Numerical Simulation of Air Sampling in Micro Scale Rising Bubbles

Capturing particles, mainly bio-particles, such as viruses and spores, from the air into a liquid is critical for air purification and analysis of the sampling systems. In the conventional impinger sampling systems, the air bubbles carrying the particles are injected into a liquid column. The particles are captured from the air into the liquid as the bubbles travel to the surface due to the buoyancy forces. This paper focuses on the numerical simulation of a newly developed microfluidic air sampling system. In this system, a microscale liquid column, equipped with an array of microchannels as pressure controlled gas injection points, is used for gas bubbling process for air sampling purpose. The air bubbles containing the particles are injected into the bottom of the microscale column filled with a liquid (such as water). As the microscale bubbles (of about 250∼500 microns) travel to the surface, a shear driven flow will develop within the bubbles and the airborne particles will follow the flow toward the boundary of the bubble. The inertia force of particles will result in the departure of particles from the flow into the liquid. The fraction of the particles departed from air bubble and trapped in the liquid represents the collection efficiency of the air sampler. For tracking the liquid/gas interface, the Volume-of-Fluid (VOF) method along with Youngs’ algorithm for geometric reconstruction of the free surface is used. The particle trajectories are predicted by integrating the force balance on each particle, which is based on a Lagrangian frame of reference. It is found that in addition to the particle departure during the bubble rising, a considerable amount of particles are trapped in the liquid during the air injection and bubble formation which may be due to the high velocity of the injected air and oscillation of the bubble interface. It is worth noting that this is the first time that the numerical simulation is performed to understand the whole air sampling process which was never addressed in the past. As a comparison, an experimental study was conducted to measure the collection efficiency. The fluorescent polystyrene latex particles with different diameters (diameters: 0.5, 1, 2 microns) were used in the experiment. The experimental results were compared with that of numerical simulation. Both the experiments and computations revealed that a collection efficiency of 90% can be achieved by the microfluidics based impinger, which is much higher than that of conventional, macro-scale, impinger systems. In addition to verifying the existing experimental work, this numerical simulation method provides a valuable tool to design and improve new generations of air sampling systems.