Although the basic principles of fibrous filters have been well understood for capture of micron and submicron sized particles, questions arise when they are applied to nanoscale particles. In the first part of this review, the classical theory of fibrous filters is described with focus on the principles that are applicable to nanoparticle collection. The areas of recent developments reviewed include thermal rebound of nanoparticles and the effects of particle shape, aggregate morphology, flow regime, humidity, fiber size, and particle loading. One of the outstanding questions in nanoparticle collection is the particle size at which the effect of thermal rebound on collection efficiency can be observed. Theoretical calculations indicate that the effect probably can be observed only for particles smaller than 1 nm, but experimental confirmation is difficult at present because of lack of instruments for classifying and counting subnanoscale particles. Two promising devices based on filtration principles have been studied in recent years: multilayer filters and inertial fibrous filters. Multilayer filters, which are composed of nanofiber and microfiber mats, have potential to become an efficient and economical device for removing nanoparticles from gas streams. The inertial fibrous filter operates at high flow rates and relatively low pressure drop, thereby offering an attractive alternative to lowpressure impactors for nanoparticle sampling. Further development of these two types of filtration devices is needed to make them simple and reliable.

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Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 ym in diameter and at different charging states. A single fiber collection efficiency qEl, was found to be expressible as a function of dimensionlegs parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K I,, as, here, A, B, C, and 1) are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 pm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 pm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.

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