There was a time when Plasma Physics was all about the behaviour of plasmas at millions of degrees of temperature trapped in magnetic confinement fusion devices, or those plasmas in stars enjoined with the stellar magnetic fields undergoing violent upheavals, or those streams of plasma ejected out of the sun as solar wind and compressing the geomagnetic field. Then plasma physicists started to look for plasmas useful in applications that make our lives comfortable and exciting. Creating reactive plasmas which etch silicon wafers into computer chips, building arrays of millions of pixels that glow and produces patterns to form the visual images in Plasma TVs, creating plasma lamps that glow and make your homes bright at the night. I am one of those plasma physicists who got excited by the promise of the pervasive presence of plasma as a versatile industrial tool for 21st-century manufacturing.
Many societally beneficial applications require plasmas where the ions and gas molecules remain close to the ambient temperature: cold plasmas. Low-pressure electrical discharges, like neon signs and fluorescent lamps, produce cold plasmas. Discharges in atmospheric pressure, like welding arcs, produce hot plasmas where gas temperatures are in thousands of degrees. This is because the electrons in these discharges absorb energy from the electric field and pass it on to the neutrals and ions by collisions and make them hot. In low pressures, collisions between electrons and neutral are rare so that electrons can remain energetic while the ions and neutral remain cold. Supplied with enough driving voltage, plasmas can be formed at atmospheric pressure which can be made cold by turning off the electric field after the plasma is formed. This causes the transfer of energy from electrons to neutrals to get switched off. This is called pulsed plasma. A repetitive train of pulses will create repetitive bursts of short-lived cold plasma, with energetic electrons and cold neutrals. Liberation of the plasma from the confines of the vacuum chamber has opened a myriad of new applications. Processing of matter which is not vacuum compatible has now become possible. Biological matter, fluids, fluffy materials, and powders belong to this category.
Cold plasma streams generate reactive charged species like electrons and ions, excited neutral atoms and molecules. They also emit ultraviolet radiation and have space charge electric fields. The reactive oxygen and nitrogen species (RONS) created within the plasma can mediate in many interactions, which promote the rate of seed germination, enhance plant growth, and increase agricultural yields. NTP applications in agriculture possess advantages over conventional treatments such as short treatment time and easy accessibility during operations.
Scientists are investigating a range of potential applications of cold plasma in agriculture. These applications can be classified as pre-harvest and post-harvest. Cold plasma technology used in pre-harvest includes sterilization of seeds, improving seed germination, reducing pathogen invasion in soil, etc. The use of cold plasma in post-harvest processes includes the preservation of food by eliminating pathogenic bacteria.
Seeds exposed to plasma and immersed in water for a day showed larger radicals or starter roots. When looked through an atomic force microscope the exposure had roughed up the surface of the seeds increasing water absorptivity. MRI images of treated beans showed more water inside, compared to untreated beans. In the context of seed technology, exposing the seeds to plasma changes the seed morphology. Genetic effects like gene expression and protein level can also be affected. These physical changes are beneficial to increased germination and enhancement of growth.
Plasma jets contain radicals that are chemically active and cause disinfection and sterilization of surfaces. An exposure of fruits for less than a minute to plasma can destroy disease-causing bacteria, such as E-coli, Salmonella and Listeria.
Water exposed to results in the production of different reactive oxygen and nitrogen species (RONS). Plasma treated water (PTW) therefore, has a mixture of different RONS, mainly those that have a long lifetime, e.g., H2O2, NO2−, NO3−, etc. There has been much observation that shows the potential for beneficial use of PTW in the agriculture and food industry.
In the last century, the number of humans supported per hectare of arable land increased from 1.9 to 4.3 persons; this increase was mainly possible due to Haber−Bosch nitrogen fertilizers. The most significant increase occurred in the 1980s, where improvements in farming practices and increased fertilizer contributed to the rise in crop yields. At the end of the 20th century, about 40% of the world’s population depended on fertilizer inputs to produce food. Trends in global nitrogen (N) application over the past 50 years shows that crop yield saturates with further N input.
The impact of synthetic N manufacture and fertilization on greenhouse gas emissions is raising concern. The emissions from synthetic N manufacture and fertilization in China were estimated to be 52.11 and 80.40 Mt CO2-eq year−1 for wheat and maize, respectively. In addition to the harmful environmental effects, the global production of N fertilizer is energy intensive. The possibility of producing synthetic fertilizers by in-situ plasma processes, which can reduce the carbon footprint of fertilizers is yet another area of investigation. One of the most attractive uses of plasma is as means to produce and deliver nitrogen fertilizer instead of using ammonia. Plasma-assisted nitrogen fixation (in the form of NOx) has many advantages, among which the possible use of air as feedstock, and the fact that NOx can be used in many other applications. More importantly, non-thermal plasma for NOx production has a lower limit of energy consumption than the H-B process.
Relevant to all these applications, there is the issue of finding a way to deliver plasma to plants and seeds on a large scale. Basic research is still oriented towards figuring out what the charged soup of plasma is doing to plants. When the plasma is added to the water, the reactive species contained in the plasma dissolve in water. The resulting plasma-infused water, with its biologically available nitrogen, will then be used to irrigate the plants. It will do the same job as ammonia: Nitrogen, which plants require for growth, is delivered as ions, excited molecules and compounds in the water. PTW as a carrier of nitrogen is effective and safe.