Plasma Activated Liquids (PALs)

Plasma-activated liquids (PALs) are categorized based on the base liquid modified by the plasma process, allowing for the creation of solutions engineered to specific applications. Below are detailed descriptions of common PALs and the key molecular interactions involved in their formation.

Plasma-Activated Water (PAW): As the most studied PAL, PAW is created by treating water with plasma.

• Molecular Interaction: Plasma primarily interacts with water (H₂O) molecules and, if air is used as the plasma gas, dissolved atmospheric nitrogen (N₂) and oxygen (O₂). This interaction dissociates the molecules, leading to a cascade of reactions that form a rich mixutre of reactive species.
• Key Species & Applications: The result is an aqueous solution containing reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂) and ozone (O₃), and reactive nitrogen species (RNS) like nitrate (NO₃⁻) and nitrite (NO₂⁻). These make PAW a powerful, eco-friendly agent for agricultural applications, food decontamination, and disinfection.

Plasma-Activated Saline (PAS) and Ringer's Solution (PARS): These PALs are created by treating biocompatible, isotonic solutions for medical purposes.

• Molecular Interaction: In addition to interacting with water molecules, plasma also interacts with the electrolytes present, most notably chloride ions (Cl⁻).
• Key Species & Applications: This produces not only the standard ROS/RNS but also reactive chlorine species (e.g., OCl⁻), which add another layer of therapeutic action. The primary benefit is a sterile, antimicrobial liquid that is safe for direct application in wound healing and medical sterilization without damaging healthy tissue.

Plasma-Activated Oil: This involves treating various oils, from edible types like olive oil to mineral oils.

• Molecular Interaction: The plasma's energy primarily targets and modifies the long-chain fatty acids that constitute the bulk of the oil.
• Key Species & Applications: This interaction creates carbon-based radicals and lipid peroxides (LOOH). These new species can enhance the oil's oxidative stability and provide antimicrobial properties, making it useful for non-thermal food preservation to extend shelf life or for developing enhanced industrial lubricants.

Plasma-Activated Juice and Milk: Plasma offers a form of non-thermal pasteurization for beverages like juices and dairy products.

• Molecular Interaction: The plasma generates primary reactive species from the liquid's high water content. These species then interact with the complex organic molecules present, such as sugars, proteins (e.g., casein in milk), fats, and vitamins.
• Key Species & Applications: The goal is to eliminate harmful microbes using ROS and RNS, thereby extending shelf life while better preserving the sensitive flavors, colors, and nutritional components that are often degraded by traditional heat-based pasteurization.

Plasma-Activated Alcohol: This involves treating alcohols like ethanol or isopropanol.

• Molecular Interaction: Plasma acts on the alcohol molecules themselves (e.g., CH₃CH₂OH for ethanol) as well as any residual water.
• Key Species & Applications: This process generates highly potent reactive species that are distinct from those in water, including alkoxyl radicals (like ethoxy, CH₃CH₂O•) and hydroxyalkyl radicals (•CH₂CH₂OH). These species significantly enhance the liquid's disinfectant properties for advanced sterilization and create a highly reactive medium for specialized chemical synthesis.

Plasma-activated water (PAW) is water treated with plasma-generated reactive species, resulting in modified chemical properties that enhance its functionality. The primary active agents and characteristics in PAW include: 

• Reactive Oxygen Species (ROS): Such as hydrogen peroxide (H₂O₂), ozone (O₃), and hydroxyl radicals (•OH). 
• Reactive Nitrogen Species (RNS): Including nitrate (NO₃⁻), nitrite (NO₂⁻), and peroxynitrite (ONOO⁻). 
• Acidification and Increased Conductivity: PAW typically has a lower pH and higher oxidation-reduction potential (ORP), contributing to its effectiveness in disinfection and other applications. 

PAW has demonstrated promising uses in diverse fields, such as:  

• Antimicrobial Applications: For food safety and medical disinfection.  
• Agriculture: Promoting seed germination and plant growth.  
• Environmental Remediation: In wastewater treatment and pollutant degradation. 

While PAW is one of the most studied PALs, the concept of plasma activation can extend to other liquids. Selecting the correct base liquid enables plasma treatment for various applications, including sterilization, enhanced chemical reactivity, or targeted chemical transformations.  

PALs, including plasma-activated water (PAW), represent a groundbreaking advancement at the intersection of science, technology, and sustainability. Their significance extends far beyond their immediate applications as they offer innovative solutions that are important for addressing some of the most pressing global challenges. As industries increasingly prioritize sustainable practices and seek safer, more environmentally friendly alternatives, PALs emerge as powerful tools across various sectors, including agriculture, healthcare, sanitation, and environmental remediation. 

One of the defining advantages of PALs is their ability to reduce dependency on harsh chemicals, which have long been associated with adverse environmental and health effects. By promoting practices rooted in environmental safety, PALs help mitigate the ecological impact caused by conventional chemical use. For example, in agriculture, plasma-activated water can serve as an eco-friendly alternative to chemical fertilizers and pesticides, supporting healthier soil, cleaner water systems, and ultimately, a more robust ecosystem. Similarly, in healthcare, PALs offer groundbreaking potential for sterilization and disinfection without the use of toxic or hazardous substances, significantly improving safety in clinical and medical environments. 

Beyond their immediate applications, PALs contribute to the health of broader ecosystems. Their role in environmental remediation is particularly noteworthy, shedding light on the untapped potential of plasma technology in addressing critical ecological issues. PALs have effectively treated water pollution by breaking down harmful contaminants and pathogens, thus offering a sustainable and chemical-free approach to maintaining clean and safe water resources. This capability underscores the transformative potential of PALs in addressing broader environmental challenges such as waste management, soil recovery, and even air purification in the future. 

As industries and researchers continue to explore the potential of PALs, they uncover new ways to harness plasma technology in support of global sustainability goals. PALs represent a significant step forward in reducing environmental harm and pave the way for innovative and practical applications that can create safer, healthier environments worldwide. By developing healthier ecosystems and addressing urgent ecological challenges, plasma-activated liquids exemplify how science can be leveraged to drive meaningful and sustainable change across multiple domains. 

The chemical and physical properties of plasma-activated liquids (PALs), including plasma-activated water (PAW), are influenced by the liquid's composition, plasma generation method, and the reactive species produced. These characteristics are essential for various applications, including disinfection, pollutant degradation, and biomedical uses. Key features include: 

Reactive Species Generation: Plasma treatment produces a variety of reactive oxygen and nitrogen species (RONS), such as hydrogen peroxide (H₂O₂), ozone (O₃), hydroxyl radicals (•OH), nitrates (NO₃⁻), nitrites (NO₂⁻), and peroxynitrites (ONOO⁻), depending on the liquid and gases used. These species drive the antimicrobial, anti-biofilm, and contaminant-degrading effects of PALs. 

Lower pH: Acidification occurs due to the formation of acidic species (e.g., nitric acid), which enhances the liquid's antimicrobial and oxidative potential. 

High Oxidation-Reduction Potential (ORP): PALs exhibit elevated ORP, enabling strong oxidative activity, which contributes to the effective neutralization of pathogens, degradation of organic pollutants, and inactivation of harmful microorganisms. 

Enhanced Conductivity: Increased conductivity results from dissolved ions introduced during plasma treatment. This characteristic can influence the liquid's interaction with biological systems and chemical reactivity. 

Short- and Long-Lived Reactive Species: PALs contain transient and stable reactive species. Short-lived species (e.g., hydroxyl radicals) promote immediate reactions, while long-lived species (e.g., hydrogen peroxide, nitrates) provide sustained activity, contributing to their prolonged efficacy. 

Thermal Effects: Plasma treatment can trigger heating, which could influence the reaction kinetics of chemical species, although this effect is typically minimal in low-temperature plasma systems. 

Selective Reactivity: PALs demonstrate specificity in reactions, selectively targeting harmful microorganisms, biofilms, or organic pollutants without significantly affecting surrounding healthy tissues or non-target materials. This is key in biomedical and environmental applications. 

Compatibility with Broad Ranges of Liquids: Plasma activation can be applied to various liquids, including water, saline, and other solutions. This versatility allows it to be used in diverse fields, such as healthcare, agriculture, and industrial cleaning. 

Biocompatibility: Depending on the plasma parameters and liquid composition, PALs can exhibit biocompatible properties, enabling their safe use in medical applications such as wound healing and tissue sterilization. 

Decomposition of Harmful Compounds: Reactive species in PALs effectively break down toxic organic or inorganic chemicals, contributing to water and environmental purification efforts. 

Shelf-Life and Stability: Long-lived species in PALs allow for stored activity, making them practical for applications with time delays between production and use. However, stability depends on storage conditions. 

Eco-Friendliness: Plasma activation is a clean technology that avoids harmful chemical additives, making PALs environmentally sustainable for disinfection and decontamination. 

The production of reactive species in plasma-activated liquids (PALs) depends on two primary elements:  

• The gas used during plasma generation, such as air, oxygen, or nitrogen plasmas, produces unique reactive molecules.  
• The liquid medium was exposed to plasma. Water and other liquids interact with these reactive species to create an engineered combination of chemical products. 

As a result, each gas-liquid combination generates distinct reactive oxygen species (ROS) and reactive nitrogen species (RNS), making PALs versatile agents for specialized applications. For example: 

• The plasma treatment of water with air can produce ROS, such as hydrogen peroxide and ozone.  
• Plasma treatment involving nitrogen gas can form reactive nitrogen species, such as nitrogen oxides (NOx), which may lead to downstream products like ammonia. 

Plasma-activated liquids (PALs), including plasma-activated water (PAW) and other solutions treated with plasma (such as plasma-activated saline or plasma-treated organic solvents), have been widely studied for their diverse applications. Below are some prominent fields where PALs, particularly PAW, show significant potential: 

Medical and Healthcare Applications 

• Disinfection: Plasma-Activated Water (PAW) demonstrates strong antimicrobial properties, effectively inactivating bacteria, viruses, and fungi. This makes it a promising, environmentally friendly alternative to conventional chemical disinfectants, especially in clinical and hospital settings. Other PALs, such as plasma-activated saline, have also shown efficacy in similar disinfection roles.  
• Wound Healing: PAW promotes accelerated cell proliferation and exhibits strong antimicrobial action, creating an optimal environment for tissue regeneration. This dual action is particularly beneficial in treating chronic wounds and burns. Plasma-activated saline has also been proposed as a similar alternative for wound care, depending on its compatibility with specific tissues.  
• Cancer Therapy: Preliminary evidence suggests that certain PALs, including PAW, may selectively induce oxidative stress in cancer cells while sparing healthy cells. This selective cytotoxicity, attributed to reactive oxygen and nitrogen species (RONS), is an emerging area of research with promising implications for non-invasive cancer treatments. 

Agriculture and Food Industry 

• Seed Germination and Plant Growth: PAW has been shown to enhance seed germination rates, improve seed viability, and stimulate early plant growth by directly delivering nitrogen and oxygen species to seeds and seedlings. Beyond PAW, other plasma-treated nutrient solutions may offer specific benefits based on crop requirements.  
• Pesticide Reduction: PAW's antimicrobial properties help reduce the dependency on synthetic pesticides by suppressing microbial pathogens on crops. Some studies have also explored plasma-activated liquid fertilizers, which combine disinfection with nutrient enhancement.  
• Food Safety: PAW provides a non-thermal method for deactivating harmful microorganisms on fresh produce, meats, and seafood. This application not only ensures food safety but also helps extend the shelf life of perishable items. The use of other PALs targeted for specific food types, such as plasma-activated saline for seafood, is also being explored. 

Environmental Remediation 

• Wastewater Treatment: PALs have shown remarkable potential for degrading organic pollutants, removing heavy metals, and eliminating microbial pathogens in industrial and municipal wastewater treatment systems. PAW and other plasma-treated liquids, such as plasma-activated organic solvents, have been used depending on the target contaminants.  
• Air Purification: Plasma-activated liquids (PALs) have demonstrated significant potential for absorbing and neutralizing airborne pollutants, including volatile organic compounds (VOCs), chemical toxins, and particulate matter. The reactive species within PALs enable the breakdown of harmful airborne contaminants, offering a promising approach for enhancing indoor and outdoor air quality in industrial and urban environments. 
• Soil Remediation: PALs, particularly PAW with its reactive species, facilitate the degradation of harmful chemical residues, such as pesticides and hydrocarbons, in contaminated soil. These capabilities make them a promising tool for sustainable land management and restoration efforts. 

Industrial and Chemical Applications 

• Nanoparticle Synthesis: PALs provide a controlled environment for producing nanoparticles and nanostructures. 
• Surface Modification: Plasma-treated solvents can enhance material coatings and adhesion properties. 
• Catalysis: PALs serve as reactive media in organic synthesis and chemical manufacturing. 
• Nanoparticle Synthesis: PALs, including PAW and plasma-treated solvents, provide controlled environments rich in reactive species, enabling the synthesis of nanoparticles with precise sizes and shapes. This application is valuable for the electronics, biomedicine, and catalysis industries. 
• Surface Modification: Plasma-activated liquids, particularly plasma-treated organic solvents, modify surface properties, enhance adhesion, and improve material coatings. This is particularly important in applications such as flexible electronics and medical device manufacturing. 
• Catalysis and Chemical Manufacturing: PALs, with their high concentrations of reactive oxygen and nitrogen species, act as reactive media for organic synthesis and chemical production. Replacing harsher chemical precursors can lower energy requirements and enable cleaner manufacturing processes. 

The interaction between plasma and liquids is a fundamental process that underpins the unique properties and applications of plasma-activated liquids (PALs) or plasma-activated water (PAW). These complex interactions generate various physical, chemical, and biological effects, making plasma-liquid systems highly versatile for multiple applications. Key mechanisms and their implications include: 

Generation of Reactive Species: One of the defining outcomes of plasma-liquid interactions is the production of reactive species with significant chemical activity. These reactive species—oxygen and nitrogen species (RONS)—emerge from the interactions between energetic plasma particles and the liquid. For example, plasma exposure induces ionization and dissociation of liquid molecules, enhancing oxidative and reductive capacities in various processes. These species include:  

• Hydroxyl radicals (•OH): Highly reactive and central to advanced oxidation processes (AOPs).  
• Hydrogen peroxide (H₂O₂): Contributes to oxidative stress in biological and environmental applications.  
• Ozone (O₃): Facilitates pollutant degradation and microbial inactivation. 

Enhanced Mass Transfer and Reaction Dynamics: Plasma modifies the liquid environment to improve mass transfer and accelerate reactions:  

• Gas Dissolution: Plasma interactions can dissolve reactive gases (e.g., ozone, nitrogen oxides) into the liquid, increasing their chemical potency.  
• Bubble Formation and Cavitation: Plasma creates micro- or nanobubbles that amplify gas-liquid interaction. Cavitation events, caused by rapid pressure changes, intensify mixing, reaction kinetics, and even mechanical effects like pollutant disintegration. 

Decontamination and Environmental Cleanup: PALs are highly effective for environmental applications due to their ability to neutralize contaminants. 

• Pollutant Degradation: Plasma-generated reactive species break down complex organic pollutants into less harmful byproducts. This is particularly relevant in wastewater treatment, where plasma decomposes volatile organic compounds (VOCs) and toxic organics.  
• Microbial Inactivation: Reactive species exert antimicrobial effects by disrupting cell membranes and biomolecules in pathogens, making PALs ideal for water decontamination and food safety. 

Chemical Synthesis and Catalysis: Plasma-liquid systems serve as innovative platforms for chemical reactions:  

• Nanomaterial Synthesis: Plasma-activated liquids provide precursors for nanoparticle formation, facilitating controlled nucleation and growth processes.  
• Electrochemical Enhancement: Plasma reduces activation energy and accelerates chemical transformations in organic synthesis or electrolysis applications. 

Biological Effects and Agricultural Applications: Plasma-liquid systems also show promise in biological and agricultural applications: 

• Stimulation of Biological Processes: Plasma-activated liquids stimulate biological processes by promoting growth, improving tolerance, and encouragingmination through reactive oxygen and nitrogen species.  
• Biocompatibility and Biomedical Applications: The antimicrobial and bio-stimulatory effects of PALs hold immense potential in healthcare and wound treatment. 

Thermal and Surface Effects: Beyond chemical activity, plasma-liquid interactions induce other valuable effects: 

• Localized Heating: Plasma interactions can generate small zones of thermal activity, which is advantageous in applications like thermal plasma spraying.  
• Surface Modification: Plasma-generated species can functionalize or modify solid surfaces in contact with the liquid to improve wettability or adhesion, which is useful in coatings, manufacturing, or biomedical device development. 

Research and advancements continue to expand our understanding of PALs' capabilities. Investigating various liquid interactions, optimizing production methods, and exploring new applications are important for the future of plasma technology. Different studies focus on optimizing PAW production processes and exploring their diverse applications. Recognizing PALs' potential in nitrogen fixation and environmental remediation holds promise for industry standards and widespread shifts toward sustainability and safety in agriculture and food processing sectors. 

The growing emphasis on sustainable and environmentally friendly solutions highlights the importance of PALs in addressing global challenges. Ongoing research aims to optimize plasma-liquid interactions, scale production methods, and expand applications, particularly in nitrogen fixation, agriculture, and waste management. 

PALs remain at the forefront of research due to their unique properties and potential applications across various industries. Significant advancements in recent years have deepened our understanding of the interactions between plasma and liquids and refined the processes used to create PALs. Researchers are exploring key areas such as the dynamics of plasma-liquid interactions, the chemical and physical changes induced by plasma treatment, and the factors that influence the stability and efficacy of PALs. These insights drive innovations in production technologies, allowing for greater precision, scalability, and efficiency in PAL synthesis.  

One of the most promising research areas focuses on the diverse applications of PALs, particularly in nitrogen fixation and environmental remediation. Nitrogen fixation, the conversion of atmospheric nitrogen into biologically usable forms, is a critical agricultural process. Current research highlights the potential of PAL-based approaches to perform this function more sustainably and efficiently than traditional chemical methods. This has the potential to revolutionize modern agriculture and addresses pressing global concerns such as food security and the environmental impact of synthetic fertilizers.   

Environmental remediation explores PALs as practical tools for breaking down pollutants, reducing hazardous waste, and improving water treatment processes. Their ability to generate reactive species that can neutralize contaminants positions PALs as environmentally friendly solutions for waste management and pollution control. These applications align strongly with the global shift toward sustainable practices and reduced environmental impact.   

The broader societal implications of PALs also extend to industries such as healthcare, food processing, and energy production. For example, in food safety, PALs demonstrate potential as antimicrobial agents, reducing the need for harmful chemical preservatives. In healthcare, ongoing studies aim to harness the antimicrobial and wound-healing properties of PALs for treatment applications.   

As research progresses, the potential for PALs to set new benchmarks in sustainability, safety, and efficiency is becoming increasingly evident. Future advancements will likely focus on optimizing plasma-liquid interactions, improving the stability of activated liquids, and developing cost-effective, scalable production methods. By expanding the range of practical applications and enhancing their efficiency, PALs stand to make significant contributions to industries prioritizing environmental stewardship and technological innovation.   

LPP is generated by applying external energy sources to the liquid medium. The following are the primary methods for producing liquid-phase plasma: 

Electrical Discharges: High-voltage electrodes are submerged in the liquid, where electrical energy creates high-energy plasma at localized regions within the medium.  

Ultrasound-Induced Plasma: Intense ultrasound waves generate cavitation bubbles in the liquid. Upon compression, these rapidly collapsing bubbles create localized plasma.  

Laser-Induced Plasma: Focused laser beams ionize liquid molecules, generating plasma directly within the liquid environment. 

Plasma generation in liquids is inherently more complex than in gases due to the challenges of maintaining stability in a high-density medium. Consistent plasma generation requires precise control over variables such as energy input, liquid composition, and electrode configuration. 

Bubble dynamics are essential in determining the efficacy and efficiency of LPP processes. Introducing bubbles into the liquid phase provides several advantages that enhance plasma generation, improve stability, and maximize interactions between the plasma and the liquid. Key contributions of bubbles include: 

• Increased Surface Area: Bubbles significantly expand the surface area available for interaction between the reactive species produced by plasma and the liquid medium. This increased contact facilitates faster and more efficient reactions, which are critical for chemical degradation and water treatment. 
• Induced Cavitation: The rapid collapse of bubbles can produce localized shock waves and intense micro-mixing within the liquid. This cavitation effect enhances the dispersion of reactive species and intensifies the breakdown of contaminants, leading to improved cleaning, degradation, or remediation performance. 
• Stabilized Plasma Formation: Bubbles serve as a stabilizing medium that aids in maintaining uniform plasma conditions. By modulating energy dissipation and providing a controlled environment for plasma discharge, bubbles contribute to the overall stability and reproducibility of the process. 

Comparison with Gas-Phase Plasma Diffusion (GPPD) Bubble Dynamics: In contrast to LPP, where bubble dynamics primarily emphasize physical effects such as cavitation and shock wave generation, bubble dynamics in gas-phase plasma diffusion (GPPD) focus mainly on enhancing the diffusion of reactive species into the liquid phase. In GPPD, the interaction between the reactive plasma and liquid is diffusion-driven, as reactive species generated in the gas phase (e.g., radicals, ions) transfer into the liquid through the bubble interface. This diffusion process is essential for determining reaction efficiency and is a key consideration for optimizing GPPD-based systems. 

The direct generation of plasma in liquids offers several key benefits: 

High Reactivity: The in-situ generation of various reactive species at high concentrations allows for enhanced reactivity and a broader range of chemical transformations.  

Efficient Liquid Modification: LPP can alter the liquid's chemical and physical properties while maintaining process simplicity.  

Precise Reaction Control: The ability to control the plasma parameters, such as energy input and liquid composition, ensures greater precision in influencing reaction pathways.  

Environmental Sustainability: Compared to conventional chemical methods, LPP is more sustainable. It generates fewer harmful by-products and minimizes reliance on toxic chemicals.  

Economic Benefits: LPP processes are often energy-efficient and enable faster reaction times, making them cost-effective for research and industrial applications. 

Despite its numerous advantages, the implementation of LPP faces several challenges, including:  

Scalability: Although LPP is highly effective in small-scale setups, scaling systems to industrial levels can be difficult due to the need to maintain uniform plasma properties and reaction efficiencies.  

Operational Complexity: LPP's sensitive and intricate nature requires precise control over parameters such as voltage, liquid composition, and electrode configurations. This requires advanced monitoring and regulation systems. 

Liquid-phase plasma (LPP) represents a significant advancement in plasma technology, unlocking innovative solutions for environmental remediation, chemical synthesis, and material modification. Its capability to generate reactive species directly in liquid media provides substantial efficiency, sustainability, and versatility benefits. However, challenges must be addressed to expand its industrial applicability, scalability, and operational complexity.  

As research and development in LPP technology evolve, its potential to revolutionize various industries—from materials science to wastewater treatment—will likely grow significantly. This will offer novel pathways for addressing modern scientific and industrial challenges. 

GPPD can be implemented using several approaches, including:  

Dielectric Barrier Discharge (DBD): Plasma is generated above the liquid surface using high-voltage electrical discharges within a plasma-gap setup. The reactive plasma species are transferred to the liquid through diffusion, enriching it over time.  

Plasma Jets: Streams of high-energy plasma (often called atmospheric-pressure plasma jets) are directed onto the liquid surface. This localized exposure facilitates the dissolution of plasma species into the liquid.  

Bubbling Systems: Plasma-activated gas is bubbled directly through the liquid using diffuser systems. The plasma species interact with the liquid during their contact within the bubbles, ensuring efficient mass transfer of reactive species. 

Unlike Liquid Phase Plasma (LPP) systems, where bubble dynamics are critical for plasma generation and sustaining the plasma-liquid interface, GPPD bypasses bubble formation by introducing pre-generated plasma directly into the liquid. 

GPPD relies on the efficient diffusion of plasma-produced species (such as reactive oxygen and nitrogen species, ions, and radicals) into the liquid. The success of this method depends more on factors like plasma composition, flow dynamics, gas solubility, and liquid properties (e.g., viscosity and polarity), which control how well plasma-generated species dissolve and interact in the liquid phase. 

Several methods exist for diffusing gas-phase plasma into liquids in GPPD, each designed to maximize plasma-liquid interactions intended for the application. Methods, such as Venturi, nanobubble generation, sparging, and trickling, vary in their advantages, limitations, and suitability for producing specific plasma-activated liquids (PALs), such as plasma-activated water (PAW), plasma-activated saline (PAS), or other reactive solutions.  

Venturi method: The Venturi method leverages fluid dynamics principles to introduce gas-phase plasma into a liquid. A Venturi tube creates a localized low-pressure region by forcing the liquid through a constricted section, simultaneously increasing liquid velocity and drawing gas into the flow stream. This approach effectively dissolves the gas plasma into the liquid.  710. 

Advantages

• High Gas Dissolution Efficiency: The Venturi tube mechanism enhances gas dissolution rates, allowing efficient plasma-gas transport into the liquid phase. This is particularly effective for generating reactive oxygen species (ROS) and reactive nitrogen species (RNS) in water-based systems. 1.  
• Scalability: The method is suitable for industrial-scale applications due to its adaptability to larger setups, making it a common choice for water treatment and large-volume liquid processing. 
• Ozone (O₃) Capability: The Venturi method is especially efficient for dissolving ozone (O₃), a critical ROS often applied in water purification systems. 1. 

Disadvantages

• Inconsistent Dissolution Rates: Flow-induced turbulence can lead to uneven gas distribution in the liquid, resulting in inconsistent particle size distributions that may hinder uniform treatment outcomes. 1. 
• Energy Costs Due to Pressure Loss: The method requires careful management of pressure differentials to maintain efficient flow, which can increase energy demand to maintain system stability. 
• Scaling Complexity: While scalable, large-scale implementations face logistical and engineering challenges, such as maintaining optimal pressure, flow uniformity, and precision in gas-liquid mixing. 

Applications

• Plasma-Activated Water (PAW): Commonly used to generate plasma-activated water (PAW) due to its efficiency with ozone (O₃) and other reactive species.  
• Large Volume Applications: The method is best suited for liquids requiring uniform plasma exposure over a larger volume. 

Nanobubble generation: Nanobubbles are gas-filled cavities smaller than one μm in diameter, often generated by methods such as high-pressure dissolution, swirl-type flow devices, or ejector mechanisms. They create a large surface area for plasma interactions within the liquid phase. 19. 

Advantages

• Enhanced Gas Solubility and Stability: The nanoscale size of the bubbles increases the gas-liquid interface surface area, promoting more effective gas absorption and dissolution, especially for ROS and RNS species. Additionally, nanobubbles exhibit longer retention in the liquid, prolonging their reactivity. 5. 
• Improved Plasma-Liquid Interactions: Nanobubbles can enhance chemical reactions between plasma-generated species and the liquid because they interact at the microscopic level, potentially increasing the generation of long-lived reactive species. 
• Suitability for Biological Applications: Nanobubbles have shown efficacy in boosting biological processes, such as accelerating reactive oxygen interactions or enhancing bioavailability in medical and environmental applications. 4. 2. 

Disadvantages

• Energy-Intensive Production: Generating robust nanobubbles requires specialized equipment and processes that consume significant amounts of energy, driving up operational costs.  4.  
• Potential Mischaracterization: Debate persists about whether the produced bubbles meet strict nanoscale definitions, which may complicate predictions of their behavior in specific applications. 5. 
• Over-Saturation Risks: Improper management of nanobubble concentrations may oversaturate the liquid, reducing its capacity for dissolving additional plasma gases. 

Applications

• Stability and Reactivity: Nanobubble generation is versatile and can be applied to various plasma-activated liquids (PAW, PAS, and others), particularly when enhanced stability and reactivity of the dissolved plasma species are desired for medical or agricultural applications. 

Sparging: Sparging involves bubbling gas-phase plasma directly through the liquid. This creates gas-liquid interfaces via bubble formation, facilitating the dissolution of plasma-generated reactive species into the liquid phase. 8. 

Advantages

• Enhanced Mass Transfer Rates: Sparging increases the interaction surface between gas and liquid, making it effective for transferring and maintaining high concentrations of plasma-generated chemical species in the liquid. 
• Reduced Mechanical Agitation: Direct bubbling provides continuous gas-liquid mixing, minimizing the need for additional mechanical agitators and simplifying operation. 

Disadvantages  

• Foaming Issues: Increased bubble formation can lead to problematic foaming, complicating liquid handling and reducing the efficiency of plasma species dissolution. 8. 
• Inconsistent Gas Distribution: Variability in bubble size and dissolution behavior can result in non-uniform plasma species distribution across the liquid. 
• Efficiency Limitations: Compared to methods like nanobubbles, sparging offers lower interaction efficiency for nanoscale plasma species, leading to shorter-lived or inconsistent reactivity in the liquid. 

Applications

• Lab-Scale PAW Applications: Commonly used for generating plasma-activated water (PAW) in lab-scale systems.  

Tricking and Spraying: These methods involve introducing liquid into a gas-phase plasma through trickling (passing liquid over a packed bed) or spraying (atomizing liquid into droplets), increasing gas-liquid surface area. 8. 

Advantages

• Improved Contact Surface Area: Spraying and trickling maximize the gas-liquid interface, facilitating the absorption of plasma components. This is particularly effective for producing reactive oxygen species (ROS) and increasing ozone retention in liquids. 
• Flexible and Simple Setup: Both methods are relatively simple to implement and can be easily adapted to experimental or industrial conditions. 

Disadvantages

• Inefficient for Nanoscale Interaction: These methods typically produce larger bubbles or droplets, which reduce the efficacy of plasma species dissolution compared to other techniques.  3. 
• Inconsistent Gas Distribution: The uniformity of dissolved plasma depends on droplet size and system configurations, leading to potential variability in liquid treatment outcomes. 
• Higher Maintenance Requirements: Equipment such as spray nozzles or packed beds requires careful monitoring and maintenance to avoid clogging or performance degradation. 

Applications

• Simple Plasma-Activated Liquid (PALs) Applications: Trickling and spraying are versatile but generally suited for simpler plasma-activated liquids (e.g., PAW or plasma-treated cleaning solutions). Their efficiency decreases in applications requiring higher plasma-liquid interaction precision, such as medical-grade plasma solutions. 

Comparison of Methods 

Different plasma-liquid exposure methods must be selected based on the desired application, scalability, efficiency, and required characteristics of the plasma-activated liquid (PAL). Methods like Venturi and nanobubble generation are more adaptable to various plasma-activated liquids and applications. At the same time, simpler approaches like sparging and spraying may be better suited for basic processes or lab-scale setups.  

Key Consideration 

Nanobubble generation or carefully optimized sparging may be preferred for applications requiring plasma-activated liquids with long-lived reactive species (e.g., therapeutic or agricultural applications). Conversely, ozone-heavy solutions for water treatment (PAW) often benefit from Venturi methods or trickling techniques focused on maximizing localized reactive oxygen species (ROS) levels.  

The diffusion of plasma into liquids offers several key benefits:

Simpler Setup: Unlike liquid-phase plasma (LPP) systems, GPPD setups avoid direct contact between electrodes and the liquid. This minimizes the risk of electrode corrosion or contamination, simplifying long-term operation and maintenance.  

Scalability: GPPD methods are highly scalable, making them suitable for large-scale industrial applications such as producing plasma-activated water (PAW) for agricultural or medical use.  

Selective Chemistry: GPPD allows better control over the reactive species generated by varying the gas composition and plasma parameters. This control enables customization of the liquid's chemical profile for specific applications.  

Reduced Electrode Fouling: Since the plasma is generated in the gas phase above the liquid, there is little or no direct interaction between the electrical components and the liquid, reducing wear and maintenance challenges. 

The implementation of GPPD faces several challenges, including:  

Lower Reactivity and Efficiency: Compared to liquid-phase plasma (LPP), where plasma is generated directly in the liquid, GPPD often has a lower transfer efficiency of reactive species into the liquid. This can result in slower activation or reduced concentrations of active compounds.  

Limited Uniformity: Depending on the size and depth of the liquid volume, reactive species may distribute unevenly. The effective scaling of GPPD processes for large reactors may require careful engineering to ensure homogeneity.  

Energy Efficiency: Generating plasma in the gas phase and transferring the species to the liquid often requires more energy than LPP, making GPPD less energy-efficient in specific applications.  

Gas Dependency: The nature and concentration of reactive species depend heavily on the type of gas used. For example, air and oxygen gases generate a mix of ROS and RNS, while argon produces fewer chemically reactive species. Thus, the choice of gas must align carefully with the intended application.