Vinutha H A, Marchand Manon, Caggioni Marco, Vasisht Vishwas V, Del Gado Emanuela, Trappe Veronique
Department of Physics, Institute for Soft Matter Synthesis and Metrology, Georgetown University, Washington, DC, USA.
Department of Physics, University of Fribourg, Fribourg, Switzerland.
PNAS Nexus. 2024 Oct 4;3(10):pgae441. doi: 10.1093/pnasnexus/pgae441. eCollection 2024 Oct.
Cessation of flow in yield stress fluids results in a stress relaxation process that eventually leads to a finite residual stress. Both the rate of stress relaxation and the magnitude of the residual stresses systematically depend on the preceding flow conditions. To assess the microscopic origin of this memory effect, we combine experiments with large-scale computer simulations, exploring the behavior of jammed suspensions of soft repulsive particles. A spatiotemporal analysis of particle motion reveals that memory formation during flow is primarily governed by the emergence of domains of spatially correlated nonaffine displacements. These domains imprint the configuration of stress imbalances that drive dynamics upon flow cessation, as evidenced by a striking equivalence of the spatial correlation patterns in particle displacements observed during flow and upon flow cessation. Additional contributions to stress relaxation result from the particle packing that reorganizes to minimize the resistance to flow by decreasing the number of locally stiffer configurations. Regaining rigidity upon flow cessation drives further relaxation and effectively sets the magnitude of the residual stress. Our findings highlight that flow in yield stress fluids can be seen as a training process during which the material stores information of the flowing state through the development of domains of correlated particle displacements and the reorganization of particle packings optimized to sustain the flow. This encoded memory can then be retrieved in flow cessation experiments.
屈服应力流体中的流动停止会导致应力松弛过程，最终导致有限的残余应力。应力松弛速率和残余应力大小都系统地取决于先前的流动条件。为了评估这种记忆效应的微观起源，我们将实验与大规模计算机模拟相结合，探索软排斥颗粒的堵塞悬浮液的行为。对颗粒运动的时空分析表明，流动过程中的记忆形成主要由空间相关非仿射位移域的出现所控制。这些域印记了流动停止时驱动动力学的应力不平衡配置，流动过程中和流动停止时观察到的颗粒位移中的空间相关模式的显著等效性证明了这一点。应力松弛的额外贡献来自颗粒堆积，颗粒堆积通过减少局部更硬配置的数量进行重组，以最小化流动阻力。流动停止时恢复刚性会驱动进一步的松弛，并有效地设定残余应力的大小。我们的研究结果突出表明，屈服应力流体中的流动可被视为一个训练过程，在此过程中，材料通过相关颗粒位移域的发展以及为维持流动而优化的颗粒堆积重组来存储流动状态的信息。然后，这种编码记忆可以在流动停止实验中被检索到。
