In this work, experimental observations of the microstructure of neutralized polyacrylic acid (Carbopol) in water by confocal microscopy under both static and flow conditions are presented. In the former case, a Carbopol-rich phase made by swollen particles dispersed in a water-rich continuous phase is found, so that the system will be henceforth referred to as a suspension, as long as particles are observed. The swollen particles form dendritic-like aggregates, which span the entire solution volume above a critical concentration. In such conditions, a percolated network can be formed, leading to the onset of a yield stress behavior. By separating the dispersed and continuous phase through centrifugation, we provide evidence of a miscibility gap in the phase behavior of Carbopol in water. When the Carbopol suspensions flow in a microfluidic capillary, a particle-concentrated plug core can be distinguished from a less concentrated layer corresponding to a steep velocity decrease. Confocal imaging also shows that the apparent slip found in Carbopol suspensions is due to a particle-concentrated near-wall region, where no flow is observed. Such flow-induced microstructure is responsible for the different nature of the yield stress values measured by classical rheometry and by flow velocimetry. While the yield stress measured by the former can be here related to the presence of a percolated network, the yield stress obtained from the velocity profile is due to the heterogeneous particle distribution along the capillary radius. These results provide a novel insight on the mechanisms governing yield stress in complex fluids.

在这项工作中，通过共聚焦显微镜在静态和流动条件下，对水中中和的聚丙烯酸（Carbopol）的微观结构进行了实验观察。在前一种情况下，发现了由分散在富水连续相中的溶胀颗粒制成的富Carbopol相，因此，只要观察到颗粒，该系统便称为悬浮液。溶胀的颗粒形成树突状聚集体，其跨越整个溶液体积超过临界浓度。在这种情况下，会形成渗滤网络，导致屈服应力行为的开始。通过离心分离分散相和连续相，我们提供了Carbopol在水中的相行为的混溶性缺口的证据。当Carbopol悬浮液在微流体毛细管中流动时，可以将颗粒集中的塞芯与浓度较低的层（对应于急剧的速度下降）区分开。共聚焦成像还表明，在Carbopol悬浮液中发现的明显滑移是由于颗粒集中的近壁区域所致，在该区域没有观察到流动。这种由流动引起的微观结构是由经典流变法和流动测速法测得的屈服应力值不同性质的原因。尽管前者测量的屈服应力在此处可能与渗透网络的存在有关，但从速度曲线获得的屈服应力是由于沿毛细管半径分布的异质颗粒引起的。这些结果为控制复杂流体屈服应力的机理提供了新颖的见解。可以将颗粒集中的塞子芯与相应于陡峭的速度下降的较少集中的层区分开。共聚焦成像还表明，在Carbopol悬浮液中发现的表观滑移是由于颗粒集中的近壁区域所致，在该区域没有观察到流动。这种由流动引起的微观结构是由经典流变法和流动测速法测得的屈服应力值不同性质的原因。尽管前者测量的屈服应力在此处可能与渗透网络的存在有关，但从速度曲线获得的屈服应力是由于沿毛细管半径分布的异质颗粒引起的。这些结果为控制复杂流体屈服应力的机理提供了新颖的见解。可以将颗粒集中的塞子芯与相应于陡峭的速度下降的较少集中的层区分开。共聚焦成像还表明，在Carbopol悬浮液中发现的表观滑移是由于颗粒集中的近壁区域所致，在该区域没有观察到流动。这种由流动引起的微观结构是由经典流变法和流动测速法测得的屈服应力值不同性质的原因。尽管前者测量的屈服应力在此处可能与渗透网络的存在有关，但从速度曲线获得的屈服应力是由于沿毛细管半径分布的异质颗粒引起的。这些结果为控制复杂流体屈服应力的机理提供了新颖的见解。