Micro-molecular tagging velocimetry of internal gaseous flow

Dual-laser micro-molecular tagging velocimetry (μMTV) for internal gaseous flows on the microscale has been successfully demonstrated. MTV is a non-intrusive optical technique suitable for gaseous flow measurement by using molecules as tracers. In our dual-laser μMTV technique, seeded NO₂ molecules in a flow were tagged by photodissociation, producing NO molecules that can be distinguished from surrounding molecules. The tagged NO molecules were traced and visualized by laser-induced fluorescence. However, the fluorescence was in the deep ultraviolet region, and a reflective objective with a finite conjugate optical system was employed for imaging on the microscale. The seeded and tagged molecules of NO₂ and NO are stable in the gas phase at around room temperature and atmospheric pressure. Thus, this technique is free from condensation at the walls and is feasible for measurements of internal gaseous flow on the microscale. To demonstrate the validity of our dual-laser μMTV technique, the cross-sectional flow velocity profile in a rectangular microchannel and flow velocities in a micronozzle were measured and compared with numerical results.