School of Fashion, Zhongyuan University of Technology, Zhengzhou , China

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1 Journal of Nano Research Online: ISSN: , Vol. 7, pp doi:1.48/ 14 Trans Tech Publications, Switzerland Lightning-like Charged Jet Cascade in Bubble Electrospinning with Ultrasonic Vibration Hong-Yan Liu 1,a, Hai-Yan Kong, Mei-Zhen Wang 3,b, Ji-Huan He 1,4,c 1 School of Fashion, Zhongyuan University of Technology, Zhengzhou 457, China National Engineering Laboratory for Modern Silk, College of Textile and Engineering, Soochow University, 199 Ren-ai Road, Suzhou 1513, China 3 Zhejiang Textile & Fashion College, Ningbo 31511, China 4 Nantong Textile Institute of Soochow University, 58 Chong-chuan Road, Nantong 618, China a phdliuhongyan@gmail.com b meizhengwang@1cn.com c hejihuan@suda.edu.cn (corresponding author) [Submitted: August, 13; accepted: December 9, 13] Keywords: Bubble electrospinning, surface charge, Coulomb force, daughter cascade, two dimensional nanoweb Abstract. Lightning is a natural phenomenon caused by an atmospheric electrical discharge, and lightning strikes are of hierarchical structure. Similar phenomenon is first observed in a charged jet in the presence of a high electrostatic field, the process is widely adopted for fabrication of superfine fibers, and its mechanism of lightning-like charged jet is still unknown. Our observation reveals that a daughter jet can be ejected from the surface of a micro/nano scale charged jet to form an initial two-stage cascade, whereby the daughter charged jet can reduce size over three orders of magnitude, while at the ultimate stage, the jets have almost same size from several nanometers to dozens of nanometers. The origin of this phenomenon might be central to nanotechnology. Here we demonstrate that an electrostatic field can accelerate a charged jet, and the surface charge repels each other. When the repelling force reaches a threshold to overcome its surface tension, one or more daughter charged jets are ejected from the surface. The daughter jets behave similarly to the initial jet, the process is iterative, creating a lightning-like multi-stage cascade. This observation opens the door to mimicking the lightning to produce two dimensional superfine fiber web with hierarchical structure. 1. Introduction Lightning [1-4] is a massive electrostatic discharge between electrically charged regions within clouds, or between a cloud and the Earth s surface, and lightning strikes are of hierarchical structure. When a sufficiently high electric potential between a cloud and the ground is accumulated, discharge occurs through a lightning flash, developing very rapidly downwards at speeds which can exceed 17 m/s [1]. Electrospinning [5-8] or bubble electrospinning [9-17] uses an electrical field to eject a charged jet, which is accelerated and becomes thinner to form superfine fibers typically on the micro or nano scale. In the spinning process, the charged jets can be considered as charged clouds, when the surface charge is high enough, lightning-like charged jet cascade can be formed.. Experimental The experimental set-up is illustrated in Fig.1. Polyvinyl alcohol (PVA) with a degree of 175±5 and wood ashes were dissolved into distilled water with the temperature 16. o C. Then, the mixture was stirred with the aid of electromagnetic stirrer at 9 o C for 4 hours to get a homogeneous and transparent solution, and cooled to the room temperature before the experiment. The solution concentration was 1% and the ash concentration was 1%. The solution was placed in a 1 ml syringe. The needle tip with a diameter.7 mm was connected to a DC high-voltage generator via an All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-6/3/16,1:56:59)

2 11 Journal of Nano Research Vol. 7 alligator clip. A piece of aluminum foil, placed 1cm before the needle tip, was served as the collector for the electrospun fibers. During the spinning process, the voltage applied was maintained at kv and humidity 43%. A hierarchical fiber cascade obtained by the electrospinning is illustrated experimentally in Fig.1, where we also observe many spots on the fibers surface. Fig.1 A hierarchical fiber cascade by the electrospinning. The top left is an experimental setup; the top right is the SEM image; (A-E) two-stage cascade. 3. Predictions from chemical thermodynamics When normal lightning strikes soil, chemical energy is stored in nanoparticles of Si, SiO or SiC, which are ejected into the air as a filamentary network []. Similar phenomenon was observed in electrospinning and bubble electrospinning. The attempt to apply chemical thermodynamics to the problem of lightning-like spinning process was extremely poorly developed. Since humid air and electric field seemed to be involved in spinning process, net excess electric charge is formed in air or on electrodes [18]. Santos et al. found that positive (negative) water is obtained from a positive (negative) needle and its charge largely exceeds the Rayleigh limit [18]. Water in humid air or adsorbed as solvent on surface of the charged jet, under the presence of high electronic field, has always excess concentration of H + or OH - ions: RT ln a z FV (1) i i 1 where µ i is the electrochemical potential of the ion I (H + or OH - ), µ o i is the standard electrochemical potential of I, R is the gas constant, T is the temperature, a i is the activity of I, z i is the valence, F is Faraday s constant and V is the electric potential affecting i. According to (1), water should have an excess of H + under negative electric potential, and an excess of OH - under positive potential. When the concentration of OH - ion, which is attracted to the metal receptor, reaches threshold, discharge occurs 4OH H O O 4e () This will also give great risk of fire or explosion [19-1]. i

3 Journal of Nano Research Vol The electrospinning process is of intrinsic instability [7], and the jet surfaces might approach tightly to each other, similarly in the bubble electrospinning, multiple jets are formed, which are very closed to each other, but they can not combine together during the spinning process due to the same surface charge. Water in humid air or on the jet surface can be considered as charged regions. The charged jet temporarily equalizes itself through a lightning flash, and lightning-like phenomena occur. There are mainly four primary types: from a charged jet surface to itself (SI type); from a charged jet surface to air (SA type); from one jet surface to another jet surface (SS type) and finally between a jet surface and the metal receptor (SR type). The SI type generally cannot be observed in modern experiment, it occurs between the surface charge and its water in the solvent. During the spinning process, the surface charge increases greatly (see Eq. 14), concentration of H + or OH - ions around it becomes extremely high, and discharge happens. The SA type occurs only for high humid environment and high temperature and high voltage, these conditions can lead to a high electrochemical potential of H + or OH - ion. The SS type can be observed when two charged jets approach closely enough, one jet has high surface charge, while the other has high concentration of opposite ion (H + or OH - ). The SR type happens when the jet is not yet solidified. This type is similar to the natural lightning from cloud and the ground. 4. Thermal excitation The liquid surface before solidification oscillates due to thermally excited capillary waves. The capillary waves typically have small amplitudes (~1 nm) and small wavelength (~1 nm). The capillary waves are conducive to produce lightning-like strikes, because the charge concentration on the wave peak increases remarkably. Fig. Thermally excited capillary waves on the surface of the charged jet A nanoscale capillary wave is dominated by the balance between surface tension.and viscous forces. The dispersion relation for capillary waves is [] 3 4 i 1 k ( ivk ) v k (1 ) (3) vk where ρ is the liquid density while ν = η/ρ and η are the kinematic and dynamic shear viscosities, k the wavenumber, γ surface tension. The wavelength is (4) k The solutions of Eq. (3) provide the complex frequencies When the wavenumber reaches a threshold: ( k) ( k) i( k) (5) k ~ (6) k cr surface fluctuations are exponentially damped ( )at a rate given by

4 114 Journal of Nano Research Vol. 7 k ( k ) ~ (7) It is obvious that the solution viscosity and surface tension affects greatly capillary waves. When k<k cr, the dispersion relation for nanoscale capillary waves is 4. Electrostatic properties 3 k (8) Any charged surfaces are subjected to an electronic force, when two surfaces tend to micro/nano scales, according to Coulomb's law, its repelling force increases greatly. Consider a curve with length dl as illustrated in Fig. 3. It is repelled outward by the other charge on the circle: ( dl)( rd ) df e k ( r r cos) ( r sin ) (9) where is the surface charge per length, r is the radius of the jet, k is a constant. Integrating Eq. (9) results in rdl F e k d ( r r cos) ( r sin ) The Coulomb force acting the surface per length reads Fe f e ke (11) dl r where k e is a constant. (1) Fig. 3 The Coulomb force acting the surface. The spots in Fig. 1 are formed due to the Coulomb force, which repels the whole jet surface simultaneously, and many protruding cones are formed on the surface, but generally only one cone can eject its daughter jet on a close region, because the characteristic diameter scale is reduced greatly between adjacent cascades, and the other un-ejected cones become spots. The formed cones result in an unsmooth surface and remarkably increase the surface-to-volume ratio. 5. Dynamic and hydrodynamic effects During the electrospinning process, the charged jet follows the mass conservation, which requires [16] r u Q (1) where Q is the mass flow rate, is density, u the velocity of the jet. Generally the flow rate and density keep unchanged in the experiment, we, therefore, have r uc (13) where C is a constant.

5 Journal of Nano Research Vol A higher electrostatic field leads to a higher jet velocity and smaller jet radius. When the radius reduces to micro/nano scales, the Coulomb force due to the surface charges increases remarkably (see Eq. 11): On a macro scale, the Coulomb force can be ignored, but when the radius of the jet becomes smaller, the Coulomb force might be large enough to relax the surface tension. The coupling of surface charge and the Coulomb force creates a tangential stress, extruding the surface to form a cone; this can greatly affect the electronic force acting on the jet surface. Once the electric force exceeds the critical value needed to overcome the surface tension of the cone, a daughter charged jet is ejected, see Fig. 1. A similar phenomenon occurs in a daughter jet, and a sub-daughter jet can be ejected, thus a hierarchical jet cascade is formed, an event that typically occurs within milliseconds. The density of the charge on Taylor cone or a bubble surface might be high enough so that the electric force can overcome the surface tension to eject charged jets. The initial diameter of the charged jet is about several millimeters to several centimeters. Just consider a case d o = nm, and surface tension is σ o. The charged jet is accelerated and diameter reduces greatly, saying nm. Conservation of the surface charge requires d d (14) That means ( 5 / d / d mm/ nm 1 (15) The density of the surface charge increases dramatically during the spinning process. According to Eq.(11), Coulomb force on the surface reaches f / f ( / ) ( d / d) ( d / d) (16) It is obvious that it increases almost 1,,,,, times compared with that acting on the initial jet, this force is large enough to overcome the surface tension of the waved surface to eject daughter jets. The diameter of the obtained fibers in last cascade can reach as small as several nanometers, 5 nm in Fig. 1c, and 4 nm in Fig. 1a, which might be a minimum in artificial fibers. 6. Bubble electrospinning with ultrasonic vibration Bubble-electrospinning was invented in 7 [9]. It uses polymeric bubbles to produces multiple charged jets by applying a high voltage on the bubbles surfaces to overcome the surface tension [9,1]. It was originally designed to mass produce one dimensional nanofibers. In this paper, we elucidate that bubble electrospinning is extremely suitable for SS type lightning-like charged jet cascade, because a single polymer bubble can produce thousands of thousands of charged jets simultaneously, see Fig. 4. As discussed above, it is necessary to control surface tension and solution viscosity for producing lightning-like strikes. Ultrasonic vibration provides us with a good candidate for this purpose. Fig. 5 shows the schematic of an ultrasonic vibration coupled electrospinning setup. When ultrasonic vibration is applied to the polymer solution, its viscosity deceases remarkably [7]. The decrease of solution viscosity implies the increase of critical wavenumber (see Eq. (6)). This will result in a high frequency of capillary waves, which benefits charge concentration on the wave peaks. Humid air and high voltage are much suitable for our experiment as discussed above. In our experiment, the humidity is 49%, and the applied voltage is 15 kv. Polyvinyl alcohol (PVA) solution with 1 wt. % concentration is used as spun solution. The lightning-like strikes are obtained as illustrated in Fig. 6.

6 116 Journal of Nano Research Vol. 7 Fig. 4 Multiple jets in bubble electrospinning process. Fig. 5 Bubble electrospinning with ultrasonic vibration.

7 Journal of Nano Research Vol Fig. 6 Lightning-like cascade in bubble electrospinning. The bubble electrospinning can produce cylindrical fibers and hollow fibers depending upon the fragment s shape when a bubble is broken. Consider a strip of a ruptured film with the width a and thickness h. The minimization of surface energy results in a cylindrical fiber or a hollow fiber, the later will be observed as a strip (see the middle one in Fig.6). The critical width for a cylindrical fiber can be approximately expressed as [1] and the fiber diameter is a 4h (15) d 4h (16) In case a 4 h, a hollow fiber is predicted, which becomes a trip due to some external forces (see Figure 6). The maximal fiber in Fig.6 is 813nm in diameter, using Eq. (16), the maximal thickness of the bubble wall is estimated as 3.5 nm. The trip in Fig.6 is 1,53nm in width (see Fig. 6), a = x1,53 nm, we, therefore, have a/h = x153/3.5 = > 4π, satisfying the condition for a hollow fiber. 7. Discussion and Conclusions The properties of lightning-like spinning process can be preliminarily explained by thermodynamics. Detailed explanations involve rather complex interactions between the various electrical, chemical and physical effects. Temperature, humidity, surface tension and viscosity are the most crucial factors for lightning-like spinning, both ultrasonic vibration and temperature can greatly affect viscosity and surface tension of the solution [3], making the lightning-like spinning tenable. We demonstrate that when a charged polymer jet is accelerated by an electrostatic field, its diameter tends to micro/nano scale, and its surface can eject one or more daughter charged jet due to the growing Coulomb force acting on its surface. Sub-daughter jets are formed when the

8 118 Journal of Nano Research Vol. 7 Coulomb force overcomes the surface tension of the daughter charged jet, as a result, we can observe a multi-stage cascade, which is received as superfine fibers, the diameter of the minimal fiber reaches as small as 4 nm (4 Å), which might be a minimum in artificial fibers because a macromolecule usually ranges from about 1 to 1 nm. Acknowledgement The work is supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), National Natural Science Foundation of China under grant No , Project for Six Kinds of Top Talents in Jiangsu Province under grant No. ZBZZ-35, and Science & Technology Pillar Program of Jiangsu Province under grant No. BE137, Doctoral Horizon Awards of Outstanding Talent Training Plan of Soochow University (58336), and China Scholarship Council. References [1] V.P. Pasko, M.A. Stanley, J.D. Mathews, U.S. Inan, T.G. Wood, Electrical discharge from a thundercloud top to the lower ionosphere, Nature, 416() [] J. Abrahamson, J. Dinniss, Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil, Nature, 43() [3] D.J. Turner, Ball lightning and other meteorological phenomena, Physics Reports, 93(1998) -6. [4] N. Theethayi, R. Thottappillil, Some issues concerning lightning strikes to communication towers, Journal of Electrostatics, 65(7) [5] Y. Dzenis, Spinning continuous fibers for nanotechnology, Science, 34 (4) [6] M.G. McKee, J.M. Layman, M.P. Cashion, T.E. Long. Phospholipid nonwoven electrospun membranes, Science, 311(6), [7] J.H. He, Y. Liu, L.F. Mo, Y.Q. Wan, L. Xu, Electrospun Nanofibres and Their Applications (ISBN: ), Smithers Rapra Technology, Shawbury, UK, 8. [8] J.H. He, An elementary introduction to recently developed asymptotic methods and nanomechanics in textile engineering, International Journal of Modern Physics B, (8) [9] Y. Liu, J.H. He., Bubble electrospinning for mass production of nanofibers, International Journal of Nonlinear Science and Numerical Simulation, 8 (7) [1] J.H. He, Y. Liu, L. Xu, J.Y. Yu, G. Sun, BioMimic fabrication of electrospun nanofibers with high-throughput, Chaos, Solitons & Fractals, 37 (8) [11] J.H. He, Effect of Temperature on Surface Tension of a Bubble and Hierarchical Ruptured Bubbles for Nanofiber Fabrication, Thermal Science, 16 (1) [1] J.H. He, H.Y. Kong, R.R. Yang, H. Dou, N. Faraz, L. Wang, C. Feng, Review of fiber morphology obtained by bubble electrospinning and blown bubble spinning, Thermal Science, 16(1) [13] H. Dou, B.Q. Zuo, J.H. He, Blown bubble spinning for fabrication of superfine fibers, Thermal Science, 16(1) [14] H. Dou, J.H. He, Nanoparticles fabricated by the bubble electrospinning, Thermal Science, 16(1) [15] H.Y. Kong, J.H. He, Superthin combined PVA-graphene film, Thermal Science, 16(1) [16] J.H. He, L. Xu, Y. Wu, Y. Liu, Mathematical models for continuous electrospun nanofibers and electrospun nanoporous microspheres, Polymer International, 56(7) [17] H.Y. Kong, J.H. He, A modified bubble electrospinning for fabrication of nanofibers, Journal of Nano Research, 3(13)15-18

9 Journal of Nano Research Vol [18] R. Delgado-Buscalioni, E. Chacon, P. Tarazona, Capillary waves dynamics at the Nanoscale, Joutnal of Physcs: Condensed Matter, (8) [19] N. Wilson, The risk of fire or explosion due to static charges on textile clothing, Journal of Electrostatics, 4(1977) [] M. Nifuku, H. Katoh, Incendiary characteristics of electrostatic discharge for dust and gas explosion, Journal of Loss Prevention in the Process Industries, 14(1) [1] M. Glor, Ignition hazard due to static electricity in particulate processes, Powder Technology, (3) [] L.P. Santos, T.R.D. Ducati, L.B.S, Balestrin, F. Galembeck, Water with Excess Electric Charge, Proc. ESA Annual Meeting on Electrostatics, 11, [3] A. V. Brancker, Viscosity-Temperature Dependence, Nature, 166(195)

10 Journal of Nano Research Vol / Lightning-Like Charged Jet Cascade in Bubble Electrospinning with Ultrasonic Vibration 1.48/ DOI References [18] R. Delgado-Buscalioni, E. Chacon, P. Tarazona, Capillary waves' dynamics at the Nanoscale, Joutnal of Physcs: Condensed Matter, (8) / //49/4949