TY - JOUR

T1 - Correlation between ordering and shear thinning in confined OMCTS liquids

AU - Kobayashi, Yusei

AU - Arai, Noriyoshi

AU - Yasuoka, Kenji

N1 - Funding Information:
This work was supported, in part, by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) as Research and Development of Next-Generation Fields. Y.K. was supported by JSPS KAKENHI Grant No. JP21K20411. The computations were partially carried out using the computer resources offered under the category of general projects by the Research Institute for Information Technology, Kyushu University. The authors are grateful to Professor K. Kurihara (Tohoku University) and Professor M. Mizukami (Tohoku University) for helpful discussions and useful comments.
Publisher Copyright:
© 2022 Crown.

PY - 2022/9/21

Y1 - 2022/9/21

N2 - Despite decades of extensive research, the behavior of confined liquids, particularly in the mixed/boundary lubrication regime, remains unelucidated. This can be attributed to several factors, including the difficulty to make direct experimental observations of the behavior of lubricant molecules under nonequilibrium 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. In this regard, 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 shear rate, γ˙≲108s-1. However, the shear viscosities at γ˙≲108s-1 depended on the number of layered structures. Newtonian behavior was observed with further increase in the shear rate. 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 shear rate.

AB - Despite decades of extensive research, the behavior of confined liquids, particularly in the mixed/boundary lubrication regime, remains unelucidated. This can be attributed to several factors, including the difficulty to make direct experimental observations of the behavior of lubricant molecules under nonequilibrium 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. In this regard, 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 shear rate, γ˙≲108s-1. However, the shear viscosities at γ˙≲108s-1 depended on the number of layered structures. Newtonian behavior was observed with further increase in the shear rate. 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 shear rate.

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U2 - 10.1063/5.0099473

DO - 10.1063/5.0099473

M3 - Article

C2 - 36137803

AN - SCOPUS:85138400080

VL - 157

JO - Journal of Chemical Physics

JF - Journal of Chemical Physics

SN - 0021-9606

IS - 11

M1 - 114506

ER -