拥挤溶液中聚合物的动力学与构象行为
Dynamical and conformational behavior of a polymer in a crowded solution
AI总结：
本研究通过朗之万动力学与格子玻尔兹曼分子动力学模拟，揭示了拥挤溶液中胶体尺寸比、体积分数对聚合物构象紧缩及扩散行为的调控机制，识别出三种动力学区域及流体动力学相互作用的关键影响。
AI中文摘要：
我们研究了聚合物在含有半径为$R$的可移动球形胶体拥挤剂的流体中的结构与动力学特性。我们将朗之万动力学（Langevin dynamics, LD）与格子玻尔兹曼分子动力学（lattice–Boltzmann molecular dynamics, LBMD）的行为进行了对比分析，后者纳入了长程流体动力学相互作用。我们改变了胶体相对于单体半径$r$的尺寸以及体积分数$ϕ$，以确定拥挤效应如何改变聚合物的行为。体积分数的升高会导致聚合物紧缩，其作用机制强烈依赖于尺寸比$R/r$。小胶体主要改变聚合物的短波长构象，使得自回避行走（self-avoiding-walk）类行为能在更短的长度尺度下维持；而大胶体则会降低有效的长波长Flory指数，表明溶剂质量下降，这与约束 blob 图像一致。聚合物扩散在LD和LBMD中表现出截然不同的行为。在LD中，扩散速率快速下降，且强烈依赖于$R/r$；一个包含$\boldsymbol{\rm ln}(1+R/r)$的唯象标度关系可以描述这种尺寸依赖性，而引入与回转半径$R_g$相关的额外标度关系可降低数据离散度，表明拥挤效应诱导了聚合物尺度的相关性。相比之下，LBMD中的扩散遵循有效介质类的浓度指数依赖关系，由流体动力学耦合主导。Rouse模式分析识别出三种区域：低体积分数下的标度失效区、LD和LB中中等密度下的Zimm类行为区，以及高密度下LB中的流体动力学屏蔽区与LD中的约束主导动力学区。
英文摘要：
We investigate the structure and dynamics of a polymer in a fluid containing mobile spherical colloidal crowders of radius $R$. We compare and contrast the behavior with Langevin dynamics (LD) and lattice--Boltzmann molecular dynamics (LBMD), the latter incorporating long-range hydrodynamic interactions. Both the colloid size relative to the monomer radius $r$ and the volume fraction $ϕ$ are varied to determine how crowding modifies polymer behavior. Increasing volume fraction induces polymer compaction, with the mechanism strongly dependent on the size ratio $R/r$. Small colloids primarily modify the short-wavelength polymer conformation, causing self-avoiding-walk-like behavior to persist to shorter length scales, whereas large colloids reduce the effective long-wavelength Flory exponent, indicating degraded solvent quality consistent with a confinement-blob picture. Polymer diffusion exhibits distinct behavior in LD and LBMD. In LD, diffusion decreases rapidly and depends strongly on $R/r$; a phenomenological scaling involving $\ln(1+R/r)$ captures this size dependence, and additional scaling with $R_g$ reduces scatter, indicating polymer-scale correlations induced by crowding. In contrast, LBMD diffusion follows an effective-medium-like exponential dependence on concentration, governed by hydrodynamic coupling. Rouse-mode analysis identifies three regimes: scaling breakdown at low volume fraction, Zimm-like behavior at intermediate density in both LD and LB, and at high density hydrodynamic screening in LB with confinement-dominated dynamics in LD.
