Micro-extrusion of organic inks for direct-write assembly

Direct-write assembly is a highly versatile microfabrication technique used to create microfluidic networks by the robotic deposition of a fugitive ink onto a moving stage. To optimize the resulting shape of the microchannel, the translational speed of the moving stage has to closely match the linear velocity of the fugitive ink at the micro-nozzle exit. In this work, we have performed a comprehensive characterization of the micro-extrusion process of organic fugitive inks through a nozzle and characterized the rheological properties of petroleum jelly-based organic inks with various microcrystalline wax contents (10 to 40 wt%). The local microcrystal concentration has been probed using polarized optical microscopy and Raman spectroscopy. Small amplitude oscillatory shear tests in a vane geometry have revealed a solid-like structure of the organic inks, and a strong shear-thinning behavior of the complex viscosity. Particle tracking velocimetry (PTV) experiments performed in a glass microchannel have suggested the occurrence of apparent slip, showing a microcrystal depletion layer near the nozzle wall and a plug flow in the remainder of the micro-nozzle. From Raman spectroscopy and polarized microscopy performed on extruded samples, a crystal free layer was observed and estimated to be approximately 10-20 mu m thick (or 2-4% of the microcapillary diameter), explaining the strong apparent wall slip behavior.