Every high school science course focuses on the fundamental states of matter in the form of gases, liquids, and solids—states that are straightforward to study and manipulate. But there is a fourth state of matter that most people are much less familiar with because it does not exist freely on Earth.
This is plasma—a gas in which electrons have been stripped from atoms. The sun is such a mixture of ions and electrons, and much of interstellar space is filled with plasma. But on Earth, plasmas tend to occur fleetingly—in lightning, for example.
However, in the past 100 years, scientists and engineers have begun exploiting this form of matter to create light (neon lights are plasmas) and to interact with materials in a way that modifies the properties of their surfaces.
Because plasmas are generally hard to make and control, they are often confined to industrial machinery or specialized labs. But an easier way to make and control plasmas could change all that.
Enter Kausik Das of the University of Maryland Eastern Shore, and several colleagues who have found a way to create plasmas in an ordinary kitchen microwave. Their technique opens the way for a new generation to experiment with this exotic form of matter and perhaps to develop new applications.
First, some background. One way to make plasmas is to break apart molecules using powerful electric fields. This creates ions that the electric fields then accelerate, causing them to smash into other molecules. These collisions knock electrons off the atoms, creating more ions.
In the right circumstances, this process triggers a cascade that causes the entire gas to become ionized.
Das and his colleagues have worked out how to do this in a standard kitchen microwave oven (they don’t identify the brand). They also use a cheap glass flask capable of holding a vacuum as well as a seal.
Kitchen microwaves produce electromagnetic radiation with a wavelength of around 12 centimeters. These waves particularly influence polar molecules that have a positive charge at one end and a negative charge at the other.
Water is a good example of a polar molecule. As the alternating field changes, water molecules attempt to align themselves with the field. This rotation causes them to bump into other molecules, thereby raising their temperature.
But if the density of molecules is low, they do not bump into other molecules and so cannot dissipate this extra energy. In that case, the alternating field causes the water molecules to rotate ever faster and eventually rip apart.
That’s the process that triggers the formation of a plasma. Das and company exploit it by sucking air out of their flask to create a low pressure. The low-pressure gas consists mostly of nitrogen and oxygen, but a few water molecules are also inevitably present.
Das's team then places the flask in the microwave and switches it on. The microwaves rip apart the water molecules inside the flask and accelerate them. If the pressure is low enough, they gain enough kinetic energy to knock electrons off nitrogen molecules, and the cascade begins. This creates a plasma that glows with a soft blue light.
But only for a few seconds. Soon the process begins to tear apart oxygen atoms, which creates a purple light. So the plasma changes color.
Das and company observe exactly this color evolution in their experiments, although they had to experiment carefully with the pressure in the flask. Too much gas prevents the water molecules from gaining enough kinetic energy to trigger the cascade. Too little gas means that collisions are less likely, so a plasma is more difficult to form. Das and his colleagues say their goal is to operate at the sweet spot between these regimes.
To get a better idea of what is going on, the team has analyzed the spectrum of light produced by the plasma to reveal the telltale signature of oxygen and nitrogen. And voilà—they have a plasma generated in a kitchen microwave.
That turns out to be useful for a variety of things that are otherwise impossible outside specialized labs. For example, Das and company show how to use the plasma to change the properties of polydimethylsiloxane, or PDMS, a common silicon-based polymer.
This is usually hydrophyllic—it attracts water. But bathing the material in the plasma for just a few seconds makes it hydrophobic. This property can be quantified by measuring the contact angle that a drop of water makes with the surface. Before treatment, PDMS has a contact angle of 64 degrees. After treatment, the angle increases to 134 degrees.
This is probably because the various ions in the plasma become embedded in the surface of the material during exposure. Those ions repel water.
The team goes on to show how to modify surfaces so they can become more adhesive and even change their electronic properties.
That’s interesting work that can be done not just in any lab but in any kitchen. It will certainly be a useful teaching method, but it may also allow home-based makers to experiment with plasma cleaning and etching.
As Das and his colleagues conclude: “These simple techniques of plasma generation and subsequent surface treatment and modification may lead to new opportunities to conduct research not only in advanced labs, but also in undergraduate and even high school research labs.”
Ref: arxiv.org/abs/1807.06784 : Plasma Generation by Household Microwave Oven for Surface Modification and Other Emerging Applications
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