Fig.1 (a) Structure diagram of microfluidic chip with electric field; (b) Motion path of droplets with different properties in electric field[31]

Fig.2 Sequentially addressable dielectrophoretic array： (a) Concept and simulation of the SADA； (b) Schematic of SADA-based fluorescence-activated droplet sorter (SADA sorter in short)； (c) Pictures of constructed SADA sorter[33]

Fig.3 Working principle of droplet manipulation using DEP force and droplet buoyancy： (a) Cross-sectional view of IDE region; (b) Top view of droplet manipulation region; (c) Schematic showing three functional sections of droplet bandpass filter device[34]

Fig.4 Sorting of droplets in upper/lower outlet: (a) No ultrasound, droplets followed the flow to the lower outlet; (b) Excitation at 463 kHz generated a λ/2 mode, which deflected droplet paths to upper outlet; (c), (d) Droplets were sorted by switching excitation frequency between 463 kHz and 979 kHz, corresponding to λ/2 and λ mode, respectively[40]

Fig.5 Schematic of fluorescence-activated droplet sorting in detachable acoustic sorting system

Fig.6 (a) Schematic diagram of acoustofluidic chip for separation of microscale droplets by surface acoustic wave (SAW)-induced acoustic radiation force (ARF); (b) Trajectories of droplets with varying acoustic impedance, acoustic power and droplet streamwise velocity, when they are exposed to acoustic field[48]

Fig.7 Microchannel chip for magnetic droplet deflection： (a) Schematic of design; (b) Photograph of microfluidic device[52]

Fig.8 Microchannel with multiple lateral outlets (based on magnetic field)[54]

Fig.9 Droplet generation and sorting microfluidic chip based on pneumatic control[56]

Fig.10 Principle of multi-mode droplet sorting[58]

Fig.11 Schematic diagram of droplet sieving principle of inertial microfluidic device[62]

Fig.12 Droplet sieving based on droplet migration： (a)—(c) Experimental images in time series showing interfacial migration of equally viscous (λ=0.001564) droplets of size ρ=0.29和ρ=0.45； (d) Size-based sorting of droplets of size ρ=0.29和ρ=0.45； (e) Sorting of droplets of λ1=0.001564 from droplets of λ2=0.0086 of same size ρ=0.34[67]

Fig.13 Top view of typical DLD geometry [70]

Fig.14 (a) Top view schematic of DLD pillar array； (b) Microscope image of DLD device[71]

Fig.15 Schematic representation of three-step DLD-based separatorv[73]

Fig.16 Principle of pinched flow fractionation： (a) In pinched segment, particles are aligned to one sidewall regardless of their sizes by controlling flow rates from two inlets; (b) Particles are separated according to their sizes by spreading flow profile at boundary of pinched and broadened segments[74]

Fig.17 (a) Principle of five different-sized droplet sorting； (b) Result of five different-sized droplets sorting[80]

Fig.18 Schematic view of droplet sorting principle： (a) Hydrodynamic forces on droplets and different droplet behaviors in rail pair depending on volumes； (b) Scheme of multi-stage droplet sorting in cascade channel； (c) Shape recovery using dot rail for repeatable droplet transfer[81]

Fig.19 Principle of size-dependent droplet sorting by selective droplet transfer on dot-rail； (a) Transferring of large droplets; (b) Guiding of small droplets[82]

Fig.20 (a) Schematic diagram of channel configuration; (b) Sieving process of droplets with different size[86]

Fig.21 Principle diagram of droplet screening at asymmetric T-junctions： (a) Microchannel structure diagram； (b) Case of droplets flowing out from outlet 1； (c) Case of droplets flowing out from outlet 2[89]

Fig.22 (a) Schematic of constricted and parallel microchannels； (b) Effect of throat length on droplet sieving； (c) Effect of channel contraction ratios on droplet sieving[91]