软玻璃材料中剪切介导的拉伸流动和屈服
Shear Mediated Elongational Flow and Yielding in Soft Glassy Materials
摘要：
在这项工作中，我们研究的各种软玻璃材料的薄膜，同时受到两个蠕变流场，旋转剪切流通过施加扭矩和拉伸流通过施加法向力的变形行为。研究了发胶、乳胶漆、剃须泡沫和粘土悬浮液等不同微观结构的软质玻璃材料在复合电场作用下的一般行为。一个应力分量的强度增加而另一个应力分量保持恒定，不仅导致在其自身方向上的预期增强的变形，而且导致在另一个方向上的更大的应变。HerschelBulkley模型定性地解释了这一现象。在材料中引起的伸长流动最终导致材料失效。有趣的是，观察到失效时间不仅强烈依赖于法向力，而且还依赖于所施加的旋转剪切应力。我们认为，三维堵塞结构的存在，其中可以通过施加具有足够大小的应力而不考虑方向来诱导整体解除堵塞，从而导致观察到的行为。此外，我们观察到自相似性的拉伸以及旋转应变时间曲线对应于两个领域的各种组合。这一观察结果表明，仅仅是时间尺度上的变化，就涉及保持过程中所遵循的路径不变。一个相图也被构造为各种软玻璃质材料，通过确定不同的正交应力的组合，超过该材料屈服。在由施加在顶板上的拉力主导的流动极限中，估计的屈服应力显示出分散性，这可能源于指进不稳定性。除此之外，当应力张量的不变量超过屈服应力时，观察到屈服，验证了Von Mises准则。
英文摘要：
In this work, we study the deformation behavior of thin films of various soft glassy materials that are simultaneously subjected to two creep flow fields, rotational shear flow by applying torque and elongational flow by applying normal force. The generic behavior under the combined fields is investigated in different soft glassy materials with diverse microstructure such as: hair gel, emulsion paint, shaving foam and clay suspension. Increase in strength of one stress component while keeping the other constant, not only leads to an expected enhanced deformation in its own direction, but also greater strain in the other direction. Herschel Bulkley model is observed to explain this behavior qualitatively. Elongational flow induced in the materials eventually causes failure in the same. Interestingly time to failure is observed to be strongly dependent not just on normal force but also on the applied rotational shear stress. We believe that the presence of a three dimensional jammed structure, in which overall unjamming can be induced by applying stress having sufficient magnitude irrespective of the direction leads to the observed behavior. In addition, we observe self-similarity in the elongational as well as rotational strain time curves corresponding to various combinations of both the fields. This observation suggests a mere shift in the time-scales involved keeping the path followed in the process unchanged. A phase diagram is also constructed for various soft glassy materials by determining different combinations of orthogonal stresses beyond which materials yield. Estimated yield stress in the limit of flow dominated by applied tensile force on the top plate demonstrates scatter, which might be originating from fingering instability. Except this deviation, yielding is observed when the invariant of stress tensor exceeds yield stress, validating the Von Mises criterion.
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