We perform extensive molecular dynamics simulations of an ensemble of realistic microgel particles in swollen conditions in a wide range of packing fractions ζ. We compare neutral and charged microgels, where we consider charge distribution adherent to experimental conditions. Through a detailed analysis of single-particle behavior, we are able to identify the different regimes occurring upon increasing concentration: from shrinking to deformation and interpenetration, always connecting our findings with available experimental observations. We then link these single-particle features with the collective behavior of the suspension, finding evidence of a structural reentrance that has no counterpart in the dynamics. Hence, while the maximum of the radial distribution function displays a nonmonotonic behavior with increasing ζ, the dynamics, quantified by the microgels’ mean-squared displacement, always slows down. This behavior, at odds with the simple Hertzian model, can be described by a phenomenological multi-Hertzian model, which takes into account the enhanced internal stiffness of the core. However, also this model fails when deformation enters into play, whereby more realistic many-body models are required. Thanks to our analysis, we are able to unveil the key physical mechanisms, shrinking and deformation, giving rise to the structural reentrance that holds up to very large packing fractions. We further identify key similarities and differences between neutral and charged microgels, for which we detect at high enough ζ the fusion of charged shells, previously invoked to explain key experimental findings, and responsible for the structural reentrance. Overall, our study establishes a powerful framework to uncover the physics of microgel suspensions, paving the way to tackle different regimes, e.g., high temperature, and internal architectures, such as for hollow and ultralow-cross-linked microgels, where experimental evidence is still limited. Published by the American Physical Society 2024

中文翻译：

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中性和带电微凝胶悬浮液的数值研究：从单粒子到集体行为


我们在膨胀条件下对各种堆积组分的真实微凝胶颗粒的集合进行了广泛的分子动力学模拟ζ。我们比较了中性微凝胶和带电微凝胶，我们认为电荷分布符合实验条件。通过对单粒子行为的详细分析，我们能够识别浓度增加时发生的不同状态：从收缩到变形和相互渗透，始终将我们的发现与可用的实验观察结果联系起来。然后，我们将这些单粒子特征与悬浮液的集体行为联系起来，找到在动力学中没有对应物的结构再进入的证据。因此，虽然径向分布函数的最大值随着ζ的增加而表现出非单调行为，但由微凝胶的均方位移量化的动力学总是减慢。这种行为与简单的赫兹模型不一致，可以用现象学多赫兹模型来描述，该模型考虑了核心内部刚度的增强。但是，当变形开始发挥作用时，此模型也会失效，因此需要更逼真的多体模型。由于我们的分析，我们能够揭示关键的物理机制，即收缩和变形，从而产生结构重入，该回切可以容纳非常大的堆积分数。我们进一步确定了中性微凝胶和带电微凝胶之间的关键相似之处和不同之处，为此我们在带电壳的融合ζ足够高地检测到这些微凝胶，以前用于解释关键的实验发现，并负责结构重新进入。 总体而言，我们的研究建立了一个强大的框架来揭示微凝胶悬浮液的物理学，为处理不同的状态（例如高温）和内部结构（例如空心和超低交联微凝胶）铺平了道路，这些结构的实验证据仍然有限。美国物理学会 2024 年出版