The solid–fluid transition in a yield stress shear thinning physical gel
The solid–fluid transition in a yield stress shear thinning physical gel
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DOI：
10.1007/s00397-009-0365-9
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发表时间：
2009-05
期刊：
影响因子：
2.3
通讯作者：
A. Putz;T. Burghelea
中科院分区：
文献类型：
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作者：
A. Putz;T. Burghelea
我们提出了在剪切下的屈服应力剪切稀化物理凝胶（Carbopol®940）中的固-流转变的实验研究。在逐渐增加的外部强迫，我们观察到三个不同的变形制度：弹性固体状制度（其特征在于线性应力-应变依赖），固-流体相共存制度（其特征在于破坏和重组之间的竞争的凝胶），和一个纯粹的粘性制度（其特征在于由幂律应力应变率依赖）。通过动态弹性模量（作为应力，振幅和温度的函数）和非稳态流量斜坡的系统测量的凝胶的破坏和改造之间的竞争进行了研究。从固体行为到流体行为的转变显示出明显的滞后后，增加和减少值的外部强迫。我们发现，对应于滞后区域的变形功率与材料被强迫的速率（流动不稳定的程度）成线性比例。在小强迫率的渐近极限，我们的结果与以前的稳态调查屈服过渡。基于这些实验结果，我们提出了固-流相变和一级相变之间的类比，例如，铁磁体的磁化，其中不可逆性和滞后性作为相共存状态的结果出现。为了进一步了解固-液转变，我们的实验结果是由一个简单的动力学模型，定性地描述了在我们的流变实验中观察到的结构滞后补充。该模型是相当好的验证对振荡流数据的部分重建的Pipkin空间的材料的响应及其非线性光谱行为。
We present an experimental investigation of the solid–fluid transition in a yield stress shear thinning physical gel (Carbopol®940) under shear. Upon a gradual increase of the external forcing, we observe three distinct deformation regimes: an elastic solid-like regime (characterized by a linear stress–strain dependence), a solid–fluid phase coexistence regime (characterized by a competition between destruction and reformation of the gel), and a purely viscous regime (characterized by a power law stress-rate of strain dependence). The competition between destruction and reformation of the gel is investigated via both systematic measurements of the dynamic elastic moduli (as a function of stress, the amplitude, and temperature) and unsteady flow ramps. The transition from solid behavior to fluid behavior displays a clear hysteresis upon increasing and decreasing values of the external forcing. We find that the deformation power corresponding to the hysteresis region scales linearly with the rate at which the material is being forced (the degree of flow unsteadiness). In the asymptotic limit of small forcing rates, our results agree well with previous steady state investigations of the yielding transition. Based on these experimental findings, we suggest an analogy between the solid–fluid transition and a first-order phase transition, e.g., the magnetization of a ferro-magnet where irreversibility and hysteresis emerge as a consequence of a phase coexistence regime. In order to get further insight into the solid–fluid transition, our experimental findings are complemented by a simple kinetic model that qualitatively describes the structural hysteresis observed in our rheological experiments. The model is fairly well validated against oscillatory flow data by a partial reconstruction of the Pipkin space of the material’s response and its nonlinear spectral behavior.