Effect of droplet deposition on a charged fiber
In contrast to solid particles, liquid droplets behave very differently when they are captured by a fiber as they can deform and spread on and between the fibers. In this work, we have developed a 3-D model to simulate the interactions between liquid aerosols (DOP and water) and charged fibers and to also quantify the aerosol capture efficiency of a droplet-loaded charged fiber. Our simulations of droplet deposition on electret fibers took advantage of the volume of fluid (VOF) method in the presence of an electrostatic field. More specifically, a body force term (electrical stress) was added to the Navier–Stokes equations to account for the impact of the electrostatic field on droplet motion. The electric field was obtained by solving the electrostatic potential equation (assuming neutral droplets). The electric field around a charged fiber and volumetric charge density around the droplet surface was calculated by solving the Poisson equation and the charge conservation equation respectively.
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The magnitude of the electrostatic field decreases on the surface of the fiber where a DOP droplet is deposited (see the blue-colored droplets on the surface of the fibers in the figure below). This effect is due perhaps to the differences between the permittivity of DOP and air. In addition, for the case of parallel dipole, the maximum reduction in electrostatic field intensity was observed on the side of the fiber facing the air flow, while for the perpendicular dipole orientation, it was on the side of the fiber perpendicular to the air flow.
Increasing the relative permittivity of the droplets resulted in a decrease in the magnitude of the electrostatic field. This decrease is because droplets with higher relative permittivity values can become more polarized and consequently, weaken the electric field around the fiber. This effect is often referred to as the electrostatic shielding. The deposition of high-conductivity droplets severely weakens the performance of the fiber. The performance reduction seems to be due to the weakened electrostatic forces (polarization and Coulomb) caused by electrostatic shielding and the charge neutralization.
Our simulations revealed that the deposition of such droplets on an electret fiber tends to decrease its particle capture efficiency. It was also concluded that droplet’s electrical conductivity (due to severe charge neutralization) and droplet’s permittivity (via charge shielding) have the most negative effect on fiber’s efficiency. This is in contrast to the droplet’s wetting properties that seemed to have a minimal effect on fiber performance.