Breakup of extruded filaments of yield-stress fluid*

The extrusion through a cylindrical nozzle of continuous and uniform filaments of cell-laden hydrogel is important to ensure accurate tissue constructs in 3D bioprinting. The hydrogel typically exhibits yield stress and shear-thinning properties, so we choose Carbopol as a model viscoplastic fluid to perform extrusion experiments. We study the gravity flow of an extruded filament and its breakup to explore how to control filament length and generate uniform filaments. Breakage occurs at an approximately constant distance from the nozzle for modest extrusion flow rates which typically produces short filament segments. In this limit, the experiment provides a ‘syringe test’ to simply measure the extensional yield stress [1]. We focus on larger extrusion flow rates for which we find that the breakup distance from the nozzle rapidly increases with both increasing flow rate and increasing yield stress. We find a similar trend for the length of the detached filament segment. We identify a threshold flow rate for each fluid, beyond which the breakup distance increases rapidly, and interpret this transition in terms of shifts in the balance of viscoplastic, capillary and inertial forces within the filament for low and high values of yield stress.

[1] A. Geffrault, H. Bessaies-Bey, N. Roussel & P. Coussot (2023) J. Rheol., 67 (2), 305–314.