Dueling rheology in a cross-slot flow instability

In polymeric fluid flows, the dominance of elastic stresses over viscous stresses (large Weissenberg number, Wi) leads to viscoelastic instability, characterized by symmetry breaking and unsteady flow. Recently, the importance of shear-thinning in regulating viscoelastic instabilities has been highlighted, but the rheological conditions triggering the instability remain to be fully understood. Here, we begin to address this knowledge gap by probing the stability of a microfluidic cross-slot flow having two opposing fluids with contrasting rheological properties from either inlet. The fluids span purely elastic, purely shear thinning, viscoelastic, and Newtonian materials, where their respective properties are tuned and characterized using shear and extensional rheology. The flow topology and velocity fluctuations for different combinations of these miscible fluids are quantified using micro-PIV across a range of Weissenberg numbers (Wi) and shear thinning parameter values. Our results show that opposing flows of purely elastic and purely shear thinning fluids remain stable for the tested Wi, whereas the presence of one fluid exhibiting both shear-thinning and elasticity appears crucial to facilitating instability. These findings are potentially important for processes including remediation and mucus flows, where variations in fluid rheology have macro-scale implications for transport properties.