S13-4
Numerical Modeling and Simulation Analysis of Plasma-Liquid Interaction
at Atmospheric Pressure
大気圧下におけるプラズマ－液体相互作用の数値モデリングとシミュレーション解析
Satoshi Uchida
内田 諭
Graduate School of Science and Engineering, Tokyo Metropolitan University
1-1, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
首都大学東京理工学研究科 〒192-0397 東京都八王子市南大沢1-1
Atmospheric pressure plasma has been widely applied to water cleanup treatment, material synthesis,
medicine and so on. However, the discharge dynamics including the interaction with liquid and biologicalinterfaces are extremely complex and not understood well. For further development of the plasma
applications, theoretical analyses based on numerical modeling and simulations are essential. In this paper,
the analytical schemes using fluid models and molecular dynamics were briefly described. The numericalresults with the interaction between atmospheric pressure plasma and liquid-biological interface werereviewed for recent topics of plasma application. Moreover, the future prospects for the numericalsimulations in this research field were also discussed.

1. Introduction
Presently, we can easily bring plasma radicals
into contact with various substances such as fluids
and biomaterials by means of non-equilibrium
plasma under atmospheric pressure. This plasma
technology has been applied to water purification,material synthesis, and sterilization. In particular,
plasma medicine is a recent attractive application[1]. However, the interactions among multi-phaseinterfaces are extremely complicated, and cannot bewell understood with only experimental validation.
The theoretical modeling and numerical simulation
are essential for further insights of the mechanism.
In this paper, the modeling of multiphaseincluding plasma, gas, liquid and biomaterial underatmospheric pressure, and typical numericalmethods are briefly described. Some analyticalexamples of liquid-biological interface are reviewed
in the latest topical applications. The prospects ofnearest future simulations in this research field arealso discussed.
2. Simulation Models and Techniques
In order to model the multiphase of plasma, gas,liquid and biomaterial numerically, we need toconsider various complicated and specificconditions and cheese appropriate numerical
schemes. As is shown in Fig. 1, fluid models andmolecular dynamics are available for above
simulations.
Fluid model for plasma physics is a macroscopic
method describing statistical average dynamics

Fig.1. Schematics of an analytical multi-phase modelsuch as spatio-temporal distributions of particles
and fields. This analytical system generally consistsof the continuity equations for density of electrons,ions and neutral particles, and Poisson's equation.
Transport parameters and reaction rate coefficientsare required in advance. In order to refer electronic
swarm parameters, the local field, pseudo-thermal
equilibrium or relaxation continuum approximationis adopted.
Fluid model is suitable for faster calculations ofhigh-density plasma behavior under a wide range ofelectromagnetic field. However, various conditions
should be suitably approximated for boundary,energy conservation and interaction.
Molecular dynamics (MD) is a deterministic
model of particle behavior in atomic scale. The
motion equations of all individual particles areintegrated by time step in the order of afemtosecond.
In typical simulations, atoms move by interatomic

force based on quantum atomic potentials. These
forces are obtained from classical interatomic
potentials referred to as force field or are directly
derived from quantum mechanical calculations.
MD can deal with accurate behavior of eachparticle in fine-grained time and length scale
without arbitrary fitting parameters because of theself-consistency in principle. On the other hand, it
is difficult to perform long time and large size
simulations since the computational load is limited.
3. Examples of Simulations
The streamer propagation through bubble inliquids was simulated using two-dimensional fluid
model for treatments of volatile organic compounds
by Babaeba and Kushner [2]. Spatial evolution ofstreamer along the surface of bubble was promotedwith an increase of the relative permittivity andconductivity of liquids. The transition of streamer
path from axial to surface of bubble strongly
depended on the size of bubble, applied voltage andmean free path of photon.
They investigated filament formation inatmospheric pressure dielectric barrier discharges(DBD) and surface charging of human skin tissue
with similar numerical technique [3]. The
maximum value of intracellular electric fieldexceeded 100 kV cm-1. This model was upgraded
for wound skin filled with blood serum [4].
Shirafuji et al. numerically investigated charged
particle behavior through gas and liquid phases in
DBD with water [5]. Ion accumulation in surface
layer of liquid was much larger than that without
plasma irradiation. Takeuchi considered masstransfer through gas-liquid interface for radicals [6].
Further detailed framework of multiphase modelingfor plasma-biofilm and plasma-tissue interactions
was developed by Chen et al. [7].
With respect to MD simulations, Minagawa et al.
investigated the physical behavior of watermolecules by the bombardment of positive oxygenion on plasma-liquid surface [8]. The number ofsputtered water molecules, liquid temperature, andpenetration depth in liquid were quantified to be afunction of various incident energy and electricfield strength.
Using reactive force field referred as to ReaxFF,
Yusupov et al. simulated the interaction of oxygen
radicals with water. Their computational results
showed that OH, HO2, and H2O2 could diffuse
deeply to the liquid layer [9]. They also modeledinteraction of radical oxygen species withbiological surface such as bacterial cell walls [10]and lipids in skin barrier [11] by similar MD
technique. The abstraction of hydrogen played akey role on the degradation of biological surface.
Abolfath et al. examined quantum mechanical andmolecular mechanical MD (QM/MM-MD) for thereactive interaction of OH radicals with DNA [12].
The selective promotion of hydrogen abstraction
from guanine was shown for different spin states ofOH – guanine system.
4. Conclusions
For numerical analyses with interactions between
plasma and liquid or biological interface underatmospheric pressure, we described the outline of
fluid model and molecular dynamics, andoverviewed the latest analytical results.
The numerical simulations are available fortheoretical and quantitative analyses of further
complicated system with respect to boundary shape,
interfacial interaction and changes of flow and heatin multiphase.
In the nearest future, the establishment of generalmodels and techniques for multi-physics will berequired. We should verify the space division at
appropriate time in synthesized multiphasesimulations.
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