Yield stress fluids in microfluidics: research, applications and opportunities.

Microfluidic technology enables scalable solutions in precision medicine, diagnostics, drug delivery, organ-on-a-chip models, single-cell analysis, high-throughput screening, and environmental monitoring. In fact, many of these applications process fluids with complex rheology, e.g., blood, mucus, bioinks, and polymer or particle suspensions, exhibiting yield stress, viscoelasticity, and shear-thinning that govern transport, mixing, and interfacial dynamics at small scales. However, the interplay of such rheology with microscale confinement, roughness, and surface patterning remains underexplored. In this review, we highlight microfluidic applications where yield stress (and related rheological) properties are pivotal. For instance, we discuss how blood rheology shapes the design of devices for circulating tumor cell separation, and how bioink viscoelasticity balances flowability and shape fidelity in extrusion and embedded bioprinting. We also examine electrorheological fluids as field-tunable media for microvalves, pumps, and mixers, and analyze microorganism and microrobot locomotion in complex fluids, linking physics to biology and targeted delivery. We further consider microscale slip, microrheology platforms, and viscous fingering instabilities, to specifically highlight how rheology controls transport and enables fabrication of channels and hydrogel structures. We conclude that yield-stress (and viscoelastic) effects are not mere complications but they are powerful design variables, and we outline future opportunities for leveraging these properties to advance microfluidic science and technology.