Systems driven far from equilibrium often retain structural memories of their processing history. This memory has, in some cases, been shown to dramatically alter the material response. For example, work hardening in crystalline metals can alter the hardness, yield strength, and tensile strength to prevent catastrophic failure. Whether memory of processing history can be similarly exploited in flowing systems, where significantly larger changes in structure should be possible, remains poorly understood. Here, we demonstrate a promising route to embedding such useful memories. We build on work showing that exposing a sheared dense suspension to acoustic perturbations of different power allows for dramatically tuning the sheared suspension viscosity and underlying structure. We find that, for sufficiently dense suspensions, upon removing the acoustic perturbations, the suspension shear jams with shear stress contributions from the maximum compressive and maximum extensive axes that reflect or “remember” the acoustic training. Because the contributions from these two orthogonal axes to the total shear stress are antagonistic, it is possible to tune the resulting suspension response in surprising ways. For example, we show that differently trained sheared suspensions exhibit (1) different susceptibility to the same acoustic perturbation, (2) orders of magnitude changes in their instantaneous viscosities upon shear reversal, and (3) even a shear stress that increases in magnitude upon shear cessation. We work through these examples to explain the underlying mechanisms governing each behavior. Then, to illustrate the power of this approach for controlling suspension properties, we demonstrate that flowing states well below the shear jamming threshold can be shear jammed via acoustic training. Collectively, our work paves the way for using acoustically induced memory in dense suspensions to generate rapidly and widely tunable materials.

中文翻译：

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将记忆塞入经过声学训练的剪切力密集悬浮液中

远离平衡的系统通常会保留其处理历史的结构记忆。在某些情况下，这种记忆已被证明可以极大地改变材料的反应。例如，晶体金属的加工硬化可以改变硬度、屈服强度和拉伸强度，以防止灾难性故障。处理历史的记忆是否可以在流动系统中得到类似的利用，在流动系统中结构可能发生更大的变化，但仍然知之甚少。在这里，我们展示了一种嵌入此类有用记忆的有前途的途径。我们的工作表明，将剪切致密悬浮液暴露于不同功率的声扰动可以显着调整剪切悬浮液粘度和底层结构。我们发现，对于足够稠密的悬浮液，在消除声学扰动后，悬浮液剪切会堵塞反映或“记住”声学训练的最大压缩轴和最大扩展轴的剪切应力贡献。由于这两个正交轴对总剪切应力的贡献是相反的，因此可以以令人惊讶的方式调整所得的悬浮响应。例如，我们表明，经过不同训练的剪切悬浮液表现出（1）对相同声学扰动的不同敏感性，（2）剪切反转时其瞬时粘度发生数量级的变化，以及（3）甚至剪切应力的大小也会增加剪切停止。我们通过这些例子来解释控制每种行为的基本机制。然后，为了说明这种方法控制悬浮特性的能力，我们证明远低于剪切干扰阈值的流动状态可以通过声学训练进行剪切干扰。总的来说，我们的工作为在致密悬浮液中使用声感应记忆来生成快速且广泛可调的材料铺平了道路。