Resonator-Based Air Microfluidic Sensing Systems for Monitoring Airborne Particulate Matter

Particulate matter (PM) are small solid particles or liquid droplets suspended in air. These small particles can be generated by various sources, like diesel engines and mining operations. Exposure to respirable dust and elevated diesel exhaust concentrations in underground coal mines can cause life-threatening airway diseases such as coal worker's pneumoconiosis (CWP), silicosis, and lung cancer. Hence, the concentration of respirable PM in mines has to be monitored to make sure it is below regulatory standards. Here, we present the design, fabrication, and experimental evaluation of a novel wearable respirable dust monitor (WEARDM) which uses a dual-resonator gravimetric sensing approach for continuous measurement of respirable PM concentrations. The sensor aerodynamically separates the respirable fraction from the air stream using a virtual impactor and removes moisture for accurate mass measurement. WEARDM uses a quartz crystal microbalance (QCM) to measure the mass concentration of coarse particles generated from coal mining operations (typically greater than 2.5 micrometers), separated using inertial impaction, and uses thermophoretic precipitation to deposit the fine particles emitted from diesel sources (typically smaller than 0.1 micrometers) on a film bulk acoustic resonator (FBAR) and measures the mass concentration in real-time. This allows the WEARDM to maintain an almost uniform collection efficiency (CE) across the respirable fraction in underground coal mines. The WEARDM system is optimized for a low flow rate of 250 ml/min, which results in low power usage and a small form factor, and is an order of magnitude smaller and less expensive than comparable devices. Additionally, we introduced a novel method to enhance the sensitivity of impaction-based sensors. Inertial impaction is frequently used for the collection and subsequent measurement of aerosol particles in resonator-based airborne particulate matter sensors. However, particle bounce is known to significantly reduce particle CE on surfaces exhibiting low roughness, such as those present in quartz crystal microbalance PM sensors. We show that the addition of micro-pillars to impaction surfaces can significantly enhance their particle collection. Similarities in particle capture mechanisms between fibrous filters and pillar-enhanced surfaces are explained, and we show the adaptability of fibrous filter theory to pillared surface collection efficiency. Experiments confirm that micro-pillar cross-section and spacing have a significant role in particle capture. Pillars with circular, rectangular, and cross-shaped horizontal cross-sections with 15 micrometers height and 12 micrometers (dense), 20 micrometers (nominal), and 27 micrometers (sparse) center-to-center spacings were printed using two-photon micro-stereolithography. Efficiency increased 35%-52% in the dense case, while the effect of pillar shape was negligible. At nominal spacing, CE depended heavily on pillar shape. The cross-shaped and circular pillars improved the CE by 26%-29%, although rectangular pillars were as efficient as the bare surface. No significant difference between the bare and pillar-enhanced surfaces was visible in sparse spacing. We further show that, upon addition of a nominal distribution of micro-pillars to the surface of a QCM sensor for real time mass measurements, the sensor response improves significantly (approximately 2.5 times) compared to a QCM with a bare surface. We also studied the effect of different micro-pillar array configurations on the particle collection efficiency of resonators for gravimetric airborne PM sensors. Inline and radially staggered arrays of micro-pillars with rectangular and circular cross-sections were 3D-printed on a surface of fused silica glass using two-photon stereolithography and the particle collection efficiency of these pillar-covered surfaces due to impaction was evaluated. The experimental results show that in some cases the CE is highly dependent on the angle of incidence and array configuration of the pillars. The CE of an inline array of rectangular micro-pillars (60%) was similar to the CE of the bare surface with no pillars. However, the radially staggered arrangement of the same pillar size and geometry improved CE by 25% on average. The presented result shows the strong influence of the incident angle, array configuration, and pillar shape on particle capture behavior of the pillared-enhanced impaction surfaces. The results presented in this work pave the way for highly sensitive real-time gravimetric PM mass sensors.