From paints to food products, solvent evaporation is ubiquitous and critically impacts product rheological properties. It affects Newtonian fluids by concentrating any nonvolatile components and viscoelastic materials, which harden up. In both of these cases, solvent evaporation leads to a change in the volume of the sample, which makes any rheological measurements particularly challenging with traditional shear geometries. Here, we show that the rheological properties of a sample experiencing “slow” evaporation can be monitored in a time-resolved fashion by using a zero normal force controlled protocol in a parallel-plate geometry. Solvent evaporation from the sample leads to a decrease of the normal force, which is compensated at all times by a decrease of the gap height between the plates. As a result, the sample maintains a constant contact area with the plates, despite the significant decrease of its volume. We validate the method under both oscillatory and continuous shear by accurately monitoring the viscosity of water–glycerol mixtures experiencing evaporation and a relative volume decrease as large as 70%. Moreover, we apply this protocol to drying suspensions. Specifically, we monitor a dispersion of charged silica nanoparticles undergoing a glass transition induced by evaporation. While the decrease in gap height provides a direct estimate of the increasing particle volume fraction, oscillatory and continuous shear measurements allow us to monitor the evolving viscoelastic properties of the suspension in real time. Overall, our study shows that a zero normal force protocol provides a simple approach to bulk and time-resolved rheological characterization for systems experiencing slow volume variations.

中文翻译：

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干燥液体和悬浮液的时间分辨流变

从油漆到食品，溶剂的蒸发无处不在，并严重影响产品的流变性。它会浓缩所有硬化的不挥发成分和粘弹性材料，从而影响牛顿流体。在这两种情况下，溶剂的蒸发都会导致样品体积的变化，这使得任何流变学测量对于传统的剪切几何结构都特别具有挑战性。在这里，我们表明可以通过在平行板几何形状中使用零法向力控制协议，以时间分辨的方式监测经历“缓慢”蒸发的样品的流变特性。样品中溶剂的蒸发导致法向力的减小，而法向力的减小始终可通过减小板之间的间隙高度来补偿。因此，尽管样品的体积显着减小，但样品与板的接触面积保持恒定。我们通过精确监测经历蒸发的水-甘油混合物的粘度和最大70％的相对体积下降，来验证该方法在振荡剪切和连续剪切下的有效性。此外，我们将此协议应用于干燥悬浮液。具体而言，我们监测带电的二氧化硅纳米粒子的分散体，该分散体经历了蒸发引起的玻璃化转变。间隙高度的减小可以直接估计增加的颗粒体积分数，而振荡和连续剪切测量使我们能够实时监控悬浮液的不断发展的粘弹性质。全面的，