uniquely defined at the nanoscale. We report a new method that allows us to calculate the local pressure tensor and its spatial integral using an arbitrary contour definition of the “virial-route” local pressure tensor. We show that by integrating the local tangential pressure over a small region of space, roughly the range of the intermolecular forces, it is possible to define a coarse-grained tangential pressure that appears to be unique and free from ambiguities in the definition of the local pressure tensor. We support our argument by presenting the results for more than ten types of contour definitions of the local pressure tensor. By defining the coarse-grained tangential pressure, we can also find the effective thickness of the adsorbed layer and, in the case of a porous material, the statistical pore width. The coarse-grained in-layer and in-pore tangential pressures are determined for Lennard-Jones argon adsorbed in realistic carbon slit pores, providing a better understanding of the pressure enhancement for strongly wetting systems.}, number={8}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Shi, Kaihang and Santiso, Erik E. and Gubbins, Keith E.}, year={2021}, month={Feb} }
@article{shi_santiso_gubbins_2020, title={Conformal Sites Theory for Adsorbed Films on Energetically Heterogeneous Surfaces}, volume={36}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.9b03633}, abstractNote={We present a conformal sites theory for a solid substrate whose surface is both geometrically and energetically heterogeneous and that interacts with an adsorbed film. The theory is based on a perturbation expansion for the grand potential of a real system with a rough surface about that of a reference system with an ideal reference surface, thus mapping the real system onto a much simpler interfacial system. The expansion is in powers of the intermolecular potential parameters, and leads to mixing rules for the potential parameters of the reference system. Grand canonical Monte Carlo simulations for the adsorption of argon at 87.3 K, carbon dioxide at 273 K, and water vapor at 298 K on heterogeneous carbon surfaces are investigated to explore the limits of applicability of the theory. Simulation results indicate that the theory works well with typical asymmetry of the potential parameters in the force field. However, care should be taken when applying the theory to strongly associating fluids and in the low-pressure region where the active surface sites play an important role. The conformal sites theory can be used to predict the adsorption properties and to characterize the solid substrate by taking advantage of the corresponding states principle. Other possible applications are also discussed.}, number={7}, journal={LANGMUIR}, author={Shi, Kaihang and Santiso, Erik E. and Gubbins, Keith E.}, year={2020}, month={Feb}, pages={1822–1838} }
@article{shi_shen_santiso_gubbins_2020, title={Microscopic Pressure Tensor in Cylindrical Geometry: Pressure of Water in a Carbon Nanotube}, volume={16}, ISSN={["1549-9626"]}, DOI={10.1021/acs.jctc.0c00607}, abstractNote={The microscopic pressure tensor plays an important role in understanding the mechanical stability, transport, and high-pressure phenomena of confined phases. The lack of an exact formulation to account for the long-range Coulombic contribution to the local pressure tensor in cylindrical geometries prevents the characterization of molecular fluids confined in cylindrical pores. To address this problem, we first derive the local cylindrical pressure tensor for Lennard-Jones fluids based on the Harasima (H) definition, which is expected to be compatible with the Ewald summation method. The test of the H-definition pressure equations in a homogeneous system shows that the radial and azimuthal pressure have unphysical radial dependence near the origin, while the axial pressure gives physically meaningful values. We propose an alternative contour definition that is more appropriate for cylindrical geometry and show that it leads to physically realistic results for all three pressure tensor components. With this definition, the radial and azimuthal pressures are of Irving–Kirkwood (IK) type, and the axial pressure is of Harasima type. Because of the practical interest in the axial pressure, we develop a Harasima/Ewald (H/E) method for calculating the long-range Coulombic contribution to the local axial pressure for rigid molecules. As an application, the axial pressure profile of water inside and outside a (20, 20) single-wall carbon nanotube is determined. The H/E method is compared to the IK method, which assumes a spherically truncated Coulombic potential. Detailed analysis of the pressure profile by both methods shows that the water confined in the nanotube is in a stretched state overall in the axial direction.}, number={9}, journal={JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, author={Shi, Kaihang and Shen, Yifan and Santiso, Erik E. and Gubbins, Keith E.}, year={2020}, month={Sep}, pages={5548–5561} }
@article{schneible_shi_young_ramesh_he_dowdey_dubnansky_libya_gao_santiso_et al._2020, title={Modified gaphene oxide (GO) particles in peptide hydrogels: a hybrid system enabling scheduled delivery of synergistic combinations of chemotherapeutics}, volume={8}, ISSN={["2050-7518"]}, DOI={10.1039/d0tb00064g}, abstractNote={Composite material enabling the delivery of synergistic combination of doxorubicin and gemcitabine against breast cancer with molar and kinetic precision.}, number={17}, journal={JOURNAL OF MATERIALS CHEMISTRY B}, author={Schneible, John D. and Shi, Kaihang and Young, Ashlyn T. and Ramesh, Srivatsan and He, Nanfei and Dowdey, Clay E. and Dubnansky, Jean Marie and Libya, Radina L. and Gao, Wei and Santiso, Erik and et al.}, year={2020}, month={May}, pages={3852–3868} }
@article{cutright_brotherton_alexander_harris_shi_khan_genzer_menegatti_2020, title={Packing density, homogeneity, and regularity: Quantitative correlations between topology and thermoresponsive morphology of PNIPAM-co-PAA microgel coatings}, volume={508}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2019.145129}, abstractNote={This study investigates the formation of monolayers of microgel particles comprising poly[(N-isopropylacrylamide)-co-(acrylic acid)] on solid substrates, their surface morphology, and stimuli-responsiveness. Crosslinked microgels with different chemical composition were produced to show a broad range of responses in hydrodynamic radius and degree of self-assembly with temperature and pH. Microgels were deposited on silicon wafers primed with a bilayer of poly(octadecene-alt-maleic anhydride) and polyethyleneimine by either incubation or spin coating of aqueous suspension of microgels at different temperature and pH. The characterization of the microgel-coated wafers led to the identification of three metrics describing microgel arrangement: density (ρ); heterogeneity (H), which correlates strongly with ρ and depends on deposition temperature and pH with statistical significance, but not on microgel composition; and packing efficiency (PE), which portrays the regularity of microgel arrangement and exhibits no correlation with ρ nor H. The values of ρ, H, and PE calculated for in silico models of microgel coatings confirmed that these three metrics portray distinct characteristics of surface topology. Finally, profilometry analysis showed that microgel coatings respond to thermal stimuli with sensible variations in surface roughness; notably, the thermal variation of roughness correlates strongly with ρ and H, and to a lesser extent with PE.}, journal={APPLIED SURFACE SCIENCE}, author={Cutright, Camden and Brotherton, Zach and Alexander, Landon and Harris, Jacob and Shi, Kaihang and Khan, Saad and Genzer, Jan and Menegatti, Stefano}, year={2020}, month={Apr} }
@article{long_palmer_coasne_shi_sliwinska-bartkowiak_gubbins_2020, title={Reply to the 'Comment on "Pressure enhancement in carbon nanopores: a major confinement effect"' by D. van Dijk, Phys. Chem. Chem. Phys., 2020, 22, DOI: 10.1039/C9CP02890K}, volume={22}, ISSN={["1463-9084"]}, DOI={10.1039/c9cp04289j}, abstractNote={By calculating the unique effective tangential pressure and discussing recent evidence from experiment and simulations, we show that the high pressure and strong compression in adsorbed layers for wetting systems on carbon are significant.}, number={17}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Long, Yun and Palmer, Jeremy C. and Coasne, Benoit and Shi, Kaihang and Sliwinska-Bartkowiak, Malgorzata and Gubbins, Keith E.}, year={2020}, month={May}, pages={9826–9830} }
@article{hijes_shi_noya_santiso_gubbins_sanz_vega_2020, title={The Young-Laplace equation for a solid-liquid interface}, volume={153}, ISSN={["1089-7690"]}, DOI={10.1063/5.0032602}, abstractNote={The application of the Young–Laplace equation to a solid–liquid interface is considered. Computer simulations show that the pressure inside a solid cluster of hard spheres is smaller than the external pressure of the liquid (both for small and large clusters). This would suggest a negative value for the interfacial free energy. We show that in a Gibbsian description of the thermodynamics of a curved solid–liquid interface in equilibrium, the choice of the thermodynamic (rather than mechanical) pressure is required, as suggested by Tolman for the liquid–gas scenario. With this definition, the interfacial free energy is positive, and the values obtained are in excellent agreement with previous results from nucleation studies. Although, for a curved fluid–fluid interface, there is no distinction between mechanical and thermal pressures (for a sufficiently large inner phase), in the solid–liquid interface, they do not coincide, as hypothesized by Gibbs.}, number={19}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Hijes, P. and Shi, K. and Noya, E. G. and Santiso, E. E. and Gubbins, K. E. and Sanz, E. and Vega, C.}, year={2020}, month={Nov} }
@article{gubbins_gu_huang_long_mansell_santiso_shi_sliwinska-bartkowiak_srivastava_2018, title={Surface-Driven High-Pressure Processing}, volume={4}, ISSN={["2096-0026"]}, DOI={10.1016/j.eng.2018.05.004}, abstractNote={The application of high pressure favors many chemical processes, providing higher yields or improved rates in chemical reactions and improved solvent power in separation processes, and allowing activation barriers to be overcome through the increase in molecular energy and molecular collision rates. High pressures—up to millions of bars using diamond anvil cells—can be achieved in the laboratory, and lead to many new routes for chemical synthesis and the synthesis of new materials with desirable thermodynamic, transport, and electronic properties. On the industrial scale, however, high-pressure processing is currently limited by the cost of compression and by materials limitations, so that few industrial processes are carried out at pressures above 25 MPa. An alternative approach to high-pressure processing is proposed here, in which very high local pressures are generated using the surface-driven interactions from a solid substrate. Recent experiments and molecular simulations show that such interactions can lead to local pressures as high as tens of thousands of bars (1 bar = 1 × 105 Pa), and even millions of bars in some cases. Since the active high-pressure processing zone is inhomogeneous, the pressure is different in different directions. In many cases, it is the pressure in the direction parallel to the surface of the substrate (the tangential pressure) that is most greatly enhanced. This pressure is exerted on the molecules to be processed, but not on the solid substrate or the containing vessel. Current knowledge of such pressure enhancement is reviewed, and the possibility of an alternative route to high-pressure processing based on surface-driven forces is discussed. Such surface-driven high-pressure processing would have the advantage of achieving much higher pressures than are possible with traditional bulk-phase processing, since it eliminates the need for mechanical compression. Moreover, no increased pressure is exerted on the containing vessel for the process, thus eliminating concerns about materials failure.}, number={3}, journal={ENGINEERING}, author={Gubbins, Keith E. and Gu, Kai and Huang, Liangliang and Long, Yun and Mansell, J. Matthew and Santiso, Erik E. and Shi, Kaihang and Sliwinska-Bartkowiak, Malgorzata and Srivastava, Deepti}, year={2018}, month={Jun}, pages={311–320} }