Shaping suspensions: Stabilizing anisotropy in viscoelastic media using oscillations

An easy process to generate highly anisotropic structures and properties is desired for many applications, such as the formation of meat analogues, conductive polymer composites or directed assembly of microparticles. However, the required processing parameters for viscoelasticity-induced alignment preclude such implementations, necessitating additional and cumbersome processing steps. Traditionally, string formation of particles in viscoelastic liquids occurs only at either high shear rates or in highly elastic fluids. We demonstrate how oscillatory flows can induce particle alignment at shear rates nearly two orders of magnitude lower than for steady shear, to easily reach long particle strings in comparison to the limitations of steady shear. Furthermore, we establish that alignment in oscillation is cumulative because string breakup is prevented by flow reversal, which explains the potentially unlimited anisotropic assembly, while showing that Weissenberg number-dependent string stability in steady shear leads to non-cumulative alignment. Finally, we show that this mechanism enables the extrusion of aligned structures using superposed oscillatory and steady shear, and demonstrate its broader applicability with non-ideal particles.