Faculty Profiles - AZUMA Naoki

Papers - AZUMA Naoki

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Coarse-grained Molecular Dynamics Study of Effect of Surface Roughness on Adsorption Behavior of Fatty Acid Additives in Lubricating Oil

TANG Jiahao, ZHANG Hedong, FUKUZAWA Kenji, ITOH Shintaroh, AZUMA Naoki

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： J13109 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

Coarse-grained molecular dynamics simulations were performed to compare the adsorption of organic friction modifier (OFM) added in lubricating oil to flat and corrugated surfaces. Stearic acid, dodecane, and hematite were used as the OFM, lubricating oil, and solid surfaces, respectively. The simulation results showed that the increase in the surface number density of adsorbed stearic acid molecules in 100 ns simulations at 353 K was approximately four times faster on the corrugated surface than on the flat surface. We infer that surface roughness may promote adsorption of OFM additives, owing to the enhanced liquid–solid interactions at the corners formed between adjacent layers of solid surfaces.

DOI： 10.1299/jsmemecj.2020.j13109

CiNii Research

Reactive Molecular Dynamics Study of Thermal Decomposition and Transfer of Perfluoropolyether Lubricant in Heat-assisted Magnetic Recording

CHEN Xingyu, ZHANG Hedong, FUKUZAWA Kenji, ITOH Shintaro, AZUMA Naoki

The Proceedings of Mechanical Engineering Congress, Japan Vol. 2020 ( 0 ) page： S16102 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

To achieve reliable heat-assisted magnetic recording (HAMR), it is crucial to understand the decomposition of perfluoropolyether (PFPE) lubricant films. In this study, we performed reactive all-atom molecular dynamics simulations to investigate the thermal decomposition of PFPE D-4OH using our original ReaxFF force field. The results showed that the thermal decomposition of D-4OH molecules is mainly attributed to the dissociation of C–O bond in the polar end groups and the dissociated fragments transfer from the disk to head and form smears. On the other hand, by quantifying the decomposition time constants at 700 and 800 K, we infer that the thermal decomposition of D-4OH films hardly occurs within the HAMR heating time of ~1 ns.

DOI： 10.1299/jsmemecj.2020.s16102

CiNii Research

Reactive Molecular Dynamics Study of Mechano-chemical Decomposition of Perfluoropolyether Lubricants in Heat-assisted Magnetic Recording

CHEN Xingyu, KAWAI Kento, ZHANG Hedong, FUKUZAWA Kenji, ITOH Shintaro, AZUMA Naoki

The Proceedings of the Conference on Information, Intelligence and Precision Equipment : IIP Vol. 2020 ( 0 ) page： 1A06 2020

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

To achieve effective lubrication of head-disk interface in heat-assisted magnetic recording (HAMR),
mechano-chemical decomposition of nonpolar and polar perfluoropolyether (PFPE) lubricant films, i.e., Z, Z-tetraol, Demnum, and D-4OH, was investigated using reactive molecular dynamics. By fitting with quantum calculation results, the reactive force field (ReaxFF) for PFPE was developed, allowing simulation of chemical reactions of PFPE lubricants. The results showed that at a temperature of 700 K, pure thermal decomposition of PFPE hardly occurs within the nanosecond heating time of HAMR, whereas mechano-chemical decomposition is highly possible to occur when normal pressure and shear are applied by the head and disk. We confirmed that thermal decomposition is mainly caused by the dissociation of C–O bond at the polar end groups and mechano-chemical decomposition is caused by dissociation of C–O bonds at both the polar end groups and main chains.

DOI： 10.1299/jsmeiip.2020.1a06

CiNii Research

Dynamic Viscoelasticity of Hydrated Phospholipid Polymer Brush Measured by Fiber Wobbling Method Invited Reviewed

S. Itoh, S. Aoyama, Y. Higashizaka, K. Fukuzawa, H. Zhang, N. Azuma

Proceedings of the STLE/ASME International Joint Tribology Conference 2019.10

Gap and Shear Rate Dependence of Viscoelasticity of Lubricants with Polymer Additives Sheared in Nano Gaps Invited Reviewed

S Itoh, R Aoki, K Fukuzawa, H Zhang, N Azuma, Y Onumata

Proceedings of the STLE/ASME International Joint Tribology Conference 2019.10

Measurement of Trapping Time of DNA Molecule in front of Nanogap Using Concentration Method

AZUMA Naoki, FUKUZAWA Kenji, ITOH Shintaro, ZHANG Hedong

The Proceedings of the Symposium on Micro-Nano Science and Technology Vol. 2019.10 ( 0 ) page： 21pm1PN319 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

When a DNA molecule passes through a nanochannel which depth is smaller than its size, the molecule is trapped in front of the nanochannel based on an entropy trap. Trapping time τ is a parameter to characterize the entropy trap and measuring trapping time τ accurately is required to design DNA separation and concentration methods based on the entropy trap. In this study, we proposed an experimental measurement method of trapping time τ using DNA concentration method. We fabricated a chip device with a nanogap of 25-nm depth in a microchannel using microfabrication technique and succeeded in determining the voltage V dependence of trapping time τ of 48.5-kbp DNA by experiments with 0.8 - 1.2 V.

DOI： 10.1299/jsmemnm.2019.10.21pm1pn319

CiNii Research

Determination of optimal voltage by theoretical model for DNA concentration using nanoslit

AZUMA Naoki, FUKUZAWA Kenji, ITOH Shintaro, ZHANG Hedong

The Proceedings of the Conference on Information, Intelligence and Precision Equipment : IIP Vol. 2019 ( 0 ) page： 2C01 2019

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

On-chip concentration is a method of concentrating biomolecules using a nanometer-sized structure created into a microchannel, which is effective for improving detection sensitivity for microfluidic devices. As an on -chip concentration method for DNA molecules, we proposed a concentration method using nanoslit, which was a gap smaller than a diameter of a single molecule. Since this concentration method was based on the principle of entropy trap, its concentration amount depended on applied voltage. In this study, we proposed a theoretical model of concentration using nanoslit and determined an optimal voltage at which maximizing the concentration amount. We fabricated a chip device with 25 nm-depth nanoslit using microfabrication technique and experimentally verified the validity of our model. We achieved the concentration of λ DNA molecules at 17 times when we applied 0.6 V for 15 s.

DOI： 10.1299/jsmeiip.2019.2c01

CiNii Research

Separation of large DNA molecules by applying pulsed electric field to size exclusion chromatography-based microchip Reviewed

Naoki Azuma, Shintaro Itoh, Kenji Fukuzawa, Hedong Zhang

Japanese Journal of Applied Physics Vol. 57 ( 2 ) page： 8 2018.2

Optimization of applied voltages for on-chip concentration of DNA using nanoslit Reviewed

Naoki Azuma, Shintaro Itoh, Kenji Fukuzawa, Hedong Zhang

Japanese Journal of Applied Physics Vol. 56 ( 12 ) page： 8 2017.12

Fast size separation of large DNA molecules by applying pulsed electric field to size exclusion chromatography-based electrophoresis microchip

AZUMA Naoki, ITOH Shintaro, FUKUZAWA Kenji, ZHANG Hedong

The Proceedings of the Conference on Information, Intelligence and Precision Equipment : IIP Vol. 2017 ( 0 ) page： E-01 - 1 2017

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

Size separation of large DNA molecules (10 kbp – 1000 kbp) is one of the fundamental procedures for identifying bacterium’s genotypes, which should be fast for quick defection of infectious diseases. In our conventional study, we developed an electrophoresis microchip based on size exclusion chromatography. This separation has an advantage that it does not cause large deformation of DNA, which is important for separation of large DNAs because they can be easily fragmented. We achieved separation of large DNAs (48 kbp + 166 kbp) driven by a direct electric field as electrophoretic condition of DNA. However, it caused long separation time (3000 s) because DNAs migration was slow due to low applied voltage for the separation. In this study, we achieved faster separation of DNAs using a pulsed electric field under which applied voltage can be higher.

DOI： 10.1299/jsmeiip.2017.e-01

CiNii Research

On-chip concentration of DNA samples using a nano-slit fabricated in a microchannel

Azuma Naoki, Itoh Shintaro, Fukuzawa Kenji, Zhang Hedong

The Proceedings of the Symposium on Micro-Nano Science and Technology Vol. 2017.8 ( 0 ) page： PN-106 2017

Language：Japanese Publisher：The Japan Society of Mechanical Engineers

On-chip concentration of samples using nanometer-sized structures fabricated in a microchannel is an effective method to improve detection sensitivity in μTAS. We proposed on-chip concentration of DNA samples using a nano-slit, which is a channel with a depth smaller than a diameter of DNA molecule. The concentration is based on an entropy trap at the boundary between the microchannel and the nano-slit. Therefore, concentration is achieved only by adjusting applied voltages. In this study, we verified the feasibility and the performances of the proposed concentration experimentally. When we applied 0.3 V for 100 s by using 30-nm deep nano-slit, we succeeded in concentration of λ DNA at 25 times the concentration rate.

DOI： 10.1299/jsmemnm.2017.8.pn-106

CiNii Research

Separation of large DNA molecules by size exclusion chromatography-based microchip with on-chip concentration structure Reviewed

Naoki Azuma, Shintaro Itoh, Kenji Fukuzawa, Hedong Zhang

Japanese Journal of Applied Physics Vol. 55 ( 06GN01 ) page： 8 2016.4

Improvement of resolution in SEC-type DNA separation microchip by on-chip concentration using nano-slit Reviewed

Naoki Azuma, Shintaro Itoh, Yusuke Sugimoto, Kenji Fukuzawa, Hedong Zhang

Proceedings of the JSME-IIP/ASME-ISPS Joint International Conference on Micromechatronics for Information and Precision Equipment Vol. 2015 ( MoB-2-4 ) 2015.6

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Papers - AZUMA Naoki