Fig. 3.

SEM images of PDEP and NDEP experiments. a Clausius–Mossotti factor (${\mathit{f}}_{\mathbf{CM}}$) of nanoparticles used for experiments. Several characteristics (${\mathbf{\epsilon }}_{\mathbf{m}/\mathbf{p}},{\mathbf{\sigma }}_{\mathbf{m}/\mathbf{p}},{\mathbf{\sigma }}_{\mathbf{bulk}},{\mathbf{K}}_{\mathbf{s}}$) of Au and silica nanoparticles are from reference [29]. The X-axis is a log scale of the frequency (Hz). Signs of ${\mathit{f}}_{\mathbf{CM}}$ change at ~2 MHz (100 nm silica), ~4 MHz (50 nm silica), and ${\mathit{f}}_{\mathbf{CM}}$ of 40 nm Au always has a plus sign in the 0.1–100 MHz range. Arrows show the frequencies used, and colors indicate PDEP (yellow) or NDEP (green). b PDEP result of Au nanoparticles with 40 nm diameter Au nanoparticles, ~2 µm diameter pipette with ${\mathit{R}}_{\mathbf{inner}}/{\mathit{R}}_{\mathbf{outer}}$ ratio = 0.7, ${\mathit{V}}_{\mathbf{peak}}=$ 1.0 V, and 15 MHz frequency. Particles are attracted to surface electrodes of the substrate and the pipette. c PDEP result of silica nanoparticles with 100 nm diameter silica nanoparticles, ~ 2 µm diameter pipette, ${\mathit{V}}_{\mathbf{peak}}=$ 2.5 V and 1 MHz frequency. Particles show the same tendency as in a. The concentration and surfactants of nanoparticle solutions result in differences between b and c. d NDEP result of silica nanoparticles with 50 nm diameter silica nanoparticles, ~1.5 µm diameter pipette, ${\mathit{V}}_{\mathbf{peak}}=$ 2.5 V, and 20 MHz frequency. Particles move off to the surface electrodes of the substrate and pipette and gather around the center of the pipette. e Sorting result of the mixed solution with 100 nm diameter silica nanoparticles and 20 nm diameter Au nanoparticles, ${\mathit{V}}_{\mathbf{peak}}=$ 2.5 V, and 30 MHz frequency. Silica nanoparticles in the green circle show NDEP and Au nanoparticles next to the yellow arrows show PDEP