The summer holidays are over, the kids are back to school, and everyone is heading back to the office. An integral part of the morning routine for most is coffee. So, what's the best plasma to stir your coffee? A peculiar question to ask, but for PhD researcher Calum T. Ryan - who happens to be a dedicated coffee drinker - it's a question worth answering. For his PhD research, Ryan explored the key physics of plasma-induced liquid flows, which could have implications for improving medicines and growing healthier crops, and yes - even for your morning coffee. He defended his PhD thesis on September 10th.
"Plasmas are so cool!" That's how Calum T. Ryan, PhD researcher at the Department of Applied Physics and Science Education, views plasma - the fourth state of matter alongside solids, liquids, and gases. "Of course, I'm being ironic here, because in reality most plasmas are the opposite of cool."
Ryan's correction is merited, especially when plasmas are typically found in extremely hot conditions on Earth (think of lightning) and the universe at large (inside almost every single star).
The physics of plasmas
If you add energy (in the form of heat for example) to matter it leads to a change of state. For instance, heating up a solid causes it to first melt into a liquid and then evaporate into a gas. Just think of the phases of water - ice blocks, liquid in a glass, and liquid turning to steam.
However, adding even more energy causes electrons to break free from their host atoms and then turn into a 'fluid' of charged particles. This is known as a plasma - the 4th state of matter.
Plasmas can be found in lightning and in stars, but they also have industrial applications. You can meet plasmas in neon lights and fusion reactors. In addition, plasmas are used in microchip production, with ASML playing a leading role in harnessing the power of plasmas.
[Photo: iStockphoto]
Room temperature super water!
However, there is a twist, as Ryan explains. "Certain processes create plasmas at room temperature. These plasmas are not so dangerous; they can contact your skin, and they won't burn you."
When these 'cold' plasmas interact with liquids, some reactive chemistry leads to molecules known as reactive oxygen and nitrogen species (also known as RONS).
"These molecules give the liquid certain properties that turns it into plasma-activated water - a 'super water' if you will. Plasma activated water is used in a wide variety of industries, from medicine to agriculture, where these applications use RONS in different ways."
Stirring towards chaos
In plasma-liquid interactions, the plasma creates a flow in the liquid, which stirs the RONS throughout the solution differently depending on the flow type.
If the flow is very turbulent, the RONS will mix more uniformly in the liquid. "But if the flow is focused to one point in the liquid, the RONS will collect at that point and they won't be spread throughout the liquid," says Ryan.
This can create a problem for the pharmaceutical and agricultural industries where specific RONS distributions in a liquid are needed.
"Controlling plasma-induced liquid flow can lead to the optimization of plasma-liquid interactions and therefore lead to better medicines and healthier crops," notes Ryan. "In agriculture, plasma-activated water is used to make better fertilizers and thus enhance plant health."
Cleaning 'dirty' liquids
"The physics behind how room temperature plasma-liquid interactions drive flow in liquids has been a mystery, but my work sheds new light on the physics of these flows," says Ryan.
First, Ryan found that when a plasma interacts with pure water (without any minerals or impurities), the movement of the plasma's ions push the fluids to create a flow in the liquid. "Adding certain impurities to the liquid causes the flow to change direction," says Ryan.
For instance, when salts are added, the electric field produced by the plasma causes the salt ions to move within the liquid itself, and this drives a force in the opposite direction to the plasma ions. When soap is added, a similar flow results but driven by a different process.
"Plasmas can be used to clean 'dirty' liquids, and as the plasma removes the soap from the liquid, the surface tension of the water changes, which produces a flow towards the region with the highest tension. So, as the plasma removing the soap increases the surface tension at the interaction, it drives the flow."
Plasma-based coffee machines?
But what does this all mean for your morning coffee? And what is the best plasma to stir your coffee?
"When comparing flows driven by plasmas with different polarities (positive or negative voltages), we found that positive plasmas drive faster flows in liquids in comparison to negative plasmas."
To visualize how positive and negative voltage plasmas stir liquids, Ryan added instant coffee to a plasma liquid mixture. "We discovered that the faster flow of the positive plasma mixed the instant coffee more uniformly, so we can expect the same behavior for a plasma-mixed liquid containing RONS."
While Ryan's discovery is a fundamental finding about plasma flows, does the researcher think that his work could revolutionize the coffee ritual in the morning?
"I'm not sure about that. I don't think that there'll be plasma-based coffee machines any time soon! I never drank any of the coffee though. The plasma adds nitrates to the liquid (which are very useful for fertilizer production), you could argue that it might taste like bad alcohol!"
Despite the bad taste, there could be valuable lessons here for anyone who wants to make a nice cup of uniformly mixed coffee every morning.
"These lessons would come at the cost of a nice, tasty cup of coffee though - at the moment anyway!" jests Ryan.
