The role of water content in triboelectric charging of wind-blown sand

Size dependent polarities in tribocharged dust aggregates

It is long known that particles of the same material but with different sizes charge with different polarities in mutual collisions. In most cases, the smaller grains become negative. Here, we study tribocharging of (sub-)mm dust aggregates in the course of microgravity experiments by determining the charges of particles through their motion within an electric field. Similar experiments were already conducted with monolithic grains. Here, the constituent dust grains in an aggregate add complexity to the process of tribocharging in various ways. This ranges from the dust size scale, setting local surface curvatures, over shifting grains during collisions, altering the outer surfaces and potentially generating sub-surface tribocharging, to material-dependent tribocharging with a non-homogeneous dust composition. Nevertheless, in concert with the usual size dependence, the small aggregates predominantly charge negatively, the large population charges predominantly positively.

Volcanic electrification: recent advances and future perspectives

The electrification of volcanic plumes has been described intermittently since at least the time of Pliny the Younger and the 79 AD eruption of Vesuvius. Although sometimes disregarded in the past as secondary effects, recent work suggests that the electrical properties of volcanic plumes reveal intrinsic and otherwise inaccessible parameters of explosive eruptions. An increasing number of volcanic lightning studies across the last decade have shown that electrification is ubiquitous in volcanic plumes. Technological advances in engineering and numerical modelling, paired with close observation of recent eruptions and dedicated laboratory studies (shock-tube and current impulse experiments), show that charge generation and electrical activity are related to the physical, chemical, and dynamic processes underpinning the eruption itself. Refining our understanding of volcanic plume electrification will continue advancing the fundamental understanding of eruptive processes to improve volcano monitoring. Realizing this goal, however, requires an interdisciplinary approach at the intersection of volcanology, atmospheric science, atmospheric electricity, and engineering. Our paper summarizes the rapid and steady progress achieved in recent volcanic lightning research and provides a vision for future developments in this growing field.

Surfactant–solid complex for enhancing the flow in pipelines: an experimental approach

Background

Viscoelastic soluble polymeric additives have been used successfully for a long time as drag reducers in pipelines carrying commercial liquids like crude oil. Most of these polymers suffer from irreversible degradation when exposed to high shearing zones as in valves, elbows, and pumps which reduces, or eliminates, its flow enhancement effect. Insoluble additives were proven to be an effective drag reducer that overcomes the degradation drawback of soluble additives. On the other hand, insoluble additives suffer from the lack of viscoelasticity which limits their use as flow enhancers. The creation of complexes from soluble and insoluble additives is a field of research that is rarely explored despite its importance in introducing new flow enhancement methods for a higher drag reduction performance. The present work introduces a new surfactant–solid complex as a drag-reducing agent for turbulent flow in pipelines.

Results

The surfactant, solid, and their complexes’ drag reduction performance was tested in a closed-loop turbulent flow liquid circulation system, while rheological characteristics of the soluble additives were tested using a standard rheometer. All the surfactant solutions showed non-Newtonian shear thinning behavior in all the investigated concentrations that ranged between 500 and 1300 wppm. The initial experimental result indicated that the surfactant solution's drag reduction performance was higher than that of the solid suspensions. On the other hand, the drag reduction performance was enhanced by 52% when creating a 1300 wppm surfactant–2000 wppm solid complex. This improvement in the drag reduction performance is due to the formation of surfactant–solid-enforced aggregates with high resistance to shear forces and high turbulence suppression efficiency.

Conclusions

The present work introduces a new drag reduction solid–surfactant complex by creating aggregates combining the viscoelastic properties of surfactants with the resistance to high shear forces exerted by the solid particles. The polar nature of the surfactant micelles that form in single-phase flow systems contributed significantly to trapping the solid's micro-particles as enforcement to resist the shearing forces applied by the turbulent flow system.

Study of a dust deposition mechanism dominated by electrostatic force on a solar photovoltaic module

Solar energy is one of the most promising renewable energy sources to solve the energy crisis. Dust deposition on solar photovoltaic (PV) modules significantly reduces the power generation of PV power plants. In this paper, the motion characteristics of the gas phase and charging mechanism of dust particles and solar PV glass are investigated by means of the computational fluid dynamics-discrete element model (CFD-DEM) method. In addition, the mechanism and characteristics of dust deposition on a solar PV module as dominated by electrostatic force are discussed. The research results show that frequent collisions between dust particles and PV glass or between dust particles lead to charging. The dust deposition mechanism on a solar PV module is a gas-solid-electrical multi-directional coupling process. There is a great electrostatic field near the solar PV glass, causing charged dust particle deposition. The dust deposition density decreases when the air inlet velocity increases and when the tilt angle of the solar PV module or the number of particle collisions decreases. Different particle dynamics have different dust deposition ratios for different predominant deposition forces (such as the electrostatic force, van der Waals force, and gravity force). The research findings provide an important theoretical basis for dust deposition prevention and removal from solar PV modules.

Slide electrification: charging of surfaces by moving water drops

We investigate the charge separation caused by the motion of a water drop across a hydrophobic, insulating solid surface. Although the phenomenon of liquid charging has been consistently reported, these reports are primarily observational, results are difficult to reproduce, and no quantitative theory has been developed. In this work, we address both the experimental and theoretical sides of this problem. We reproducibly measure the charge gained by water drops sliding down a substrate, and we outline an analytical theory to describe this charging process. As an experimental system, we choose water drops moving down an inclined plane of glass hydrophobized with perfluoro octadecyltrichlorosilane (PFOTS). On this surface, sliding drops gain a positive charge. We observe charge saturation in three variables: increasing drop number, increasing interval between drops, and increasing drop-sliding length. These charge saturations indicate a limited "storage capacity" of the system, as well as a gradual discharging of the surface. To explain these results, we theorize that some fraction of the charge in the Debye layer is transferred to the surface rather than being neutralized as the drop passes. This fraction, or "transfer coefficient", is dependent on the electric potentials of surface and drop. All of our experimental charge saturation results can be interpreted based on the proposed theory. Given that nearly every surface in our lives comes in contact with water, this water-dependent surface charging may be a ubiquitous process that we can begin to understand through the proposed theory.

Contribution of Binary Organic Layers to Soil Water Repellency: A Molecular Level Perspective

Soil water repellency (SWR) is an extensively occurring phenomenon on natural and agricultural soils with a severe impact on soil water relations and thus crop yields and ecosystem productivity. It is caused by long chain amphiphilic compounds that originate from plant cuticular waxes. However, the severity of SWR varies with soil physical properties and the concentration of the compounds closely associated with producing hydrophobic coatings on soil surfaces. The induction of SWR by hexadecane, isopropyl tetradecanoate, and palmitic acid (PA), as pure (individual) coatings and as coatings composed of binary mixtures, was investigated by applying a range of loadings on acid-washed sand (AWS) (300-500 μm diameter) and AWS with 5% kaolinite. Molarity of ethanol droplet (MED) tests were conducted to assess the severity of SWR. Palmitic acid was very effective at inducing SWR at loadings of >0.5 × 10^-6 mol g^-1. Hexadecane and isopropyl tetradecanoate had no effect on SWR when applied as single component coatings. However, when hexadecane was combined with palmitic acid, it enhanced the SWR effect of palmitic acid. In comparison, isopropyl tetradecanoate was found to partially mitigate the SWR caused by palmitic acid. The experimental measurements of SWR were complemented by fully atomistic molecular dynamics simulations that suggested variations of SWR could be explained through molecular level interactions, packing on different soil mineral surfaces and the surface characteristics of the mineral surfaces. In addition, H-donor interactions of PA were found to be instrumental in intermolecular and surface interactions. Furthermore, cohesion and packing of hydrocarbon chains were found to be important parameters favoring surface adhesion, which in turn led to the formation of hydrophobic molecular coatings. The finding that ester derivatives of long chain fatty acids do not induce water repellency suggests that the introduction of chemical or biological processes that promote esterification of fatty acids could be a mechanism for reducing soil water repellency in agricultural soils.

Spreading of triboelectrically charged granular matter

We report on the spreading of triboelectrically charged glass particles on an oppositely charged surface of a plastic cylindrical container in the presence of a constant mechanical agitation. The particles spread via sticking, as a monolayer on the cylinder's surface. Continued agitation initiates a sequence of instabilities of this monolayer, which first forms periodic wavy-stripe-shaped transverse density modulation in the monolayer and then ejects narrow and long particle-jets from the tips of these stripes. These jets finally coalesce laterally to form a homogeneous spreading front that is layered along the spreading direction. These remarkable growth patterns are related to a time evolving frictional drag between the moving charged glass particles and the countercharges on the plastic container. The results provide insight into the multiscale time-dependent tribolelectric processes and motivates further investigation into the microscopic causes of these macroscopic dynamical instabilities and spatial structures.

Theoretical modeling of relative humidity on contact electrification of sand particles

Contact electrification of identical insulating particles has crucial significance for industrial and environmental science, especially in wind-blown granular systems. At the same time, the experimental phenomena of charge transfer first increased and then decreased with the increase of relative humidity has attracted the interest of many researchers. Humidity can affect the charge transfer has been early observed in the experiment, but the reason always puzzles researchers. In this study, based on trapped high-energy electron transfer mechanism, we introduce the effect of the water film in the charge transfer model and consider the actual situations of the sand particles in the collision process. Furthermore, charge transfer between sand particles in a single collision under different humidity conditions is investigated. The predicted results agree well with the law obtained in existing experiments qualitatively and thereby a possible explanation why humidity can affect the charge transfer is given.

Friction, tribochemistry and triboelectricity: recent progress and perspectives

Mechanochemical reactions during polymer friction or contact produce ionic fragments distributed on positive and negative domains at both surfaces.