The morphological dynamics, instabilities, and transitions of elastic filaments in viscous flows underlie a wealth of biophysical processes from flagellar propulsion to intracellular streaming and are also key to deciphering the rheological behavior of many complex fluids and soft materials. Here, we combine experiments and computational modeling to elucidate the dynamical regimes and morphological transitions of elastic Brownian filaments in a simple shear flow. Actin filaments are used as an experimental model system and their conformations are investigated through fluorescence microscopy in microfluidic channels. Simulations matching the experimental conditions are also performed using inextensible Euler–Bernoulli beam theory and nonlocal slender-body hydrodynamics in the presence of thermal fluctuations and agree quantitatively with observations. We demonstrate that filament dynamics in this system are primarily governed by a dimensionless elasto-viscous number comparing viscous drag forces to elastic bending forces, with thermal fluctuations playing only a secondary role. While short and rigid filaments perform quasi-periodic tumbling motions, a buckling instability arises above a critical flow strength. A second transition to strongly deformed shapes occurs at a yet larger value of the elasto-viscous number and is characterized by the appearance of localized high-curvature bends that propagate along the filaments in apparent “snaking” motions. A theoretical model for the as yet unexplored onset of snaking accurately predicts the transition and explains the observed dynamics. We present a complete characterization of filament morphologies and transitions as a function of elasto-viscous number and scaled persistence length and demonstrate excellent agreement between theory, experiments, and simulations.

粘性流中弹性丝的形态动力学、不稳定性和转变是从鞭毛推进到细胞内流动的丰富生物物理过程的基础，也是破译许多复杂流体和软材料流变行为的关键。在这里，我们结合实验和计算模型来阐明简单剪切流中弹性布朗丝的动力学状态和形态转变。肌动蛋白丝用作实验模型系统，并通过微流体通道中的荧光显微镜研究其构象。在存在热波动的情况下，还使用不可扩展的欧拉-伯努利梁理论和非局域细长体流体动力学进行了与实验条件相匹配的模拟，并与观测结果定量一致。我们证明，该系统中的细丝动力学主要由无量纲弹粘数控制，将粘性阻力与弹性弯曲力进行比较，而热波动仅发挥次要作用。虽然短且刚性的细丝执行准周期性翻滚运动，但在临界流动强度之上会出现屈曲不稳定性。向强烈变形形状的第二次转变发生在弹粘数的更大值处，其特征是出现局部高曲率弯曲，这些弯曲沿着细丝以明显的“蛇行”运动传播。尚未探索的蛇行开始的理论模型准确地预测了这种转变并解释了观察到的动态。我们提出了作为弹粘数和缩放持久长度函数的细丝形态和转变的完整表征，并证明了理论、实验和模拟之间的良好一致性。