Multi-level Computational Chemistry Approach forPhase Change Behavior of Water induced by Laser Irradiation

H. Takaba*, A. Nomura*, Y. Sasaki*, K. Chiba*, H. Hata*, K. Okushi*, A. Suzuki**, H. Tsuboi*,M. Koyama*,N. Hatakeyama*, A. Endou*, M. Kubo*, C.A. Del Carpio*, M. Kitada***,

H. Kabashima*** and A. Miyamoto**,*

*Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,6-6-11-1302 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

**New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba,Aramaki, Aoba-ku, Sendai 980-8579, Japan

***Automobile R&D Center, Honda R&D Co., Ltd., 4630 Shimotakanezawa,Haga-machi, Haga-gun, Tochigi 321-3393, Japan

ABSTRACT

Recently, laser irradiation involving chemical reactionis particularly interesting in the field related to advancedmaterial design. In this study, a phenomenon of rapid phasechange of water induced by infrared laser irradiation wasinvestigated by means of multi-level computationalchemistry approach. Our recent work using quantumchemical molecular dynamics of laser irradiation on waterrevealed the possibility that the O-H bond of watermolecule is dissociated by infrared laser irradiation. Basedon the result, bubbling formation induced by laserirradiation is modeled by using a kinetic Monte Carlo(KMC) method. KMC calculation enables to estimate atime evolution of micro-bubble generation. The KMCtechnique was combined with flow dynamics simulation onwater reservoir which is solved by a finite volume methodbased on Navier-Stokes equations. Our simulation resultpredicts that the water bubbling occurs and that the rapidconvectional flow of micro-bubble takes place by applyinginfrared laser irradiation to the bulk water. This resultpromises to open the door of novel technology utilizinginfrared laser irradiation.

Keywords: ultra accelerated quantum chemical moleculardynamics, computational fluid dynamics, meso-scalekinetic Monte Carlo, infrared laser, evaporation

1 INTRODUCTION

Water is indispensable for our life and plays animportant role in various kinds of processes, e.g. flowdynamics, bio-metabolism, catalytic reaction system,electrochemical process, tribology, extraction process andseparation process. Therefore, properties of water areparticular interest in various field not onlyphysicochemistry but also chemical industry, biochemistry,and nanotechnology. Recently, a number of papers havebeen reported on laser-induced reaction [1-3]. Most of themfocused on heat or pressure effect of laser irradiation using

high energy laser. Few studies investigate chemical reactioninvolved by infrared (IR) laser irradiation. IR laserirradiation is like to be absorbed by chemical bonding inmolecules; therefore, the excitation of frequency mode ofchemical bonding is expected and is speculated to influenceon chemical reaction. Because IR laser irradiationaccompanies heat generation, it is difficult to distinguish aneffect of change of vibration mode on chemical reaction.Therefore, it is still difficult in experimental and theoreticalstudies for direct investigation of the effect of laser-inducedreaction. Thus, novel approach by experimental orsimulation technique for laser induced reaction is stronglydemanded.On the other hand, our group has developed ultra-

accelerated quantum chemical molecular dynamicstechnique (UA-QCMD) simulator, "New-Colors" [4,5],based on our original tight-binding approximation. "New-Colors" has an advantage in computation cost compared tothe first-principles molecular dynamics (FPMD). Weapplied this technique to investigate the chemical reactiondynamics of IR laser induced reaction in bulk water, andrevealed that the possibility of dissociation of watermolecules resulting in the evaporation [6].Moreover, theoretical findings obtained by large-scale

simulation are worthwhile that is realized by combinedatomistic simulation with macroscopic simulations such askinetic Monte Carlo (KMC) and computational fluiddynamics (CFD). This combination of simulation methods,which is called as �multi-level computational chemistryapproach�, makes possible entirely understanding of laser-induced chemical reaction. In this study, we attempted toestablish the multi-level simulation methodology based onthe atomistic- and macro- scale simulation technique toinvestigate chemical reaction dynamics of water induced byIR laser irradiation.

2 METHODS

Fig. 1 shows our strategy for multi-level modeling oflaser-induced water phase change behavior. Particularly, in

487Clean Technology 2008, www.ct-si.org, ISBN 978-1-4200-8502-0

this study, we modeled IR laser irradiation to bulk waterwhere energy from the irradiated laser would be absorbedby O-H bond in water molecules resulting in the excitationof vibration model of O-H bonding. This phenomenon wastheoretically investigated by us from the point of view ofmolecular level using quantum molecular dynamics, andthe detail of the results will be presented in another paper[6]. In this theoretical study, the possibility of reaction ofdissociation of water molecule induced by IR-laser wasevaluated. The reaction possibility from quantum moleculardynamics is used in the meso-scale bubble simulator. Thesimulator plays a role to bridge knowledge from micro-level simulation to that from macro-level CFD result. CFDsimulation is carried out to analyze an effect of laserirradiation on flow dynamics of water in the reservoir. Themeso-scale bubble simulator modeled a bubble formationinduced by laser irradiation, and it could evaluate both sizedistribution of bubbles and amount of generated bubbles.This properties related to the laser-induced bubble iscombined with the result of flow dynamics obtained fromCFD by using a KMC method. KMC simulates bubble flowdynamics in the reservoir that is directly comparable withexperimental observation. By using this strategy, weinvestigate a laser-induced water phase change behaviorbased on quantum molecular simulation technique.

Figure 1: Strategy for multi-level modeling of laser-inducedphase change behavior of water.

2.1 Computational Fluid Dynamics (CFD)

The differential equations for the continuous phase weresolved numerically based on the finite volume methodembedded in the PHOENICS package [7]. The geometry ofa cell is shown in Fig. 2. The area of the cell was dividedinto the 40 (20 mm) × 41 (20 mm) ×50 (50 mm) meshes.Inside the cell is filled with liquid water at atmospherecondition. IR laser is assumed to be irradiated at the rightside of the cell with the shape of circle having 1 mm inradius. Laser irradiation tuned at O-H bonding frequencyexcites the O-H bonding selectively and it might cause theincrement of pressure at the local region. Such local

pressure increment would generate a shock wave ofpressure in the reservoir. This phenomenon is representedin CFD calculation by setting the high pressure cell next tothe cell that the laser is irradiated. The outlet for pressurerelease is set at the bottom of the cell.

Figure 2: The geometry of a cell used in CFD calculations.

2.2 Bubble Simulator

We investigated meso-scale dynamics of laser-inducedbubble formation by KMC technique. Fig. 3 shows theschematic of bubble simulator based on KMC. It isassumed that laser irradiates from one side of a unit cell inwhich water particle is filled. If the laser collides with awater particle, the water is assumed to be excited andevaporated with 13 % of possibility. Water particle andevaporated water particle randomly move in the reservoirand if two bubbles collides each other, an aggregation ofbubbles is simulated by uniting them into one particle withthe increase in the particle size.

2.3 Kinetic Monte Carlo (KMC)

We carried out KMC simulation using the data of thesize distribution of bubbles obtained from a bubblesimulator to investigate dynamics of generated bubbles inmacro-scale. In this simulation, time evaluation of theposition of generated bubbles is determined withconsideration of flow velocity in the reservoir that isobtained from CFD calculation. A bubble represented bysphere model is generated at the laser irradiated point. Thefrequency and size of the generated bubble is determined

objective：Quantitative analysis of bubble formationfrom water by laser irradiation.

3.Meso-scale bubble simulator

objective：Analysis of flow dynamics ofwater in the reservoirinduced by laser irradiation.

Fluid velocity distribution inthe water reservoir

Size distribution of bubbles

objective：Prediction of experimental observable phenomenaof water reservoir by laser irradiation.

4.Kinetic Monte Carlo

1.Quantum molecular dynamics

Information of reaction of waterinduced by laser irradiation

2.Computational fluid dynamics

objective：Quantitative analysis of bubble formationfrom water by laser irradiation.

3.Meso-scale bubble simulator

objective：Analysis of flow dynamics ofwater in the reservoirinduced by laser irradiation.

Fluid velocity distribution inthe water reservoir

Size distribution of bubbles

objective：Prediction of experimental observable phenomenaof water reservoir by laser irradiation.

4.Kinetic Monte Carlo

1.Quantum molecular dynamics

Information of reaction of waterinduced by laser irradiation

2.Computational fluid dynamics

Pressure

controlled

region

Laserirradiatedregion

Pressure

release region

20 mm

20 mm

50 mm

Pressure

controlled

region

Laserirradiatedregion

Pressure

release region

20 mm

20 mm

50 mm

488Clean Technology 2008, www.ct-si.org, ISBN 978-1-4200-8502-0

based on the result of bubble simulator. Generated bubblechange its moving direction randomly where randomwalking model is assumed, and its velocity is changedbased on the flow velocity. Buoyancy effect is consideredfor the bubble movement.

Figure 3: Schematic of a bubble simulator implementedalgorithm of KMC for bubble formation.

3 RESULTS AND DISCUSSION

3.1 Flow Dynamics induced by IR Laser

One of the CFD simulation result is shown in Fig. 4.The velocity distribution viewed from the right side of thecell is presented. It is noted that jet-like flow is generatedfrom the irradiation point to the opposite face of the cell.The flow reaches to the opposite face of the cell andoverspread the cell surface that causes the continuousconvection flow in the reservoir. Calculated flow velocity atmaximum is approximately 100 m/s � 200 m/s. The instantrelease of high pressure from local region would generatethe observed jet-like flow. The left side figure in Fig. 4represents the velocity vector at each cell around the laserirradiated point. As shown in the figure, rather largevelocity distribution are observed around the pressurecontrolled region, while incoming flow towards thepressure controlled region is observed, which suppliesamount of water enough to generate jet flow. This resultindicates that the rapid flow is observed if high pressureregion is generated by laser irradiation. High pressureregion would be originated in the locally rapid evaporation.Fig. 5 shows the flow dynamics when the pulsed laser is

irradiated to the reservoir. We simulated sequence of fourpulsed laser as shown in Fig. 5 (a) and each pulsedirradiation gives the 20 mJ of the heat to the reservoir cell.Jet-like flow is observed after two pulsed laser irradiation.

This implies that the pulsatory motion of pressure isnecessary for the generation of the jet-like flow. Fig. 5 (b)shows observed velocity distribution in the reservoir fromthird pulse of irradiation. The generated flow run over thecell and reached to the opposite face of the cell. Then, theflow disappeared during the interval of the irradiation.

Figure 4: CFD result of the velocity distribution viewedfrom the right side of the cell.

Figure 5: (a) Schematic of pulsed laser irradiation and (b)velocity distribution in the reservoir.

3.2 Bubble Formation

CFD calculation reveals jet-like flow dynamics.However, in a continuum simulation technique like CFD,detail discussion of the bubble formation from molecularlevel reaction mechanism is difficult. Therefore, we carriedout a bubble simulator. Fig. 6 shows the time evolution offormed bubbles calculated by the bubble simulator. Asshown in this figure, many bubbles are generated aroundthe surface where the laser is assumed to be irradiated. Asincreasing the time, the bubbles grow and become larger.From this simulation, the bubble size distribution andamount of bubble can be evaluated as shown in Fig.7.

generation

①

move

②

D�taggregation

③

Laser irradiation

(a)

(b)

generation

①

move

②

D�taggregation

③

Laser irradiation

generation

①

generation

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③③③③③③③③③③③③③③

aaaaaaaaaaaaaggggggggggggggggggggggggggrrrrrrrrreeeeeeeeeegggggaggregation

③

Laser irradiation

(a)

(b)

0.0014 s

20ｍＪ 20ｍＪ 10ｍＪ20ｍＪ

(a)

0 0.0004 0.0006 0.0010 0.0012 0.0016 0.0018 [s]

(b)

0.0016 s 0.0018 s0.0014 s

20ｍＪ 20ｍＪ 10ｍＪ20ｍＪ

(a)

0 0.0004 0.0006 0.0010 0.0012 0.0016 0.0018 [s]

(b)

0.0016 s 0.0018 s

flow

Velocity [m/s]

flow

Velocity [m/s]

489Clean Technology 2008, www.ct-si.org, ISBN 978-1-4200-8502-0

Laser irradiation

bubble

0 ns

10 ns

30 ns

100 ns

Laser irradiation

bubble

0 ns

10 ns

30 ns

100 ns

Figure 6: The snapshots of dynamics of bubble formation.

Figure 7: Evaluated bubble size distribution and amount of

bubble.

(a) (b)

(c) (d)

(a) (b)

(c) (d)

Figure 8: Snapshot of KMC simulation result of bubble

dynamics in the reservoir cell. (a) 0.02 s, (b) 0.045 s, (c)

0.405s and (d) 2.005 s.

3.3 Bubble Dynamics in the Reservoir Cell

We carried out the KMC simulation of bubble dynamics

in the reservoir using the size distribution of bubbles

obtained from the bubble simulator. In this simulation, time

evaluation of position of generated bubbles is calculated

with the consideration of flow dynamics in the reservoir

evaluated by the CFD calculation. Fig. 8 shows the

calculated result by KMC. The bubbles are generated at the

laser irradiation point and diffuse the entire cell conveyed

by the water flow. By this analysis, direct comparison of

simulation result with experimental one is realized.

4 SUMMARY

In this study, we attempted to establish the multi-level

simulation methodology based on the atomistic- and

macroscale simulation technique to investigate chemical

reaction dynamics of water induced by IR laser irradiation.

Bubble formation induced by laser irradiation is modeled

by using a kinetic Monte Carlo (KMC) method. KMC

calculation estimated the size distribution and amount of

the microbubble generation. The KMC technique is

combined with CFD simulation to reveal the flow dynamics

simulation on water reservoir. Our simulation result

predicts that the water bubbling occurs and that the rapid

convectional flow of micro-bubble takes place by applying

infrared laser irradiation to the bulk water. This result

promises to open the door of novel technology utilizing

chemical reaction induced IR laser irradiation.

REFERENCES

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1972.

[4] M. Elanany, P. Selvam, T. Yokosuka, S. Takami, M.

Kubo, A. Imamura, and A. Miyamoto, J. Phys. Chem.

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[5] P. Selvam, H. Tsuboi, M. Koyama, A. Endou, H.

Takaba, M. Kubo, C. A. Del Carpio, and A. Miyamoto,

Rev. Eng. Chem., 22, 377, 2006.

[6] A. Endou et al., to be presented in NSTI Nanotech 2008.

[7] S. V. Patankar, Numerical Heat Transfer and Fluid

Flow, Hemisphere Publishing Corporation, New York,

1980.

490Clean Technology 2008, www.ct-si.org, ISBN 978-1-4200-8502-0