Abstracts of STEP-1
Analysis of Electrostatic Force on Non-uniformly Charged Particles
(PDF of PPT: 1.3MB)
Boonchai Techaumnat
Chulalongkorn University, Thailand
The electrostatic adhesion between a charged particle and a planar substrate is utilized for various applications such as electrophotography, precipitators, and power coating. The adhesion varies with properties of media involved, charge amount, and the distribution of charge. The charge distribution on a dielectric particle depends on the charging method. An estimate based on uniform charging usually fails to agree with the measurement results. In this work, models of discrete charge distribution are considered on a dielectric particle. Numerical field calculation is applied to determine
the electric field and force on the particle. From the results, the variation of the force with charged position and with the permittivity of the particle can be clarified.
Anatomy of Toner Adhesion with SPA Measurement and Electrostatic Simulation
(PDF of PPT: 9.6MB)
Masami Kadonaga
Ricoh Company, Japan
Numerical prediction of electrostatic adhesion force of toner particle is carried out with charge distribution estimated from SPM (Scanning Probe Microscope) measurement. The prediction of toner adhesion calculated with three dimensional electrostatic simulation offers an appropriate value corresponding to that of measurement. Adhesion force distribution obtained with various rotating positions with the same charge distribution shows a reasonable distribution. The relation between the position of the charged area on a toner surface and electrostatic adhesion force is investigated. The results show that charge on the southern hemisphere plays a significant role for toner adhesion.
Mechanistic-based multi-scale modeling of tribocharging of powders during pneumatic transport
(PDF of PPT: 2.2MB)
Khashayar Saleh
Université de Technologie de Compiègne, France
The presentation deals with the modeling of simultaneous tribo-charging of the wall and particles during pneumatic transport of powders in dilute phase. The concepts of “simple-condenser model” and “effective work function” are used to take into account the local charge transfer between the two materials during particle-wall collisions. Exact analytical solutions are provided for the model. It is shown that, the tribo-charging process is mainly governed by the ratio between the charge transfer constants of the wall and particles. The results show that for high tribo-charging conditions of wall with respect to particles, the accumulated charge of the particles tends to an asymptotic limit imposed by the charge saturation of the wall. For low charging walls, the particle charge is no more limited by the charge of the wall and increases continuously. Quantitative analyses show that the predominant regime (i.e. particles or wall limited) could be fairly predicted using a dimensionless group based on the volume fraction of the powder, the pipe diameter, D, and the particles mean diameter, dp. Furthermore, a dimensionless criterion is established allowing the prediction of tribo-charging regimes and the trend of space evolution of particles charges.
Tribo-Electric Charging of Particles in a Shaker
(PDF of PPT: 3.8MB)
Mojtaba Ghadiri
Institute of Particle Science and Engineering, University of Leeds, Leeds, UK
Powder flow could give rise to tribo-electrification, causing segregation and adhesion of particles to the containing walls [1]. This leads to operational problems and efficiency reduction in processing of particulate solids. In order to mitigate the undesirable effects of tribo-electric charging, a fundamental understanding of charge transfer is essential. For this, we have developed three different methods of assessment of charge transfer: single particle impact testing [2], shaking a mass of particles in a container [3], and aerodynamic dispersion of particles, sandwiched between two metal foils, by a pressure pulse causing bursting of the foils [4]. Charge transfer due to collisions and sliding of particles against the walls is then modelled, based on single particle charging, and incorporated into numerical simulations by the Distinct Element Method (DEM) to predict the bulk behaviour [5]. The results of the simulations and experiments are presented and compared.
[1] Šupuk, E., Hassanpour, A., Ahmadian, H., Ghadiri, M., Matsuyama, T. (2011). Tribo-Electrification and Associated Segregation of Pharmaceutical Bulk Powders, Kona Powder and Particle Journal.
[2] Watanabe, H., Ghadiri, M., Matsuyama, T., Ding, Y., Pitt, K., Maruyama, H., Matsusaka, S., Masuda, H. (2007). "Triboelectrification of pharmaceutical powders by particle impact", International Journal of Pharmaceutics, 334(1-2), 149-155.
[3] Supuk, E., Seiler, C., and Ghadiri, M. (2009). "Analysis of a Simple Test Device for Tribo-Charging of Bulk Powders", Particle and Particle Systems Characterization, 26, 7-16.
[4] Zarrebini, A., Ghadiri, M., Dyson, M., Kippax, P., and McNeil-Watson, F. (2013). "Tribo-electrification of Powders Due to Dispersion", Powder Technology, 250, 75-83.
[5] Imba, M., Zarrebini, A., Matsuyama, T., Ghadiri, M. (2013). Tribo-Electric Charging of Particles in a Shaker, PARTEC.
Contact Electrification of Polymers due to Electron Transfer Among Mechano Anions, Mechano Cations and Mechano Radicals Which Were Produced by Mechanical Fracture of Polymers
Masato Sakaguchi
University of Shizuoka, Japan
e-mail: sakaguchi@u-shizuoka-ken.ac.jp
Contact-electrified polydimethylsiloxane showed random "mosaics" of positively charged (+) and negatively charged (–) regions of nanoscopic dimensions on a surface which was accompanied by a material transfer [1]. We have revealed that the mosaic pattern of charges on the electrified surface is due to mechanoradicals, mechanoanions, and mechanocations, which are induced by mechanical scissions of the covalent bonds comprising the polymer main chain [2]. The contact between the polytetrafluoroethylene (PTFE) beads and a polystyrene (PS) dish generated negatively charged PTFE beads and positively charged PS dish accompanying the material transfers. Each sign of charge of the PTFE beads or PS dish reflected the overall or “net” charge on the surface comprising the mosaic pattern in nanoscopic domains [3]. We have revealed that the sign of the charge in nanoscopic domains after the contact and separation reflects electron transfer among mechanoanions, mechanocations, and mechanoradicals [2]. The direction of electron transfer depends on its HOMO or LUMO energy level, which can be estimated by the MO calculations of its model structure. Negative net charge in PTFE after the contact and separation reflects that the number of paths for electron transfer from PS to PTFE is larger than that from PTFE to PS. Furthermore, we elucidated the relative signs of net charge for PP, PE, PVC, PVF, PVDF, BC and PTFE based on the proposed mechanism. The order of polymers from positive to negative sign, i.e., triboelectric series was obtained as follows; Positive(+), PP < PE < PVC < PVF ≅ PVDF < BC < PTFE, Negative(-).
[1] H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, B. A. Grzybowski: The Mosaic of Surface Charge in Contact Electrification. Science, 333, 308-312, 2011
[2] M. Sakaguchi, M. Makino, T. Ohura, T. Iwata: Contact electrification of polymers due to electron transfer among mechano anions, mechano cations and mechano radicals. Journal of Electrostatics, 72, 412-416, 2014
[3] H. T. Baytekin, B. Baytekin, J. T. Incorvati, B. A. Grzybowski: Material Transfer and Polarity Reversal in Contact Charging. Angewandte Chemie International Edition, 51, 4843-4847, 2012
Keywords: Contact electrification, Charge, Mechanoanion, Mechanocation, Mechanoradical, Electron transfer, Triboelectric series
Contact Electrification due to Different Contact Mode for Identical Materials
Li Xie
Lanzhou University, China
Moving sand particles can be charged and carry some positive charges or negative charges, which have been found for a long time. Since the charged sand particles moving in air, a strong electric will be produced. It will not only change the trajectory's equations of sand particles, but also makes the sand particle attaching on the measurement instruments to affect the measurement results. However, the charging mechanism of sand particles is not clearly known, but just 7 charging mechanisms are summarized by Kanagy and Mann. We conducted out experiment studies on the collision electrification, triboelectrification, thermoelectrification, piezoelectrification, contact electrification and polarization electrification to investigate which charging mechanism is dominant. By comparison between the surface charge densities due to different charging models, it can be found that the collision electrification and trioelectrification are important charging mechanisms for sand particles. When the electric field is very strong, the polarization electrification is also an important charging mechanism.
Charge Transfer Limited by Gas Breakdown of Air
(PDF of PPT: 4.3MB)
Tatsushi Matsuyama
DEES, Soka University, Japan
e-mail: tatsushi@t.soka.ac.jp
The process of contact- or tribo-charging consists of two sub-cascade-processes of (i) contact and charge transfer between two surfaces, and (ii) separation and charge fixation. In the separation process with fixed charge, as gap increases the capacitance between two separating surfaces decreases, and in its consequence, the potential difference between the two surfaces increases, which may reach to gas break down limit voltage in ambient condition easily. Once the gas discharge takes place, the fixed charge may relaxe. In this case the amount of charge observed after a total separation becomes that of the residual after the relaxation. Experimental and theoretical works showed that the charge relaxation process dominates the amount of charge on a single spherical polymer particles (for 1-3 mm in diameter) when they impact on a metal plate [1, 2]. Recently the idea was extended to estimate the maximum charge of powder particles in a pneumatic conveyer, as a function of particle diameter, dp, pipe diameter, D, and volume fraction of powder, φ. As the results, the maximum charge, q1, of an isolated particle is given as:
\(q_1/C = 6.43 \times 10^{-6} \, (d_p/m)^{1.5}, \tag{1}\)
and the maximum charge, q, per single particle in the pipe flow is:
\begin{align*} q/C = \frac {1.10 \times 10^{-4} \, (d_p/m)^3}{\sqrt{\{\phi (D/m)\}^2+\{17.1(d_p/m)^{1.5}\}^2}}. \tag{2} \end{align*}
Note that these should be functions of relative dielectric constant of particle in more details [1]. Eq.(1) showed good quantitative agreements with experimental results [1], although eq.(2) seemed to give over estimations comparing to results reported in other works in one or two order of magnitude [3]. The electric field strength at the inner pipe wall was about constant of 6 MV/m for the high volume fraction case, when Paschen's law was referred for the gas break down limit. Because this field strength is somehow higher than 3 MV/m, as the value accepted normally for the macroscopic space of air, studying the characteristics of micro-gap discharge in this context will be in more interest in the future study. It is also interesting to note that eq.(3) suggested that the power index of particle diameter, dp, on the particle charge can transit from 1.5 to 3.0 as powder volume fraction increases.
[1] T.Matsuyama, H.Yamamoto: Charge relaxation process dominates contact charging of a particle in atmospheric conditions, Journal of Physics D: Applied Physics, 28, 2418-2423, 1995
[2] T.Matsuyama, H.Yamamoto: Charge relaxation process dominates contact charging of a particle in atmospheric conditions II. general model, Journal of Physics D: Applied Physics, 30, 2170-2175, 1996
[3] T.Matsuyama, H.Yamamoto: Maximum electrostatic charge of powder in pipe flow, Advanced Powder Technology, 21, 350-335, 2010
Key words: impact charge, charge relaxation, gas discharge, maximum charge
Electrostatics of Granular Flow in Pneumatic Systems
(PDF of PPT: 4.1MB)
Jun Yao
Xiamen University, China
The phenomenon of electrostatic charge generation and its effects on granular flow behavior in a pneumatic conveying system was studied. The main parameters used for quantitative characterization of the phenomenon were the induced current, particle charge density and equivalent current of the charged granular flow. These were measured using a Digital Electrometer, Faraday Cage and Modular Parametric Current Transformer (MPCT) respectively. In addition, granular distribution was measured using an electrical capacitance tomography (ECT) and granular velocities were cross referenced with those using particle image velocimetry (PIV). Three different flow patterns corresponding to different electrostatic effects within the pneumatic conveying system were observed and these were named the disperse flow, half-ring flow and ring flow patterns. It was found that the induced current, particle charge density and equivalent current increased with decreasing flow rates. Electrostatic effects generally become stronger with time and this may lead to clustering behavior occurring even in the disperse flow regime. The electrostatic force at low air flow rates is found to be the primary cause for granules sticking to the pipe wall and results in the formation of the half-ring or ring structure.
Key words: granular material, electrostatics, charge, pneumatic conveying
Triboelectric Charging of Granular Systems -- Separating Effects from Particle-Wall and Particle-Particle Interactions
(PDF of PPT: 2.5MB)
Daniel Lacks
Case Western Reserve University, USA
Triboelectric charging occurs when two materials are brought into contact and then are separated – as a result of the contact, charge is transferred such that one material becomes charged positively and the other becomes charged negatively. In our research, we use a combined experimental-theoretical approach to study the factors affecting contact charging, with a focus on the charging that occurs as granular materials flow. Charging in granular materials is especially difficult to study systematically for two reasons: First, the particles can charge to different polarities, such that the net charge of the granular material is not meaningful -- i.e., the particles can be very highly charged but have a net charge of zero. Second, the charging can occur by contact with any surface, so it is difficult to separate the effects of particle-particle and particle-wall interactions, which can be very different. We developed an apparatus to study the charging due only to particle-particle interactions. We show that the particle-particle interactions can lead to a particle-size-dependent charge transfer, such that smaller and larger particles tend to charge to opposite polarities.
Electrostatic formation of liquid-particle agglomerates
Peter M. Ireland
University of Newcastle, Australia
We report on experiments in which electrostatically-charged particles of various materials in an external electric are transferred to a pendent water drop to form liquid-particle agglomerates. When all the particles are hydrophilic ballotini, this transfer consists of a sudden, violent ‘avalanche’ of particles, filling the drop in a fraction of a second and causing detachment. A simple quantitative argument involving electrostatic stresses in the particle bed is provided to explain the sudden onset of the particle 'avalanche'. A particle bed consisting of 10% (mass) hydrophobic PMMA particles and 90% ballotini behaves in a similar way, although the PMMA visibly coat the outside of the drop prior to mass transfer of the ballotini into the drop. Particle beds with higher proportions of PMMA appear more cohesive than those with less PMMA, and the particles transfer in clumps. The observed behaviour offers the prospect of producing layered agglomerates with a hydrophilic core stabilised by a hydrophobic shell (or vice-versa), with potential applications in pharmaceuticals, waste management and mineral processing.
