Kobayashi, Yusei

 Affiliation Faculty of Science and Technology, Department of Mechanical Engineering （Yagami） Position Assistant Professor (Non-tenured)/Research Associate (Non-tenured)/Instructor (Non-tenured) External Links

### Papers 【 Display / hide 】

• Y Kobayashi, N Arai, K Yasuoka

arXiv preprint arXiv:2203.03228  2022

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Despite the extensive research that has been conducted for decades on the
behavior of confined liquids, detailed knowledge of this phenomenon,
particularly in the mixed/boundary lubrication regime, remains limited. This
can be attributed to several factors including the difficulty of direct
experimental observations of the behavior of lubricant molecules under
non-equilibrium conditions, the high computational cost of molecular
simulations to reach steady state, and the low signal-to-noise ratio at
extremely low shear rates corresponding to actual operating conditions. To this
end, we studied the correlation between the structure formation and shear
viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in
a mixed/boundary lubrication regime. Three different surface separations
corresponding to two-, three-, and five-layered structures were considered to
analyze the effect of confinement. The orientational distributions with one
specific peak for $n=2$ and two distributions, including a parallel orientation
with the surface normal for $n>2$, were observed at rest. The confined liquids
exhibited a distinct shear-thinning behavior independent of surface separations
for a relatively low sliding velocity, $V_{\rm x}\lesssim 10^{-1}\,{\rm m/s}$.
However, the shear viscosities at $V_{\rm x}\lesssim 10^{-1}\,{\rm m/s}$
depended on the number of layered structures. Newtonian behavior was observed
with a further increase in the sliding velocity. Furthermore, we found a strong
correlation between the degree of molecular orientation and the shear viscosity
of the confined liquids. The magnitude of the shear viscosity of the confined
liquids can primarily be determined by the degree of molecular orientation, and
shear-thinning originates from the vanishing of specific orientational
distributions with increasing sliding velocity.

• T Sato, Y Kobayashi, T Michioka, N Arai

Soft Matter 17 (15), 4047-4058 （ROYAL SOC CHEMISTRY)  17 （ 15 ） 4047 - 4058 2021.04

ISSN  1744683X

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In this study, a coarse-grained molecular simulation was performed to investigate the morphologies and phase diagrams of self-assembled polymer-tethered nanoparticles (NPs) confined in nanotubes (NTs). Unlike ordinary NPs, polymer-tethered NPs have two distinct characteristic lengths, which are key factors that determine their self-assembly. Herein, two distinct types of NT walls and three types of polymer-tethered NPs were considered: hydrophilic and hydrophobic walls, and hydrophilic, hydrophobic, and Janus surfaces. First, the qualitative phase diagrams of the axial pressure, P-z, versus the ratio of the NT radius to the NP radius, L, were derived. The results revealed that diverse self-assembled morphologies, which are not formed in non-tethered NPs, were observed in the polymer-tethered NPs. For example, three types of ordered structures with different structural characteristic lengths, depending on P-z, were obtained. In addition, the effect of the chemical nature of the polymer-tethered NP surface on the self-assembled morphology confined in NTs was investigated. Clusters of water molecules were formed, particularly in the hydrophobic polymer-tethered NPs, and these clusters caused the structural distortion of the NP. Moreover, in the polymer-tethered NPs with the Janus amphiphilic surface, the hydrophobic and hydrophilic polymer tethered NPs assembled in the axial direction to form an ordered structure, and a double-helix structure was formed at L = 3.0 in the hydrophobic NT. The results of these simulations indicate that the self-assembly behaviours of polymer-tethered NPs can be qualitatively predicted based on the chemical nature of the NT walls and the surface design of the polymer-tethered NP.

• Structural and rheological properties of Janus colloid-polymer mixtures in dilute solution under shear

Y Kobayashi, N Arai, A Nikoubashman

Bulletin of the American Physical Society （TEPCO Memorial Foundation)   2021

• S Tanaka, N Arai, Y Kobayashi

Chemical Physics Letters 785, 139129 （ELSEVIER)  785 2021

ISSN  00092614

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The mechanisms underlying the thermal conductivity behavior of nanofluids have not been completely clarified thus far. This is due to the various competing factors and the lack of a molecular-level understanding of the heat transfer enhancement of nanofluids. In this study, energy-conserving dissipative particle dynamics simulations were conducted to investigate the effects of the self-assembly of nanoparticles (NPs) on the nanoscale heat transfer properties. We demonstrated that considering the balance between the effects of the distance between the NPs and the solvent and the enhancement in thermal conductivity on adding NPs is important for controlling the thermal conductivity of nanofluids.

• Y Kobayashi, H Gomyo, N Arai

International Journal of Molecular Sciences 22 (14), 7573 （MDPI)  22 （ 14 ）  2021

ISSN  16616596

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The phenomenon of drag reduction (known as the "Toms effect") has many industrial and engineering applications, but a definitive molecular-level theory has not yet been constructed. This is due both to the multiscale nature of complex fluids and to the difficulty of directly observing self-assembled structures in nonequilibrium states. On the basis of a large-scale coarse-grained molecular simulation that we conducted, we propose a possible mechanism of turbulence suppression in surfactant aqueous solution. We demonstrate that maintaining sufficiently large micellar structures and a homogeneous radial distribution of surfactant molecules is necessary to obtain the drag-reduction effect. This is the first molecular-simulation evidence that a micellar structure is responsible for drag reduction in pipe flow, and should help in understanding the mechanisms underlying drag reduction by surfactant molecules under nonequilibrium conditions.

### Research Projects of Competitive Funds, etc. 【 Display / hide 】

• Structural control and mechanism of mechanical properties of polymer nanocomposites

2021.08
-
2023.03

MEXT,JSPS, Grant-in-Aid for Scientific Research, Grant-in-Aid for Research Activity Start-up , Principal investigator

### Courses Taught 【 Display / hide 】

• MECHANICAL ENGINEERING PROJECT

2022

• LABORATORY IN SCIENCE

2022

• LABORATORIES IN SCIENCE AND TECHNOLOGY

2022

• EXPRESSION OF MECHANICAL PRODUCTS

2022

• MECHANICAL ENGINEERING PROJECT

2021