Abstracts of STEP-2

List of Invited Speakers:

Analysis of Single Particle Tribo-Electric Charging in G3 Disperser

M. Ali and M. Ghadiri*
Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT, UK

G3 disperser is used for dispersion of particles by injecting a pulse of pressurised air. The dispersed particles are then examined using Malvern Morphologi G3 for size and shape analysis. The high transient air velocity inside the disperser arising from the air pressure pulse causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause tribo-electric charging of the particles. The charge transfer to the particle depends on a large number of factors, some of which are related to particulate solids and containing wall material properties, such as the work function, particle physical, mechanical and chemical properties, whilst others such as impact velocity and number of collisions are related to process operation conditions. Moreover, weak particles are prone to breakage on the application of the pressure pulse, and cohesive particles might not get fully dispersed, if the pressure pulse is not sufficiently high.
In this study, we analyse the turbulent pulsed-air flow and its influence on particle trajectories and impact velocities as a function of the inlet air pressures by CFD. The particles are tracked using Lagrangian approach using transient conditions. The charge transfer to particles is then predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. The results are compared with experimental measurements.

The Impact of Processing on Interfacial Properties and Electrostatic Chargeability of Powders

Karolina Biegaj, Tim Lukas, Martin Rowland, Jerry Heng

†Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, United Kingdom
‡Pfizer Pharmaceutical Sciences, Discovery Park, Sandwich, Kent, CT13 9NJ, United Kingdom

Generation of electrostatic charge as a result of contact friction between powders and surfaces has been frequently observed during many pharmaceutical processing operations. These range from simple powder pouring and sieving operations to more intense processes like micronisation[1]. Despite growing awareness and increasing interest in powder electrostatic phenomena, there are still many uncertainties regarding the process of charge generation and which factors determine the polarity and magnitude of the charge generated[2].
The aim of this research is to characterise the relationships between the physicochemical properties and electrostatic properties of powders and how the latter will be influenced by altering certain particle characteristics. This could potentially help to predict the electrostatic behaviour during a number of industrial processes. Furthermore, a number of processing induced transformations, such as surface amorphisation, could possibly rationalise the unexpected electrostatic behaviour of powders during processing.
The present study involved independently investigating the following particle properties: surface chemistry, presence of the amorphous state and the influence of particle shape. Two methods are used to assess powder chargeability and subsequent charge decay rates: an in house built system involving Capacitive Probe coupled with Dynamic Vapor Sorption[3] and a commercial instrument: the JCI Chilworth Charge Decay Time Analyser (CDTA). The work is conducted using surface modified glass beads and two pharmaceutical excipients – lactose and mannitol.
The results obtained show strong correlations between the physicochemical and electrostatic properties of powders. This demonstrates that the inherent properties of powders influence their electrostatic properties and thus by suitable characterisation, the electrostatic properties of powders may be predicted.

The authors acknowledge Dr Jin Wang Kwek and Dr Daryl Williams for their contribution to this work.

[1] Matsusaka, S.; Maruyama, H.; Matsuyama, T.; Ghadiri, M., Triboelectric charging of powders: A review. Chem. Eng. Sci. 2010, 65 (22), 5781-5807.
[2] Šupuk, E.; Zarrebini, A.; Reddy, J. P.; Hughes, H.; Leane, M. M.; Tobyn, M. J.; Timmins, P.; Ghadiri, M., Tribo-electrification of active pharmaceutical ingredients and excipients. Powder Technol. 2012, 217, 427-434.
[3] Biegaj, K.; Kwek, J. W.; Lukas, T.; Rowland, M.; Heng, J. Y. Y., Novel Coupling of a Capacitive Probe with a Dynamic Vapor Sorption (DVS) Instrument for the Electrostatic Measurements of Powders. Ind. Eng. Chem. Res. 2016, 55 (19), 5585-5589.

Charge transfer and discharge in contact and separation

Tatsushi Matsuyama
Faculty of Science and Engineering, Soka University, Japan

Contact and tribe-electrification consists of the processes of contact-charge transfer-separation-charge fixation, therefore, the final amount of 'generated' static electricity is determined by the charge fixation process by separation of two surfaces. We have been arguing that the amount of charge on a particle is limited by gas-discharge in the separation process in the particle charging due to impact. As separated, due to an increase of surface gap, the capacitance between the two surfaces decreases. Charge fixed condition with the decrease of capacitance results in an increase of potential difference between the two surfaces and which can easily reach to the gas breakdown limit of air, even for small particles. A question to be answered was whether the charge can remain, or all the charge can be relaxed when once the charge relaxation takes place due to such gas discharge. Recently we showed an experimental evidence that the gas discharge can take place serially and not-all the charge will relax in a separation process of polymer film and metal plate after contact, with using a ultra high impedance contact voltmeter (Trek Model 820). The gas discharges took place randomly in each separation process, but after multiple runs all the on-set voltages were consistent to give a certain discharge limitation line as a function of gap. The each amount in each charge relaxation event was small enough to give the surface voltage just below the limitation line. How the results can be interpreted to particle charging will be discussed in details.

Measurement of electrostatic charge of pharmaceutical aerosols

Matti Murtomaa
Department of Physics and Astronomy, University of Turku, Finland

Dry powders are widely used in the treatment of asthma and other respiratory diseases. In dry powder inhales (DPIs), the powder usually consists of micronized drug particles and coarser carrier particles. As the particles contact each other and the walls of the inhaler device, they acquire charge due to tribo-electrification. Static electrification has been shown to have a significant effect on the function of an inhaler, separation of drug particles from the carrier particles and also on the lung deposition. Thus, reliable measurement of the electrostatic charge of pharmaceutical aerosol is very important. Due to many different contacting materials, the charge of pharmaceutical aerosol is bipolar. While the net charge of emitted powder might be of interest in some cases, the charge distribution together with particle size distribution provides important information for formulation and device development. In this presentation, various methods for charge measurement are reviewed and discussed.

Movement of nonspherical conducting particles under an electric field

Boonchai Techaumnat
Chulalongkorn University, Thailand

The movement of particles in an insulation system magnifies their undesirable effects on the insulating capability of the system. In this work, we investigate the motion of nonspherical particles under electric field under different charging conditions. The purpose of this work is to identify the difference in electromechanical behavior of particles due to their profiles. Spheroidal and wired-shape particles are used as the samples. In the experiments, the particles are subjected to nonuniform electric field in a diverging electrode system. The particles are either in direct contact with the lower electrode or separated from the electrode by insulating layers. From the results, we observe a variety of particle behaviors according to the shapes and the charging conditions.

Modelling of Partial Electrocoalescence: Linear Dynamics Approach and Level-Set Method

V.Vivacquaa, M.Ghadirib*, A.M.Abdullaha, A.Hassanpourb, M.J.F.A.Al-Marric, B.Azzopardid, B.Hewakandambyd, B.Kermanie

a Center for Advanced Materials, Qatar University, Qatar
b Institute of Particle Science and Engineering, University of Leeds, UK
c Gas Processing Center, Qatar University, Qatar
d Department of Chemical and Environmental Engineering, University of Nottingham, UK
e Keytech, UK

The coalescence of a water drop in a dielectric oil phase at a water layer interface in the presence of an electric field can lead to the formation of a secondary droplet in some cases. In order to understand the conditions which can favour incomplete coalescence, two different modelling approaches are developed. A linear dynamic model of water droplet deformation in the presence of an electric field is first proposed and analytical solutions of the differential equation of motion are provided with different waveforms as forcing functions. This model predicts that the waveform affects the response of the droplet to the electric field stimulus and qualitative agreement with experiments in the literature suggests a strong relationship between the formation of secondary droplets and the amplitude of oscillation of the mother drop. The process is also simulated by solving the Navier-Stokes and charge conservation equations with the finite element method and tracking the oil-water interface by the Level-Set method. The sensitivity of the model with respect to some input parameters is assessed and it is shown that a good quantitative agreement with the experiments can be obtained if the parameters are suitably tuned. According to this model, the water phase conductivity affects the quality of coalescence, revealing the importance of taking into account the physics of charge relaxation.

A study on contact electrification of particles under different relative humidity

Li XIE and Jun ZHOU
Lanzhou University, China

We carried out an experimental study of the contact electrification (CE) that happens when glass spheres of identical materials collide under different ambient relative humidity (RH) conditions. The experimental results indicate that the net charge on a sphere from a single collision is significantly altered by varying the RH level; the charge increases with increasing RH at low humidity, and then decreases at high RH conditions. The net charge reaches a maximum in the 20%–40% RH range. To explain the dependence of the CE on RH, we propose a model which yields predictions in agreement with the experimental data. The model also reveals how CE can be affected by temperature and surface absorption energy.

Evaluation of a New Technique for Triboelectric Charging of Particles

F.Alfano1, U.Zafar, M.Ghadiri*

Institute of Particle Science and Engineering, University of Leeds, UK
1 On Erasmus Exchange from Dipartimento di Ingegneria per l’Ambiente ed il Territorio ed Ingegneria Chimica, Università della Calabria, Italy

In powder handling processes, triboelectric charging is encountered in many operations such as pneumatic conveying, comminution, mixing and sieving, in which the charged particles cause problems like segregation, agglomeration, particle deposition or adhesion. Moreover, if particles are excessively charged, an electrostatic discharge may occur, which can result in fire or dust explosion. For dielectric solids it is difficult to predict triboelectric charging propensity, so it is highly desirable to be able to quickly evaluate it, and where possible using the smallest powder quantity possible, as in a number of applications such as pharmaceutical powders there is often a very small powder quantity available for testing.

In the present work, a dispersion device (the disperser of Malvern© Morphologi G3) is adopted to cause triboelectric charging of powders. A very small powder quantity (as small as 0.1 mg) is dispersed by a pressure pulse of compressed gas such as air or nitrogen. The high transient gas velocity causes collisions of the particles with the containing walls resulting in dispersion, but also causing tribo-electric charging of the particles. The process is quick, compared to the current charge characterisation devices. In this presentation we focus on the effect of surface functional groups on the tribo-electric charge transfer process by using a number of organic crystalline particles (aspirin, paracetamol, α-lactose monohydrate) and model materials with a well-defined shape (glass beads) but with different silane groups deposited on their surfaces. The aim is to develop a relationship between the surface properties and triboelectric charging characteristics.

Keywords: Triboelectric charging, Malvern Dispersion unit, Pharmaceutical powders, Surface properties

Electromagnetic waves scattering by a partially charged spherical particles

Jun ZHOU and Li XIE
Lanzhou University, China

We derived the scattering filed of electromagnetic (EM) waves by a charged particle based on Mie theory. The change of scatering coefficient, absorption coefficient and extinction coefficient in surface charge density carried on a particle is investigated in detail. The results show that the charges carried on the particle have an effect on the scattering coefficient, absorption coefficient and extinction coefficient, which is correlated with amount of charges, the charged zone and particle size number. In the case of small particle size number, the ratio of the extiction coefficient of charged particle to the one of the uncharged particle increases as the area of charged zone increasing. In the full charged particle, whole surface of the particle uniformly charged, the ratio is far higher than one, in case of samll particle size number and the larger charge surface density.

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