Microrheology has been a hotspot in rheology research for the past 20 years, and researchers at the forefront are also increasingly interested in rheological characterization. Different from macrorheology, microrheology starts from the microstructure of the sample and obtains the viscoelastic parameters of the system by tracking the movement of colloidal tracer particles in the dispersed system. The characteristic of micro-rheology is that it can characterize the viscoelasticity of very low viscosity samples (such as polymer solutions or protein solutions), and the measurement range and sensitivity far exceed the reach of the most sophisticated modern mechanical rheometer. One of the most important advantages of micro-rheological technology is that it can measure and characterize the viscoelasticity of fluids with weak structures. In addition, the amount of sample required is very small, the operation is simpler, and the change process of viscoelasticity can be continuously tracked.
1.1 The movement of particles in a dispersed system
The Brownian motion of particles is a disordered motion. Microrheology studies its statistical distribution law. The root mean square displacement (MSD=Mean Square Displacement) is used to describe the motion of particles, that is, the area of particle motion per unit time. This can eliminate the irregularities in the movement of particles.
For a purely viscous non-Newtonian fluid, the particle movement is free, and the root mean square displacement MSD increases linearly with the increase of the particle movement time, as shown in Figure 1. The particles in the viscoelastic non-Newtonian fluid will be blocked and entangled by the macromolecular framework after moving for a certain distance. As time continues to increase, they break through the entanglement and enter the macroscopic solvent at the relaxation time TR, and continue to move to expand the root mean square area. Therefore, the plateau area of the root mean square displacement curve shows the elasticity of the sample, and the slope area after TR shows the viscosity of the sample.
Results and discussion
MSD curve analysis of viscoelastic equilibrium state
The above figure is a comparison of the root mean square displacement curves of three different formulations of high-concentration dispersion systems in viscoelastic equilibrium. 1# is red, 2# is blue, and 3# is yellow. First, all three samples have obvious elastic platforms, all of which are viscoelastic fluids. The elastic platform area of sample 3# (yellow) is closer to the bottom, indicating that the microstructure is more elastic, and the viscous slope area of sample 2# (blue) is more to the right, indicating that its viscosity is strongest.
The viscosity factor and relaxation time changes of the three samples in the static state were analyzed, and the stability of each drilling fluid was sorted by monitoring the rheology of each drilling fluid with the static time. The viscosity factor is calculated from the reciprocal of the slope of the viscosity zone of the root mean square displacement MSD curve, which can intuitively reflect the change process of drilling fluid viscosity
It can be seen from the above that the viscosity of the three samples continued to increase due to the structural recovery process and gradually reached a viscosity balance during the initial time, but the viscosity of the 1# sample (red) began to decrease after 2.5h, and the 2# sample (blue) after 22h The decrease in viscosity indicates that the structure has also collapsed. The 3# sample (black) is the most stable, and the viscosity has not decreased within the 30h test time.
The high-molecular polymer in the high-concentration dispersion system can form a network structure for the skeleton of suspended particles. When the dispersed system is stirred, the network structure is broken up, the structure begins to recover when it is left standing, and the viscosity gradually increases. The poorly stable dispersion system begins to lose stability after being left for a period of time, which may cause serious particle settling problems.
Microrheology can accurately describe the viscoelastic change process of a high-concentration dispersion system, which is of great significance for further research and characterization of the complex end-use performance of the dispersion system