Self-organization in complex plasmas — lane formation and beyond

Complex plasmas consist of micron-sized particles inside a low temperature plasma usually generated by a rf or dc discharge in a gas at a pressure around 1 mbar. The particles collect negative charges of several thousand electrons and interact with themselves, the plasma, and (confining) electric fields. By adjusting the plasma parameters complex plasmas can be put in nearly any state from crystalline ('plasma crystal') to super-critical. This can be visualized by cameras and allows to study the behavior of matter on the kinetic level of individual particles. To get rid of the disturbing influence of gravity complex plasma experiments were performed onboard the space station ISS and on parabolic flights. The PK-4 setup uses a dc discharge in a glass tube to create complex plasmas to study fluids on the particle level. Experiments on parabolic flights exhibit effects of self- organization in complex plasma flows, such as 'lane formation'. Particles in interpenetrating clouds form lanes to minimize the energy to pass through each other. These effects are also known from granular media and even moving crowds. The behavior of complex plasma flows in PK-4 is compared to these cases.