INVESTIGATION OF SOLVATION OF RADICALS PRODUCED BY MICROPLASMAS IN BUBBLES IN WATER* Wei Tiana) and Mark J. Kushnerb) a)Department of Nuclear Engineering and Radiological Science University of Michigan, Ann Arbor, MI 48109, USA bucktian@umich.edu b)Department of Electrical Engineering and Computer Science mjkush@umich.edu International Workshop on Microplasmas May 2013 Good morning everyone, I am Wei Tian, working with Prof. MJK at UMich. Welcome to my presentation today. I am going to talk about the discharges in bubble in water, my work title is “INVESTIGATION OF SOLVATION OF ….” This work is supported by the Department of Energy, Office of Fusion Energy Science. * Work supported by Department of Energy Office of Fusion Energy Science and the National Science Foundation.

University of Michigan Institute for Plasma Science & Engr. AGENDA Introduction to plasma interactions with liquid water Description of models Optical emission compared with experiments in gas phase Reactions on water surface Reactions in bulk water Concluding Remarks I will begin with ... ...will be briefly described. My work bases on the comparison in OES between model and experiments. The model will be extended to Reactions in liquid water Concluding Remarks are emphasized. University of Michigan Institute for Plasma Science & Engr. IWM2013

DISCHARGES IN BUBBLES IN WATER Plasmas in bubbles in water are of interest for water purification, especially in destruction of organic compounds and microorganisms. Chemical: produces reactive species such as hydroxyl radical, hydrogen peroxide, reactive oxygen species Radiative: UV and visible emission Mechanical: Shockwave production Plasmas in bubbles in water are of interest for water purification and biomedicine. Mechanisms of producing active species in water are poorly understood, having timescales of nanoseconds to hours. As shown in this figure, The radical production mechanism consists of three part at different time scales, reactions in gas phase, ns - us reactions on the water surface, us - ms reactions inside the liquid water. ms - longer (This part can be referred to the paper) Electrode configurations used to study tdischarges in bubbles in liquid. P. Bruggeman et al, J. Phys. D: Appl. Phys. 42(2009) 053001 University of Michigan Institute for Plasma Science & Engr. IWM2013

TRANSPORT OF REACTIVE SPECIES: GAS TO LIQUID In the absence of discharges in water , reactive species produced in the gas phase must penetrate through the plasma–liquid interface. Transport and solvation occurs on timescales of ns to s. Plasmas in bubbles in water are of interest for water purification and biomedicine. Mechanisms of producing active species in water are poorly understood, having timescales of nanoseconds to hours. As shown in this figure, The radical production mechanism consists of three part at different time scales, reactions in gas phase, ns - us reactions on the water surface, us - ms reactions inside the liquid water. ms - longer (This part can be referred to the paper) Gas Phase (ns ~ ms) Water Surface (ms ~ ms) Liquid Phase (ms ~ longer) University of Michigan Institute for Plasma Science & Engr. P. Bruggeman et al, J. Phys. D: Appl. Phys. 45, 253001 (2012) IWM2013

IN PRACTICE, NON-PRISTINE WATER Most biological fluids are not pristine water but are suspensions and solutions of organic solids. Dissolved gases, such as O2 and CO2 “RH”(alkane-like hydrocarbons) Suspensions of proteins, blood cells, bacteria. In this presentation, we present results from a computational investigation of micro-plasmas sustained in water. Production of reactive species in gas phase in bubbles. Transport of reactants into water (solvation) Production and transport of reactants in water. Interaction with model “bacteria”. University of Michigan Institute for Plasma Science & Engr. IWM2013

MODELING PLATFORM: nonPDPSIM Poisson’s equation: Transport of charged and neutral species: Charged Species:  = Sharffeter-Gummel Neutral Species:  = Diffusion Surface Charge: Electron Temperature (transport and rate coefficients from 2-term spherical harmonic expansion solution of Boltzmann’s Eq.): The model we used here, nonPDPSIM, is shown here. The fundamental equations are solved here, Poisson’s equation for electric potential, the continuity equation for the transportation of charged and neural species. the surface charge is also included, The electron temperature is obtained by solving the electron energy equation. University of Michigan Institute for Plasma Science & Engr. IWM2013

MODELING PLATFORM: nonPDPSIM Radiation transport and photoionization: Bubbles are assumed to be static (not deformed) over tens of milliseconds. Liquid water represented as dielectric (no reactions) or as a “real plasma” (with reactions). Water vapor diffuses into the bubble from the water-boundary where its density is given by the saturated water vapor ( ≈27 Torr at 1 atm). Oxygen naturally dissolved in liquid water ( equivalent to 3 Torr at 1 atm) through diffusion from the surrounding air. After the charge density and potential have been updated, Radiation transport is addressed using a Green’s function approach. Bubbles are assumed to be static (not deformed). Liquid water represented as dielectric (no reactions) or as a “real plasma” (with reactions). Water vapor is allowed to diffuse into the bubble from the water-boundary where its density is given by the saturated water vapor ( ≈ 27 Torr at 1 atm). Oxygen will be naturally dissolved in liquid water ( ≈ 3 Torr at 1 atm) through diffusion from the surrounding air. University of Michigan Institute for Plasma Science & Engr. IWM2013

University of Michigan Institute for Plasma Science & Engr. TREATMENT OF LIQUID Liquid plasma is treated identically to gas as a partially ionized substance. Higher density Specified susceptibility/atom to provide known permittivity Surface tension is addressed by specifying species able to pass through vapor/liquid interface. Diffusion into water is limited by Henry’s law equilibrium at the surface layer. Solvated species “stay in liquid”. Liquid evaporates into gas with source given by its vapor pressure. Evaporation Ions Dissolved gases Solvated Liquid University of Michigan Institute for Plasma Science & Engr. IWM2013

WATER REACTION MECHANISM Plasma H,OH,O,O3, O2,NO,NO2 Photons, M* M+ e M- Water R RH H H2 H OH H2O+ (H2O)e O2- OH- H,OH,O,O3, O2,NO,NO2 OH O,O- H3O+ O3,O3- RH R H2O2 H3O+ RH N2O3,N2O4 R R RH O2 H HO2 H3O+ NO2- NO3- = reaction with H2O University of Michigan Institute for Plasma Science & Engr. IWM2013

DISCHARGES IN GAS PHASE First part, the discharges in gas phase. DISCHARGES IN GAS PHASE IWM2013

DISCHARGES IN BUBBLES IN WATER Model results compared to experiments by Tachibana et al. A reproducible single self-standing bubble with synchronized discharge enables wavelength resolved optical emission. Model results are compared to experiments by Tachibana et al. A reproducible single self-standing bubble with synchronized discharge enables wavelength resolved optical emission. The bubble is born by injecting gas through the nozzle and grow slowly. The size of bubble is controllable. This picture shows the discharges inside the bubble. K. Tachibana et al, Plasma Sources Sci. Technol. 20, 034005 (2011) University of Michigan Institute for Plasma Science & Engr. IWM2013

University of Michigan Institute for Plasma Science & Engr. MODEL GEOMETRY Bubble Radius 1.0 mm Inside bubble, He, Ar, N2 at 1 atm with H2O evaporating from the interface. Outside bubble – Pure liquid water as dielectric (no reactions) with negligible conductivity. Cylindrical symmetry. The left figure shows the geometry of the model. Bubble Radius 1.0 mm Inside bubble, He, Ar, N2 at 1 atm with H2O released from the interface. Outside bubble – Pure liquid water as dielectric (No reactions) with negligible conductivity. Powered electrode and grounded electrode are shown in the figure. The geometry is cylindrically symmetric with centerline of left boundary. University of Michigan Institute for Plasma Science & Engr. IWM2013

WATER VAPOR PRESSURE IN INJECTED GASES Water vapor evolves from the gas-water boundary. In He bubble, water vapor has a more uniform distribution compared to Ar and N2. Larger diffusion coefficient in He enables more rapid filling of bubble. Water vapor evolves from the gas-water boundary in different gases. Time up to 1 ms before pulse. Saturated water vapor pressure, that is the water vapor pressure on the interface, 27 Torr at 300 K. In He bubble, water vapor has a more uniform distribution, while for Ar and N2, water vapor is confined along the interface. Larger diffusion coefficient in He enables more rapid filling of bubble. Time up to 1 ms before pulse. Saturated water vapor pressure 27 Torr at 300 K. Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX IWM2013

University of Michigan Institute for Plasma Science & Engr. DISCHARGE EVOLUTION ne (cm-3) Te (eV) Here are animations showing the evolution of the discharges. Discharges in bubbles act as DBD. Electrons are strongly confined along the interface. He discharge has much higher Te than the other two. The discharges reach quasi-steady state after 10 ns. Discharges in bubbles act as DBD. Time ~ 10 ns Electrons are generated along the gas-water interface due to electric field enhancement resulting from change in dielectric constant. He discharge has much higher Te. The discharges reach quasi-steady state after 10 ns. Animation Slide University of Michigan Institute for Plasma Science & Engr. MIN MAX IWM2013

University of Michigan Institute for Plasma Science & Engr. EMISSION INTENSITY This slide gives the model results with comparison to experiments. The left figure shows the total emission intensity and the right one shows the Ha emission intensity. The top line gives the photos from the experiments, and the bottom line gives the results from our model. Generally, they agree very well. For the total emission, Discharges are confined to the gas-water interface, while penetrating into the volume for He and Ar. Ha emission are largely confined to the interface. It is consistent with the water vapor distribution. Discharge confined to the gas-water interface, while penetrating into the volume for He discharge. Ha emission largely confined to interface. Ha emission is strongest in He discharge and hardly seen in N2 discharge. University of Michigan Institute for Plasma Science & Engr. MIN MAX IWM2013

ELECTRON ENERGY RELAXATION LENGTH:  4/21/2019 The electron energy relaxation length is shown in the left figure. It is calculated from the solutions of Boltzmann’s equation for gases with 3% water vapor, which represents the situation in the right figure. The typical electron temperature for the discharges in our cases is from 1 to 10 eV. We can see that the lamda for N2 is very short, much smaller than the bubble size, so the electrons are confined along the interface, where they are generated. The lamda for He is much longer for a large energy range, so the electrons can travel further to the volume of the bubble and make ionizations. In the case of Ar, lamda drops very much as Te goes beyond 4 eV, which is the average electron temperature. As a result, Ar discharge confines itself along the surface compared to the He discahrge, but still has a further penetration compared to N2 discharge. This partly explains the volumetric discharge in He and the surface discharge in Ar and N2, shown in Fig. 4(a). Electron energy relaxation lengths  were obtained from solution of Boltzmann equation. For He and Ar, there is a peak (larger ) in the range of 2 ~ 6 eV. In N2  decreases monotonically with Te. Penetration of plasma into bubble from interface correlates with . University of Michigan Institute for Plasma Science & Engr. IWM2013 16

ELECTRON ENERGY RELAXATION LENGTH:  4/21/2019 With increasing water vapor concentration,  decreases due to inelastic collisions with water molecules. H2O has significant energy losses for 0.05 eV to 2 eV. Smaller effect in N2 which already has low energy inelastic losses. The lamda for the cases with different vapor densities are shown here, so we can see clearly the impact of water vapor. The lamda with pure water is also shown as dashed line as reference. Without water vapor, He and Ar have very large lamda at low Te. The lamda decreases sharply for N2 case due to the molecular vib and rot excitation. With 3% water vapor, the lamda has a significant drop at low Te for He and Ar, because water consumes a lot of energy from the eons. With more water vapor, 30%, the lamda drops for entire Te range. In this case, discharges will be all confined to the interface. University of Michigan Institute for Plasma Science & Engr. IWM2013 17

University of Michigan Institute for Plasma Science & Engr. EXCITATION REACTIONS e + H2O  H* + OH + e (18.3 eV) e + H2O  H + OH* + e (9.0 eV) He*(19.8 eV)+ H2O  H* + OH + He He*(19.8 eV)+ H2O  H + OH* + He Ar*(11.6 eV)+ H2O  H + OH* + Ar N2*(11.0 eV) + H2O  H + OH* + N2 The column bars represent the relative emission intensity of Ha and OH(A-X). The dominating production reactions are shown above it, including electron direct impact exciting dissociation and excitation transfer from injected gases to water molecules. In the figure, solid bars represent the experimental results and the dashed bars represent the model results. The agree well. For Ha emission, we have very intensive emi in He, but much less in Ar and N2. For OH emission, they are generally in the same level. These can be explained by the reactions above. The electron impact dissociation dominates during pulse, and the excitation transfer from excited species dominates during afterglow. The threshold for H* production is 18.3 eV, but OH* production is only half of that, 9.0 eV. For the excitation transfer processes, only He* can do it to H*, while all excited species can do it to OH*. These make differences on the emission intensities. Emission from Ha and OH(A-X), indicates electron impact dissociation dominates during pulse. Excitation transfer from M* dominates during afterglow. University of Michigan Institute for Plasma Science & Engr. IWM2013

University of Michigan Institute for Plasma Science & Engr. EMISSION VS POWER The relative emission intensity with increasing applied voltage is shown here, Ha emission in the left and OH emission in the right. The excited H atoms increase with pulse voltage. For Ar and N2 discharges, H* atoms has a linear increase in the log scale of y-axis. In Ar and N2 discarges, H* atoms production mainly comes from the electron impact dissociation of H2O. The rise of pulse voltage results in the rise of electron density and therefore in the rise of H* atom production. For He discharge, although both electron impact and excitation transfer processes contribute to the production of H* atoms, the well-established eon density quickly expelled the EF and therefore weakened the effect of voltage increase. The excited OH molecules also increase with pulse voltage. But N2 has a more significant rise than the other two gases. In N2 discharge, N2* molecule density increases very much due to the voltage rise and then transfers to the dissociative excitation of H2O. The Ha emission increases with voltage. Due to the already high Te in He, emission is less sensitive to voltage. N2 discharge has a sharp increase in OH(A-X) emission due to its efficient excitation transfer processes. University of Michigan Institute for Plasma Science & Engr. MIN MAX IWM2013

FLUENCES OF ROS TO WATER SURFACE Fluence (time integrated flux) of reactive oxygen species (ROS) to surface of water. 15 kV, 1 sec. The fluences of ROS are shown in this figure. The fluence is the time integrated fluxes onto the surface. The H2O2 fluence is much larger than the corresponding OH for each gas. The OH comes from the dissociation of H2O and dominates in the pulse. But after the pulse, OH turns into H2O2, a more stable radical, which eventually becomes dominant. Among the three gases, the fluence of OH and H2O2 is highest for Ar discharge and lowerest for He discharge, which is quite from the produced densities of OH and H2O2. The He discharge produced wide-spread distribution of OH and H2O2 in the pulse. After the pulse, the recombinative loss of OH and H2O2 played a significant role in the loss mechanism, so less is received by the interface. The Ar and N2 produce a narrow distribution and the diffusion dominates the loss. That is, majority of OH and H2O2 are received by the interface, which results in a relative high fluence. H2O2 results from OH association: OH + OH + M  H2O2 + M In N2 discharge, the H2O2 fluence is higher at the bottom of the bubble due to the more confined discharge. University of Michigan Institute for Plasma Science & Engr. IWM2013

DISCHARGES EXTENDED TO LIQUID PHASE The dissociation induced by electron and excited species in gas phase can also happen in liquid phase. Now, Let's move to liquid phase. IWM2013

REACTIONS ON AND INSIDE WATER On Water Surface Solvation (“aq” solvated in water) e (M±) + H2Oaq  eaq (M±aq) Dissociative Recombination eaq + H2O+aq  Haq + OHaq Charges Induced Dissociation eaq + H2Oaq Haq + OH-aq H2O+aq + H2Oaq  H2Oaq + H+aq + OHaq Penning Dissociation M* + H2Oaq  Haq + OHaq Photo-Ionization and -Dissociation hv + H2Oaq  eaq + H2O+aq hv + H2Oaq  Haq + OHaq In Bulk Water Negative Charge Transfer eaq + O2aq  O2-aq O-aq + O2aq  O3-aq eaq + OHaq  OHaq- Long-Lived Species Formation Haq + Haq  H2aq OHaq + OHaq  H2O2aq Haq + O2aq  HO2aq HO2aq  H+aq + O2-aq This slide gives a list of reactions on the water surface on the left and in the bulk water on the right. On the water surface All the charges will become solvated when they hit on water surface. Most of the species including charged and excited species result in dissociation of water. Photons induce both dissociation and ionization. In the bulk water, Mose eons are eventually attached to solvated O2, some to O3- and OH-. and long lived neutral species are H2, H2O2. HO2 automatically dissociates to H+ and O2-. It plays as a source of H+ in our model. G. V. Buxton, J. Phys. Chem. Ref. Data. 17, 513 (1988) D. J. Creasey, Geophysical Research Letters. 27, 1651(2000) S. Staehelin, Environ. Sci. Technol. 16, 676 (1982) R. E. Buhler, J. Phys. chem. 88, 2560 (1984) IWM2013

GLOBAL MODELING OF REACTIONS IN WATER Here is the results from a simple global model on reactions in water. Liquid water density is as high as 1022 cm-3 with dissolved O2 3ppm. We assume initial solvated electrons and H2O+ with density of 1015 c-3. The electrons are quickly attached to O2 to make O2-. Meanwhile, electron attached to aqueous water will dissociate water to OH-. H2O+ will lead to H3O+ with by production of OH, which eventually will become H2O2. The long lived species are While some species such as can also live for miliseconds. H2Oaq 1022 cm-3, O2aq 3 ppm Initial conditions: eaq and H2O+aq 1015 cm-3 The long lived species are H3O+aq, O2-aq, O3-aq, H2O2aq Some species such as OH-aq, OHaq, and HO2aq can live for milliseconds. University of Michigan Institute for Plasma Science & Engr. IWM2013

WATER PLASMA WITH “BACTERIA” Bubble Radius 1.5 mm Inside bubble uniform Ar/H2O = 97/3, 1 atm. Water treated as “real plasma” – simply a very “high pressure” plasma. Dissolved O2 ≈3 ppm Dielectric islands represent LARGE bacteria. /0≈10 and no conductivity. Here is our new geometry, similar to previous one, but the water now is treated as “real plasma” - simply a very “high pressure” plasma so that we can have reactions in it. It is set to be the liquid density as much as 1022 cm-3 O2 will be naturally dissolved in liquid water, in our case , 3 Torr or 1017 cm-3. In order to show the disinfection effect, two LARGE bacteria represented by dielectric islands are shown here. The fluxes onto the surface of the bacteria will be recorded. A single pulse of 15 kV is applied for 5 ns to start the plasma and then let the species react and diffuse up to 1 sec. Species are allowed to across the interface into the liquid water and become solvated. So they will eventually reach the bacteria. University of Michigan Institute for Plasma Science & Engr. IWM2013

BUBBLE DISCHARGE INTERACTING WITH WATER Ar/H2O = 97/3, 1 atm, 15 kV, 5 ns ne up to 2.4 x 1016 cm-3 Te up to 6 eV sustains ionization sources of 4.4 x 1024 cm-3-s-1 Streamer is launched with initial electron cloud of 1010 cm-3 near powered electrode Streamer is sustained by photoionization ahead of the streamer. Here are the animations showing the discharge inside the bubble. The water now is plasma. Applied by a 15 kV pulse and 5 ns duration, the discharge looks very similar to the previous cases and at the end it reaches quasi-steady state. Although not shown here, the water vapor was evaporating for 1 ms before the discharge, the same with previous case. Electron density increases up to cm-3 along the interface. Electron temperature has a maximum value of about 6 eV, also along the interface. The bottom is an enlarge view of the bacteria. Animation Slide University of Michigan Institute for Plasma Science & Engr. IWM2013 MIN MAX

University of Michigan Institute for Plasma Science & Engr. ION DENSITIES Ar/H2O=97/3, 1 atm, 15kV Time up to 1 sec Virtually all positive ions incident onto the water produce H2O+ . H2O+ ultimately leads to H3O+ while producing OH. H3O+ participates in equilibrium reactions which maintains its density. Here are animations showing the positive ions being produced inside the bubble and then diffuse into the liquid water. The maximum density is labeled at the top right. The time computed for this case is up to 1 second, long enough to see the reaction activities and diffusion in liquid water. Ar+ is produced in the pulse and then diffuse onto the bubble interface after pulse. The Ar+ has higher potential than H2O+, so Ar+ will charge exchange to aqueous H2O+. The lifetime of aqueous H2O+ is very short. In about microseconds, aqueous H2O+ leads to H3O+ and meanwhile OH is produced. H3O+ lived very long, at least up to 1 sec. It is the dominant positive ion. Animation Slide University of Michigan Institute for Plasma Science & Engr. IWM2013 MIN MAX

HYDROXYL AND ROS DENSITIES Ar/H2O=97/3, 1 atm, 15kV Time up to 1 sec Majority of OH are produced in the bubble along the interface. OH into water by diffusion and photo- dissociation. OH rapidly produces H2O2, which diffuses to the bacteria. HO2 formed by H reacting with dissolved O2, then decomposing to H+ and O2-. Here are animations showing hydroxyl species in liquid water. The hydroxyl species are the longest lived species in liquid phase. Aqueous OH is produced in many ways. The incident from the gaseous OH. The photodissociation of H2O at the interface. The dissociation of H2O by the charge exchange. Solvated OH quickly recombines to H2O2, which is less reactive than OH but will reach much further to the bacteria. As a result, OH only exists in a very thin layer under the interface, shown in the bottom enlarged view. H2O2 is a common oxidizing radical existing in liquid water. It is able to reach the bacteria 2. HO2 is also formed through H combining with dissolved O2, but in our model will quickly decompose to H3O+ and O2-. It is another channel to produce H3O+ in the liquid water. If the buffed water was used, the HO2 can maintain its density since the recombination increases a lot. Animation Slide University of Michigan Institute for Plasma Science & Engr. IWM2013 MIN MAX

NEGATIVE ION DENSITIES Ar/H2O=97/3, 1 atm,15 kV Time up to 1 sec O2 dissolved in water, 3 ppm O- is produced in gas phase, diffuses into bulk water, and then forms O3- with dissolved O2. O2- ALSO forms through electron attachment and dominates the solvated negative ions. Here are animations showing the negative ions in liquid water. O- is produced in gas phase and diffuse into bulk water, and then form O3- with dissolved O2. O2- forms through electron attachment. O2- and O3- are also produced in gas phase but too little, not shown here. Animation Slide University of Michigan Institute for Plasma Science & Engr. IWM2013 MIN MAX

University of Michigan Institute for Plasma Science & Engr. ROS DENSITY WITH RH “RH” (alkane-like hydrocarbon) 30 ppm. All reactions with ROS are the equivalent of being nearly gas kinetic OH + RH  H2O + R H2O2 + RH  H2O + OH + R HO2 + RH  H2O2 + R Even small amount of RH will consume the vast majority of ROS, converting to R (alkyl-like radicals) Only H2O2 and R reaches the bacteria. The water is very likely to contain RH (), for this case, 30 ppm. Animation Slide University of Michigan Institute for Plasma Science & Engr. IWM2013 MIN MAX

FLUENCE TO “BACTERIA” SURFACE (1 sec) The fluences of radicals onto the bacteria are shown in the figure. The bacteria 1 is 100 um away from the bubble and bacteria 2 is 400 um away. The radical fluences onto bacteria 1 are much higher than that onto bacteria 2. It shows the rapid decay of radicals with distance away from the bubble. In the case of bacteria 1, the H2O2 dominates the fluence, up to 1018 cm-2. The OH fluence is two orders lower. In the case of bacteria 2, the O3- dominates the radicals, with fluence of only 1015 cm-3. H2O2 and OH fluences are reduced by 4 orders. HO2 is below 1010 cm-3. O3- decays slower than H2O2. So we found that some radicals can reach further. The fluence of ROS onto “bacteria” strongly depends on surface position (but, of course, real bacteria move around!). The ROS decays with distance from bubble. H2O2 dominates fluence onto close bacteria 1. O3- dominates fluence onto far bacteria 2. University of Michigan Institute for Plasma Science & Engr. IWM2013

University of Michigan Institute for Plasma Science & Engr. CONCLUDING REMARKS The optical emission of plasmas in bubbles predicted by the model are in good agreement with experimental results. The electron impact dissociative excitation and excitation transfer processes dominate the production of excited states of H and OH. Discharge produced species diffuse onto the water surface and become solvated. The reactive gas phase species are quickly consumed liquid-phase reactions. OH, H2O2 and ROS are able to diffuse in water for 1 sec and finally reach the “bacteria”. Dissolved O2 aids in the production of ROS. RH (alkane-like hydrocarbon) consumes radicals rapidly producing large fluxes of R. University of Michigan Institute for Plasma Science & Engr. IWM2013