We present a molecular dynamics study of the flow of rigid spherical nanoparticles in a simple fluid. We evaluate the viscosity of the dispersion as a function of shear rate and nanoparticle volume fraction. We observe shear-thinning behavior at low volume fractions; as the shear rate increases, the shear forces overcome the Brownian forces, resulting in more frequent and more violent collisions between the nanoparticles. This in turn results in more dissipation. We show that in order to be in the shear-thinning regime the nanoparticles have to order themselves into layers longitudinal to the flow to minimize the collisions. As the nanoparticle volume fraction increases there is less room to form the ordered layers; consequently as the shear rate increases the nanoparticles collide more, which results in turn in shear thickening. Most interestingly, we show that at intermediate volume fractions the system exhibits metastability, with successions of ordered and disordered states along the same trajectory. Our results suggest that for nanoparticles in a simple fluid the hydroclustering phenomenon is not present; instead the order-disorder transition is the leading mechanism for the transition from shear thinning to shear thickening.

中文翻译：

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球形纳米颗粒分散体中的剪切变稀和增稠。

我们提出了在简单流体中刚性球形纳米颗粒流动的分子动力学研究。我们评估分散液的粘度与剪切速率和纳米颗粒体积分数的关系。我们观察到低体积分数下的剪切稀化行为。随着剪切速率的增加，剪切力克服了布朗力，导致纳米粒子之间发生更频繁，更猛烈的碰撞。这进而导致更多的耗散。我们表明，为了处于剪切稀化状态，纳米粒子必须将其自身排列成与流动垂直的层，以最大程度地减少碰撞。随着纳米颗粒体积分数的增加，形成有序层的空间变少了。因此，随着剪切速率的增加，纳米颗粒会更多地碰撞，从而导致剪切增厚。最有趣的是，我们表明，在中等体积分数下，系统表现出亚稳性，沿着同一轨迹具有连续的有序和无序状态。我们的研究结果表明，对于简单流体中的纳米颗粒，不存在水力成簇现象。相反，有序-无序过渡是从剪切稀化到剪切增厚过渡的主要机制。