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On the spontaneous electric-bipolar nature of aerosols formed by mechanical disruption of liquids
Colloids and Interface Science Communications 7 (2015) 7–11
Contents lists available at ScienceDirect
Colloids and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
On the spontaneous electric-bipolar nature of aerosols formed by mechanical disruption of liquids Thiago A.L. Burgo ⁎, Fernando Galembeck Institute of Chemistry, University of Campinas, Campinas, SP 13083-970, Brazil National Nanotechnology Laboratory at the National Center for Energy and Materials Research, Campinas, SP 13083-970, Brazil
a r t i c l e
i n f o
Article history: Received 31 July 2015 Received in revised form 31 October 2015 Accepted 13 November 2015 Available online 7 January 2016 Keywords: Aerosol formation Bipolar charging Water splashing Balloelectric effect Spray charging
a b s t r a c t Aqueous aerosols are widely found on the Earth's atmosphere and they participate from any anthropic environment. We show here that aerosols produced by a nebulizer based on splashing contain both positive and negative droplets, this means, it is bipolar but the overall aerosol charge is usually non-zero. Charge distribution within the aerosol is by itself fractal, as previously observed in other cases of mechano-chemical charge formation that is an important mechanism for producing triboelectricity in solids. The present information added to the other authors' work leads to a particle charging mechanism based on charge partition at interfaces combined to the variability of interfacial area/volume ratio in aqueous droplets, thus explaining the formation of bipolar aerosol. The demonstration of charge bipolar distribution within aerosols contributes to explain the ubiquity of electric charge patterns in solid surfaces that is receiving growing evidence and contributes to understand both beneficial and damaging outcomes of electrostatic charging. © 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Most natural and anthropic gaseous environments contain water droplets with diameters ranging from a few nanometers (e.g. close to waterfalls) to a few millimeters (e.g. in rain). They display significant surface areas (e.g. 30 m2/g for 100 nm droplets) that produce a host of interfacial phenomena. Interfaces of condensed phases are always important sites for charge accumulation and exchange [1] and charge at the gas–liquid interfaces [2] and also in bulk water has been previously detected [3] but reproducible procedures for imparting net charge to water drops and observing its effect on the liquid surface and bulk properties were only recently published [4]. Many scientific disciplines study water and other liquid droplets in the atmosphere paying greater or lesser attention to their electrification but there is no consensus on the mechanisms for droplet charging and divergent views are found in recent literature [4,5]. Aerosols have paramount importance in atmospheric phenomena [6], including a large number of chemicals discharged in the atmosphere by natural and anthropic phenomena. Aqueous aerosols are particularly relevant for cloud formation, stability and rain precipitation, while clouds are also important as precursors of atmospheric aerosols formed by other chemical substances [7,8]. Many authors in this area relate atmospheric electricity to liquid water or ice particles and they assign the origins of atmospheric electricity to diverse events, like atmospheric gas ionization due to radiation and ice particle breakdown [9]. Water ⁎ Corresponding author at: Institute of Chemistry, University of Campinas, Campinas, SP 13083-970, Brazil. E-mail address: [email protected] (T.A.L. Burgo).
splashing is another source of charged aerosol particles [10,11,12]. The formation of electrified drops by mechanical disruption of liquids is named balloelectric effect, arising from statistical fluctuations of ion density in the liquid [13] and presumably contributing to cloud electrification [14]. Many researchers undertook laboratory studies of aerosol formation, properties and stability, often related to important practical problems in industrial, energy and health contexts. However, aerosol charge is not even mentioned in many publications on aerosols. This situation is quite different from the literature on liquid sols, where zeta potentials or particle charge data appear in nearly every paper. Moreover, hydrophobic [15] and hydrophilic [16] solid surfaces acquire charge when exposed to water vapor or liquid, due to asymmetric partitioning of hydroxide ions at water-hydrophobic interfaces [17] or to Brønsted acid or base character of solid surfaces [18], as in hygroelectricity [19]. The atmosphere is thus a reservoir of electric charge. An additional factor for water ion partition is the electrochemical potential [1] of hydronium or hydroxide ions that predicts excess concentration of H+ under a negative potential and of OH− under a positive potential. Since the Earth's surface and atmosphere are electrified environments displaying large electric fields [20], excess positive or negative charge in water is expected in water [19]. There is abundant but often uncorrelated information on charge appearance in aqueous aerosols and droplets, where a detailed laboratory investigation by Takahashi [21] led to a mechanism for charge generation on thunderstorms but depending on ice crystal breakdown. Here we describe unprecedented reproducible laboratory experiments
http://dx.doi.org/10.1016/j.colcom.2015.11.002 2215-0382/© 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
8
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
showing the formation of spontaneously bipolar-charged aerosols due to the mechanical disruption of water and aqueous NaCl solutions, together with the effect of composition, followed by a discussion of its implications and proposing a mechanism for charge formation. The literature describes elaborate procedures for measuring particle size and charge, in aerosols, targeting nanoparticles [22]. These instruments provide detailed information but they involve significant particle handling, including exposure to radioactive charge emitters. In an alternative robust and reproducible method to study charge in aerosol produced by a nebulizer, the flowing aerosol passes in the interior of a Faraday cup while measuring the current between the electrodes, as schematically shown in Fig. 1. The Faraday cup is made of two concentric copper cylinders and connected to an electrometer (Keithley 6514) through a low-noise triaxial cable. Current measurements are made under high-speed acquisition rate (800 readings/s), using an USB-to-GPIB interface (Keithley KUSB-488b) at binary transferring format to a computer (NI LabVIEW 2011). Positive current means that
the net charge of the set of particles entering the Faraday cup is positive, and vice-versa. Another way to demonstrate the co-existence of positive and negative charges is electrophoresis, in the gas phase. Parallel copper disks (20 cm diameter) were connected to high voltage sources, up to ±30 kV (Spellman CZE1000R) and they were video recorded (Canon EOS 60D illuminated with a blue LED), as schematically shown in Fig. 1(b). These two procedures do not give detailed information on particle size and charge distribution but they yield information on particles as they leave the aerosol source, with minimal particle manipulation. Reading current peaks instead of sorting out individual particles to measure their charges is analogous to using energy-dispersive (EDX) techniques in X-ray spectrometry, instead of using wavelength dispersive (WDX) methods. Surprisingly, every experiment made in this laboratory to detect excess electric charge in aerosols formed by mechanical means or in water vapor revealed an intrinsic electrostatic charging behavior. In
Fig. 1. Setup to measure aerosol charging and electrophoresis. (a) The electrostatic charge of the liquid/reservoir set is measured with a Kelvin probe while the current or charge of the released aerosol is measured with a Faraday cup. (b) Set-up for observing aerosol motion within the electric field created by two copper disk electrodes connected to a high voltage DC power supply.
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
some experiments, short pulses of water and ethanol spray (produced using a laboratory device used for spraying staining solutions) were driven into a Faraday cup and the resulting current measurements are shown in Fig. 2a–b. Each water spray pulse introduced a positive charge in the cup, detected as a short current peak reaching 15 nA. Ethanol spray pulses also introduced charge in the Faraday cup but this is bipolar, as seen in the inset of Fig. 2b. Rinsing or spraying with ethanol was recently recognized [23] as an effective solution to neutralize electrostatic charges on insulator surfaces and this may derive from the coexistence of positive and negative water droplets, as evidenced in this experiment. Aerosol formed by the condensation of water vapor produced in a boiler and driven to the input of the Faraday cup also produces oscillating current as shown in Fig. 2(c). Stopping the entrance of vapor flow into the cup produces larger and less noisy current that is probably related to temperature change in the cup, allowing further vapor condensation. When aerosol from deionized water stemming out of a nebulizer passes through the inner electrode of the Faraday cup it produces initially highly variable positive current (Fig. 3(a)), changing to a time series of sharp positive and negative current peaks. When the aerosol is made using an aqueous NaCl solution, a similar result is obtained but the initial current is negative (Fig. 3(b)). Each plot in this figure contains 240,000 experimental readings and thus each peak is drawn using ca. 40 experimental points, sufficient to define properly its shape. The noise level is typically 1/100 of the recorded peak signals and peak charge is generally lower than in the plots acquired at low speed. The positive and negative peaks evidence the successive entrance of positive and negative droplet clusters within the Faraday cup, at different moments. Most peaks are well resolved but the plots also contain a few doublets or broader bands. Current peak half-widths are in the range of 0.01 to 0.1 s. Also, as expected from larger ion density fluctuations [13] in NaCl
9
Fig. 3. Electric current between the electrodes of a Faraday cup, produced by passing aerosol from (a) deionized water and (b) sodium chloride solution (1 mmol L−1) measured at a high-speed acquisition rate (800 readings/s). Positive and negative peaks show that oppositely charged particles or particle clusters coexist in the aerosol.
Fig. 2. Electric current produced by introducing aerosol in a Faraday cup: (a) demineralized water spray jets with a photo of the spraying device used, (b) ethanol spray jets, and (c) vapor and aerosol from boiling water. The insets in (a) and (b) are close-ups of the current peaks measured soon after 31 s.
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T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
solution, more positive and negative peaks are counted per unit time in the NaCl aerosols, when compared to water (insets of Fig. 3). Together, these observations show that the aerosol is divided in clusters of water droplets carrying net positive or negative charge. Current vs. time plots display fractal profiles (Dwater = 1.44 and DAqNaCl = 1.42, calculated using Higuchi's method) evidencing that positive and negative clusters are subdivided in smaller sub-sets that can probably be detected by increasing the spatial and time resolution of the detector, this means, by using smaller Faraday cups and higher aerosol speeds (work in progress). However, the present combination of particle number density and aerosol convective velocity already allows the detection of aerosol domains with excess charge passing through the Faraday cup. The amount of net charge per domain, obtained by integrating different peaks, is in Supplementary information Table S1. On the other hand, this information is not obtained by separating particles prior to charge measurements and it was not, in previously published work. Aerosol droplet size was measured 6.9 ± 1.6 μm, using an optical microscope (see Supplementary information Fig. S2). Using the volumetric flow of water droplets (0.333 mL/min), we obtain the number of droplets per peak, typically 4.21 × 106. Thus, each peak contains a large number of droplets and the average charge per volume of liquid can be calculated (Supplementary information Table S1). The results show an interesting and surprising fact: irrespective of the liquid used, charge per volume is roughly 17 μC L− 1 averaging 184 charges per single drop, either positive or negative. Droplet charge can be related to results presented in previous work on the electrostatic charging of individual water drops [4]. Following these, water drops with charge excess greater than 40 nC/cm2 undergo spontaneous elongation followed by Coulomb explosion at even higher voltages. Aerosol droplet charge surface density is much lower, 1.92 pC/ cm2 allowing droplets to retain spherical shape. However, the low average charge density obtained here is probably a minimum estimate because each peak results from the summation of positive and negative charges in each aerosol domain. Observation of aerosol motion within an electric field confirms the bipolar charging behavior: separate aerosol currents migrate towards
both electrodes, showing the coexistence of positive and negative charge domains, as shown in the Supplementary information (video and Fig. S3). Some relevant movie frames are in Fig. 4. Aerosol flows in between grounded electrodes, moving straight away from the nebulizer exit. However, under ± 15 kV potential difference between the electrodes, the aerosol from deionized water deviates largely towards the negative electrode while a smaller current migrates to the positive electrode. Aerosol from NaCl solution also deviates while passing through the biased electrodes sending streams to both, but the bulkier stream deviates towards the positive pole. This is in agreement with the sign of the electric current measured at shorter times, in Fig. 3. Absorption of the aerosol droplets on separate disks of filter paper (Schleicher & Schuell ME 24, 142 mm diameter and 0.2 μm pore size) placed in front of each electrode followed by weighing allows quantification of the amount of aerosol deflected towards each electrode. The results appear in Table 1, confirming that aerosol from deionized water contains a higher amount of positive than negative droplets while the opposite happens for NaCl solution. Although the weight of aerosol absorbed on the electrodes during nebulization of deionized water is lower than that from NaCl solution, the respective ratios (higher deposited mass/lower deposited mass) are similar, 2.5. This is an intriguing result that should be verified for other liquids, to see if this is just a coincidence, or not (work in progress). Charge partition during the formation of aerosol is tentatively explained as the result of two basic effects: i) OH− accumulation at water–air interfaces [15] and ii) the larger ratio between interfacial area and volume ratio of small droplets, compared to larger drops or bulk liquid [24]. The first point is supported by evidence in the literature [2], obtained by detecting excess concentration of hydroxyl ions at the interfaces of water with any hydrophobic phase, especially gas bubbles and oil droplets immersed in pure water. The same argument was used in Ref. [25] to explain the negative charge detected on fine aerosol particles formed by bursting water bubbles. Moreover, salt solution– gas interfaces display negative excess ion concentration [25] and the more polarizable chloride ions have a surface propensity, as compared to sodium ions. Another relevant information is that the foaminess of
Fig. 4. Deviation of aerosols immersed in an electric field. When a voltage is applied to the electrodes, aerosol formed from deionized water deflects mainly towards the negative copper electrode, unlike the aerosol from NaCl solution that deflects mainly towards the positive electrode. Liquid is deposited on both electrodes, revealing that both positive and negatively charged droplets coexist in the aerosol. These images are taken from a movie shown in Supplementary information (see SI Fig. S3).
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11 Table 1 Weight of liquid collected on quantitative filter paper coating the electrodes, collected under ± 15 kV (exposure time: 3 min). Results are from at least five independent experiments. Aerosol
Deionized water NaCl solution
Collected liquid weight (g) Electrode (+)
Electrode (−)
0.053 ± 0.025 0.205 ± 0.066
0.139 ± 0.040 0.085 ± 0.017
sea water (relative to fresh water) is assigned to the effect of salt in reducing bubble coalescence due to the local influence of the ions on water structure [25]. There is also abundant evidence in the literature showing that water or solution splash produces various domains with peculiar geometry (elongated water jets, large and small droplets, bubbles). Thus, the currently available information allows us to expect that the fragmentation of pure water produces initially negative small droplets but larger positive drops, since the former have a larger area/volume ratio. For the salt solutions, lower charge at surface films is to be expected, due to compression of the Debye layer. An additional factor is charge partition at the interfaces between water and the solids contacting it. In the case of hydrophobic solids like the nebulizer body made from polypropylene, we may hypothesize that the interface acquires negative charge (as well as air or oil) leaving the bulk liquid with excess positive charge. This agrees with the positive charge of the exiting aerosol from pure water. There is not currently sufficient information on the electric properties of the polypropylene–NaCl solution interface but it is most likely also negative. However, the net charge of the aerosol is slightly negative and this may be related to lower chloride ion depletion at droplet surfaces, since they are less strongly hydrated than sodium ions. This should indeed contribute to an excess of negative charge in the aerosol. These results are relevant for many scientific areas, from atmospheric chemistry to industrial safety. Charge distribution in clouds is an important and debated topic in atmospheric sciences, with the current competition between dipole and tripole models [14]. One intriguing feature that is not explained by either model is the tendency of clouds to display rounded-off boundaries enclosing compact shapes that can only be understood considering that there is some cohesive force keeping droplets together, even at significant distances. This difficulty is eliminated assuming a model in which small and large droplets with opposing charge coexist within fractal charge domains, mutually attracting each other in a cooperative fashion and contributing a Madelung-like term to the overall aerosol energetics. Another stabilizing factor is the contribution of charge non-uniformity to increase aerosol entropy. These questions will be dealt with separately, in future work. The different experiments presented in this paper show that aerosols produced by different spraying procedures but in the absence of any intentional electrical input always display bipolar charging. In the cases of pure water and NaCl solution, positive and negative droplets coexist in different concentrations, conferring respectively net positive and negative charges to the aerosol while leaving behind opposite charge in the residual liquid. Time-resolved current measurements show large local charge fluctuations and the current–time curves have fractal dimension 1.42 for pure water. Overall net charge and timeresolved charge variation of flowing aerosols depend reproducibly on the actual liquid composition. These results show that liquid fragmentation by often-used means always introduces electrostatic charge into the environment, contributing to explain the ubiquity of charge distribution patterns observed in insulators. On the other hand, it shows that other often-quoted factors,
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as ionization caused by high-energy particles crossing the atmosphere are not essential for the appearance of atmospheric electricity. Moreover, it provides an explanation for the rounded-off shapes of atmospheric clouds, since the interspersion of positive and negative particles in the aerosol produces cohesion due to long-range electrostatic forces. Finally, this new understanding on charge bipolarity in aerosols may contribute to progress in scavenging electrical energy from the atmosphere (work in progress) and to increase safety while handling liquids in industrial environments. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.colcom.2015.11.002.
Acknowledgments The authors thank the support from CNPq (309072/2014-0) and FAPESP (2008/57867-8)(Brazil) through Inomat, National Institute (INCT) for Complex Functional Materials.
References [1] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, 6th ed. John Wiley & Sons, New York, 1997. [2] T.W. Healy, D.W. Fuerstenau, The isoelectric point/point-of-zero-charge of interfaces formed by aqueous solutions and nonpolar solids, liquids and gases, J. Colloid Interface Sci. 309 (2007) 183–188. [3] K. Ovchinnikova, G.H. Pollack, Can water store charge? Langmuir 25 (2009) 542–547. [4] L.P. Santos, T.R.D. Ducati, L.B.S. Balestrin, F. Galembeck, Water with excess electric charge, J. Phys. Chem. C 115 (2011) 11226–11232. [5] G. Pyrgiotakis, et al., Mycobacteria inactivation using Engineered Water Nanostructures (EWNS). Nanomedicine nanotechnology, Biol. Med. 10 (2015) 1175–1183. [6] Y.J. Kaufman, D. Tanré, O. Boucher, A satellite view of aerosols in the climate system, Nature 419 (2002) 215–223. [7] A. Hirsikko, et al., Atmospheric ions and nucleation: a review of observations, Atmos. Chem. Phys. 11 (2011) 767–798. [8] R. Zhang, A. Khalizov, L. Wang, M. Hu, W. Xu, Nucleation and growth of nanoparticles in the atmosphere, Chem. Rev. 112 (2012) 1957–2011. [9] G. Heinecke, Tribochemistry, Verlag, Berlin, 1984. [10] P. Lenard, About the electricity of waterfalls, Ann. Phys. (Leipzig) 46 (1982) 584–636. [11] P. Kolarž, M. Gaisberger, P. Madl, W. Hofmann, M. Ritter, A. Hartl, Characterization of ions at alpine waterfalls, Atmos. Chem. Phys. 12 (2012) 3687–3697. [12] E.W.B. Gill, G.F. Alfrey, Production of electric charges on water drops, Nature 169 (1952) 203–204. [13] G.L. Natanson, The problem of balloelectric phenomenon, Dokl. Akad. Nauk SSSR 73 (1950) 975–978. [14] E.R. Williams, The tripole structure of thunderstorms, J. Geophys. Res. 94 (1989) 13151–13167. [15] L.S. McCarty, G.M. Whitesides, Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets, Angew. Chem. Int. Ed. 47 (2008) 2188–2207. [16] R.F. Gouveia, F. Galembeck, Electrostatic charging of hydrophilic particles due to water adsorption, J. Am. Chem. Soc. 31 (2009) 11381–11386. [17] K.N. Kudin, R. Car, Why are water-hydrophobic interfaces charged? J. Am. Chem. Soc. 130 (2008) 3915–3919. [18] R.F. Gouveia, J.S. Bernardes, T.R.D. Ducati, F. Galembeck, Acid base site detection and mapping on solid surfaces by Kelvin Force Microscopy (KFM), Anal. Chem. 84 (2012) 10191–10198. [19] T.R.D. Ducati, L.H. Simões, F. Galembeck, Charge partitioning at gas–solid interfaces: humidity causes electricity buildup on metals, Langmuir 27 (2010) 13763–13766. [20] R.C. Weast, M.J. Astle, CRC Handbook of Chemistry and Physics, 61st ed. CRC Press, Boca Raton, FL, 1982 1980–1981. [21] T. Takahashi, Riming electrification as a charge generation mechanism in thunderstorms, J. Atmos. Sci. 35 (1978) 1536–1548. [22] S.H. Kim, K.S. Woo, B.Y.H. Liu, M.R. Zachariah, Method of measuring charge distribution of nanosized aerosols, J. Colloid Interface Sci. 282 (2005) 46–57. [23] T.A.L. Burgo, C.A. Silva, L.B.S. Balestrin, F. Galembeck, Friction coefficient dependence on electrostatic tribocharging, Sci. Rep. 3 (2013) 2384 1–8. [24] I. Bhattacharyya, J.T. Maze, G.E. Ewing, M.F. Jarrold, Charge separation from the bursting of bubbles on water, J. Phys. Chem. A 115 (2011) 5723–5728. [25] V.S.J. Craig, B.W. Ninham, R.M. Pashley, Effect of electrolytes on bubble coalescence, Nature 364 (1993) 317–319.
Contents lists available at ScienceDirect
Colloids and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
Rapid Communication
On the spontaneous electric-bipolar nature of aerosols formed by mechanical disruption of liquids Thiago A.L. Burgo ⁎, Fernando Galembeck Institute of Chemistry, University of Campinas, Campinas, SP 13083-970, Brazil National Nanotechnology Laboratory at the National Center for Energy and Materials Research, Campinas, SP 13083-970, Brazil
a r t i c l e
i n f o
Article history: Received 31 July 2015 Received in revised form 31 October 2015 Accepted 13 November 2015 Available online 7 January 2016 Keywords: Aerosol formation Bipolar charging Water splashing Balloelectric effect Spray charging
a b s t r a c t Aqueous aerosols are widely found on the Earth's atmosphere and they participate from any anthropic environment. We show here that aerosols produced by a nebulizer based on splashing contain both positive and negative droplets, this means, it is bipolar but the overall aerosol charge is usually non-zero. Charge distribution within the aerosol is by itself fractal, as previously observed in other cases of mechano-chemical charge formation that is an important mechanism for producing triboelectricity in solids. The present information added to the other authors' work leads to a particle charging mechanism based on charge partition at interfaces combined to the variability of interfacial area/volume ratio in aqueous droplets, thus explaining the formation of bipolar aerosol. The demonstration of charge bipolar distribution within aerosols contributes to explain the ubiquity of electric charge patterns in solid surfaces that is receiving growing evidence and contributes to understand both beneficial and damaging outcomes of electrostatic charging. © 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Most natural and anthropic gaseous environments contain water droplets with diameters ranging from a few nanometers (e.g. close to waterfalls) to a few millimeters (e.g. in rain). They display significant surface areas (e.g. 30 m2/g for 100 nm droplets) that produce a host of interfacial phenomena. Interfaces of condensed phases are always important sites for charge accumulation and exchange [1] and charge at the gas–liquid interfaces [2] and also in bulk water has been previously detected [3] but reproducible procedures for imparting net charge to water drops and observing its effect on the liquid surface and bulk properties were only recently published [4]. Many scientific disciplines study water and other liquid droplets in the atmosphere paying greater or lesser attention to their electrification but there is no consensus on the mechanisms for droplet charging and divergent views are found in recent literature [4,5]. Aerosols have paramount importance in atmospheric phenomena [6], including a large number of chemicals discharged in the atmosphere by natural and anthropic phenomena. Aqueous aerosols are particularly relevant for cloud formation, stability and rain precipitation, while clouds are also important as precursors of atmospheric aerosols formed by other chemical substances [7,8]. Many authors in this area relate atmospheric electricity to liquid water or ice particles and they assign the origins of atmospheric electricity to diverse events, like atmospheric gas ionization due to radiation and ice particle breakdown [9]. Water ⁎ Corresponding author at: Institute of Chemistry, University of Campinas, Campinas, SP 13083-970, Brazil. E-mail address: [email protected] (T.A.L. Burgo).
splashing is another source of charged aerosol particles [10,11,12]. The formation of electrified drops by mechanical disruption of liquids is named balloelectric effect, arising from statistical fluctuations of ion density in the liquid [13] and presumably contributing to cloud electrification [14]. Many researchers undertook laboratory studies of aerosol formation, properties and stability, often related to important practical problems in industrial, energy and health contexts. However, aerosol charge is not even mentioned in many publications on aerosols. This situation is quite different from the literature on liquid sols, where zeta potentials or particle charge data appear in nearly every paper. Moreover, hydrophobic [15] and hydrophilic [16] solid surfaces acquire charge when exposed to water vapor or liquid, due to asymmetric partitioning of hydroxide ions at water-hydrophobic interfaces [17] or to Brønsted acid or base character of solid surfaces [18], as in hygroelectricity [19]. The atmosphere is thus a reservoir of electric charge. An additional factor for water ion partition is the electrochemical potential [1] of hydronium or hydroxide ions that predicts excess concentration of H+ under a negative potential and of OH− under a positive potential. Since the Earth's surface and atmosphere are electrified environments displaying large electric fields [20], excess positive or negative charge in water is expected in water [19]. There is abundant but often uncorrelated information on charge appearance in aqueous aerosols and droplets, where a detailed laboratory investigation by Takahashi [21] led to a mechanism for charge generation on thunderstorms but depending on ice crystal breakdown. Here we describe unprecedented reproducible laboratory experiments
http://dx.doi.org/10.1016/j.colcom.2015.11.002 2215-0382/© 2015 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
8
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
showing the formation of spontaneously bipolar-charged aerosols due to the mechanical disruption of water and aqueous NaCl solutions, together with the effect of composition, followed by a discussion of its implications and proposing a mechanism for charge formation. The literature describes elaborate procedures for measuring particle size and charge, in aerosols, targeting nanoparticles [22]. These instruments provide detailed information but they involve significant particle handling, including exposure to radioactive charge emitters. In an alternative robust and reproducible method to study charge in aerosol produced by a nebulizer, the flowing aerosol passes in the interior of a Faraday cup while measuring the current between the electrodes, as schematically shown in Fig. 1. The Faraday cup is made of two concentric copper cylinders and connected to an electrometer (Keithley 6514) through a low-noise triaxial cable. Current measurements are made under high-speed acquisition rate (800 readings/s), using an USB-to-GPIB interface (Keithley KUSB-488b) at binary transferring format to a computer (NI LabVIEW 2011). Positive current means that
the net charge of the set of particles entering the Faraday cup is positive, and vice-versa. Another way to demonstrate the co-existence of positive and negative charges is electrophoresis, in the gas phase. Parallel copper disks (20 cm diameter) were connected to high voltage sources, up to ±30 kV (Spellman CZE1000R) and they were video recorded (Canon EOS 60D illuminated with a blue LED), as schematically shown in Fig. 1(b). These two procedures do not give detailed information on particle size and charge distribution but they yield information on particles as they leave the aerosol source, with minimal particle manipulation. Reading current peaks instead of sorting out individual particles to measure their charges is analogous to using energy-dispersive (EDX) techniques in X-ray spectrometry, instead of using wavelength dispersive (WDX) methods. Surprisingly, every experiment made in this laboratory to detect excess electric charge in aerosols formed by mechanical means or in water vapor revealed an intrinsic electrostatic charging behavior. In
Fig. 1. Setup to measure aerosol charging and electrophoresis. (a) The electrostatic charge of the liquid/reservoir set is measured with a Kelvin probe while the current or charge of the released aerosol is measured with a Faraday cup. (b) Set-up for observing aerosol motion within the electric field created by two copper disk electrodes connected to a high voltage DC power supply.
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
some experiments, short pulses of water and ethanol spray (produced using a laboratory device used for spraying staining solutions) were driven into a Faraday cup and the resulting current measurements are shown in Fig. 2a–b. Each water spray pulse introduced a positive charge in the cup, detected as a short current peak reaching 15 nA. Ethanol spray pulses also introduced charge in the Faraday cup but this is bipolar, as seen in the inset of Fig. 2b. Rinsing or spraying with ethanol was recently recognized [23] as an effective solution to neutralize electrostatic charges on insulator surfaces and this may derive from the coexistence of positive and negative water droplets, as evidenced in this experiment. Aerosol formed by the condensation of water vapor produced in a boiler and driven to the input of the Faraday cup also produces oscillating current as shown in Fig. 2(c). Stopping the entrance of vapor flow into the cup produces larger and less noisy current that is probably related to temperature change in the cup, allowing further vapor condensation. When aerosol from deionized water stemming out of a nebulizer passes through the inner electrode of the Faraday cup it produces initially highly variable positive current (Fig. 3(a)), changing to a time series of sharp positive and negative current peaks. When the aerosol is made using an aqueous NaCl solution, a similar result is obtained but the initial current is negative (Fig. 3(b)). Each plot in this figure contains 240,000 experimental readings and thus each peak is drawn using ca. 40 experimental points, sufficient to define properly its shape. The noise level is typically 1/100 of the recorded peak signals and peak charge is generally lower than in the plots acquired at low speed. The positive and negative peaks evidence the successive entrance of positive and negative droplet clusters within the Faraday cup, at different moments. Most peaks are well resolved but the plots also contain a few doublets or broader bands. Current peak half-widths are in the range of 0.01 to 0.1 s. Also, as expected from larger ion density fluctuations [13] in NaCl
9
Fig. 3. Electric current between the electrodes of a Faraday cup, produced by passing aerosol from (a) deionized water and (b) sodium chloride solution (1 mmol L−1) measured at a high-speed acquisition rate (800 readings/s). Positive and negative peaks show that oppositely charged particles or particle clusters coexist in the aerosol.
Fig. 2. Electric current produced by introducing aerosol in a Faraday cup: (a) demineralized water spray jets with a photo of the spraying device used, (b) ethanol spray jets, and (c) vapor and aerosol from boiling water. The insets in (a) and (b) are close-ups of the current peaks measured soon after 31 s.
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T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11
solution, more positive and negative peaks are counted per unit time in the NaCl aerosols, when compared to water (insets of Fig. 3). Together, these observations show that the aerosol is divided in clusters of water droplets carrying net positive or negative charge. Current vs. time plots display fractal profiles (Dwater = 1.44 and DAqNaCl = 1.42, calculated using Higuchi's method) evidencing that positive and negative clusters are subdivided in smaller sub-sets that can probably be detected by increasing the spatial and time resolution of the detector, this means, by using smaller Faraday cups and higher aerosol speeds (work in progress). However, the present combination of particle number density and aerosol convective velocity already allows the detection of aerosol domains with excess charge passing through the Faraday cup. The amount of net charge per domain, obtained by integrating different peaks, is in Supplementary information Table S1. On the other hand, this information is not obtained by separating particles prior to charge measurements and it was not, in previously published work. Aerosol droplet size was measured 6.9 ± 1.6 μm, using an optical microscope (see Supplementary information Fig. S2). Using the volumetric flow of water droplets (0.333 mL/min), we obtain the number of droplets per peak, typically 4.21 × 106. Thus, each peak contains a large number of droplets and the average charge per volume of liquid can be calculated (Supplementary information Table S1). The results show an interesting and surprising fact: irrespective of the liquid used, charge per volume is roughly 17 μC L− 1 averaging 184 charges per single drop, either positive or negative. Droplet charge can be related to results presented in previous work on the electrostatic charging of individual water drops [4]. Following these, water drops with charge excess greater than 40 nC/cm2 undergo spontaneous elongation followed by Coulomb explosion at even higher voltages. Aerosol droplet charge surface density is much lower, 1.92 pC/ cm2 allowing droplets to retain spherical shape. However, the low average charge density obtained here is probably a minimum estimate because each peak results from the summation of positive and negative charges in each aerosol domain. Observation of aerosol motion within an electric field confirms the bipolar charging behavior: separate aerosol currents migrate towards
both electrodes, showing the coexistence of positive and negative charge domains, as shown in the Supplementary information (video and Fig. S3). Some relevant movie frames are in Fig. 4. Aerosol flows in between grounded electrodes, moving straight away from the nebulizer exit. However, under ± 15 kV potential difference between the electrodes, the aerosol from deionized water deviates largely towards the negative electrode while a smaller current migrates to the positive electrode. Aerosol from NaCl solution also deviates while passing through the biased electrodes sending streams to both, but the bulkier stream deviates towards the positive pole. This is in agreement with the sign of the electric current measured at shorter times, in Fig. 3. Absorption of the aerosol droplets on separate disks of filter paper (Schleicher & Schuell ME 24, 142 mm diameter and 0.2 μm pore size) placed in front of each electrode followed by weighing allows quantification of the amount of aerosol deflected towards each electrode. The results appear in Table 1, confirming that aerosol from deionized water contains a higher amount of positive than negative droplets while the opposite happens for NaCl solution. Although the weight of aerosol absorbed on the electrodes during nebulization of deionized water is lower than that from NaCl solution, the respective ratios (higher deposited mass/lower deposited mass) are similar, 2.5. This is an intriguing result that should be verified for other liquids, to see if this is just a coincidence, or not (work in progress). Charge partition during the formation of aerosol is tentatively explained as the result of two basic effects: i) OH− accumulation at water–air interfaces [15] and ii) the larger ratio between interfacial area and volume ratio of small droplets, compared to larger drops or bulk liquid [24]. The first point is supported by evidence in the literature [2], obtained by detecting excess concentration of hydroxyl ions at the interfaces of water with any hydrophobic phase, especially gas bubbles and oil droplets immersed in pure water. The same argument was used in Ref. [25] to explain the negative charge detected on fine aerosol particles formed by bursting water bubbles. Moreover, salt solution– gas interfaces display negative excess ion concentration [25] and the more polarizable chloride ions have a surface propensity, as compared to sodium ions. Another relevant information is that the foaminess of
Fig. 4. Deviation of aerosols immersed in an electric field. When a voltage is applied to the electrodes, aerosol formed from deionized water deflects mainly towards the negative copper electrode, unlike the aerosol from NaCl solution that deflects mainly towards the positive electrode. Liquid is deposited on both electrodes, revealing that both positive and negatively charged droplets coexist in the aerosol. These images are taken from a movie shown in Supplementary information (see SI Fig. S3).
T.A.L. Burgo, F. Galembeck / Colloids and Interface Science Communications 7 (2015) 7–11 Table 1 Weight of liquid collected on quantitative filter paper coating the electrodes, collected under ± 15 kV (exposure time: 3 min). Results are from at least five independent experiments. Aerosol
Deionized water NaCl solution
Collected liquid weight (g) Electrode (+)
Electrode (−)
0.053 ± 0.025 0.205 ± 0.066
0.139 ± 0.040 0.085 ± 0.017
sea water (relative to fresh water) is assigned to the effect of salt in reducing bubble coalescence due to the local influence of the ions on water structure [25]. There is also abundant evidence in the literature showing that water or solution splash produces various domains with peculiar geometry (elongated water jets, large and small droplets, bubbles). Thus, the currently available information allows us to expect that the fragmentation of pure water produces initially negative small droplets but larger positive drops, since the former have a larger area/volume ratio. For the salt solutions, lower charge at surface films is to be expected, due to compression of the Debye layer. An additional factor is charge partition at the interfaces between water and the solids contacting it. In the case of hydrophobic solids like the nebulizer body made from polypropylene, we may hypothesize that the interface acquires negative charge (as well as air or oil) leaving the bulk liquid with excess positive charge. This agrees with the positive charge of the exiting aerosol from pure water. There is not currently sufficient information on the electric properties of the polypropylene–NaCl solution interface but it is most likely also negative. However, the net charge of the aerosol is slightly negative and this may be related to lower chloride ion depletion at droplet surfaces, since they are less strongly hydrated than sodium ions. This should indeed contribute to an excess of negative charge in the aerosol. These results are relevant for many scientific areas, from atmospheric chemistry to industrial safety. Charge distribution in clouds is an important and debated topic in atmospheric sciences, with the current competition between dipole and tripole models [14]. One intriguing feature that is not explained by either model is the tendency of clouds to display rounded-off boundaries enclosing compact shapes that can only be understood considering that there is some cohesive force keeping droplets together, even at significant distances. This difficulty is eliminated assuming a model in which small and large droplets with opposing charge coexist within fractal charge domains, mutually attracting each other in a cooperative fashion and contributing a Madelung-like term to the overall aerosol energetics. Another stabilizing factor is the contribution of charge non-uniformity to increase aerosol entropy. These questions will be dealt with separately, in future work. The different experiments presented in this paper show that aerosols produced by different spraying procedures but in the absence of any intentional electrical input always display bipolar charging. In the cases of pure water and NaCl solution, positive and negative droplets coexist in different concentrations, conferring respectively net positive and negative charges to the aerosol while leaving behind opposite charge in the residual liquid. Time-resolved current measurements show large local charge fluctuations and the current–time curves have fractal dimension 1.42 for pure water. Overall net charge and timeresolved charge variation of flowing aerosols depend reproducibly on the actual liquid composition. These results show that liquid fragmentation by often-used means always introduces electrostatic charge into the environment, contributing to explain the ubiquity of charge distribution patterns observed in insulators. On the other hand, it shows that other often-quoted factors,
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as ionization caused by high-energy particles crossing the atmosphere are not essential for the appearance of atmospheric electricity. Moreover, it provides an explanation for the rounded-off shapes of atmospheric clouds, since the interspersion of positive and negative particles in the aerosol produces cohesion due to long-range electrostatic forces. Finally, this new understanding on charge bipolarity in aerosols may contribute to progress in scavenging electrical energy from the atmosphere (work in progress) and to increase safety while handling liquids in industrial environments. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.colcom.2015.11.002.
Acknowledgments The authors thank the support from CNPq (309072/2014-0) and FAPESP (2008/57867-8)(Brazil) through Inomat, National Institute (INCT) for Complex Functional Materials.
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