At a certain shear rate, the shear stress of the thixotrope decreases with time, and the viscosity of the system decreases with time.
The generation of thixotropy is related to a certain internal structure existing in the system. When agitated or shaken, the structure between the particles may be destroyed. But the shearing action stops, and its internal structure will resume again. Of course, the destruction of the structure requires a certain amount of energy, and it will only occur after the action of the external force exceeds the structural destruction energy. So its flow curve often does not start from the origin.
The rheological curve of the thixotropic system is roughly the same as that of the pseudoplastic fluid, but a hysteresis ring appears on the rheological curve, and the change of the shear stress recorded when the shear rate is increased from low to high and then gradually decreased from high to high does not follow the original trend. traces of. Due to the effect of time, the shear stress on the return is less.
Figure 2-21 Time-dependent rheological curve of fluid flow during thixotropic flow
The thixotropy of the system can be measured with a drum viscometer. It includes two methods, one is to continuously increase the rotating speed w of the rotary (equivalent to increasing the shear rate), and at the same time measure the torque M of the torsion wire (related to the shear stress), and then make an upward flow curve (in the figure ABC). At this time, due to the continuous increase of the rotational speed, the re-formation speed of the particle structure always lags behind the destruction speed of the structure, and the system is in a state of non-equilibrium. When it reaches a certain maximum speed, then gradually reduce the speed, because the damaged structure has no time to re-form, and the shear stress is only used to make the material system flow, so the downward line CA declines in a straight line, thus forming The ABCA hysteresis circle shown in 21a is a shear rate hysteresis circle; the other is to maintain a constant shear rate after reaching the maximum rotational speed point C, then the required shear stress will decrease along the CD line to reach the equilibrium value D, Then reduce the shear rate to get the downward line DA, which is the hysteresis circle ABCDA obtained in the equilibrium state, also known as the time lag circle.
