Experimental study on injecting highly viscous liquids by using a reciprocating needle dispensing system

The reciprocating needle dispensing technique is capable of patterning functional inks with a wide range of viscosities. As injections are performed by extruding an ink driven by the reciprocating needle motion, the mechanical specifications and the ink properties are expected to influence the amount of ink conveyed by the needle, thereby affecting the size of drops formed on a substrate. Needle dispensing tests were performed by using Newtonian silicone oils of different viscosities and a pseudoplastic conductive ink with various needle speeds and positions of the needle and the ink reservoir. Time-resolved analysis of the dispensing motions indicated that the formation of the dynamic meniscus around the needle was primarily described by the Landau–Levich–Derjaguin ink-coating theory. However, a slight modification that accounted for the initial gap between the needle and the glass capillary was needed to precisely describe the ink extrusion behaviors. The scaling analysis presented herein provides a perspective for the precise control of drop formation via optimal tuning of the ink injection. Regarding ink filament pinch-off, we discovered that the pseudoplastic liquid exhibited a faster pinch-off at higher needle retraction speed, which is attributed to a reduction of the apparent viscosity due to shear thinning. This behavior is preferable for needle dispensing with better stability.

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