Roosen-Runge Felix, Schurtenberger Peter, Stradner Anna
Division of Physical Chemistry, Lund University, Naturvetarvägen 14, 22100 Lund, Sweden.
Department of Biomedical Sciences and Biofilms-Research Center for Biointerfaces (BRCB), Faculty of Health and Society, Malmö University, Sweden.
J Phys Condens Matter. 2021 Feb 18;33(15). doi: 10.1088/1361-648X/abdff9.
Modeling diffusion of nonspherical particles presents an unsolved and considerable challenge, despite its importance for the understanding of crowding effects in biology, food technology and formulation science. A common approach in experiment and simulation is to map nonspherical objects on effective spheres to subsequently use the established predictions for spheres to approximate phenomena for nonspherical particles. Using numerical evaluation of the hydrodynamic mobility tensor, we show that this so-called effective sphere model fundamentally fails to represent the self-diffusion in solutions of ellipsoids as well as rod-like assemblies of spherical beads. The effective sphere model drastically overestimates the slowing down of self-diffusion down to volume fractions below 0.01. Furthermore, even the linear term relevant at lower volume fraction is inaccurate, linked to a fundamental misconception of effective sphere models. To overcome the severe problems related with the use of effective sphere models, we suggest a protocol to predict the short-time self-diffusion of rod-like systems, based on simulations with hydrodynamic interactions that become feasible even for more complex molecules as the essential observable shows a negligible system-size effect.
尽管对非球形颗粒扩散进行建模对于理解生物学、食品技术和配方科学中的拥挤效应非常重要，但它仍然是一个尚未解决的重大挑战。实验和模拟中的一种常见方法是将非球形物体映射到有效球体上，随后使用已建立的球体预测来近似非球形颗粒的现象。通过对流体动力学迁移率张量进行数值评估，我们表明这种所谓的有效球体模型从根本上无法表示椭球体溶液以及球形珠子的棒状组装体中的自扩散。有效球体模型将自扩散的减慢严重高估到体积分数低于0.01的情况。此外，即使在较低体积分数下相关的线性项也是不准确的，这与有效球体模型的一个基本误解有关。为了克服与使用有效球体模型相关的严重问题，我们提出了一种协议，用于预测棒状系统的短时间自扩散，该协议基于具有流体动力学相互作用的模拟，即使对于更复杂的分子，这种模拟也变得可行，因为基本可观测量显示出可忽略不计的系统尺寸效应。
