TABLE 2.

Filtration mechanisms of different electret filtration processes [17].

Particle	Fibre	E‐field	Electrostatic force	Equation
Charged	Charged	None	Electrostatic force	$N_E=\frac{qQ}{2\pi {\epsilon }_{0}r}$
Charged	Neutral	None	Image force	$N_E=\frac{1}{16\pi {\epsilon }_0}\left(\frac{{\epsilon }_f-1}{{\epsilon }_f+1}\right)\frac{q^2}{{\left(r-r_f\right)}^2}$
Neutral	Charged	None	Induction force	$N_E=\frac{d_p^3}{8\pi {\epsilon }_0}\left(\frac{{\epsilon }_p-1}{{\epsilon }_p+2}\right)\frac{Q^2}{r^2}$
Neutral	–	External electric field	Polarisation force	$N_E=\frac{{\pi {\epsilon }_{0}d}_p^3}{4}\left(\frac{{\epsilon }_p-1}{{\epsilon }_p+2}\right)\nabla {\left|E\right|}^2$
Charged	–	External electric field	Image force	$N_E=qE$

Note: ${\epsilon }_0$ = permittivity of free space; ${\epsilon }_p$ = dielectric constant of the particle; ${\epsilon }_f$ = dielectric constant of fibre; $d_p$ = particle diameter; $d_f$ = fibre diameter; $r_f$ = fibre radius; q = net charge on particle; Q = net charge on a unit length of fibre; E = externally applied electric field; E = electric field with components; θ = polar angular coordinate; and r = radial distance from fibre axis.