大幅振荡拉伸流中的囊泡动力学
Vesicle dynamics in large amplitude oscillatory extensional flow
AI总结：
本研究通过实验与边界积分模拟，揭示囊泡在大幅振荡拉伸流中的三种动力学状态（脉动、重定向、对称），构建相图并量化应力-应变率非线性关系，为悬浮流变学提供新见解。
摘要：
尽管流体填充囊泡在稳态流中的行为已被广泛研究，但对其在时间依赖性振荡流中的形状动力学了解甚少。在此，我们通过实验和边界积分（BI）模拟，研究囊泡在大幅振荡拉伸（LAOE）流中的非线性动力学。我们的结果以减缩体积（囊泡非球形度）、毛细数（$\Ca$，无量纲流动强度）和戴博拉数（$\De$，无量纲流动频率）为参数，表征了LAOE中囊泡的瞬态膜变形、动力学状态和应力响应。单囊泡实验结果与BI模拟在广泛参数范围内吻合良好。我们的结果揭示了基于囊泡变形的三种不同动力学状态：脉动、重定向和对称状态。我们构建了相图，在由$\De$和$\Ca$定义的二维Pipkin空间中，表征囊泡形状在脉动、重定向和对称状态之间的转变。与清洁牛顿液滴的观察相反，在重定向和脉动状态下，囊泡在每个应变率周期内不会达到两次最大长度。每个状态中观察到的不同动力学源于流动频率、流动时间尺度和膜变形时间尺度之间的竞争。通过计算粒子应力矩，我们量化了平均囊泡应力与应变率之间的非线性关系。此外，我们展示了管状囊泡在多个应变周期内经历形状转变的结果。总的来说，我们的工作为时间依赖性流动中囊泡的瞬态动力学提供了新信息，直接为体悬浮流变学提供参考。
英文摘要：
Although the behavior of fluid-filled vesicles in steady flows has been extensively studied, far less is understood regarding the shape dynamics of vesicles in time-dependent oscillatory flows. Here, we investigate the nonlinear dynamics of vesicles in large amplitude oscillatory extensional (LAOE) flows using both experiments and boundary integral (BI) simulations. Our results characterize the transient membrane deformations, dynamical regimes, and stress response of vesicles in LAOE in terms of reduced volume (vesicle asphericity), capillary number ($\Ca$, dimensionless flow strength), and Deborah number ($\De$, dimensionless flow frequency). Results from single vesicle experiments are found to be in good agreement with BI simulations across a wide range of parameters. Our results reveal three distinct dynamical regimes based on vesicle deformation: pulsating, reorienting, and symmetrical regimes. We construct phase diagrams characterizing the transition of vesicle shapes between pulsating, reorienting, and symmetrical regimes within the two-dimensional Pipkin space defined by $\De$ and $\Ca$. Contrary to observations on clean Newtonian droplets, vesicles do not reach a maximum length twice per strain rate cycle in the reorienting and pulsating regimes. The distinct dynamics observed in each regime result from a competition between the flow frequency, flow time scale, and membrane deformation timescale. By calculating the particle stresslet, we quantify the nonlinear relationship between average vesicle stress and strain rate. Additionally, we present results on tubular vesicles that undergo shape transformation over several strain cycles. Broadly, our work provides new information regarding the transient dynamics of vesicles in time-dependent flows that directly informs bulk suspension rheology.
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