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arXiv 1802.07356cond-mat.stat-mechcond-mat.soft
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准二维扩散中的流体动力学涨落

Hydrodynamic fluctuations in quasi-two dimensional diffusion

R. P. Peláez, F. Balboa Usabiaga, S. Panzuela, Q. Xiao, R. Delgado-Buscalioni, A. Donev

摘要:

我们使用粒子模拟和波动流体力学研究流体-流体界面上的胶体扩散。二维界面上的扩散与三维流体力学是已知的异常,与集体扩散系数发散一样的波数的倒数。这种不寻常的集体效应是由于界面平面内流体流动的可压缩性而产生的,并导致在整体平均浓度的扩散方程中出现非线性非局部卷积项。我们扩展了以前的流体动力学理论,考虑到一个物种/颜色标记的颗粒,作为必要的模型实验的基础上荧光技术。我们使用一种新的布朗动力学算法,以及波动流体动力学理论和模拟研究密度和颜色密度波动的幅度和动态。我们发现,一个单一的标记粒子和集体密度波动之间的流体动力学耦合导致的长期自扩散系数的减少,即使是非相互作用的粒子的理想气体。使用线性化波动流体力学理论,我们表明,对于流体-流体界面上的扩散,总密度的非平衡波动是小的相比,平衡波动,但颜色密度的波动是巨大的,并表现出一个频谱,衰减为波数的立方幂的倒数。我们证实了这些预测,通过布朗动力学模拟扩散混合两个不可区分的物种。我们还研究了二维流体力学系统中的非平衡涨落,如真空中的近晶薄膜。我们发现,非平衡波动是巨大的和可比的幅度的平均值,可以准确地模拟使用数值求解器的非线性方程的波动流体动力学。

英文摘要:

We study diffusion of colloids on a fluid-fluid interface using particle simulations and fluctuating hydrodynamics. Diffusion on a two-dimensional interface with three-dimensional hydrodynamics is known to be anomalous, with the collective diffusion coefficient diverging like the inverse of the wavenumber. This unusual collective effect arises because of the compressibility of the fluid flow in the plane of the interface, and leads to a nonlinear nonlocal convolution term in the diffusion equation for the ensemble-averaged concentration. We extend the previous hydrodynamic theory to account for a species/color labeling of the particles, as necessary to model experiments based on fluorescent techniques. We study the magnitude and dynamics of density and color density fluctuations using a novel Brownian dynamics algorithm, as well as fluctuating hydrodynamics theory and simulation. We find that hydrodynamic coupling between a single tagged particle and collective density fluctuations leads to a reduction of the long-time self-diffusion coefficient, even for an ideal gas of non-interacting particles. Using linearized fluctuating hydrodynamics theory, we show that for diffusion on a fluid-fluid interface, nonequilibrium fluctuations of the total density are small compared to the equilibrium fluctuations, but fluctuations of color density are giant and exhibit a spectrum that decays as the inverse cubed power of the wavenumber. We confirm these predictions through Brownian dynamics simulations of diffusive mixing with two indistinguishable species. We also examine nonequilibrium fluctuations in systems with two-dimensional hydrodynamics, such as thin smectic films in vacuum. We find that nonequilibrium fluctuations are colossal and comparable in magnitude to the mean, and can be accurately modeled using numerical solvers for the nonlinear equations of fluctuating hydrodynamics.

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