Enhancing pressure consistency and transferability of structure-based coarse-graining.

Coarse-graining, which models molecules with coarse-grained (CG) beads, allows molecular dynamics simulations to be applied to systems with large length and time scales while preserving the essential molecular structure. However, CG models generally have insufficient representability and transferability. A commonly used method to resolve this problem is multi-state iterative Boltzmann inversion (MS-IBI) with pressure correction, which matches both the structural properties and pressures at different thermodynamic states between CG and all-atom (AA) simulations. Nevertheless, this method is usually effective only in a narrow pressure range. In this paper, we propose a modified CG scheme to overcome this limitation. We find that the fundamental reason for this limitation is that CG beads at close distances are ellipsoids rather than isotropically compressed spheres, as described in conventional CG models. Hence, we propose a method to compensate for such differences by slightly modifying the radial distribution functions (RDFs) derived from AA simulations and using the modified RDFs as references for pressure-corrected MS-IBI. We also propose a method to determine the initial non-bonded potential using both the target RDF and pressure. Using n-dodecane as a case study, we demonstrate that the CG model developed using our scheme reproduces the RDFs and pressures over a wide range of pressure states, including three reference low-pressure states and two test high-pressure states. The proposed scheme allows for accurate CG simulations of systems in which pressure or density varies with time and/or position.

Anisotropic Shear Viscosity of Photoaligned Liquid Crystal Confined in Submicrometer-to-Nanometer-Scale Gap Widths Revealed with Simultaneously Measured Molecular Orientation.

In the context of the use of liquid crystals (LCs) as lubricants and lubricant additives, this study investigates the anisotropic shear viscosity of LCs confined in nanometer-sized gap widths subject to both shearing and photoalignment. The photoalignment is achieved using anisotropically dimerized polyvinyl cinnamate (PVCi) films coated on substrates. We simultaneously measure the viscosity and order parameter of a liquid crystal (4-cyano-4'-pentylbiphenyl) confined and sheared in the gap range of 500 nm down to a few nm. We achieve this simultaneous measurement using an original method that combines a highly sensitive viscosity measurement and a sensitive birefringence measurement. When the LC is sheared in the same direction as the photoalignment (parallel shearing), the order parameter, which is around 0.3 in the bulk state, increases up to around 0.4 at a gap width of less than 50 nm and the viscosity is smaller than half the bulk viscosity. We consider that this increase in the order parameter is due to the highly ordered photoaligned LC layer near the PVCi film, and the viscosity decrease is due to shear thinning of this layer enhanced by both confinement and molecular ordering. In addition, we observe a gradual decrease in viscosity starting at a gap of less than around 300 nm in the parallel shearing. Based on the apparent slip model, we show that the LC layer near the PVCi film can also cause this gradual viscosity decrease. In contrast, when the LC is sheared in the direction perpendicular to the photoalignment direction (perpendicular shearing), the viscosity increases as the gap decreases. We speculate that this is due to the rotational motion of the LC molecules caused by the competing effect of shear alignment and photoalignment. We believe our findings can significantly contribute to a better understanding of the confined LCs utilized for lubrication.

Structure-based coarse-graining for inhomogeneous liquid polymer systems.

The iterative Boltzmann inversion (IBI) method is used to derive interaction potentials for coarse-grained (CG) systems by matching structural properties of a reference atomistic system. However, because it depends on such thermodynamic conditions as density and pressure of the reference system, the derived CG nonbonded potential is probably not applicable to inhomogeneous systems containing different density regimes. In this paper, we propose a structure-based coarse-graining scheme to devise CG nonbonded potentials that are applicable to different density bulk systems and inhomogeneous systems with interfaces. Similar to the IBI, the radial distribution function (RDF) of a reference atomistic bulk system is used for iteratively refining the CG nonbonded potential. In contrast to the IBI, however, our scheme employs an appropriately estimated initial guess and a small amount of refinement to suppress transfer of the many-body interaction effects included in the reference RDF into the CG nonbonded potential. To demonstrate the application of our approach to inhomogeneous systems, we perform coarse-graining for a liquid perfluoropolyether (PFPE) film coated on a carbon surface. The constructed CG PFPE model favorably reproduces structural and density distribution functions, not only for bulk systems, but also at the liquid-vacuum and liquid-solid interfaces, demonstrating that our CG scheme offers an easy and practical way to accurately determine nonbonded potentials for inhomogeneous systems.

Response properties of trigeminal ganglion mechanosensitive neurons innervating the temporomandibular joint of the rabbit.

The primary mechanosensitive neurons innervating the temporomandibular joint (TMJ neurons) may play an important role in controlling mandibular movement and position. The purpose of the study was to investigate the neurophysiological properties of TMJ neurons during passive movement of the isolated condyle in 55 rabbits and the intact condyle in 29 rabbits. Discharges of TMJ neurons from the trigeminal ganglion were recorded with a microelectrode as the isolated condyle was moved manually and by a computer-regulated mechanostimulator and as the intact condyle was manually stimulated. A total of 237 TMJ neurons were recorded rostrocaudally from the mandibular nerve area lateral to the maxillary region in the dorsal half of the trigeminal ganglion. Of the recorded TMJ units, 97% were slowly adapting (SA) and 67% of the SA units had an accompanying ongoing discharge. The proportion of adaptation types and appearance of ongoing discharges for the isolated condyle did not differ significantly from those for the intact condyle. Most of the TMJ units (89%) responded multidirectionally to the rostral and ventral movements of the isolated condyle. The discharge frequencies of the TMJ units increased as the condylar displacement and velocity increased within a 5-mm anterior displacement of the isolated condyle. Displacement of the isolated condyle influenced the discharge frequency of the units to a greater extent than the velocity of the condyle movement. No responses of TMJ units were observed during the descending ramp. Based on these results, we conclude that sensory information is transmitted by TMJ neurons encoding joint position, displacement and velocity in a physiological range of mandibular displacement.

Motion picture imaging of a nanometer-thick liquid film dewetting by ellipsometric microscopy with a submicrometer lateral resolution.

We visualized the detwetting of a nanometer-thick unstable liquid film on a nanotextured solid surface with a high lateral spatial resolution. The dewetting was imaged as a motion picture at a submicrometer spatial resolution and a frame rate of 4 frames/s, using ellipsometric microscopy in a vertical objective configuration. The observation revealed that the dewetting process significantly depends on the sign of the disjoining pressure Pi. When Pi is negative, the film rupture due to the spinodal dewetting proceeds to droplet formation in a single step, whereas, when Pi is positive, the film rupture due to the spinodal dewetting stops when the pressure of the thicker region balances with that of the thinner region, and then the heterogeneous grooves are nucleated and grow. The dewetting process dependence on the sign of Pi can be found in systems other than that reported here because the sign of Pi changes at the local maximum of the surface energy.

Control of wettability of molecularly thin liquid films by nanostructures.

The patterning of liquid thin films on solid surfaces is very important in various fields of science and engineering related to surfaces and interfaces. A method of nanometer-scale patterning of a molecularly thin liquid film on a silicon substrate using the lyophobicity of the oxide nanostructures has recently been reported (Fukuzawa, K.; Deguchi, T.; Kawamura, J.; Mitsuya, Y.; Muramatsu, T.; Zhang, H. Appl. Phys. Lett. 2005, 87, 203108). However, the origin of the lyophobicity of the nanostructure with a height of around 1 nm, which was fabricated by probe oxidation, has not yet been clarified. In the present study, the change in thickness of the liquid film on mesa-shaped nanostructures and the wettability for the various combinations of the thickness of the liquid films and the height of ridge-shaped nanostructures were investigated. These revealed that lyophobicity is caused by a lowering of the intermolecular interaction between the liquid and silicon surfaces by the nanostructure and enables the patterning of a liquid film along it. The tendency of the wettability for a given liquid film and nanostructure size can be predicted by estimating the contributions of the intermolecular interaction and capillary pressure. In this method, the height of the nanostructure can control the wettability. These results can provide a novel method of nanoscale patterning of liquid thin films, which will be very useful in creating new functional surfaces.

Diffusive motion of molecules in submonolayer liquid films on a solid surface.

The spreading of submonolayer liquid polymer films on a solid surface that consist of molecules with different mobilities was investigated. The molecular conformations of the adsorbed and free molecules were estimated from the dispersive surface energy measurement and the relationship between the diffusion coefficient of the free molecules and the coverage of the adsorbed molecules were obtained from the spreading profile measurement. A free mobile molecule lies flat on the solid surface and an adsorbed molecule lies less flat than a mobile one. The relation between the diffusion coefficient and the coverage can be explained by the percolation model at a small coverage and it can be explained by the reptation model at a large coverage.