Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland.
Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland.
Proc Natl Acad Sci U S A. 2017 Nov 14;114(46):12150-12155. doi: 10.1073/pnas.1712266114. Epub 2017 Oct 30.
Dense colloidal suspensions can propagate and absorb large mechanical stresses, including impacts and shocks. The wave transport stems from the delicate interplay between the spatial arrangement of the structural units and solvent-mediated effects. For dynamic microscopic systems, elastic deformations of the colloids are usually disregarded due to the damping imposed by the surrounding fluid. Here, we study the propagation of localized mechanical pulses in aqueous monolayers of micron-sized particles of controlled microstructure. We generate extreme localized deformation rates by exciting a target particle via pulsed-laser ablation. In crystalline monolayers, stress propagation fronts take place, where fast-moving particles ( approximately a few meters per second) are aligned along the symmetry axes of the lattice. Conversely, more viscous solvents and disordered structures lead to faster and isotropic energy absorption. Our results demonstrate the accessibility of a regime where elastic collisions also become relevant for suspensions of microscopic particles, behaving as "billiard balls" in a liquid, in analogy with regular packings of macroscopic spheres. We furthermore quantify the scattering of an impact as a function of the local structural disorder.
密集的胶体悬浮液可以传播和吸收大的机械应力，包括冲击和震动。波的传输源于结构单元的空间排列和溶剂介导的效应之间的微妙相互作用。对于动态微观系统，由于周围流体的阻尼，胶体的弹性变形通常被忽略。在这里，我们研究了在具有受控微观结构的微米尺寸颗粒的水单相层中局部机械脉冲的传播。我们通过用脉冲激光烧蚀来激发目标颗粒来产生极端的局部变形率。在晶体单相层中，会发生应力传播前沿，其中快速移动的颗粒（约每秒几米）沿着晶格的对称轴排列。相反，粘性较大的溶剂和无序结构会导致更快和各向同性的能量吸收。我们的结果表明，在弹性碰撞也适用于微观颗粒悬浮液的情况下，可以达到一种状态，这些悬浮液在液体中表现为“弹球”，类似于宏观球体的规则堆积。此外，我们还定量地研究了冲击的散射作为局部结构无序的函数。
