A plasma - an ionized gas consisting of electrons and ions - is the fourth state of matter and, besides solid (crystalline), liquid and gaseous, also the most disordered state. Plasmas exist mainly at high temperatures and expose a good electric conductivity. In our daily life we see plasmas in the flame of a candle, in the illuminating gas of neon lights, or in the shining surface of our sun. More than 99% of the visible matter in our universe is in the plasma state.
The crystallization of a plasma - without losing the plasma state - can be achieved with an additional component: micro-particles or 'dust'. These particles (actually melamine-formaldehyde spheres provided by Microparticles GmbH, see microscopic image) with the size of several microns (1 µm = 1/1000 mm) get charged inside the plasma by the bombardment with free electrons and ions. Due to the much lower mobility of the heavier ions, the particles are hit on average by more electrons. They accumulate a negative charge of several 1000 - 10,000 electron charges. This negative charge is, at a certain distance, screened by a positive cloud of ions around the particle. (See figure below.)
At a certain particle density the particles start to interact with each other by the repulsive Coulomb force. Together with the surrounding plasma they form a so-called 'complex (dusty) plasma'. The interaction may lead to a strong coupling of the particles, resembling a fluid phase, and even to the crystallization of the particles with a typical spacing of some 1/10 mm in a so-called 'plasma crystal'. Compared to the size of the particles the inter-particle spacing in a plasma crystal is large.
In general, the properties of complex plasmas and plasma crystals offer a variety of unique opportunities:
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