3.6.1. Particle focusing
Particle/cell focusing inside a microchannel is a crucial pre-step for downstream particle/cell counting, detection and sorting (Kung, Huang, Chong, & Chiou, 2016; X. Wang, Zandi, Ho, Kaval, & Papautsky, 2015). Jia et al. (Y. Jia, Ren, & Jiang, 2015), for the first time, demonstrated particle focusing in generated MVs under the AC electric field around the planar floating electrode placed at the bottom of a microchannel (Fig. 5A ). Chen et al. (Xiaoming Chen et al., 2017) employed this focusing method for the pre-focusing of particles in different solutions prior to their dielectrophoretic separation. Song et al. (Song, Wang, Li, Pan, & Li, 2016) demonstrated particle focusing under DC electric field at the desired location in the microchannel by the induced MVs around a polarizable plate located near the microchannel walls (Fig. 5B ). Ren et al. (Ren et al., 2016) developed a continuous-flow microfluidic device in which the desired number of focused particles were isolated at the target outlets using the generated MVs around asymmetric electrodes placed at the bottom of the microchannel (Fig. 5C ). Liu et al. (W. Liu et al., 2016) presented a flow-focusing device in which a suspension of particles was forcused within a cylindrical volume with a diameter range of 1-8 µm (nominal diameter of 4 µm) and conducted toward the desired outlet by the induced MVs around a floating electrode (Fig. 5D ).