Lopez de la Cruz Ricardo Arturo, Haward Simon J, Shen Amy Q
Micro/Bio/Nanofluidics Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa 904-0495, Japan.
PNAS Nexus. 2024 Dec 23;4(1):pgae571. doi: 10.1093/pnasnexus/pgae571. eCollection 2025 Jan.
Canopy flows occur when a moving fluid encounters a matrix of free-standing obstacles and are found in diverse systems, from forests and marine ecology to urban landscapes and biology (e.g. cilia arrays). In large-scale systems, involving Newtonian fluids (like water or air), canopy flows typically exhibit inertial turbulence due to high Reynolds numbers (Re). However, in small-scale systems like cilia, where Re is low, but the fluid can be viscoelastic (like mucus), the relevant control parameter is the Weissenberg number (Wi), quantifying elastic stresses in the flow. Here, we investigate the flow of a viscoelastic polymer solution over a microscopic canopy within a microfluidic device. As the Weissenberg number increases, the flow undergoes distinct transitions, eventually becoming unstable beyond a critical Wi. At high Wi, we observe the emergence of elastic turbulence (ET), a chaotic flow regime that, despite differing underlying mechanisms, exhibits striking similarities to large-scale canopy inertial turbulence. Similar to canopy inertial turbulence, ET within the canopy can be spatially divided into distinct regions: a porous layer within the canopy, a mixing layer at the canopy tips, a transitional region just above the canopy, and a Poiseuille-like flow further up. The separation of the flow into different regions reveals a new analogy between inertial turbulence and ET, providing a fresh insight into ET flows and expanding their potential for innovative microfluidic designs and real-world applications.
当流动的流体遇到一系列独立障碍物时，就会出现冠层流，这种现象存在于从森林、海洋生态到城市景观和生物学（如纤毛阵列）等各种系统中。在涉及牛顿流体（如水或空气）的大规模系统中，由于雷诺数（Re）较高，冠层流通常表现出惯性湍流。然而，在像纤毛这样的小规模系统中，雷诺数较低，但流体可能是粘弹性的（如黏液），相关的控制参数是魏森贝格数（Wi），它量化了流动中的弹性应力。在这里，我们研究了微流控装置中粘弹性聚合物溶液在微观冠层上的流动。随着魏森贝格数的增加，流动经历明显的转变，最终在超过临界Wi时变得不稳定。在高Wi时，我们观察到弹性湍流（ET）的出现，这是一种混沌流动状态，尽管其潜在机制不同，但与大规模冠层惯性湍流有显著的相似之处。与冠层惯性湍流类似，冠层内的ET在空间上可分为不同区域：冠层内的多孔层、冠层尖端的混合层、冠层上方的过渡区域以及更高处类似泊肃叶流的区域。将流动分离为不同区域揭示了惯性湍流和ET之间的新类比，为ET流动提供了新的见解，并拓展了其在创新微流控设计和实际应用中的潜力。
