Liquid lace: under-extruding viscous threads to build periodic structures

3D printing requires careful calibration of extrudate flow rate, nozzle speed, and layer height to create visibly solid layers. Our work takes up a common problem in such extrusion processes: under-extrusion, i.e. when too little liquid is extruded too quickly. We find that this leads to the formation of defects in printed layers consisting of an alternating pattern of solid blobs and finely drawn threads. Here we rationalize the physics at play and show how to harness the instability to create periodic lattice structures.