This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Molecular dynamics (MD) simulation is a very effective tool that gives a microscopic insight into the mechanisms of complex physical phenomena. This paper uses MD simulation to study the evaporation of a liquid from a heated surface. As for the argon/platinum model, a group of simulations starts from a fixed lower wall with the temperature of 110K. In this system, argon molecule numbers of 784, 1200, 1440 are
simulated respectively. Additional simulations for argon models are based on superheat conditions, which indicate that the variation of ultra-thin liquid film thickness is very small with the different numbers of argon molecules. Also, it shows that if the argon molecule numbers increase, the extra molecules accumulate near the cooling wall. In terms of the MD simulation for the water/magnesium model, water evaporates from a magnesium heating wall at different temperatures and an initial study has been carried out. Moreover, further
and more accurate simulations will be improved in the near future

2nd Micro and Nano Flows Conference (MNF2009)

Ji, C

Yan, Y

Yang, X

English

Brunel University Research Archive

2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 1 -Molecular Dynamics Simulation forMicroscope Insight of Liquid Evaporation on a Heated SurfaceXiao YANG, Chaoyue JI, Yuying YAN ** Corresponding author: Tel.: ++44 (0)115 951 3168; Fax: ++44 (0)115 951 3159;Email: yuying.yan@nottingham.ac.ukSchool of the Built Environment, University of Nottingham, UKAbstract Molecular dynamics (MD) simulation is a very effective tool that gives a microscopic insight intothe mechanisms of complex physical phenomena. This paper uses MD simulation to study the evaporation ofa liquid from a heated surface. As for the argon/platinum model, a group of simulations starts from a fixedlower wall with the temperature of 110K. In this system, argon molecule numbers of 784, 1200, 1440 aresimulated respectively. Additional simulations for argon models are based on superheat conditions, whichindicate that the variation of ultra-thin liquid film thickness is very small with the different numbers of argonmolecules. Also, it shows that if the argon molecule numbers increase, the extra molecules accumulate nearthe cooling wall. In terms of the MD simulation for the water/magnesium model, water evaporates from amagnesium heating wall at different temperatures and an initial study has been carried out. Moreover, furtherand more accurate simulations will be improved in the near future.Keywords: Molecular dynamics, Water, Evaporation, Magnesium, Argon1. IntroductionIn recent years, microscopic calculationapproaches are frequently used to investigatethe physical phenomena of fluid–solidinteractions at the very small scale [1–8],particular methods include moleculardynamics (MD) and Monte Carlo (MC)simulation. Using non-equilibrium moleculardynamics (NEMD) simulation with atemperature gradient imposed, Xue et al. havedemonstrated that the layering of a simplemono-atomic liquid does not have anysignificant effect on liquid–solid interfacialthermal resistance [1]. Freund studied a twodimensional (in the mean) liquid drop centredon a cold spot on an atomically smooth solidwall with evaporating menisci extending fromit onto hotter regions of the wall [2]. Oharaand Suzuki did the numerical simulation onliquid argon (without vapour phase) betweentwo solid walls and studied the intermolecularenergy transfer at the solid–liquid interface [3].Yi et al. performed MD simulations of thevaporisation phenomena of an ultra-thin layerof argon on a platinum surface. Theysimulated the entire vaporisation process fortwo temperature cases and the condensationprocess after complete evaporation [4]. Theeffects of surface wettability on the behaviourof liquid atoms near a solid boundary werealso studied by using MD simulation methods[5, 6]. Wemhoff et al. explored a hybridapproach to investigate a thin liquid argon filmon a solid surface by combining adeterministic MD simulation of the liquidregions with a stochastic treatment of the far-field vapour region boundary [7]. Liu [8] hasemployed the MC method to study Lennard-Jones liquid film and its vapour with aconsideration of attractive and short-rangedwall potential near the liquid–solid interface.From the previous studies noted above, itcan be seen that the MD method is often usedto investigate the chemical reactions inbioscience and physical phenomena of fluid-solid interactions at the nano scale. [9]Moreover, based on the MD investigations ofthe typical argon model, in the past few years,many people have started to apply the MDmethod to simulate the heat transfer processesof other fluids, such as water, CO2 etc.Mitsuhiro Matsumoto [10-11], used the MD2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 2 -method to simulate the phase change of argon,water, CO2, and SO2, in order to find thethermal properties of these fluids. Hans W.Horn et al [12] simulated the liquid-vapourphase change of water by using the TIP4P(transferable intermolecular potential function)model. Swaroop Chatterjee [13] used a simplepoint charge Statistical Process Control (SPC)model to investigate the thermodynamics,structure, dynamics and behaviour of water.Yang Zhen [14] investigated the structure anddynamic properties of water near an Au nano-particle under room temperature, so that thisinvestigation presumed the heat transferprocess between water and metal.Although many people have madesignificant efforts on the heat transfer processby using the MD method, so far, comparedwith the MD studies of argon, the MDsimulations based on water and organicworking fluids with metals or alloys in thethermal area are still infrequent and are at thebasic stage of the heat transfer study area.Therefore, this paper aims to investigatethe boiling process of water in order tosimulate the phase change of heat pipes in thefuture based on Ji and Yan’s works [15-16] ofPt-Ar MD simulations. However, in terms ofthis initial MD simulation, there are a fewdifficulties existing in the whole process, forexample the reactions between molecules.Thus, for my early work, consideration willonly be given for processes without anychemical reactions, which means the Mg alloywall is just a heating wall and thus simulatesthe movement trend of water molecules undertemperature rises. Further and more accuratesimulation will be carried out in the future.2. MethodologyMolecular dynamic simulation starts froma set of molecules occupying a region of space,with each assigned a random velocitycorresponding to the Boltzmann distribution atthe temperature of interest. The interaction ofthe molecules is prescribed, commonly in theform of a two-body potential energy, and thetime evolution of the molecular positions isobtained by integrating Newton’s equations ofmotion.iii amF  (1)Based on the integration over time, thebehaviours of the molecules such as theaverages of density, velocity, stress,temperature, fields etc. can be calculated.The Lennard-Jones (L-J) potential formulais a general and important method for MDsimulation, which is regarded as a suitablereference fluid for modelling properties of realfluids [8]. Generally, it shows the transitionsbetween vapour-liquid, solid-liquid and solid-vapour phases and the critical and triple points,which is expressed as follow:])()([4)( 612ijijijijijijijij rdrcr  . (2)The first term of the equation is an arbitrarybut convenient short-range repulsion whichprevents overlap of the atoms in space; whilethe second term represents the attractivepolarised interaction of neutral spherical atoms.rij is the intermolecular separation distancebetween particles i and j.  ij and ij are theminimum energy and the zero energyseparation distance relative to the pair,respectively. cij and dij are adjustableparameters which can be chosen to control themolecular interactions, for example, thewetting characteristic of fluid-solidinteractions. For the distances smaller than ij,the resulting force is repulsive; whereas it isattractive for larger distances.In terms of argon simulation, the L-Jformula gives accurate simulation for thesenon-polar molecules. The L-J potential isknown to give a reasonably quantitativedescription of liquid argon, with parameters: = 1.67×10-21 J, = 0.3405 nm,and c = d = 1.With regards to the water MD simulation,although it belongs to polar molecules, the L-Jpotential still can be used as the generalmethod for the following study. However, the2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 3 -force fields are quite different to argon.According to previous research, [9-14,17]taking water as an example, there are mainlythree kinds of force fields frequently used:central force (CF), transferable intermolecularpotential function (TIPS) and the simple pointcharge (SPC).The central force (CF)CF force field is given by the expressionH+ -H+ interaction:)]05.2(40exp[1181345.36)(rrrVHH])45251.1(61277.7exp[17 2 r(2)O2- -H+ interaction:)]05.1(40exp[11023403.6269.72)( 19912.0rrrrVOH)]2.2(49305.5exp[14r(3)O2- - O2- interaction:])4.3(4exp[25.02.26758538.144)( 28591.8  rrrrVOO])5.4(5.1exp[25.0 2 r (4)where r is distance between molecules.Transferable intermolecular potentialfunction (TIPS)The TIPS force field evolved from the MonteCarlo method and it has been constructed witha rigid theoretical model. AaBb abbaabbaabbaAB rCCrAAreqqV )( 6122(5)where a and b represent atoms in differentmolecules. q0 =-0.80, qH =0.40 is charge ofoxygen atom and charge of hydrogen atomrespectively. Based on the force field above, aseries of similar force fields have beenapproached, such as TIP4P, TIP5P and others.Simple point charge (SPC)The water model is also assumed as a rigidtheoretical model:O2- - O2- interaction:6122)(OOOOOOOOOOOO rCrAreqqrV  (6)O2- -H+ interaction:OHHOOHOH reqqrV2)(  (7)H+ -H+ interaction:HHHHHHHH reqqrV2)(  (8)It can represent reasonable radial distributionfunction and thermodynamic properties byusing SPC model. However, the SPC modelleads to a large deviation with diffusionconstant between theoretic data andexperimental data (normally 200%-300%) [17].Moreover, the following table shows acomparison of the parameters between SPCand TIPS models.Table1: Parameters comparison of SPC andTIPS models.Parameters SPC TIP3P TIPS2 TIP4Pr(OH) /。A1.0 0.9572 0.9572 0.9572∠HOH/(。) 109.47 104.52 104.52 104.52A×10-3/(。A 12.kcal/mol)629.4 582.0 695.0 600.0C/(。A 6.kcal/mol)625.5 582.0 695.0 600.0qO -0.82 -0.834 0.0 0.0qH 0.41 0.417 0.535 0.52qm 0.0 0.0 -1.07 -1.04r(OM) /。A0.0 0.0 0.15 0.152nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 4 -Based on the comparison above, TIP4P modelwill be used in this paper.In addition, as for the integration time step,it can be selected as follows: the time stepshould be smaller than 10% of the systemlongest movement period. According toInfrared spectrometry method, the fastestfrequency of the vibration is v = 1.08×1014 s-1and period of the vibration can be calculated asT = 1/v = 0.92×10-14s [17]. Therefore, theintegration time step can be determined asv×10% = 0.92×10-14×10% = 0.92×10-15 s ≈ 1fs.3. Results and discussionIn the study of liquid–vapour–solidsystems near the triple-phase contact line offlow boiling in a microchannel, Ji and Yancarried out MD simulation of Argon and foundthat systems with different numbers of argonmolecules, the film thickness remains almostthe same if the heater temperature is constant.Fig. 1 shows the ultra-thin liquid film of thesystem with different numbers of argonmolecules under the same temperatureconditions. For a fixed lower wall temperatureat 110 K, the systems, respectively, with 784,1200 and 1440 molecules are simulated. It canbe seen that for a given superheat, thevariation of ultra-thin liquid film thicknesswith different number of molecules is verysmall. With the increase of argon number,extra molecules are accumulated near thecooling wall (not shown in Fig. 1).Fig. 1: Snapshots of different Ar moleculenumber system.Additional simulations also mentioned thatthe liquid film thickness decreases with theincrease of wall heating temperature. Fig. 2shows the liquid film of a system of 1200molecules under different temperature. Whenthe heating temperature reaches 210 K, theliquid film still exists but its thickness isdecreased to the order of only one layer ofargon molecules.Fig. 2: Snapshots of 1200 Ar at different heattemperatures.As for the water-magnesium model,regarding this initial simulation, the TIP4Pmodel was chosen for the water model and anMg wall was used in a hexagonal close-packed(hcp) lattice structure. The size of thecomputational domain is 5nm×5nm×8nm,which contains the magnesium atoms solidwall and water molecules. The Mg wall isplaced at z=0nm and the initial systemtemperature is the same as the experimentalstarting temperature (34ºC). The domain ispresented in Fig. 3 and relevant time step setup as 1 fs.Fig. 3: Sketch of initial domain.3D Snapshots of the computational domainare shown at different temperatures in Figure 4.The Mg wall atoms are denoted by the circlesymbols in green, the H atoms and O atomsforming the liquid film are shown in silver and2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 5 -red respectively. Fig. 4 (a) presents theequilibrium period at the start. As theoperating temperature increases, the liquidwater molecules become vapour moleculesand keep rising, then they escape from liquidfilm (see Fig. 4 (b) & (c)). Fig. 4 (c) and (d)illustrates the evaporation process of the waterliquid film. From this process, it can be seenthat the stable system is distorted with thetemperature growth and then becomes unstable;finally the system becomes saturated when thetemperature of the heated wall passes thesaturated point and boiling starts. Meanwhile,Fig. 4 (d) also indicates that under theLennard-Jones [33] potential, there exists agap between the H2O liquid film and the Mgheated wall. Although this initial simulationdoes not provide accurate density valuechanges, the trend still clearly shows that withrising temperature, the more liquid vaporizedthe more vapour molecules presented invapour phase.(a) 34 ºC(b) 55 ºC(c) 90 ºC(d) 100 ºCFig. 4: Snapshots of water 3D simulationdomain at different heating temperatures.From the Ar-Pt studies (Ji’s work), weknow that the liquid film thickness is definedas the distance from the wall to the centre ofliquid–vapour interfacial region and can becalculated from the density distributions. InFig. 5 which is the density distribution ofargon molecules, it can be seen that thedensities in the z direction exhibit anoscillation structure and different regions ofthe liquid, interface and vapour. Fig. 5 showsthat the region of liquid–vapour interface isspecified by density distributions which willlead to some uncertainty; and also points outthat the liquid film thickness is within 2 nmand has a tendency to decrease with anincrease of heating temperature. This meansthat more liquid molecules may escape fromthe attractions of solid as the heater walltemperature continue to rise.2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 6 -Fig. 5: Density distribution of 1200 Armolecules system (90–110 K).Compared with Ar-Pt density chart,indicated in Fig. 6, is the variation in thenumber density of water along the height in Zdirection. It can be seen that with thetemperature increase, the number density ofwater molecules decrease gradually in therange of height from 0.1nm to 1.2nm, whichshows that the liquid molecules reduces withmore evaporation happen. Moreover, at aheight of 1.2nm, the number density presents ahuge difference, which can be recognized asthe boundary between the liquid phase and thevapour phase. According to some literaturewhen compared with this result, we mightconsider that the liquid film exists around thisposition. And then, after 1.2nm in height theliquid and vapour interface should be found,and then the vapour phase. Thus, the wholetrend of density variation is similar with thechange shown in previous sketches.Fig. 6: Variation of water molecules numberdensity with different height in Z direction.4. ConclusionAccording to the MD simulations of argonand water, which are based on NEMD, someconclusions can be made as follow:1. For the Ar-Pt system, under the systemwith different numbers of argon molecules,the film thickness remains almost the sameif the heater temperature is constant.Moreover, the liquid film thicknessdecreases with the increase of wall heatingtemperature if molecule numbers areconstant.2. For the H2O-Mg system, the liquid filmposition and vapour phase can be estimatedbased on NEMD.3. Therefore, comparing these two systems, itcan be seen that the trends of liquid filmchanges are similar. The liquid filmthickness of H2O-Mg system decreaseswhen heating temperature increases as well.However, compared with the Ar-Pt work,this initial study of H2O-Mg MD simulation isnot sufficient to reach firm conclusions aboutthe use of magnesium alloys in heat pipes.Therefore, further work will be carried outusing MD simulation, which will involve moredetailed simulation and further accuracycorrection.NomenclatureA constanta accelerationB ConstantC ConstantF force, NM centre of massm massq charger radiusV forceGreek symbols potential energy energy parameter of L-J potential length parameter of L-J potential surface tension2nd Micro and Nano Flows ConferenceWest London, UK, 1-2 September 2009- 7 -Subscriptsa atom on different H2O moleculesb atom on different H2O moleculesc atom on different H2O moleculesOO force between two oxygen atomsOH force between hydrogen and oxygenatomsHH force between two hydrogen atomsOM force between oxygen atom and centreof massReference[1] L. Xue, P. Keblinski, S.R. Phillpot, et al.Effect of liquid layering at the liquid–solidinterface on thermal transport. InternationalJournal of Heat and Mass Transfer. 47 (2004)4277–4284.[2] J.B. Freund. The atomic detail of anevaporating meniscus. Physics of Fluids. 17(2005) 22104.[3] T. Ohara, D. Suzuki. Intermolecular energytransfer at a solid–liquid interface. MicroscaleThermophysical Engineering. 4 (2000) 189–196.[4] P. Yi, D. Poulikakos, J. Walther, G.Yadigaroglu. Molecular dynamics simulationof vaporization of an ultra-thin liquidargonlayer on a surface. International Journalof Heat and Mass Transfer. 45 (2002) 2087–2100.[5] G. Nagayama, P. Cheng. Effects ofinterface wettability on microscale flow bymolecular dynamics simulation. InternationalJournal of Heat and Mass Transfer. 47 (2004)501–513.[6] J.A. Thomas, A.J.H. McGaughey. Effect ofsurface wettability on liquid density, structure,and diffusion near a solid surface. Journal ofChemical Physics. 126 (2007) 34707.[7] A.P. Wemhoff, V.P. Carey, Moleculardynamics exploration of thin liquid films onsolid surfaces. 1. Monatomic fluid films.Microscale Thermophysical Engineering. 9(2005) 331–349.[8] K.S. Liu. Phase separation of Lennard-Jones systems: a film in equilibrium withvapour. Journal of Chemical Physics. 60 (1974)4226–4230.[9] Rapaport, D. C. Art of MolecularDynamics Simulation. West Nyack, NY, USA:Cambridge University Press. (2004).[10] Mitsuhiro Matsumoto. Moleculardynamics of fluid phase change. Fluid PhaseEquilibria. 144 (1998) 307–314.[11] Mitsuhiro Matsumoto. Moleculardynamics simulation of interphase transport atliquid, surfaces. Fluid Phase Equilibria. 125(1996) 195-203.[12] Hans W. Horn, William C. Swope, andJed W. Pitera. Characterization of the TIP4P-Ew water model: Vapor pressure and boilingpoint. The Journal of Chemical Phsics. 123(2005) 194504.[13] Swaroop Chatterjee, Pablo G.Debenedetti, Frank H. Stillinger. Acomputational investigation ofthermodynamics, structure, dynamics andsolvation behavior in modified water models.The Journal of Chemical Phsics. 128 (2008)124511.[14] YANG Zhen, YANG XiaoNing and XUZhiJun. Molecular Dynamics Simulation ofStructure and Dynamics Properties of WaterNear an Au Nanoparticle. Acta Phys. -Chim.Sin. 24 (2008) 2047-2052.[15] C.Y. Ji, Y.Y. Yan. A molecular dynamicssimulation of liquid–vapour–solid system neartriple-phase contact line of flow boiling in amicrochannel. Appiled Thermal Engineering.28 (2008) 195-202.[16] Y.Y. Yan, C.Y. Ji. Molecular DynamicsSimulation of Behaviours of Non-PolarDroplets Merging and Interactions withHydrophobic Surfaces. Journal of BionicEngineering. 5 (2008) 271–281.[17] Chen, C. L. Theory and Application ofMolecular Dynamics (Handout). (1998)

Molecular dynamics simulation for microscope insight of liquid evaporation on a heated surface

Abstract

Similar works

Full text

Available Versions

Brunel University Research Archive