Anisotropic shear relaxation is an interesting but rarely discussed issue in polymer dynamics under confinement. According to the earlier study of bead spring simulations for an unentangled polymer melt confined between two flat plates, the shear relaxation modulus taken perpendicular to the interface is accelerated by decreasing the distance between plates, whereas the parallel component is unchanged [Abberton et al., Macromolecules, 48, 7631, 2015]. This study observed similar anisotropic shear relaxation for entangled polymer melts under confinement in multi-chain slip-link simulations (primitive chain network simulations) for shear relaxation moduli obtained by the Green-Kubo formula. The analysis demonstrated that the accelerated relaxation in the perpendicular component reflects the Rouse-type constraint release dynamics, for which the coarsening is upper-limited by the geometry. This result suggests a novel mechanism for anisotropic shear relaxation different from the modified chain statistics under confinement considered for unentangled systems unless the obtained relaxation moduli reflect artifacts from the use of the Green-Kubo formula under confinement.

The diffusion of lightweight molecules in cementitious materials is an extremely important phenomenon, as it determines the lifetime and long-term reliability of structures made from cement-based materials, including reinforced concrete structures. Conventionally, the field in cement and concrete has employed a diffusion equation with an effective constant diffusion coefficient to predict the dynamics of the molecule, although this approach cannot treat non-Gaussian diffusion. Recently, the authors initially applied the framework of the fluctuating diffusivity, which can describe the non-Gaussian diffusion in heterogeneous nature, to the diffusion of oxygen molecules in dry cement pastes [Nakai and Ishida, Construction and Building Materials, 407, 133411, 2023]. In addition to oxygen molecules, low molecular gases such as hydrogen, nitrogen, and helium have traditionally been collected as benchmark data for discussing diffusivity in cementitious materials. This paper discusses the non-Gaussian nature of hydrogen and oxygen molecules diffusing in dry cement pastes.

In silica-filled epoxy resins, amorphous silica particles are generally covered with silanol groups on their surfaces, rendering them susceptible to moisture adsorption. Thus, the importance lies in better understanding the aggregation states of epoxy resins on the surface of amorphous silica and the influence of adsorbed water on the silica surface on the curing reaction. Here, we propose a method to generate amorphous silica by inserting oxygen atoms into the Si-Si bonds of amorphous silicon, serving as a model construction for molecular dynamics (MD) simulations. The surface structure of the silica was then extracted, and a mixture of an epoxy base (diglycidyl ether of bisphenol-A, DGEBA) and an amine hardener (4,4’-diaminodiphenyl sulfone, DDS) was placed on top of it. SO₂ groups of DDS strongly adsorbed on the silica surface through hydrogen bonding and remained after curing, although this interaction was weakened due to the reduced degree of freedom caused by cross-linking. On the other hand, DGEBA did not exhibit such a strong interaction with the silica surface. After curing, OH groups generated by the ring-opening of epoxy groups formed slight hydrogen bonds with the silica surface. When a small amount of water was adsorbed onto the silica surface, it diffused slightly into the epoxy resin, enhancing the mobility of the reactants in the system, and consequently accelerating the curing reaction.

We investigated the effect of surface treatment for iron particles in the magnetorheological fluids (MRFs) on rheological properties. Silane coupling treatment with an excess amount of dodecyltrimethoxysilane (DTMS) was conducted for iron particles. MRFs were prepared by dispersing iron particles with and without the silane coupling treatment. MRFs with iron particles washed with n-hexane after the silane coupling treatment was also prepared. The rheological properties of MRFs were measured in the presence and absence of a magnetic field. Without a magnetic field, MRFs with the silane-coupled particles exhibited higher stress than the untreated particles. Under a magnetic field, MRFs with the silane-coupled particles showed lower stress than the untreated analogue. These results imply that the silane coupling treatment enhances the aggregation of particles without a magnetic field but suppresses the cluster formation under the magnetic field. MRFs with the washed iron particles exhibited almost the same rheological properties as that with the untreated particles. Thus we suggest that DTMS is mainly physically absorbed on the surface of iron particles, without forming chemical bonds.

In this short review, we report our recent research about molecular simulation approach for Tribology. First we show the history of molecular dynamics for both rheology and tribology. The branch point of two field was the interest for the solid liquid interface. Then we show molecular dynamic simulations for traction fluid, which is the fluid work under high pressure. Then we show our new approach for simulating multi-scale fluid system such as polymer solution, using multi-physics approach.

This paper explores the application of physics-informed neural networks for solving pulsatile shear-thinning flows in a two-dimensional channel. To identify an optimal model, models of varying implementations of boundary conditions, network sizes, number of training points, activation functions, and loss weights are investigated through case by case studies complemented by Gaussian-processes based Bayesian optimization. The final model demonstrates a high level of agreement with a reference numerical solution, with an error of less than 2%. This result indicates that appropriately trained PINNs can be utilized as a method for simulating transient shear-thinning flows.

For an unentangled four-arm star chain having long-lived associative groups (stickers) at the arm ends, a bead-spring analysis was conducted to examine nonlinear stress relaxation behavior under single-step strain. In the analysis, some stickers were disrupted by the strain to activate very fast intrinsic motion of the star chain, and the disrupted and undisrupted stickers, respectively, were treated as mobile and immobile beads during this fast process. This intrinsic motion leads the chain to average/equilibrate its conformation to an extent allowed by the constraint from the immobile beads, and the disrupted stickers are reformed after the intrinsic motion. Thus, the star chain is affinely deformed by the strain but this deformed conformation is further tuned on the strain-induced sticker disruption followed by the conformational averaging and the sticker reformation. For this tuned initial conformation, the bead-spring analysis of the slow relaxation process (governed by intact/reformed stickers) combined with a free energy analysis suggested that the star chain exhibits time-strain separable damping of its relaxation modulus under moderate strain whereas some instability of uniform deformation (possibly resulting in the yielding of the system) emerges under large strain. These results of the analyses were favorably compared with a preliminary experiment made for a transient gel of end-associative tetra-poly(ethylene glycol) star chains. Furthermore, the analyses were extended to a transient gel of associative linear chains to discuss differences/similarities of the transient gels of star and linear chains having topologically different backbones.

For an unentangled four-arm star chain having long-lived associative groups (stickers) at the arm ends, bead-spring analysis was conducted for the nonlinear stress relaxation under double-step strain. The analysis considered strain-induced sticker disruption activating the fast intrinsic motion and partial equilibration of the chain, as in the case of the relaxation under the single-step strain examined in the accompanying paper (Ref. 1 of this paper). However, under the double-step strain, the conformation of a given arm keeps its memory from the first step strain γ₁ (imposed at time −t_w) when the second step strain γ₂ is imposed at a later time 0. Namely, the conformation just before imposition of the second strain is different for the arms that carry the stickers having or not having been disrupted by the first strain, so that the analysis needs to be conducted for a whole cascade of the conformational memory, from the conformations on imposition of the first strain to those on imposition of the second strain. For the star chain having four arms, the conformational distribution in this cascade was still manageably narrow, and the bead-spring analysis allowed us to formulate the stress σ(t; γ₁, γ₂) at time t > 0 (after imposition of the second strain). It turned out that σ(t; γ₁, γ₂) thus deduced from the analysis captures the features of preliminary data obtained for tetra-PEG chains associating at the arm ends, confirming the importance of the cascaded memory and the isochronal cross-correlation of the arm conformations for those chains. In addition, for the case of γ₂ ≅ −γ₁/2, differences were found between the data and σ(t; γ₁, γ₂) deduced from the widely adopted BKZ constitutive equation, which further demonstrated the importance of the cascaded memory and cross-correlation not incorporated in this equation.

A mechanochemical approach using ball milling enables us to carry out organic reactions between solid-state reactants as well as those between liquid reactants. Under such mechanochemical conditions, a considerable reactivity difference between liquid and solid reactants is often observed, which may result from the poor-diffusivity of reactants in solids. The chemical reaction between reacting solids happens at their interface and forms a region rich in products. We here integrate the Landau theory of the solid-liquid phase transition into the Onsager's theory of irreversible processes to theoretically predict the reaction kinetics limited by the diffusion of reactants through the heterogeneous interfacial region. Our theory predicts that in the limit of low temperature, the reaction kinetics is mostly limited by the dissolution of reactant molecules into the product-rich phase, rather than the poor diffusivity in solids, and provides a quantitative relationship between the reaction rate and the equilibrium miscibility phase diagram.