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arXiv 2102.00771cond-mat.softcond-mat.stat-mech
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粘弹性相分离流体力学方程的系统推导

Systematic derivation of hydrodynamic equations for viscoelastic phase separation

Dominic Spiller, Aaron Brunk, Oliver Habrich, Herbert Egger, Maria Lukacova-Medvidova, Burkhard Duenweg

摘要:

本文详细推导了一个简单的流体动力学双流体模型,该模型旨在描述非纠缠聚合物溶液的相分离,其中粘弹性效应起着重要作用。它直接基于定义明确的分子模型的粗粒化,使得所有自由度都具有清晰和明确的分子解释。的考虑是基于一个自由能的功能,和动态分为保守和耗散的一部分,后者满足昂萨格关系和热力学第二定律。因此,该模型完全符合平衡态和非平衡态热力学。推导过程分为两个步骤:首先,我们推导出一个扩展的模型,包括两个标量场和四个矢量场,这样的惯性动力学的大分子和两种流体的相对运动被考虑在内。在第二步中,我们消除这些惯性的贡献,并作为替代,引入现象耗散项,这可以很容易地考虑到非平衡态热力学的原理建模。最后的简化模型包括动量守恒方程,其中包括界面和弹性应力,对流扩散方程的界面和弹性的贡献发生,以及适当的对流松弛方程的端到端的矢量场。与传统的用于推导流变学运动方程的双尺度描述相反,我们在这里将流体动力学和大分子自由度处理在相同的基础上。尽管如此,所得到的模型与前面讨论过的模型相当相似,尽管不完全相同。值得注意的是,我们发现一个流变本构方程,不同于标准的Oldrocket-B模型。...

英文摘要:

(abridged) We present a detailed derivation of a simple hydrodynamic two-fluid model, which aims at the description of the phase separation of non-entangled polymer solutions, where viscoelastic effects play a role. It is directly based upon the coarse-graining of a well-defined molecular model, such that all degrees of freedom have a clear and unambiguous molecular interpretation. The considerations are based upon a free-energy functional, and the dynamics is split into a conservative and a dissipative part, where the latter satisfies the Onsager relations and the Second Law of thermodynamics. The model is therefore fully consistent with both equilibrium and non-equilibrium thermodynamics. The derivation proceeds in two steps: Firstly, we derive an extended model comprising two scalar and four vector fields, such that inertial dynamics of the macromolecules and of the relative motion of the two fluids is taken into account. In the second step, we eliminate these inertial contributions and, as a replacement, introduce phenomenological dissipative terms, which can be modeled easily by taking into account the principles of non-equilibrium thermodynamics. The final simplified model comprises the momentum conservation equation, which includes both interfacial and elastic stresses, a convection-diffusion equation where interfacial and elastic contributions occur as well, and a suitably convected relaxation equation for the end-to-end vector field. In contrast to the traditional two-scale description that is used to derive rheological equations of motion, we here treat the hydrodynamic and the macromolecular degrees of freedom on the same basis. Nevertheless, the resulting model is fairly similar, though not fully identical, to models that have been discussed previously. Notably, we find a rheological constitutive equation that differs from the standard Oldroyd-B model. ...

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