Gradients in fluid viscosity characterize microbiomes ranging from mucus layers on marine organisms¹ and human viscera^2,3 to biofilms⁴. Although 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 rotation—consistent 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.

流体粘度梯度表征微生物组，从海洋生物¹和人类内脏^2,3的粘液层到生物膜⁴不等。虽然这种环境因其对病原体的保护作用和影响细胞运动的能力而被广泛认可^2,5，但在粘度梯度中调节细胞转运的物理机制仍然难以捉摸^6,7,8，主要是由于缺乏定量观察。通过微流控实验，我们直接观察模型双鞭毛微藻（莱茵衣藻) 在受控的粘度梯度中。我们表明，尽管它们局部降低了游泳速度，但粘性区域^9,10中预期的细胞积累被一种粘性转向运动所扼杀。这种确定性的细胞旋转 - 与鞭毛推力不平衡一致^11,12 - 将游泳者重新定向到梯度下方，导致它们在低粘度区域积聚以获得足够强的梯度。通过朗之万模拟和细胞推进的三点力模型证实，我们的结果说明了疏粘转向和粘性减速之间的竞争如何最终决定了粘度梯度中种群规模微生物运输的命运。