Particle-Microplasma Interactions

Summary form only given. We demonstrate that the potential gradients created within microplasmas are capable of trapping micro-and nanoparticles in a three-dimensional space. The microplasma is generated within a microwave-excited discharge gap. The gap is typically 150 microns across and a symmetric 900 MHz voltage is applied across the gap to initiate and sustain the microplasma [1]. Although the plasma is in direct contact with microwave-driven electrodes, the ion plasma frequency and the natural frequency of the particles are orders of magnitude less than the drive frequency. Therefore, the particles suspended in the microplasma only respond to the internal, time-average plasma potential and externally applied potentials. To demonstrate the particle trapping phenomena, melamine formaldehyde particles are released 1 cm from the microplasma. The particles acquire a negative charge and are captured within the localized potential well of the microplasma. The residence time of particles is several minutes in flowing argon at 300 Pa. The particles are illuminated by HeNe laser and are observed to form "plasma crystals" with a periodicity of 120 microns. The particle positions allow for the visualization of the microplasma\´s internal potential structure. We also manipulate the internal electric fields using auxiliary electrodes consisting of quarter-wave microstrip transmission lines. By designing and microfabricating appropriate electrode structures, particles can lie systematically expelled from the microplasma trap. Time-of-flight measurements of expelled particles allow for the determination of particle size distributions. Application of these principles to nanoparticle sensing and diagnostics will be discussed.