Viscoelastic flows transition from steady to time-dependent, chaotic dynamics under critical flow conditions, but the implications of geometric disorder for flow stability in these systems are unknown. Utilizing microfluidics, we flow a viscoelastic fluid through arrays of cylindrical pillars, which are perturbed from a hexagonal lattice with various degrees of geometric disorder. Small disorder, corresponding to ∼ 10% of the lattice constant, delays the transition to higher flow speeds, while larger disorders exhibit near-complete suppression of chaotic velocity fluctuations. We show that the mechanism facilitating flow stability at high disorder is rooted in a shift from extension-dominated to shear-dominated flow type with increasing disorder.

Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms 1 and human viscera 2,3 to biofilms 4 . While such environments are widely recognized for their protective effects against pathogens and their ability to influence cell motility 2,5 , the physical mechanisms regulating cell transport in viscosity gradients remain elusive [6][7][8] , primarily due to a lack of quantitative observations. Through microfluidic experiments, we directly observe the transport of model biflagellated microalgae (Chlamydomonas reinhardtii ) in controlled viscosity gradients. We show that despite their locally reduced swimming speed, the expected cell accumulation in the viscous region 9,10 is stifled by a viscophobic turning motility. This deterministic cell rotationconsistent with a flagellar thrust imbalance 11,12 -reorients the swimmers down the gradient, causing their accumulation in the low viscosity zones for sufficiently strong gradients. Corroborated by Langevin simulations and a three-point force model of cell propulsion, our results illustrate how the competition between viscophobic turning and viscous slowdown ultimately dictates the fate of population scale microbial transport in viscosity gradients.

Yield stress fluid (YSF) flows through porous materials are fundamental to biological, industrial, and geophysical processes, from blood and mucus transport to enhanced oil recovery. Despite their widely recognized importance across scales, the emergent transport properties of YSFs in porous environments remain poorly understood due to the nonlinear interplay between complex fluid rheology and pore microstructure. Here, we combine microfluidic experiments and nonlinear network theory to uncover an anomalous, hierarchical yielding process in the fluidization transition of a generic YSF flowing through a random medium. Percolation of a single fluidized filament gives way to pathways that branch and merge to form a complex flow network within the saturated porous medium. The evolution of the fluidized network with the flowing fraction of YSF results in a highly nonlinear flow conductivity and reveals a novel dispersion mechanism, resulting from the rerouting of fluid streamlines. The identified flow percolation phenomenon has broad implications for YSF transport in natural and engineered systems, and provides a tractable archetype for a diverse class of breakdown phenomena.

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Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms and human viscera to biofilms. While such environments are widely recognized for their protective effects against pathogens and their ability to influence cell motility, the physical mechanisms controlling cell transport in viscosity gradients remain elusive, primarily due to a lack of quantitative observations. Through microfluidic experiments with a model biflagellated microalga (Chlamydomonas reinhardtii), we show that cells accumulate in high viscosity regions of weak gradients as expected, stemming from their locally reduced swimming speed. However, this expectation is subverted in strong viscosity gradients, where a novel viscophobic turning motility - consistent with a flagellar thrust imbalance - reorients the swimmers down the gradient and causes striking accumulation in low viscosity zones. Corroborated by Langevin simulations and a three-point force model of cell propulsion, our results illustrate how the competition between viscophobic turning and viscous slowdown ultimately dictates the fate of population scale microbial transport in viscosity gradients.