勃姆石凝胶中的残余应力与剪切诱导过时效
Residual stresses and shear-induced overaging in boehmite gels
AI总结：
本研究发现勃姆石凝胶的液-固转变存在非单调应力弛豫的双重行为，残余应力与粘弹性随剪切应变对数增长，揭示的剪切诱导过时效现象可用于调控胶体凝胶流变特性。
摘要：
胶体凝胶在静止时表现得像软固体，而在外部剪切作用下则像液体一样流动。从施加剪切速率$\boldsymbol{\not\bar{\not\bar{}}}\not\bar{\not{}}_{p}$下的流化状态开始，流动的突然停止会触发液-固转变，在此过程中应力弛豫至所谓的**残余应力**$\not\bar{\not{}}_{\rm res}$，这是先前剪切历史的宏观标志。本文研究了勃姆石（一种氧化铝）凝胶中的液-固转变，发现其向残余应力$\not\bar{\not{}}_{\rm res}(\not\bar{\not{}}_{p})$的应力弛豫呈现出显著的非单调性，其特征相对于剪切速率$\not\bar{\not{}}_{p}$的临界值$\not\bar{\not{}}_{c}$表现出双重行为。当以$\not\bar{\not{}}_{p}>\not\bar{\not{}}_{c}$的速率剪切后，流动停止时形成的凝胶对剪切历史不敏感，且残余应力可忽略不计。然而，当$\not\bar{\not{}}_{p}<\not\bar{\not{}}_{c}$时，凝胶会编码部分剪切历史记忆，且$\not\bar{\not{}}_{\rm res}$随剪切速率的降低而增大，这直接有助于增强凝胶的粘弹性。此外，我们发现$\not\bar{\not{}}_{\rm res}$和凝胶粘弹性均随流动停止前剪切阶段累积的应变呈对数增长。这种剪切诱导的“过时效”现象在调控胶体凝胶的流变特性方面具有巨大潜力。
英文摘要：
Colloidal gels respond like soft solids at rest, whereas they flow like liquids under external shear. Starting from a fluidized state under an applied shear rate $\dotγ_{p}$, abrupt flow cessation triggers a liquid-to-solid transition during which the stress relaxes towards a so-called \textit{residual stress} $σ_{\rm res}$ that tallies a macroscopic signature of previous shear history. Here, we report on the liquid-to-solid transition in gels of boehmite, an aluminum oxide, that shows a remarkable non-monotonic stress relaxation towards a residual stress $σ_{\rm res}(\dotγ_{p})$ characterized by a dual behavior relative to a critical value $\dotγ_{c}$ of the shear rate $\dotγ_{p}$. Following shear at $\dotγ_{p}>\dotγ_{c}$, the gel obtained upon flow cessation is insensitive to shear history, and the residual stress is negligible. However, for $\dotγ_{p}<\dotγ_{c}$, the gel encodes some memory of the shear history, and $σ_{\rm res}$ increases for decreasing shear rate, directly contributing to reinforcing the gel viscoelastic properties. Moreover, we show that both $σ_{\rm res}$ and the gel viscoelastic properties increase logarithmically with the strain accumulated during the shear period preceding flow cessation. Such a shear-induced "overaging" phenomenon bears great potential for tuning the rheological properties of colloidal gels.
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