  When a thin stream of aqueous sodium alginate is extruded into a reacting
calcium chloride bath, it polymerizes into a soft elastic tube that
spontaneously forms helical coils due to the ambient fluid drag. We quantify
the onset of this drag-induced instability and its nonlinear evolution using
experiments, and explain the results using a combination of scaling, theory and
simulations. By co-extruding a second (internal) liquid within the aqueous
sodium alginate jet and varying the rates of co-extrusion of the two liquids,
as well as the diameter of the jet, we show that we can tune the local
composition of the composite filament and the nature of the ensuing
instabilities to create soft filaments of variable relative buoyancy, shape and
mechanical properties. All together, by harnessing the fundamental varicose
(jetting) and sinuous (buckling) instabilities associated with the extrusion of
a jelling filament, we show that it is possible to print complex
three-dimensional filamentous structures in the ambient fluid.
