S4-4
Study on Plasma-Liquid Interaction by Numerical Simulation
計算科学によるプラズマ-液相相互作用	 
Fumiyoshi Tochikubo杤久保文嘉Tokyo Metropolitan University1-1, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
首都大学東京 〒192-0397	  東京都八王子市南大沢1-1
We mainly present the results of numerical simulation on dc glow discharge with liquid electrode to reveala part of plasma-liquid interactions. The results were compared with that of conventional electrolysis todiscuss the liquid phase structure such as electric double layer formation. Charge injection from plasma toliquid largely affects not only the potential distribution in liquid but also the local pH near theplasma-liquid interface.

1. Introduction
Nonthermal atmospheric-pressure plasmas (APP)
in contact with liquids have been studied
energetically for material processing, environmental
and biomedical applications [1,2]. There are manykinds of APPs in contact with liquids such as
dielectric barrier discharge, plasma jet with noble
gas flow, and electrical discharges in bubbles. In
any types of APPs, the plasma-liquid interaction isvery important for determining the physical andchemical processes in liquid. Although liquid-phasereactions induced by high-energy particles havebeen investigated in radiation chemistry, theplasma-liquid interactions with APPs are toocomplicated because many kinds of charged andneutral particles are irradiated on the liquid surface
simultaneously. Recently, numerical simulations
are attempted to understand the plasma-liquidinteraction with APPs on the basis of simplified
model. In this paper, we mainly show thesimulation results of electrical discharge with liquid
electrode, and discuss the plasma-liquid interaction.
2. Physical Quantities in Plasma and Liquid
It is worthy to compare the physical quantities inplasma and liquid, and they are shown in Table I.
The plasma density in liquid depends on theelectrolyte concentration, and is high even in thedeionized water. This means that small unbalanceof charged species causes the strong electric fieldsuch as electric double layer (EDL) in front ofmetal electrode. Diffusion coefficient of both ionicand neutral species in liquid is on the order of
10!!   cm! s !! , thus, the diffusion is not the dominant
process for transport of chemical species in liquid.
When the electrolyte concentration is approximately less than 10 mM, the conductivity in liquid
becomes lower than that in APP. In such case, theelectric field in the liquid becomes relatively large.
Table I. Comparison of physical quantities inatmospheric pressure plasma and in liquid.
In APPIn liquid

Medium density(cm!! )

2.45×10

3.34×10!!

Plasma density(cm!! )

10!" ~10!"

6.02×10!"(1M electrolyte)
6.02×10!"(deionized water)

Diffusioncoefficient(cm! s !! )Mobility(cm! V !! s !! )~10!
(electron)~10!!

~10!     (electron)~10!!

Conductivity(S ∙ cm!! )0.01(@10!"     cm!! )0.1(1M electrolyte)
5.48×10!!
(deionized water)!"

3. Electrical discharges with liquid electrode3.1 Conventional electrolysis
When dc voltage is applied between two metalelectrodes in the liquid, EDLs are formed in front ofelectrodes due to the drift motion of ionic species asshown in Fig. 1. The EDL is similar to the Debye
shielding, and the Debye length 𝜆! is written as
𝜆! = 0.304𝑀 !!/!   nm,                                            (1)where M is the concentration of electrolyte in mol/L.
The Debye length becomes in the range of nm. The
strong electric field at the EDL promotes theelectrode reaction. Morrow et al. calculated theEDL formation process in NaCl solution using
continuity equations for ionic species coupled with
Poisson’s equation [3]. For example, with 0.01 M
NaCl solution, the concentrations of Cl! and Na!
in front of the anode were 3×10!"   cm!! and10!"   cm!! , respectively. The electric field reaches
55  MVm−1 . Since the concentrations of OH ! andH ! also drastically change in front of electrodes,local pH value changes from 7. This suggests that
local reactivity varies with location considerably.
DC source

Potential+

CathodeAnode++

electrode. However, the concentration of Na!   andCl! are almost same near the plasma-liquidinterface, and therefore, EDL-like structure is notformed. Higher H ! concentration near the plasmaliquid interface is caused from the positive ion
injection from plasma, similar to the case of DBD
in contact with the liquid.

+In liquidDouble layer

In gasDouble layer

Fig.1. Schematics of electric double layer formation andpotential distribution in conventional electrolysis.

3.3 DC glow discharge in contact with liquid
DC glow discharge with a liquid electrode is aneasy method to obtain a stable gas discharge plasma
in contact with liquid. It can be applied for thesynthesis of metallic nanoparticles in a liquid [5].
We performed numerical simulation of dc glow
discharge in atmospheric pressure He in contact
with a liquid by one-dimensional simulation. Bothgas gap and liquid depth are set to be 1 mm. The
liquid is a NaCl solution. The simulation method isessentially the same as that by Shirafuji et al [4].
Figure 2 is the potential distribution with 6 mM
NaCl solution. Typical glow discharge structurewith cathode fall and positive column is formed in
the gas. Because the conductivity of the liquid is
not so high, the voltage drop in the liquid is large.
In Fig. 3, the spatial distribution of ionic species inliquid is shown. EDL is formed in front of the metalFig.2. Potential distribution in dc glow discharge inatmospheric pressure He in contact with liquid. Liquid is
6 mM NaCl solution.

17101016
1015

Cl-3-31019
1018

1021
1020

(a)+NaConcentration (cm )

1021
1020

Concentration (cm )3.2 Barrier discharge in contact with liquid
When one of the metal electrodes is replacedwith APP, the charge injection mechanism into
liquid changes from the conventional electrolysis.
Shirafuji et al. performed numerical simulation ofdielectric barrier discharge (DBD) in atmosphericpressure Ar in contact with a liquid by onedimensional fluid model considering continuity
equations for both gas-phase and liquid-phase
species coupled with Poisson’s equation [4]. The
results predict the formation of EDL-like structure,
where concentrations of anion and cation have theirpeaks alternately coincident with the applied ac
voltage. They also predict that unipolar EDL-likestructure is possible to form by tuning the iontransport parameters. Since the EDL-like structure
is formed by the injection of electrons or positive
ions from the DBD to the liquid surface, its
formation mechanism is different from that inconventional electrolysis.

+H (x100)-1019
1018171010161015(b)Na+-Cl+H (x100)

OH (x100)OH-(x100)
1014
1014
1013
1013
0102030
2000
1000
0Distance from electrode (nm) Distance from interface (nm)Fig.3. Spatial distribution of ionic species in liquid near(a) metal electrode and (b) plasma-liquid interface.

4. Summary
Simulation results of electrical discharge withliquid electrode are shown to reveal a part ofplasma-liquid interactions. The charge injection
from plasma to liquid determines the electricalcharacteristics such as EDL-like structure.
Acknowledgments
This work is partly supported by Kakenhi (No.
21110007 and 26600128).
References[1] P. Bruggeman and C. Leys: J. Phys. D 42 (2009)053001.
[2] A. Fridman and G. Friedman: Plasma Medicine(Wiley, 2013).
[3] R. Morrow, D. R. McKenzie and M. M. Bilek: J.
Phys. D 39 (2006) 937.
[4] T. Shirafuji, A. Nakamura and F. Tochikubo: Jpn. J.
Appl. Phys. 53 (2014) 03DG04.
[5] N. Shirai, S. Uchida and F. Tochikubo: Jpn. J. Appl.
Phys. 53 (2014) 046202.