Minaturised condensation particle counters: Radial sheathing

Ultrafine particles (UFPs, <100 nm) pose significant risks to human health due to their prevolence in the environment and ability to enter into the respiratory system. Presently, the World Health Organization (WHO) have regulated optical particle mass (PM) based counters (measuring diameters >300 nm), which have resulted in wide adoption of PM sensors. However, these PM counters do not detect UFPs which represent the largest fraction of particle number (PN) in many indoor and outdoor environments. Condensation particle counters (CPCs) currently remain the gold standard instrumentation for PN-based instruments, yet current CPCs remain unsuitable for large-scale deployment due to their size constraints. Miniaturization efforts are fundamentally constrained by particle penetration and condensation dynamics, especially under low Reynolds number (Re) regimes where Brownian diffusion dominates. To address these challenges, we present a numerical and experimental investigation of a prototype miniaturized CPC growth chamber (CPC-GC) with a radially-sheathed airflow introduced through porous tube media. Building upon our previously reported non-dimensional computational framework documented in Balendra et al. (2024), we incorporate the fluid dynamics of radial sheathing to reassess growth chamber design limits. This new framework collapses the miniaturization constraints onto aspect-ratio bounds by eliminating the need to satisfy the particle penetration criterion required in unsheathed designs. Numerical simulations demonstrate that radial sheathing strongly confines UFPs to the flow centerline, substantially reducing particle losses and improving growth uniformity. Compared with unsheated CPCs, the radially sheathed design enables up to a ×6 increase in upper limit of penetration aspect ratio, Lmax,p,sh∗ and allows operation at Reynolds numbers ×19 lower, facilitating smaller device footprints and reduced flow rates. A miniaturized radial sheathing CPC was successfully fabricated via DMLS, with additional guidelines developed for researchers and designers to manufacture via both additive and subtractive methods. Experimental validation outlined an optimal sheath ratio, R≈2–3 identified for maximizing counting efficiency, mean droplet size, and size uniformity, while accommodating manufacturing tolerances such as surface roughness and wick fiber variability. Our findings indicate that radial sheathing serves as a key enabler of broader miniaturized CPC technology, supporting both scientific exploration and practical device development with important benefits for air quality monitoring and public health.