Shear flow is ubiquitous. Not only is it arguably the most widely-used deformation type to characterise complex fluids in rheological studies but also, in practice, the deformation most likely to occur in the great majority of flows, e.g. involving fluid transport through pipes or conduits. In steady simple shear flow the rheological properties of a complex fluid are completely characterised in just three material functions; the variation with shear rate of the shear viscosity and the so-called first and second normal-stress differences. Despite requiring only three material functions to be completely characterised, most shear-flow rheological characterisations are usually restricted simply to the shear viscosity and, at best, the variation of the first normal-stress difference N₁ with shear rate. The second normal-stress difference N₂ remains very much neglected. For dilute polymer solutions where this quantity may be negligibly small in comparison to the first normal-stress difference, such neglect is justified but for a whole range of complex fluids – indeed even polymer solutions outside of the dilute regime and especially melts – it is not clear that N₂ may be safely disregarded. Indeed, in this review article we spotlight a number of flows where second normal-stress differences are of importance and potentially major consequence. Following this attention, we review the many experimental techniques which have been proposed for its measurement and survey the available literature for measurements of this quantity for various complex fluids including the aforementioned polymeric solutions, melts, liquid crystals, dense non-Brownian suspensions (both with Newtonian and complex fluid bases), semi-dilute wormlike micellar fluids and magnetorheological fluids. Theoretical predictions for N₂ from various commonly-used continuum constitutive equations – primarily from the polymer literature – are also given and their asymptotic predictions at low and high shear rates compared. Finally, we end with a brief summary and outlook.

剪切流无处不在。不仅可以说是在流变学研究中表征复杂流体的最广泛使用的变形类型，而且在实践中，变形也很可能发生在绝大多数流动中，例如涉及通过管道或导管的流体传输。在稳定的简单剪切流中，仅用三种材料功能就可以完全表征复杂流体的流变特性。剪切粘度随剪切速率的变化以及所谓的第一和第二法向应力差。尽管只需要完全表征三种材料功能，但大多数剪切流变学特性通常仅局限于剪切粘度，至多只能限制第一法向应力差N ₁的变化。剪切速率。第二法向应力差N ₂仍然非常被忽略。对于稀聚合物溶液，其数量与第一法向应力差相比可以忽略不计，这种疏忽是有道理的，但是对于整个范围的复杂流体-实际上甚至是稀溶液之外的聚合物溶液，尤其是熔融的聚合物溶液，它不是清除N ₂可能会被安全地忽略。确实，在这篇综述文章中，我们重点介绍了第二次法向应力差异具有重要意义并且可能产生重大后果的许多流程。在关注之后，我们回顾了已提出的用于其测量的许多实验技术，并调查了可用于测量各种复杂流体（包括上述聚合物溶液，熔体，液晶，致密的非布朗悬浮液（均含））的量的现有文献。牛顿流体和复杂流体基础），半稀释蠕虫状胶束流体和磁流变流体。N _2的理论预测还给出了各种常用的连续体本构方程（主要来自聚合物文献）的结果，并比较了它们在低剪切速率和高剪切速率下的渐近预测。最后，我们以简短的总结和展望结束。