We study experimentally the rheological behavior of carbon black (CB) suspensions in water at different ionic strengths and concentrations. We show by means of standard rheometry completed by local magnetic resonance imaging-rheometry that these suspensions first appear to be thixotropic yield stress fluids: they exhibit a yield stress increasing with the time of rest, their apparent viscosity decreases under shear, and a viscosity bifurcation occurs around the yield stress, the fluid evolving either towards stoppage or to steady flow at a large shear rate for a small stress change. Then, an original effect appears when we follow the mechanical state of the material in the solid regime by measuring its apparent elastic modulus at small deformation during a creep test under various stresses. In contrast with various other yield stress fluids for which the elastic modulus under small deformation appears to be constant for any deformation in the solid regime, for CB suspensions, this modulus widely increases while deformation increases up to yielding. We suggest that this strain stiffening effect finds its origin in the specificities of the (van der Waals) interactions and of the (rough) structure (aggregates) of the particles: the slight relative rotation of particles in contact due to deformation would, on average, tend to increase the net area of contact between particles, which stiffens the whole material structure. This is supported by the observation that the relative increase of elastic modulus is approximately proportional to sample deformation, whatever the material characteristics (ionic strength, concentration) and whatever the deformation history.

中文翻译：

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水性炭黑悬浮液的屈服、触变性和应变硬化

我们通过实验研究了不同离子强度和浓度下炭黑 (CB) 悬浮液在水中的流变行为。我们通过局部磁共振成像流变学完成的标准流变测量表明，这些悬浮液首先似乎是触变屈服应力流体：它们表现出屈服应力随着静止时间而增加，它们在剪切下的表观粘度降低，以及粘度分叉发生在屈服应力附近，流体要么趋向停止，要么在大剪切速率下稳定流动以产生小的应力变化。然后，当我们通过在各种应力​​下的蠕变试验期间测量材料在小变形下的表观弹性模量来跟踪固体状态下材料的机械状态时，就会出现原始效果。与其他各种屈服应力流体相比，对于固体状态下的任何变形，小变形下的弹性模量似乎是恒定的，对于 CB 悬浮液，该模量广泛增加，而变形增加直至屈服。我们认为这种应变硬化效应起源于（范德华）相互作用和颗粒的（粗糙）结构（聚集体）的特殊性：由于变形而接触的颗粒的轻微相对旋转平均会，往往会增加颗粒之间的净接触面积，从而使整个材料结构变硬。观察结果支持这一点，即弹性模量的相对增加与样品变形大致成正比，无论材料特性如何（离子强度、