Phase-controlled field-effect micromixing using AC electroosmosis

Abstract

The exploration and application of electrokinetic techniques in micro total analysis systems have become ubiquitous in recent years, and scientists are expanding the use of such techniques in areas where comparable active or passive methods are not as successful. In this work, for the first time, we utilize the concept of AC electroosmosis to design a phase-controlled field-effect micromixer that benefits from a three-finger sinusoidally shaped electrodes. Analogous to field-effect transistor devices, the principle of operation for the proposed micromixer is governed by the source-gate and source-drain voltage potentials that are modulated by introducing a phase lag between the driving electrodes. At an optimized flow rate and biasing scheme, we demonstrate that the source, gate, and drain voltage phase relations can be configured such that the micromixer switches from an unmixed state (phase shift of 0°) to a mixed state (phase shift of 180°). High mixing efficiencies beyond 90% was achieved at a volumetric flow rate of 4 µL/min corresponding to ~13.9 mm/s at optimized voltage excitation conditions. Finally, we employed the proposed micromixer for the synthesis of nanoscale lipid-based drug delivery vesicles through the process of electrohydrodynamic-mediated nanoprecipitation. The phase-controlled electrohydrodynamic mixing utilized for the nanoprecipitation technique proved that nanoparticles of improved monodispersity and concentration can be produced when mixing efficiency is enhanced by tuning the phase shifts between electrodes.

Keywords: Microfluidics; Nanoparticles.

Fig. 1. Schematic illustration of device structure and working principle.

a Schematic illustration of the micromixer. b Minimized and c maximized mixing occur in biasing scheme 1 when Δθ_Vgs = 0° and Δθ_Vgs = 180°, respectively. For the biasing scheme 2, d minimized and e maximized mixing take place when Δθ_Vds = 180° and Δθ_Vds = 0°, respectively

Fig. 2. Mixing dependence on frequency and voltage.

MI versus a frequency and b voltage for the two biasing schemes. All experiments were performed with a 10 V_pp at the optimal phase lag and a confluent flow rate of 4 µL/min

Fig. 3. Electric field phasor analysis.

a Logscale electric field intensity for biasing scheme 1 at different phase lag values on the source-gate electrodes. b Logscale electric field intensity for biasing scheme 2 at different phase lag values on the source-drain electrodes

Fig. 4. Phase-controlled mixing.

a MI versus phase difference for the biasing scheme 1 with $\mathrm{\Delta }{\theta }_{V_{\mathrm{gs}}}$ changing. b Grayscale fluorescent images of the channel output showing the mixing of DI water (dark) and fluorescein solution (bright) corresponding to plot in (a) for the confluent flow rate of 4 µL/min. c MI versus phase difference for the biasing scheme 2 with $\mathrm{\Delta }{\theta }_{V_{\mathrm{ds}}}$ altering. d Grayscale fluorescent images of DI water and fluorescein solution corresponding to plot in (c) for the confluent flow rate of 4 µL/min

Fig. 5. Nanoscale liposome synthesis.

a Schematic illustration of the modified device for nanoparticle synthesis. b Liposome average diameter versus different FRR. The error bars indicate the average standard deviations of size distributions in three NTA runs of a single batch, c A TEM image of synthesized liposomes (FRR 5:1). The scale bar is 100 nm. d Size distribution of synthesized nanoparticles at different FRRs for phase shifts of $\mathrm{\Delta }{\theta }_{V_{\mathrm{gs}}}=180^{\circ }$ and $\mathrm{\Delta }{\theta }_{V_{\mathrm{gs}}}=0^{\circ }$. (n = 3, total flow rate: 200 µL/min)