22P002-O
Physical Action of Particle Irradiation from Atmospheric Pressure Plasma
to Liquid Surface
大気圧プラズマから液体表面へ粒子照射した際の物理的作用
Daisuke Ito, Naoki Shirai, Satoshi Uchida and Fumiyoshi Tochikubo
伊藤大祐, 白井直機，内田 諭，杤久保文嘉Tokyo Metropolitan University
1-1 Minami-Osawa, Hchioji-shi, Tokyo 192-0397, Japan
首都大学東京 〒192-0397 八王子市南大沢1-1
Although an atmospheric-pressure glow discharge with a liquid cathode is widely studied, the phenomena
at the plasma-liquid interface are not well known. In this wook, we have investigated the influence ofheavy particle irradiation from plasma to liquid surface on physical phenomena by classical molecular
dynamics simulation. The influences of the heavy particle impact with 10-100 eV such as sputtering andlocal liquid heating were studied. We found the increase in temperature of liquid and the evaporation ofseveral molecules.

1. Introduction
In late years, the studies on generations andapplications of atmospheric pressure non-thermal
plasma(APNTP)
havebeenconductedenergetically. The APNTP can be irradiated to avariety of materials such as liquid and biologicalobject in addition to the normal solid materials.
When we irradiate the APNTP to liquid, theinteraction at plasma-liquid interface is important.
However, it is currently argued only by theanalogism from experimental results. For thisreason, we focus on the study of physical action at
plasma-liquid interface by numerical simulation.
Nikiforov investigated the plasma sputtering ofwater molecules from the liquid numerically[1],however, the impinging ion energy was too large asthat of APNTP. Therefore, we studied influences of
the impingement of low energy particles.
2.ProcedureWe have carried out the numerical simulation for
two cases; (i) energetic argon atom impinges on thesurface of liquid argon, and (ii) energetic watermolecule impinges on the surface of liquid water.
Figure 1 shows geometry of molecules. In case (i)the simulation cell had a size of 29
29
115 ,and in case (ii) the cell had a size of313162 . Both simulation cells contained2000 molecules. The vacuum layers whichcorrespond to the gas phase were adjoining theupside and downside of the simulation cell and thesides of the simulation cell were subject to periodic
boundary condition. The temperature in simulation
cell was adjusted for a few picoseconds by velocity
scaling method [2]. A particle is placed in vacuum

layer at a distance of 30 from the upside of liquidsurface and moves to liquid surface at an initial
velocity. This numerical simulation was studied by
the classical molecular dynamics technique[3.4].
(a)

(b)

Fig.1. Geometry of the simulation cell; (a) argon and(b) water

In this work, the Lennard-Jones potential wasemployed as intermolecular potential function todescribe the interaction between argon atoms.

Water is modeled by SPC/E-model because it wassimple comparatively and properties described by it
were close to those of liquid water [5].
Table I presents the initial liquid temperature andthe energy of impinging particles.
Table I. Calculation condition
Case (i)Case (ii)

87 K

300 K

10 eV

10,50,100 eV

400

The local heating of the liquid and the movementof molecules near the liquid surface by impact ofparticle were investigated.

(a)temperature[K]Temperature inliquids
Energy ofimpinging particle

impact of 10 eV particle. In case of 10 eV particle
irradiation to water surface, the average temperaturerose steadily and reached a temperature of about
350 K at 10 ps after the impact of particle. The
calculated results suggest that the evaporation andthe temperature relaxation process strongly dependon the impinging energy of heavy particle.

350(a)

300

250

3. Results
Figure 2 shows the configuration of watermolecules at 5 ps after the impingement of the 100eV particle. The temperature near surface increased
and several molecules evaporated.

1020time[ps]30temperature[K]1000800(b)(b)600
400200102030time[ps]Fig.3. Temperature change of liquid in case(ii). The
impinging particle energy is 10 eV for (a) and 100 eV for(b), respectively.

Acknowledgement
This work was partly supported by Grant-in-Aidfor Scientific Research (No 21110007) from the
Ministry of Education, Culture, Sports, Science andTechnology, Japan.
Fig.2. Configuration of water molecules at 5 ps after theimpingement of the 100 eV particle.

Figure 3 presents the average temperature changein liquid water at depth in the range of 12byimpingement of 10 eV and 100 eV. The particle invacuum layer started to move at 10 ps and impacted
the liquid.
In case of 100 eV particle irradiation, the averagetemperature rose rapidly and then dropped. Similar
temperature change occurred in liquid argon by theReferences[1] A. Yu. Nikiforov: High Energy Chemistry. 42(2008) 235.
[2] L. V. Woodcock. et al.: Chem. Phys. Lett. 10(1971)257[3] A. Ueda: Molecular Simulation. Shokabo (InJapanese)[4] A. Rahman and F. H. Stillinger: J. Chem. Phys. 55(1971)3336[5] H. J. C. Berendsen, J. R. Grigera, T.P.Straatsma: J.
Phys. Chem. 91 (1987) 6269