Complex plasma with Janus particles as a model active-matter system
Active matter classifies systems consisting of self-propelled units which convert the energy stored locally or extracted from their environment into directed motion. It has recently attracted considerable attention due to rich new physics it displays and potential applications in various fields including materials science. Active matter found in nature is inherently complex, so model systems are of interest where the main relevant features can be isolated and studied in laboratory experiments. An interesting instance of active matter is a suspension of active particles (e.g., the so-called Janus particles, where the two halves have different properties) in a gas discharge plasma. Such complex plasmas with active particles are excellent model systems which can enhance our understanding of natural active matter systems not easily amenable to experiment. In the present experimental study, micrometer-size plastic microspheres with thin gold coating on one side were suspended as a single layer in the plasma sheath of a radio-frequency discharge in argon and driven by a combination of laser-induced photophoretic force and asymmetric ion drag force. Enhanced particle activity in this highly driven, inertial active-matter system leads to collective particle dynamics characterized by extended self-similarity of the velocity field, intermittency, and the emergence of a direct energy cascade with non-universal scaling exponent.