Atmospheric Dielectric Barrier Discharge Plasma Research Articles

E. Coli inactivation by humidified air barrier discharge plasmas and the chemical stability of amino acids

In this study, we use atmospheric air dielectric barrier discharge (DBD) plasma to inactivate E. coli (E. coli) by changing the air humidity levels. In order to analyze the inactivation process of E. coli during the humidified air DBD plasma treatment, we analyze the byproducts of 12 representational amino acids and compare their chemical stability. The experimental results show that the E. coli surface density (CFU cm−2) on a steel disc decreased significantly when the air humidity was increased from 20% to 60%, and humidified air DBD plasma at 60% humidity is very effective in inactivating E. coli. A consistently potent deactivation effect on E. coli can be seen in plasma afterglow treatment experiments over a storage time of two minutes, indicating that plasma-activated long-lived reactive oxygen and nitrogen species play a crucial role in controlling the E. coli inactivation. Compared to the low humidity (20%) condition, the concentration of reactive species produced by the plasma at high humidity (60%) increased 2-fold for 2-hydroxyterephthalic acid and 5-fold for hydrogen peroxide while ozone production was halved. Amino acid experiments showed that amino acids may be oxidized, hydroxylated and nitroxylated by reactive species. The reduction of amino acids by air DBD plasma was even more pronounced at high air humidity compared to low air humidity. Our analysis indicates that the significant inactivation of E. coli by humified air DBD plasma is related to the chemical instability of amino acids.

Air disinfection by nanosecond pulsed DBD plasma

Atmospheric pressure dielectric barrier discharge (DBD) plasma is an emerging and promising technique for air disinfection in public environments. Power supply is a crucial factor but it remains unclear about its impacts on the air disinfection performance of plasmas. In this work, a nanosecond (ns) pulsed power supply was applied to drive an in-duct grating-like DBD array to achieve fast single-pass air disinfection. The influence of pulse parameters and environmental factors on both the discharge characteristics and the single-pass bacterial inactivation efficiency were uncovered. At a close relative humidity (RH) level, the efficiency was dominated by the discharge power, namely, specific input energy could serve as the disinfection dose. A higher frequency, shorter pulse rising time, and suitable pulse width are preferred to obtain a higher Z value. The pulsed source was not notably superior to an alternating current source, or even worse at a low voltage frequency at the same discharge power. Airflow humidity was a predominant factor to improve the efficiency and a single-pass efficiency of ∼ 99% and a Z value of 2.2 L/J were achieved under an optimal RH of 50%–60%. This work provides fundamental knowledge of ns-pulsed DBD on discharge characteristics and air disinfection behaviors.

Surface modification of polymers by 50 Hz dielectric barrier discharge (DBD) plasma produced in air at 40 Torr

This study deals with the surface modification of polymer films utilizing a custom designed cost- effective dielectric barrier discharge (DBD) plasma produced in air at reduced pressure. We comprehensively examine diverse aspects of surface modification, encompassing electrical discharge characterization, optical signal analysis, contact angle measurements, and surface morphology assessment. Our observations unveiled the presence of distinctive filamentary streamer-based micro-discharges during the DBD process, with a power consumption of approximately 5.64 watts and an electron density of 3.4 × 1011 cm−3. Optical emission spectroscopy identifies multiple emission peaks attributed to nitrogen emissions. Notably, plasma treatment substantially reduced the water contact angle and augmented surface energy on polypropylene (PP) and polyethylene terephthalate (PET) films. Surface morphology analysis illustrated an increase in surface roughness following plasma treatment. Intriguingly, the initial rapid alterations in wettability and surface morphology attained equilibrium after approximately 30 seconds of treatment. This study highlights atmospheric DBD plasma's effectiveness in customizing polymer surfaces, improving wettability and roughness, offering promising applications for enhanced adhesion and wetting.

Surface activation of viscose textiles via air, argon, and oxygen dielectric barrier discharge plasma: Influence of peak voltage

Abstract This paper discusses the use of atmospheric pressure dielectric barrier discharge (DBD) plasma treatment to enhance the surface qualities of viscose fabrics. The study explores the effects of different plasma gases, discharge voltages, and exposure times on the treated fabrics. The findings emphasize the importance of optimizing the plasma's peak voltage to achieve desired surface treatment outcomes. The document also presents data on color strength, wettability, color fastness, and tensile strength of the treated fabrics, as well as SEM analysis of surface morphology and chemical analysis using FTIR and EDX. The results show that treatment at a peak voltage of 11.83 kV is more efficient, except for the tensile strength which is enhanced at a peak voltage of 8.92 kV. The oxygen plasma treatment significantly improves the color strength, which exhibits an increase from 11 to 18. The intensified color was attributed to the significant influence of electrostatic interactions between the charged hydroxyl groups of the oxygen plasma treated viscose textiles and the dye molecules, which enhance the printability. The oxygen dielectric barrier discharge plasma exhibits a higher ability to enhance the properties of textiles when compared to air and argon plasmas. This study presents a sustainable, economical, secure, and ecologically friendly approach to explore new fabrics for specific uses.

Germination, Physicochemical Properties, and Antioxidant Enzyme Activities in Kangkong (Ipomoea aquatica Forssk.) Seeds as Affected by Dielectric Barrier Discharge Plasma

Dielectric barrier discharge (DBD) plasma has been utilized as a sustainable technology to enhance seed germination in various plant species. The objective of this research was to identify the mechanism of physicochemical properties and antioxidant enzyme activities to promote kangkong (Ipomoea aquatica Forssk.) seed germination using different durations of DBD plasma treatments. Seeds were exposed to atmospheric DBD plasma from 5 to 20 min, compared to non-treated seeds as the control. According to SEM images, the seed surface had cracks and grew wider as a result of the prolonged DBD plasma treatments. A longer DBD plasma treatment exhibited a lower water contact angle and increased water absorption. DBD plasma treatments strongly improved germination percentages and hydrogen peroxide (H2O2) contents. Seeds treated with DBD plasma for 20 min showed the highest malondialdehyde (MDA) content and the lowest field emergence. Catalase (CAT) activity increased under DBD plasma treatments for 5 and 10 min. Ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities were not statistically different among the treatments. This finding suggested that DBD plasma treatments stimulated the germination of kangkong seeds by modifying the seed surface, and upregulating H2O2 content and CAT activity. Five minutes was an appropriate time to treat DBD plasma.

Microwave and dielectric barrier discharge atmospheric plasma interaction on guava industrial biomass for biomaterial production

AbstractThis research helps to develop a novel and economical method for improving the extraction efficiency of oil and fully valorizing guava fruit processing industrial waste into biomaterial. Dielectric barrier discharge atmospheric (DBDA) plasma is a novel non‐thermal technology that has been widely used in the food processing field of research. In this study, the synergistic effect of microwave (MW) and DBDA plasma treatment under different treatment combinations on the modification of morphological and biochemical properties of the guava pomace powder on improving the efficiency of extraction of biomaterials has been investigated. The response surface model (RSM) model was then constructed using the experimental results of Box–Behnken Design (BBD) to obtain the optimal extraction conditions. The predicted optimal extraction conditions for the microwave power level of 600 W for a treatment time of 90 s under solvent extraction at 420 min; DBDA plasma treatment as 35 kV for 15 min and solvent extraction conditions were determined as 80°C for 360 min causing an increment of the oil yield up to 16.19% when extracted using solvent extractor with ethanol as a solvent. These results indicate that the DBDA plasma treatment previous to the extraction step extraction process can contribute to reducing the duration of extraction by 120 min. The obtained data indicate that the MW and DBDA plasma pre‐treatment before the extraction can reduce the extraction duration, increase the yield, and improve the nutritional quality and functional properties.

The effect of plasma treatment on the mode II interlaminar fracture toughness of Glass/Epoxy laminates

Despite the key advantages of fiber-reinforced polymer composites, such as high specific strength and stiffness, these materials are characterized by low interlaminar fracture toughness. This study aims to investigate the effect of dielectric-barrier discharge (DBD) plasma treatment of glass fibers on the mode II interlaminar fracture toughness (GIIC) of glass/epoxy composite laminates. To this aim, first, end-notched bending (ENF) specimens were fabricated using the vacuum infusion process. During the laminating process of the specimens, the two adjacent glass layers at the top and bottom faces of the pre-delamination were exposed to an atmospheric DBD plasma for different exposure times (R: reference specimens without plasma treatment, PT2: specimens with two rounds of treatment, and PT5: specimens with five rounds of treatment). Afterward, the ENF specimens were subjected to three-point bending loading according to ASTM D7905 standard. The results showed an increase of ∼ 51% and ∼ 56% in GIIC for PT2 and PT5 specimens compared to the reference specimens (R), respectively. The Fourier-transform infrared spectroscopy (FTIR) analysis indicated that the plasma treatment did not generate new functional groups at the surface of glass fibers; therefore, there was no increase in the chemical bonding strength of the fibers to the resin. However, scanning electron microscopy (SEM) revealed that the plasma treatment increased the adhesion of the glass fibers to epoxy by removing sizing of the glass fibers and increasing the surface roughness of the fibers. The results of the present study showed that the plasma treatment can be considered as a cheap, easy-to-use, cost effective and scalable method for toughening laminated composites instead of the costly and difficult methods such as electrospun nanofibers and z-pinning.

Inhibitory effects of non-thermal atmospheric plasma on Yersinia enterocolitica and Staphylococcus aureus in the Korean traditional non-fermented kimchi “Geotjeori”

Food-borne bacteria have frequently been detected in kimchi, a representative and traditional fermented ethnic food of Korea. This study investigated the effect of atmospheric dielectric barrier discharge (DBD) plasma treatment (1.1 kV, 43 kHz, N2: 1.5 m/s, 5–60 min) on reduction of Yersinia enterocolitica and Staphylococcus aureus and on quality parameters in Geotjeroi, a non-fermented kimchi. A decrease of 0.12/0.09, 0.19/0.19, 0.34/0.45, 0.64/0.72, and 1.13/1.12 log10 CFU/g was observed by 5, 10, 20, 30, and 60 min of DBD plasma, respectively. D-value of 52.83 and 51.95 min was determined for Y. enterocolitica (R2 = 0.99) and S. aureus (R2 = 0.98) using the first order kinetics model. The quality parameters (pH, Brix, and hardness) were not significantly different (P > 0.05) between treated and untreated Geotjeori. Moreover, a decrease of >1 log10 CFU/g, for both bacteria was observed without any change in the quality of Geotjeori. These findings imply that DBD plasma treatment enhances Geotjeori safety and protects product from microbial risk.

Effects of plasma treatment on surface insulation properties of epoxy composites

The surface of epoxy composite material working in DC electric field for a long time is easy to accumulate charge and cause flashover, which seriously reduces its insulation level and poses a threat to the safe operation of electrical equipment. In this study, SiO x films were deposited on the surface of nano-SiO2/ER nanoparticles by atmospheric pressure dielectric barrier discharge plasma vapor deposition technology using ethyl nor-silicate as a precursor. The effects of different deposition time on the deposition effect and the pressure level of the film surface were studied. Scanning electron microscope and atomic force microscope were used to characterize the surface morphology of the material, and the flashover voltage before and after deposition was measured. The results show that SiO x films of different thicknesses can be formed on the surface of the material at different times of plasma deposition. After extending the deposition time of the film, the film becomes more uniform and dense, the surface roughness of the material decreases continuously, and the electrical conductivity and flashover voltage along the surface increases continuously. After 10 min plasma treatment, the conductivity increased from 1.78×10−16 S to 2.3×10−13 S, while the flashover voltage along the surface increased from 19.29 kV to 22.66 kV, an increase of about 17.5%.

A Boltzmann Electron Drift Diffusion Model for Atmospheric Pressure Non-Thermal Plasma Simulations

We introduce a fluid computational model for the numerical simulation of atmospheric pressure dielectric barrier discharge plasmas. Ion and neutral species are treated with an explicit drift diffusion approach. The Boltzmann relation is used to compute the spatial distribution of electrons as a function of the electrostatic potential and the ionic charge density. This technique, widely used to speed up particle and fluid models for low-pressure conditions, poses several numerical challenges for high-pressure conditions and large electric field values typical of applications involving atmospheric-pressure plasmas. We develop a robust algorithm to solve the non-linear electrostatic Poisson problem arising from the Boltzmann electron approach under AC electric fields based on a charge-conserving iterative computation of the reference electric potential and electron density. We simulate a volumetric reactor in dry air, comparing the results yielded by the proposed method with those obtained when the drift diffusion approach is used for all charged species, including electrons. We show that the proposed methodology retains most of the physical information provided by the reference modeling approach while granting a substantial advantage in terms of computation time.

Morphological and biochemical modification of guava processing industrial bio-mass by dielectric barrier discharge atmospheric plasma and effect on yield of bio-materials

Guava pomace, an underutilized industrial byproduct from the guava processing fruit juice industry prone to have a wide range of bioactive and functional metabolites which has not been explored. Dielectric Barrier Discharge Atmospheric (DBDA) plasma is a novel non-thermal technology that has been widely used in the food processing field of research. In this study, the effect of DBDA plasma treatment under different treatment combinations i.e, 25 kV, 30 kV, and 35 kV for 5 min, 10 min and 15 min respectively for each voltage, on modification of morphological and biochemical properties of the guava pomace powder on improving the efficiency of extraction yield of biomaterials have been investigated. The response surface model (RSM) was then constructed using the experimental results of Box–Behnken Design (BBD) to obtain the optimal extraction conditions. The predicted optimal extraction conditions for DBDA plasma treatment as 35 kV for 15 min and solvent extraction conditions were determined as 80 °C for 360 min causing an increment of the oil yield up to 16.54% when extracted using solvent extractor with ethanol as a solvent. These results indicate that the DBDA plasma treatment previous to the extraction step extraction process can contribute to reduce the duration of extraction by 120 min, increasing the oil yield by 42.75% and improving the nutritional quality of the oil. This research helps to develop a novel and economical method for improving the extraction yield of oil and to fully valorizing the guava fruit processing industrial waste into biomaterial.

Inactivation of Cercospora lactucae-sativa through Application of Non-Thermal Atmospheric Pressure Gliding Arc, Tesla Coil and Dielectric Barrier Discharge Plasmas

Cercospora leaf spot disease is a serious problem for lettuce cultivation worldwide. Cercospora lactucae-sativa, the causative agent of leaf spot disease on lettuce, was treated with non-thermal atmospheric pressure gliding arc (GA), tesla coil (TC) and dielectric barrier discharge (DBD) plasmas for the in vitro fungal inactivation of both mycelial growth and conidial germination. The fungus was exposed to the three plasmas individually for 5, 10, 15 and 20 min. The results showed that DBD plasma inactivated fungal growth during all exposure periods, and the highest inhibitory effect was caused by exposure to DBD plasma for 20 min, at 93.33% inhibition. The germination of fungal conidia was completely inactivated after exposure to all three non-thermal plasmas for 5 min, as observed 4 and 24 h after incubation. The pathogenesis of C. lactucae-sativa on lettuce after plasma treatments for 5 min was examined by spraying an inoculation of the treated conidia on lettuce. The results showed that all three plasmas reduced the disease incidence and severity compared to the non-treated control. Therefore, the non-thermal atmospheric pressure GA, TC and DBD plasmas have antifungal potential for the inactivation of C. lactucae-sativa, making them an interesting novel technology for plant disease control.

Sterilization characteristics of narrow tubing by nitrogen oxides generated in atmospheric pressure air plasma

The sterilization characteristics of active species generated by an atmospheric dielectric barrier discharge plasma using air and oxygen at the inner surface of silicone tubing were investigated. A dielectric barrier discharge torch plasma device was installed at one end of the tube and generated long-lived active species that flowed into the tube. A strip-type biological indicator with a 105-cell bacterial spore was placed at the opposite end of the 60 cm tube. Sterilization was completed within 30 min by active particles generated from the air plasma. The main factors contributing to the sterilization by air plasma were HNO3 and N2O5. When organic materials (keratin, aspartic acid, and dipicolinic acid) reflecting components of the bacterial spore, were treated by the sterilization procedure there was little effect on dipicolinic acid. Keratin was oxidized by ozone and NOx generated from the oxygen and air plasmas, respectively. Aspartic acid underwent little change in composition from ozone generated from the oxygen plasma, whereas nitro (NO2), nitroso (NO), and aldehyde (CHO) groups were formed from ozone and NOx generated from the air plasma.

Plasma catalytic non-oxidative methane conversion to hydrogen and value-added hydrocarbons on zeolite 13X

Non-thermal plasma has unfolded highly efficient, safe to operate novel routes for methane conversion to hydrogen. In this work, methane conversion is performed under atmospheric dielectric barrier discharge (DBD) plasma with and without 13X zeolite-based catalysts i.e., 13X, Ga/13X, Pd/13X, and Pd-Ga/13X. Experimental results indicate that the plasma catalytic process delivered almost twofold higher product yield than the plasma only route. The binary Pd-Ga catalyst possesses highest catalytic performance with about 40 % CH4 conversion at an input flowrate of 5 cm3min−1 and 2 W due to the formation of the Pd-Ga alloy, which acts as catalytic active centre for activating C–H bonds. Product yield can be tailored by the catalyst design where the bimetallic Pd-Ga/13X preferably favours the hydrocarbon formation while H2 is the dominant product obtained over the Pd/13X. The cleavage of C–H bonds of methane molecule over the plasma only route is mainly governed by energetic electrons in the gaseous phase the catalyst activity, and plasma-catalyst synergism play a significant role in the plasma catalysis process. The findings from this work provide significant insights into the methane activation for subsequent optimization of the methane conversion processes operated on floating production, storage, and offloading vessels (FPSOVs). Exploiting untapped offshore natural gas reserves, where conventional pipeline systems are less economical.

Hierarchical Structures Formed in an Atmospheric Pressure Plasma Jet by Polymerization from an Aerosol of Oxalic Acid Stabilized WO3–x Nanosheet Colloids: Toward High-Performance Electrochemical Devices and Sensors

The development of microstructures which combine high total surface area and high porosity is crucial for technologies such as electrocatalysis, electrochromics, and sensors. High deposition rate, composition control of deposition, and low processing temperature to retain active compositions are also desirable. To this end, this study describes combining colloidal sol chemistry with a nonthermal atmospheric pressure dielectric barrier discharge plasma jet to print hybrid inorganic/organic tungsten trioxide/oxalic acid (WO3–x/OA) microspheres. Injection of an aerosol of oxalic acid stabilized colloidal tungstic acid into an atmospheric pressure plasma jet results in the deposition of spherical structures in which the colloid is trapped within a plasma-polymerized organic shell. Subsequent low-temperature sintering produces hierarchical spherical shell-like structures comprising tungsten oxide nanosheets. Alteration of the gas flow rate changes the composition of the deposited material. The method has promise for the general preparation from colloidal precursors of porous materials of controlled morphology and composition with hierarchical microstructures, such as are required for applications in electrochemical devices and sensors which need a high ratio of surface area to volume and connectivity throughout the structure, yet also need a microstructure which is open for rapid exchange of reactants.

Mechanisms of reducing energy costs for nitrogen fixation using air-based atmospheric DBD plasmas over water in contact with the electrode

In this study, a plasma-assisted nitrogen fixation (NF) process, using dielectric barrier discharge (DBD) air and N2 plasmas ignited over a water surface is presented. We describe the influential factors on the plasma properties through plasma diagnostics such as electrode spacings, applied voltages, and feedstocks to reduce the energy cost for NF. In order to maximize the energy yield and reduce the energy cost for NF as NO3-, the gas phase NOx emitted through the gas outlet is trapped into water. Plasma diagnostics is performed to explain the underlying mechanisms for NOx formation with a reduced energy cost. The analyzed results suggest that decreasing the electrode gap over water surface can effectively reduce the energy cost for NF, and that this can be attributed to the increased number of distributed microdischarges instead of more intense plasma filaments that dominate at higher gaps. This system offers a promising result in NO3− synthesis, with a selectivity of 98 %, and an energy cost of 20.7 MJ/mol.

Grating-like DBD plasma for air disinfection: Dose and dose-response characteristics

Atmospheric pressure dielectric barrier discharge (DBD) plasma is an emerging technique for effective bioaerosol decontamination and is promising to be used in indoor environments to reduce infections. However, fundamental knowledge of the dose and dose-response characteristics of plasma-based disinfection technology is very limited. By examining the single-pass removal efficiency of S. lentus aerosol by in-duct grating-like DBD plasma reactors with varied discharge setups (gap distance, electrode size, number of discharge layers, frequency, dielectric material), it was found that the specific input energy (SIE) could be served as the dose for disinfection, and the efficiency was exponentially dependent on SIE in most cases. The corresponding susceptibility constants (Z values) were obtained hereinafter. Humidity was a prominent factor boosting the efficiency with a Z value of 0.36 L/J at relative humidity (RH) of 20% and 1.68 L/J at RH of 60%. MS2 phage showed a much higher efficiency of 2.66–3.08 log10 of reduction than those of S. lentus (38–85%) and E. coli (42%–95%) under the same condition. Using SIE as the dose, the performance of plasma reactors in the literature was compared and evaluated. This work provides a theoretical and engineering basis for air disinfection by plasma-based technology.

The effect of different cold atmospheric plasma sources and treatment modalities on the generation of reactive oxygen and nitrogen species in water

AbstractReactive oxygen and nitrogen species (RONS) can be generated in liquids by different cold plasma processes. These species can influence cell biology and pathological states, and trigger wound healing, disinfection, and cancer treatments. The optimization of plasma sources' configurations and experimental conditions is fundamental to dose‐produced RONS in liquids. This research is a case study comparing RONS generation (H2O2 and NO2−) in water, after exposure to two different atmospheric pressure cold dielectric barrier discharge (DBD) plasma sources: a plasma jet‐DBD and a planar‐DBD operating in contact with the liquid and in remote, respectively. The data demonstrate that the reactor geometry and the treatment modality strongly impact the generation of the active species in water.

Organic pollutants degradation using plasma with simultaneous ammonification assisted by electrolytic two-cell system

Atmospheric non-thermal dielectric barrier discharge (DBD) plasma has gained considerable attention due to its cost-efficiency, environmental friendliness, and simplicity. However, certain deficiencies restrict its broad application. Herein, the DBD plasma was used to disrupt three model pharmaceutically active compounds (PhACs), sulfamethoxazole (SMX), ibuprofen (IBP), and norfloxacin (NFX), by varying parameters, such as gas type (Ar, N2, O2, and air) and flow rate (1–4 L min−1). The air plasma discharge had the highest degradation efficiency, and the air flow rate was optimized at 2 L min−1. However, only 10% of IBP was removed by the sole plasma, whereas NFX and SMX were entirely removed after 30 min. Since the air plasma discharge generates reactive oxygen and nitrogen species in a chained reaction, the remaining NO2− and NO3− in the aqueous phase were problematic. Therefore, by coupling plasma with electrolysis using Cu/reduced Cu nanowire (R-CuNw) as the anode/cathode, all three PhACs were removed within 30 min, and NO2− and NO3− were completely reduced to NH3 with cathodic reduction. Moreover, the electrical energy per order (EEO, 0.04 kWh L−1) and treatment cost (0.003 USD L−1) were much lower than those of the single system. This system demonstrates great potential for water remediation, and the production of NH3 as a value-added by-product remarkably improves its practicality and is of great importance in agriculture and energy-related industries.

Area selective atmospheric pressure PECVD of organosilicon precursors: Role of vinyl and ethoxy groups on silicon oxycarbide deposition patterns - A case study

An atmospheric pressure Dielectric Barrier Discharge plasma torch was used to perform selective area plasma enhanced chemical vapour deposition of four organosilicon precurors: Triethylvinylsilane (VTES), Vinyltriethoxysilane (VTEOS), Tetraethoxysilane (TEOS) and Tetraethylsilane (TES). The precursors were chosen in such a way that they differ only by the presence or absence of vinyl and/or ethoxy groups. Experimentally, major differences have been observed on the deposition speed, coating patterns and thin film composition. Precursor with vinyl group has confined deposition at the central region of plasma torch, in contrast precursor with only ethoxy groups has circular deposition in the periphery region of plasma torch, and precursors with both the vinyl and ethoxy groups has deposition at both locations. A detailed chemical analysis revealed the film composition to be silicon oxycarbide (SiOxCyH) with high content of carbon for the central region's coating whereas the peripherical coating is close to silica like (SiOx) with a minuscule carbon content. Coatings patterns and composition have been investigated in the light of gaseous flows mixing obtained from computational Fluid Dynamic (CFD) simulation. A strong correlation is evidenced for central SiOxCyH deposition of vinyl containing precursors, to the high concentration of precursor. Whereas the peripheral SiOx deposition of ethoxy containing precursors corresponds to high concentration of plasma species. Furthermore, the role of reactive plasma species on the underlying deposition mechanisms has been identified as crucial and has been purposed as the driving factor.