Yuki Kato

Preparation of water-soluble carbon nanotubes using a pulsed streamer discharge in water

A novel technique for the preparation of water-soluble carbon nanotubes was demonstrated using a pulsed streamer discharge generated in water. The technique involved chemical reactions between radicals generated by the pulsed streamer discharge and carbon nanotubes. The pulsed streamer-treated carbon nanotubes were homogeneously dispersed and well solubilized in water for a month or longer. The mechanism of solubilization of carbon nanotubes by the pulsed streamer discharge is discussed based on FTIR spectroscopy and optical emission spectra measurements. FTIR spectroscopy revealed that -OH groups, which are known to impart a hydrophilic nature to carbon material, were introduced on the carbon nanotube surface. Optical emission spectra from the pulsed streamer plasma showed that highly oxidative O(*) and H(*) radicals were generated in water. These results suggest that the functionalization of the carbon nanotube surface by -OH group can be attributed to the O(*) and H(*) radicals. An advantage of the proposed method is that there is no need for any chemical agents or additives for solubilization. Chemical agents for solubilization are generated from the water itself by the electrochemical reactions induced by the pulsed streamer discharge.

Enhancement of microplasma-based water-solubilization of single-walled carbon nanotubes using gas bubbling in water

The authors have previously proposed a novel technique for the preparation of water-soluble carbon nanotubes (CNTs) using microplasma generated by a pulsed streamer discharge in water. This paper describes an improvement in the method of the microplasma-based CNT solubilization process by the use of gas bubbling in water. Oxygen, argon and nitrogen were used as bubbling gas in order to clarify the effects of the gas species on the single-walled CNT (SWCNT) solubilization efficiency. Ultraviolet‐visible absorption spectra of the SWCNT suspensions revealed that the SWCNT solubility was increased by more than two times by using gas bubbling together with microplasma treatment. No significant difference was observed among the three gas species tested. Fourier transform infrared (FTIR)spectroscopy and x-ray photoelectron spectroscopy (XPS) analysis showed that the number of ‐OH groups, introduced on the SWCNT surface by the microplasma treatment, was increased by gas bubbling. Optical emission measurements also showed that the number of highly oxidative oxygen and hydrogen radicals, which were generated by the microplasma, was also increased by gas bubbling. These results indicate that gas bubbling has positive effects on microplasma-based SWCNT solubilization as a result of enhanced radical formation and functionalization of the SWCNT surface. (Some figures in this article are in colour only in the electronic version)

Wave Number Downshift in Modulated Wavetrain through a Nonlinear Damping Effect

The time evolution of a nonlinear modulated wavetrain is investigated using the Dysthe equation with an additional nonlocal nonlinear damping term. It is found that the additional damping term has no significant effect on qualitative features of the modulational instability of the Stokes wave. Numerical results show that for appropriate values of the coefficient of the damping term, it is practically effective only near the crest of envelope, and causes the wave number downshift. These results suggested that two factors are essential to cause the downshift. The first is the enough nonlinearity to produce the asymmetry in spectral distribution. The second is the nonlinear dissipation that affects especially the higher components only when the wavetrain is strongly modulated.

Effects of pH on water-solubilization of carbon nanotube using microplasma in aqueous solution

The authors have previously proposed a novel technique for the preparation of water-soluble carbon nanotubes (CNTs) using microplasma generated by a pulsed streamer discharge in water. This paper describes effects of pH values on water-solubility of single-walled CNT (SWCNT) treated by the microplasma. The SWCNT treated under basic conditions showed two times higher solubility compared to that treated under neutral condition, whereas the SWCNT solubility considerably decreased under acidic conditions. Based on optical emission measurements of microplasma showed that radical formation was not pH sensitive. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis showed that the microplasma treated SWCNT was functionalized with –COO- groups with little pH dependence. In contrary, clear pH dependence was observed for zeta potential of the microplasma-treated SWCNT. The lowest zeta potential was -46 mV, which was obtained with basic solution adjusted by NH3 (pH ≈9). The SWCNT treated under basic conditions was more negatively charged due to suppressed protonation of –COO- groups. As a result, the electrostatic repulsion force between SWCNTs could overcome van der Waals force improving their solubility and dispersibility in water.

Stability of Localized Pulse Trains with a Long Tail in the Generalized Kuramoto–Sivashinsky Equation

Steadily progressive solutions of the generalized Kuramoto–Sivashinsky equation are studied by means of a numerical simulation and a stability analysis under the periodic boundary conditions. Multiple solutions are obtained by the numerical simulation, including pulse trains with an extraordinary long flat tail. A flat solution of the generalized Kuramoto–Sivashinsky equation is known to be unstable. The stability analysis however reveals that the solution with the long flat tail can be stable in a certain range of parameters.

Folded Solution Branches in Rayleigh–Bénard Convection in the Presence of Avoided Crossings of Neutral Stability Curves

Sidewall effect on the Rayleigh–Benard convection has been examined in a rectangular channel with finite aspect ratio, where the linear neutral stability curves avoid crossing on the Rayleigh number–aspect ratio plane. Numerical analysis shows that folded solution branches exist in the bifurcation diagram where the aspect ratio is adopted as the bifurcation parameter. To examine an origin of the fold, we formally derived amplitude equations. An analysis of the amplitude equations shows that certain nonlinear solutions, realized under the crossing of neutral stability curves, are split into two groups in the presence of the avoided crossing and that the folded branch arises.

Three-dimensional instabilities of capillary gravity waves of permanent form near the fourth harmonic resonance

Three-dimensional instabilities of two-dimensional periodic capillary gravity waves of permanent form near the fourth harmonic resonance are investigated numerically. It is confirmed that the unstable regions appear in the neighborhood of the linear resonance curves of sum interactions associated with a fundamental mode and its fourth harmonic. No unstable regions overlap in the wave number plane of disturbances. Two regions are frequently reconnected.

Two-Dimensional Instabilities of Weakly Nonlinear Capillary Gravity Waves of Permanent Form near the Fourth Harmonic Resonance

Three systems of weakly nonlinear envelope equations for a fundamental mode and its fourth harmonic of two-dimensional capillary gravity waves on deep water are derived. These systems are used to investigate stability of progressive periodic waves of permanent form. It is found that the permanent waves in which the fundamental mode and its fourth harmonic are of the same order are unstable. Modulational instability in which the dominant mode in the eigenfunction is near the fourth harmonic is found in the third order approximation. Furthermore, instability related to the fourth harmonic resonance is found in the fourth order approximation.

Direct simulation of unsteady capillary gravity waves on deep water

Fully nonlinear time evolution of periodic capillary gravity waves on an inviscid fluid of infinite depth is investigated numerically. The cases where the effect of surface tension is weaker than that of gravity are considered. It is found that the weakly resonant symmetric steady waves are unstable to two kinds of disturbances: the sidebands and the higher harmonics. Ripples are generated within a time of a few periods. They first appear at the point with the greatest slope on the forward face of the highest crest of the disturbed steady waves.