An experimental investigation of triboelectrification in cohesive and non-cohesive pharmaceutical powders

A detailed experimental investigation has been undertaken of the triboelectrification process of the pharmaceutical excipient alpha-lactose monohydrate and selected drugs. Particle charge studies involved the design and construction of an apparatus to incorporate pneumatic powder feed, triboelectric charging via a cyclone separator and simultaneous powder charge (Q) and mass (M) measurements using a modified Faraday well. Preliminary studies showed the charging of the selected materials to be unaffected by relative humidity up to 84%, due to the very low hygroscopicity of the powders. Experiments using lactose size fractions with brass, steel and cellulose contact charging surfaces conducted at different feeder gas pressures, gave net electronegative specific charge values (Q/M) for the metal surfaces, and electropositive values for cellulose. Specific charge increased linearly with gas pressure for the metals, and non-linearly for cellulose. Samples of micronised lactose and beclomethasone dipropionate (BDP) showed more tendency than unmicronised samples to adhere to the contact surface which resulted in more complex charging processes. A novel system to analyse the charge and mass data in further detail was developed, giving relative Q/M values at unit time intervals. It is proposed that this will overcome the limitations of reliance on the final net specific charge value. Preliminary results indicated differences in the charging mechanism for lactose in contact with the two metal surfaces.     

An experimental investigation of triboelectrification in cohesive and non-cohesive pharmaceutical powders

Abstract

A detailed experimental investigation has been undertaken of the triboelectrification process of the pharmaceutical excipient alpha-lactose monohydrate and selected drugs. Particle charge studies involved the design and construction of an apparatus to incorporate pneumatic powder feed, triboelectric charging via a cyclone separator and simultaneous powder charge (Q) and mass (M) measurements using a modified Faraday well. Preliminary studies showed the charging of the selected materials to be unaffected by relative humidity up to 84%, due to the very low hygroscopicity of the powders. Experiments using lactose size fractions with brass, steel and cellulose contact charging surfaces conducted at different feeder gas pressures, gave net electronegative specific charge values (Q/M) for the metal surfaces, and electropositive values for cellulose. Specific charge increased linearly with gas pressure for the metals, and non-linearly for cellulose. Samples of micronised lactose and beclomethasone dipropionate (BDP) showed more tendency than unmicronised samples to adhere to the contact surface which resulted in more complex charging processes. A novel system to analyse the charge and mass data in further detail was developed, giving relative Q/M values at unit time intervals. It is proposed that this will overcome the limitations of reliance on the final net specific charge value. Preliminary results indicated differences in the charging mechanism for lactose in contact with the two metal surfaces.     

Electrophoresis of a charged droplet in a dielectric liquid for droplet actuation

Electrophoretic motion of a charged droplet in a dielectric fluid under an electric field has been investigated experimentally for use as a microdroplet actuation method. The effects of the droplet size, electric field strength, and electrolyte concentration and ion species on the charging of an aqueous droplet have been examined. The amount of electrical charging has been measured by two different methods: indirect measurement using the image analysis of droplet motion and direct measurement using the electrometer. Quantitative comparison of the droplet charge measured experimentally and the theoretical value of a perfectly conductive sphere shows that an aqueous droplet is less charged than the corresponding perfectly conductive sphere. The limiting effect on electrical charging is more significant for an electrolyte droplet, and the effect is positively correlated to the electrolyte concentration rather than the ion species. This implies that the low electrical conductivity of water is not a major cause of the limiting effect. The scaling law of the charging amount for a deionized water droplet nearly follows that of the perfect conductor, whereas for an electrolyte droplet, the scaling law exponent is slightly higher. Some advantages and potentials of the current droplet actuation method are also discussed in comparison with the conventional ones.     

Advanced triboelectric materials for liquid energy harvesting and emerging application

Triboelectric properties of materials play an essential role in liquid energy harvesting and emerging application. The triboelectric properties of materials can be controlled by chemical functionalization strategy, which can improve the utilization of liquid energy resources or reduce the hazards of electrostatic effects. Herein, the latest research progress in molecular modification based on chemical functionalization to control triboelectric properties of materials is systematically summarized. By introducing the mechanism of contact electrification between liquid and solid materials and the developmental history of liquid–solid contact electrification, the influence of solid surface charge density, wettability and liquid properties on contact electrification of liquid and solid materials is described. Research progress on chemical functionalization for improving the hydrophobicity of solid materials, surface charge density of solid materials and triboelectric properties of liquid materials is highlighted. The focus then turns to the significance of enhanced liquid–solid contact electrification in energy harvesting, self-powered sensors and metal corrosion protection. Recent advances in chemical functionalization strategies for weakening the triboelectric properties of solid and liquid materials are also highlighted. Finally, an outlook of the potential challenges for developing chemical functionalization strategies in the field of solid surface modification and liquid molecular modification is presented.     

用于微机电系统的SiO2/Si3N4驻极体的制备及电荷稳定性

驻极体微型发电机是近期提出的微电子机械系统开发中的一个新领域,驻极体电荷稳定性则是影响驻极体微型发电机性能的关键.用等离子体增强化学气相沉积(PECVD)方法制备SiO2/ Si3N4双层膜,采用电晕充电和热极化方法对材料进行注极形成驻极体,探讨了器件加工工艺及存储环境对双层膜驻极体电荷稳定性的影响.结果表明,电晕充电后SiO2/ Si3N4双层膜的电荷存储稳定性明显优于SiO2单层膜;传统的电晕注极方法仅适用于大面积驻极体的制备,但对微米量级的材料表面不适用;微器件制备的工艺流程对驻极体电荷稳定性有显著影响,但存储环境对热极化驻极体电荷稳定性的影响很小.     

Limiting particle charge for electrification in a corona discharge field

An experimental study is performed of electrification of objects from a dielectric fluid within a corona discharge field. Use of jet monodispersion permits comparison of data on charging of a cylindrical surface and spherical particles, elimination of the effect of dynamic factors, refinement of the electrification mechanism and limiting charge value. It is shown that aside from other factors, the limiting charge is defined by the cross sectional area of the electrified object, and is approximately 30% less than previously accepted values.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 60, No. 4, pp. 625–632, April, 1991.     

Measuring Piezoelectric Output—Fact or Friction?

Piezoelectric polymers are emerging as exceptionally promising materials for energy harvesting. While the theoretical figures of merit for piezoelectric polymers are comparable to ceramics, the measurement techniques need to be retrofitted to account for the different mechanical properties of the softer polymeric materials. Here, how contact electrification, including friction and contact separation, is often mistaken for piezoelectric charge is examined, and a perspective for how to separate these effects is provided. The state of the literature is assessed, and recommendations are made for clear and simple guidelines in reporting, for both sample geometry and testing methods, to enable accurate determination of piezoelectric figures of merit in polymers. Such improvements will allow an understanding of what types of material manipulation are required in order to enhance the piezoelectric output from polymers and enable the next generation of polymer energy harvester design.     

Electron Transfer in Nanoscale Contact Electrification: Effect of Temperature in the Metal–Dielectric Case

The phenomenon of contact electrification (CE) has been known for thousands of years, but the nature of the charge carriers and their transfer mechanisms are still under debate. Here, the CE and triboelectric charging process are studied for a metal–dielectric case at different thermal conditions by using atomic force microscopy and Kelvin probe force microscopy. The charge transfer process at the nanoscale is found to follow the modified thermionic‐emission model. In particular, the focus here is on the effect of a temperature difference between two contacting materials on the CE. It is revealed that hotter solids tend to receive positive triboelectric charges, while cooler solids tend to be negatively charged, which suggests that the temperature‐difference‐induced charge transfer can be attributed to the thermionic‐emission effect, in which the electrons are thermally excited and transfer from a hotter surface to a cooler one. Further, a thermionic‐emission band‐structure model is proposed to describe the electron transfer between two solids at different temperatures. The findings also suggest that CE can occur between two identical materials owing to the existence of a local temperature difference arising from the nanoscale rubbing of surfaces with different curvatures/roughness.     

Numerical investigation of powder dispersion mechanisms in Turbuhaler and the contact electrification effect

Powder dispersion in dry powder inhalers (DPI) is affected by factors such as device design and flow rate, but also electrification due to particle–particle/device collisions. This work presented a CFD-DEM study of powder dispersion in Turbuhaler®, aiming to understand the effect of electrostatic charge on the dispersion mechanisms. The device geometry was reconstructed from CT-scan images of commercial Turbuhaler device. Different work functions were applied to the active pharmaceutical ingredient (API) powder and the device wall. Electrostatic charges were accumulated on the API particles due to contact potential difference (CPD) between the particles and the device wall. Results showed that both the chamber and the spiral mouthpiece played an important role in de-agglomeration of powders caused by particle–wall impactions. With increasing flow rates, the performance of the device was improved with higher emitted dose (ED) and fine particle fractions (FPF). The electrostatic charging of the particles was enhanced with higher CPD and higher flow rates, but the electrostatic charging had a minimum effect on powder dispersion and deposition with slight reduction in ED and FPF. In conclusion, the van der Waals force is still the dominant adhesive inter-particle force, and the dispersion efficiency is affected by the flow rate rather than contact electrification of particles. Future work should focus on the effect of highly charged particles emitted from the inhaler on the deposition in the airway.     

恒栅流负电晕充电的Si基SiO2驻极体的电荷稳定性

本文讨论了在不同充电参数条件下的恒恒流电晕充电的Ｓｉ基ＳｉＯ２薄膜驻极体的空间电荷储存稳定性,并和栅控恒压电晕充电的结果进行了比较。利用电容－电压（Ｃ－Ｖ）分析技术确定了空间电荷重心的漂移,并利用等温表面电位衰减测量,开路热刺激放电实验及Ｃ－Ｖ分析技术讨论了Ｓｉ基ＳｉＯ２薄膜驻极体的空间电荷储存和输动特性。     

Charge injection on insulators via scanning probe microscopy techniques: towards data storage devices

The idea of contact electrification aiming the development of nano-scale data storage devices has been explored through a careful investigation of charge injection on insulating films (SiO2 and PMMA) via scanning probe microscopy techniques. A complete route for data storage, showing simple and effective ways to write (inject the charge with an atomic force microscopy (AFM) tip), to read (detect the charge with electric force microscopy), to store (keep sample charge by changing ambient and surface conditions) and to erase the information (make the discharge process faster) is proposed and discussed. A detailed study of the influence of several parameters like AFM mode, bias voltage, relative humidity and surface hydrophobicity is also presented to optimize both charge injection and discharge processes. Results show that monitoring parameters such as ambient relative humidity and surface hydrophobic/hydrophilic character enable the control of pattern size, lateral dispersion, and storage time. The charge polarity is also dependent on the surface hydrophobicity and either positive or negative charges can become more appropriate for storage depending on the surface hydrophobic/hydrophilic character.     

Contact electrization of natural diamond crystals

The triboelectric charging of natural diamond crystals as a result of the contact electrization on a metal surface has been experimentally studied. It is established that the contact electrization of diamond crystals is related to the accumulation of a positive charge due to the filling of hole traps with an activation energy of 1.1–2.17 eV. A physical model is proposed to describe this process.     

On the Electron‐Transfer Mechanism in the Contact‐Electrification Effect

A long debate on the charge identity and the associated mechanisms occurring in contact‐electrification (CE) (or triboelectrification) has persisted for many decades, while a conclusive model has not yet been reached for explaining this phenomenon known for more than 2600 years! Here, a new method is reported to quantitatively investigate real‐time charge transfer in CE via triboelectric nanogenerator as a function of temperature, which reveals that electron transfer is the dominant process for CE between two inorganic solids. A study on the surface charge density evolution with time at various high temperatures is consistent with the electron thermionic emission theory for triboelectric pairs composed of Ti–SiO₂ and Ti–Al₂O₃. Moreover, it is found that a potential barrier exists at the surface that prevents the charges generated by CE from flowing back to the solid where they are escaping from the surface after the contacting. This pinpoints the main reason why the charges generated in CE are readily retained by the material as electrostatic charges for hours at room temperature. Furthermore, an electron‐cloud–potential‐well model is proposed based on the electron‐emission‐dominatedcharge‐transfer mechanism, which can be generally applied to explain all types of CE in conventional materials.     

Quantum chemical calculation of electron transfer at metal/polymer interfaces

Calculation of contact charging at metal/polymer interfaces were performed by a quantum chemical method (DV-Xa). In the calculation, model clusters with dangling bonds were used. The model clusters showed surface states in the density of states (DOS), the electron transfer occurred at the contact interfaces between polymer and Al. Then, 0.3 nm was a reasonable value as the contact distance in the present simulation.Contact electrifications between PTFE and six metals, such as Pt, Au, Cu, Al, Pb and Ca were simulated. The charge transferred from the metal to PTFE depended on the work function of the metals, and had a gap in range of 4.25–4.28 eV. According to the gap of metals were classified into two groups. If Fermi level of a metal is lower than the lowest unoccupied molecular orbital (LUMO) level of PTFE, the electrons of the metal transfer to the surface state (interface state). Electrons in the other metals with a higher Fermi level move into the conduction band of PTFE.     

Probing Contact-Electrification-Induced Electron and Ion Transfers at a Liquid–Solid Interface

As a well-known phenomenon, contact electrification (CE) has been studied for decades. Although recent studies have proven that CE between two solids is primarily due to electron transfer, the mechanism for CE between liquid and solid remains controversial. The CE process between different liquids and polytetrafluoroethylene (PTFE) film is systematically studied to clarify the electrification mechanism of the solid–liquid interface. The CE between deionized water and PTFE can produce a surface charges density in the scale of 1 nC cm⁻², which is ten times higher than the calculation based on the pure ion-transfer model. Hence, electron transfer is likely the dominating effect for this liquid–solid electrification process. Meanwhile, as ion concentration increases, the ion adsorption on the PTFE hinders electron transfer and results in the suppression of the transferred charge amount. Furthermore, there is an obvious charge transfer between oil and PTFE, which further confirms the presence of electron transfer between liquid and solid, simply because there are no ions in oil droplets. It is demonstrated that electron transfer plays the dominant role during CE between liquids and solids, which directly impacts the traditional understanding of the formation of an electric double layer (EDL) at a liquid–solid interface in physical chemistry.     

Electrospinning Triboelectric Laminates: A Pathway for Scaling Energy Harvesters

Herein, a new paradigm of triboelectric polymers—the triboelectric laminate—a volumetric material with electromechanical response comparable to the benchmark soft piezoelectric material polyvinylidene difluoride is reported. The electromechanical response in the triboelectric laminate arises from aligned dipoles, generated from the orientation of contact electrification in the laminates bulk volume. The dipoles form between sequential bilayers consisting of two different electrospun polymer fibers of different diameter. The loose interface between the fiber bilayers ensures friction and triboelectric charging between two polymers. The electric output from the electrospun triboelectric laminate increases with increasing density of the bilayers. This system design has clear benefits over other flexible devices for mechanical energy harvesting as it does not require any poling procedures, and the electromechanical response is stable over 24 h of continuous operation. Moreover, the electromechanically responsive electrospun laminate can be made from all types of polymers, thus providing ample room for further improvements or functionalities such as stretchability, biodegradability, or biocompatibility. The concept of a triboelectric laminate can be introduced into existing triboelectric nanogenerator form factors, to dramatically increase charge harvesting of a variety of devices.     

Electrostatic self-assembly of macroscopic crystals using contact electrification

Self-assembly of components larger than molecules into ordered arrays is an efficient way of preparing microstructured materials with interesting mechanical and optical properties. Although crystallization of identical particles or particles of different sizes or shapes can be readily achieved, the repertoire of methods to assemble binary lattices of particles of the same sizes but with different properties is very limited. This paper describes electrostatic self-assembly of two types of macroscopic components of identical dimensions using interactions that are generated by contact electrification. The systems we have examined comprise two kinds of objects (usually spheres) made of different polymeric materials that charge with opposite electrical polarities when agitated on flat, metallic surfaces. The interplay of repulsive interactions between like-charged objects and attractive interactions between unlike-charged ones results in the self-assembly of these objects into highly ordered, closed arrays. Remarkably, some of the assemblies that form are not electroneutral-that is, they possess a net charge. We suggest that the stability of these unusual structures can be explained by accounting for the interactions between electric dipoles that the particles in the aggregates induce in their neighbours.     

Experimental study of triboelectric charging of polyethylene powders: Effect of humidity,impact velocity and temperature

The manufacturing and handling of polyethylene (PE) powders is associated with undesired charging, resulting in agglomeration of charged particles, wall sheeting and eventually leading to plugging of reactors/conveyors. In this work, we measured the triboelectric charging of PE powders using both sliding and shaking apparatuses in dependence on humidity, impact velocity of colliding particles and temperature of the colliding particles or of the wall. As expected, saturation charge of PE particles is reduced with increasing air humidity. However, in a more detailed study we observed that whenever we change the humidity, the saturation charge rapidly reaches the equilibrium value and no hysteresis in charging that might be caused, for example, by adsorption/desorption thermodynamics is observed. Saturation charge is believed to be independent of impact velocity; however, we show that the saturation charge is proportional to impact velocity in the system of PE particles of non-spherical shape. The charging is more pronounced as the temperature of PE particles increases, however, it is unchanged if only the metal wall in contact with the particles is heated. That is a different trend than observed for spherical metal particles (Greason, 2000).     

Electrostatics of granules and granular flows: A review

In past decades, the electrostatics of granules and granular flows has obtained more and more attention due to many industrial problems and the associated development of new technologies. Granule-wall collision causes electrification, where charge transfer can be characterized by work function, electron transfer, ion transfer, and material transfer. Electrification is affected by many factors and increases with granule processing, and the charge amount can reach a saturated state where electrification no longer increases, which has been confirmed by single granule and granule conveying systems. In addition, the presence of electrostatic charges has profound influences in relevant areas, including chemistry, chemical engineering, energy, pharmaceuticals, and so on. The measurement technology of electrostatics used in granule conveying systems has been improved with the continuous progress of industry. Furthermore, electrostatics of granules and granular flows will be developed into a more accurate area together with other subjects as an interdisciplinary problem to be concerned. In addition, in the pneumatic conveying system, granule-wall and granule-granule collision or friction can cause material transfer due to material breakage. The working mechanism of the material transfer due to collision or friction has never been fully understood. Such problems will be solved gradually in the future.     

Evolution of potential distributions during the charging of nano-structured metal oxide films in air as response to sudden voltage application

Using metal oxide film structures, which were originally designed for gas sensing applications, we measured the charging and discharge currents and potential distributions on several metal oxide coatings after the application of an electrical potential against earth. The potential distributions show a specific charging of the surface with oxygen ions through the gas phase. The accumulated charge corresponds to that of the pseudocapacitors. Influence of air humidity has been found to be low, voltaic and temperature dependences of the charge are presented. The activation energy of discharge indicates a weak chemisorption of the charging oxygen species on the metal oxide surface.