Williams Ian, Oğuz Erdal C, Löwen Hartmut, Poon Wilson C K, Royall C Patrick
Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom.
School of Mechanical Engineering, Tel Aviv University, Tel Aviv 6997801, Israel.
J Chem Phys. 2022 May 14;156(18):184902. doi: 10.1063/5.0087444.
Colloids may be treated as "big atoms" so that they are good models for atomic and molecular systems. Colloidal hard disks are, therefore, good models for 2d materials, and although their phase behavior is well characterized, rheology has received relatively little attention. Here, we exploit a novel, particle-resolved, experimental setup and complementary computer simulations to measure the shear rheology of quasi-hard-disk colloids in extreme confinement. In particular, we confine quasi-2d hard disks in a circular "corral" comprised of 27 particles held in optical traps. Confinement and shear suppress hexagonal ordering that would occur in the bulk and create a layered fluid. We measure the rheology of our system by balancing drag and driving forces on each layer. Given the extreme confinement, it is remarkable that our system exhibits rheological behavior very similar to unconfined 2d and 3d hard particle systems, characterized by a dynamic yield stress and shear-thinning of comparable magnitude. By quantifying particle motion perpendicular to shear, we show that particles become more tightly confined to their layers with no concomitant increase in density upon increasing the shear rate. Shear thinning is, therefore, a consequence of a reduction in dissipation due to weakening in interactions between layers as the shear rate increases. We reproduce our experiments with Brownian dynamics simulations with Hydrodynamic Interactions (HI) included at the level of the Rotne-Prager tensor. That the inclusion of HI is necessary to reproduce our experiments is evidence of their importance in transmission of momentum through the system.
胶体可以被视为“大原子”，因此它们是原子和分子系统的良好模型。胶体硬磁盘因此是二维材料的良好模型，尽管它们的相行为已得到很好的表征，但流变学受到的关注相对较少。在这里，我们利用一种新颖的、基于粒子分辨的实验装置和互补的计算机模拟来测量极端受限条件下准硬磁盘胶体的剪切流变学。具体来说，我们将准二维硬磁盘限制在一个由27个粒子组成的圆形“围栏”中，这些粒子被光学陷阱捕获。限制和剪切抑制了体相中会出现的六边形有序排列，并形成了层状流体。我们通过平衡各层上的阻力和驱动力来测量系统的流变学。考虑到极端的限制条件，值得注意的是，我们的系统表现出与无限制的二维和三维硬粒子系统非常相似的流变行为，其特征是具有相当大小的动态屈服应力和剪切变稀。通过量化垂直于剪切方向的粒子运动，我们表明随着剪切速率的增加，粒子被更紧密地限制在其层中，而密度没有相应增加。因此，剪切变稀是由于剪切速率增加时层间相互作用减弱导致耗散减少的结果。我们用包含旋转-普拉格张量水平的流体动力学相互作用（HI）的布朗动力学模拟来重现我们的实验。包含HI对于重现我们的实验是必要的，这证明了它们在通过系统传递动量方面的重要性。
