Technical note The effect of capillary force on airborne nanoparticle filtration Raheleh Givehchi, Zhongchao Tan n Department of Mechanical & Mechatronics Engineering, University of Waterloo, Ontario, Canada article info Article history: Received 30 January 2015 Accepted 2 February 2015 Available online 11 February 2015 Keywords: Nanoaerosol particle Thermal rebound Capillary force Air filtration Plastic deformation abstract This paper presents a new model for airborne nanoparticle filtration by considering the effects of capillary force and plastic behavior impaction. This model was also validated using experimental data in literature and that collected by the authors. Results show that the capillary force between particle and the filter surface increases with the relative humidity level, leading to reduced rebound of nanoparticles from a filter media. Therefore, thermal rebound of nanoparticles may only occur at low relative humidity conditions. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Air filtration is the most effective method for separating nanoparticles from the air. It is widely used in air cleaning, aerosol sampling, and air monitoring devices. Conventional filtration theory states that diffusion dominates the behavior of submicron particles and the filtration efficiency increases inversely with the size of these fine particles. This theory implies that nanoparticles can be effectively captured by properly designed air filters. However, some researchers have pointed out that nanosized airborne particles (nanoaerosols) may behave like gas molecules upon impaction with the surface of a filter because the kinetic energy is greater than the adhesion energy (Wang & Kasper, 1991). As a result, such small nanoparticles may rebound from filter media upon collision through a mechanism called thermal rebound. By theoretical analysis, Wang and Kasper (1991) predicted that thermal rebound could happen for sub-10 nm particles. Since then, many experimental studies have been conducted to examine the thermal rebound theory (Alonso, Kousaka, Hashimoto, & Hashimoto, 1997; Brochot, Mouret, Michielsen, Chazelet, & Thomas, 2011; Golanski, Guiot, Rouillon, Pocachard, & Tardif, 2009; Heim, Attoui, & Kasper, 2010; Heim, Mullins, Wild, Meyer, & Kasper, 2005; Huang, Chen, Chang, Lai, & Chen, 2007; Ichitsubo, Hashimoto, Alonso, & Kousaka, 1996; Japuntich et al., 2007; Kim, Bao, Okuyama, Shimada, & Niinuma, 2006; Kim, Harrington, & Pui, 2007; Otani, Emi, Cho, & Namiki, 1995; Rengasamy, King, Eimer, & Shaffer, 2008; Scheibel & Porstendörfer, 1984; Shin, Mulholland, Kim, & Pui, 2008; Skaptsov et al., 1996; Steffens & Coury, 2007b; Van Gulijk, Bal, & Schmidt-Ott, 2009; Van Osdell, Liu, Rubow, & Pui, 1990; Wang, Chen, & Pui, 2007; Yamada, Seto, & Otani, 2011). However, very few of them have reported evidences to support the phenomena of thermal rebound in nanoparticle filtration (Ichitsubo et al., 1996; Kim et al., 2006; Otani et al., 1995; Rennecke & Weber, 2013a; Van Gulijk et al., 2009). Van Osdell et al. (1990) showed that no thermal rebound was associated with the removal efficiency of polydisperse silver and monodisperse DOP nanoparticles through fibrous glass and membrane filters. Otani et al. (1995) demonstrated that thermal rebound occurred for circular aluminum tubes for sub-2 nm particles. This experiment was conducted using Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jaerosci Journal of Aerosol Science http://dx.doi.org/10.1016/j.jaerosci.2015.02.001 0021-8502/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ1 519 888 4567x38718. E-mail address: tanz@uwaterloo.ca (Z. Tan). Journal of Aerosol Science 83 (2015) 12–24