In transfer printing, the loaded droplet on the probe has a significant influence on the dispensing resolution. A suitable loading approach for a high-viscous liquid is highly required. Herein, a novel electrostatic loading method is presented, in which the main aim is to control precisely the formation and breaking of a cone-shaped liquid bridge. An experimental device is developed. The influence of electrical and geometric parameters on the feature size of the liquid bridge is investigated in detail. In the formation of the liquid bridge, the increase of voltage or the decrease of the air gap can enhance the electric field intensity, thus reducing the formation period and increasing the initial cone tip diameter of the liquid cone. After the liquid bridge is formed, both the circuit current implying the liquid wetted area on the probe surface and the lifting velocity of the probe are utilized to further regulate the volume of the loaded droplet. Loaded droplets ranging from 60 to 600 pL are obtained via the method with a standard deviation of 4 to 30 pL. Moreover, a dot array is transferred with different loaded droplets. The minimum diameter of the printed dots is about 140 μm with a variation less than 5%. The advantages include the reduced risk of contamination, the droplet-size independent of the size of the probe, and the low cost of the device.

The breakup mechanism of a conical liquid bridge, based on the previously proposed electrostatic liquid loading method, is reported for the first time. The breakup criterion in terms of interface feature size is derived. Based on the criterion, the breakup mechanism can be categorized as either spontaneous breakup or stretching breakup. The evolution of interface and velocity for two breakup behaviors is subsequently investigated. For spontaneous breakup, the remnant volume Vd depends primarily on the top radius Rt and is proportional to the square of Rt. For stretching breakup, the remnant volume depends on the early stage of the stretching and Vd is proportional to the cube of Rt. In addition, the influence of stretching velocity U is examined. Results show that U has a weaker effect on the change of remnant volume than the top radius Rt for large capillary numbers. This study is helpful in understanding the liquid bridge breakup mechanism and improving the transfer printing process.

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Parallel microdispensing of high-viscous liquid is a fundamental task in many industrial processes. Herein, a smart printing head is developed, including the probe array, the electric control module, the contact force measurement module, and the extra force balance module. The parallel dispensing of high-viscous liquid in nL level is achieved. The interacting effect between probes on the loading process is analyzed too. According to the result, the interacting effect between probes has a strong influence on the loading process. Therefore, the strategy of serial electrical loading and parallel transfer printing is utilized. Finally, the dependency of transfer printing volume on probe size, etc., is experimentally investigated. The volume of the loaded droplet can be controlled by the lifting velocity of the probe array, and the volume of the transferred droplet can be adjusted by the size of the probe instead of the contact force. The advantage of the proposed method is to realize the highly repeatable parallel dispensing of high-viscous liquid with a relatively simple device.