Atmospheric-pressure plasmas are used in a broad range of applications, such as materials synthesis and processing, water treatment, and biomedicine. In many cases, atmospheric-pressure plasmas are formed as jets in which there is a primary plasma where the power is applied to break down the gas, and then a secondary part that expands out. The secondary part, known as a spatial afterglow, is used for applications and actually interacts with a surface, the solid material or water that is being treated.
In a new paper entitled “Charge decay in the spatial afterglow of plasmas and its impact on diffusion regimes,” published by Nature Communications, University of Illinois researchers—led by NPRE professor Mohan Sankaran—delve deeper into spatial afterglows.
“This work is the culmination of a long and arduous effort by our lab to understand a region in plasmas termed the spatial afterglow,” said Nabiel Abuyazid, the first author of the paper. Abuyazid, a recent PhD graduate in Chemical and Biomolecular Engineering, who will soon start as a process engineer at Lam Research.
“When we first tackled the work, it seemed that the plasma community had largely ignored the spatial afterglow, despite plasmas being widely used in semiconductor manufacturing and surface treatment. But recently, there has been a growing body of evidence in the literature from works by other research groups and from our own that the spatial afterglow is something that warrants closer investigation.”
“A lot of what we know about plasmas is often applicable to the primary plasma,” Sankaran said. “But this is not necessarily relevant to the afterglow. There is a need to understand the afterglow to understand how to use atmospheric pressure plasmas for applications and overcome challenges that might arise.”
In this new study, researchers carried out experiments on the spatial afterglow to measure the densities of the charged species, then supported their experiments with a model.
“We first made sure that our experimental measurements and our modeling results were in agreement,” Sankaran said. “Then, we were able to fundamentally understand and predict how the charge densities decay in the spatial afterglow.”
What they found through their investigation is that, as might be expected, the charged species, generated in the primary plasma, decay in the spatial afterglow. But as they decay, something interesting happens. In the plasma, the electrons and ions diffuse together. The electrons are more mobile, but because of electrostatic effects, the ions get dragged along with them, similar to how a dog on a leash pulls the owner. In the spatial afterglow, these electrostatic effects also decay, and eventually electrons are able to pull away from the ions, are lost, and the slower ions remain.
“Our work shows that there is an evolution of physical properties of a plasma that can be summarized as a transition from collective to individual behavior as a plasma relaxes back down to ordinary gas, and that transition occurs in the spatial afterglow,” Abuyazid said. “The broader implications are that our findings can be used to enable more intelligent design of various applications involving atmospheric-pressure plasmas, as well as to help explain various phenomena already reported in the literature.”
Sankaran also said the work will allow controlled treatment of surfaces. “For example, polymers can be modified to tune their hydrophilicity for wetting or adhesion,” he said. “Water containing organics can be degraded or converted to desired products, or a biosurface could be decontaminated or chemically activated to induce a medical outcome.”
The paper’s other co-authors are Necip Üner, a former postdoctoral researcher in Sankaran’s group now teaching at Middle East Technical University in Turkey, and Sean Peyres, an NPRE graduate student who began this work as an undergraduate.
“I am incredibly proud of my role in this hopefully high-impact work, and especially grateful to Prof. Sankaran for accepting me as a sophomore to help and keep me around,” said Peyres. “For me, this has been about two and a half years in the making––even longer for Dr. Abuyazid, Prof. Üner, and Prof. Sankaran. I credit this work for teaching me patience, research independence, and multidisciplinary thinking.
“My undergraduate education in NPRE at the University of Illinois significantly contributed to the latter: this project was a culmination of chemistry, fluid mechanics, plasma physics, electrical engineering, computation, and mathematics, and I doubt I would’ve obtained sufficient proficiency in all these subjects in any other department.”