Droplet spreading characteristics observed during 3D printing of aligned fiber-reinforced soft composites

The 3D printing of fiber-reinforced soft composites (FrSCs) is a hybrid process that combines conventional inkjet-based 3D printing with the directed deposition of electrospun polymer fiber mats. This paper investigates the spreading characteristics of droplets when deposited on fibrous substrates, under conditions relevant to 3D printing of aligned FrSCs. Both single and multi-droplet impingement studies are conducted on substrates with varying fiber number densities. High-speed imaging is used to study the characteristic time-scales and the spreading behavior of the droplets. The single droplet impingement studies on stationary substrates reveal that the presence of fibers promotes droplet spreading along the length of the fibers. Occasional surface energy variations in the fiber mats in the form of voids and fiber bundles are also seen to affect the droplet shape and the characteristic spreading times. In the case of a moving substrate, the droplets are seen to spread the most during in-line printing, i.e., when the direction of the printing velocity coincides with the direction of fiber alignment. They spread the least during orthogonal printing, i.e., when the direction of the printing velocity is perpendicular to the direction of fiber alignment. The printing of straight lines shows an interesting edge retraction phenomenon that gets accentuated the most in the case of in-line printing. The findings of the high-speed imaging studies have been confirmed by 3D printing comparable artifacts using UV curable inks. These studies indicate that for a given fiber mat and UV curable ink combination, the choice of the in-line or orthogonal printing strategy has implications for the overall printing time, fiber content, edge resolution and surface quality of the 3D printed FrSC part.