Due to the strong Coulomb repulsive force, direct writing of conductive patterns on an insulating substrate is of great difficulty for NFES. An AC electrical field has been introduced to change the transfer characteristics of the charge along the jet, by which the Coulomb repulsive force can be weakened and the stability of the charged jets can be improved. Nguyen and Byun used a nozzle that was not connected electrically to overcome the electrical breakdown in a conventional NFES system. As shown in Fig. 9.9, an AC voltage is applied to an extraction electrode and the reflection of charged droplets due to patterned geometry on the substrate decreases owing to the patterned geometry on the substrate. Under the AC voltage, positively and negatively charged droplets can be obtained. With the alternation of positive and negative voltage, the jet will be turned to an electrically neutral state, which is helpful for the continuous ejection of droplets even at the peak signal of voltage. Based on the single AC potential setup, dots with sizes ranging from 10 to 30 mm were generated on the substrate. Zheng et al. investigated the effects of process parameters on the microdroplet ejection behaviors under the AC electrical field. The deposition frequency increases and the droplet diameter decreases with increasing AC voltage frequency. In addition, the deposition frequency and droplet diameter increase with increasing duty cycle and solution supply rate. Based on the aforementioned research, Liu et al. printed a bead-on-string structure under an AC electric field. The positive voltage drags out more solution and form beads, while the negative pulse voltage provides the opposite force to stretch the jet into nanofibrous structures between two adjacent beads. The stability of the jet can be enhanced by increasing the voltage frequency. As the voltage frequency increases from 10 to 60 Hz, the diameter of the bead structure decreases from 200 to 110 mm, as presented in Fig. 9.10.
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