| In this paper,a passive microrheological experiment system based on inverted microscopic platform was built.The fluorescent microsphere with a particle size of 0.49?m was selected as the probe particle,and the continuous laser(power of1.5W,wavelength of 532nm)was used as the light source.The particle images were collected by EMCCD camera.The single particle tracking technique was employed to measure the Brownian motions of the probe particles.On the basis of measuring the Brownian motions of the probe particles,the passive microrheological experiments of Polyethylene oxide(PEO)solutions in free and confined spaces were carried out.Firstly,the Brownian motions of probe particles in pure water was measured at room temperature(20?)to verify the reliability of the experimental system and measurement method.In the experiments,the Brownian motion images of the probe particles were collected for 500 consecutive frames,and the Brownian motion trajectories of the particles were obtained by the particle tracking software,and the mean square displacements(MSDs)of the probe particles were calculated,showing a good linear relationship with time.The experimental value of particle diffusion coefficient is 0.8813?m2s-1,and the relative deviation from the theoretical value of particle diffusion coefficient(0.8527?m2s-1)based on Stokes-Einstein relation(SER)is 3.24%.The experimental result shows that the experimental system and particle Brownian motion measurement method have high measurement accuracy and reliability,which lays an important foundation for the passive microrheological experiments of PEO solutions.Next,the passive microrheology experimental studies of PEO solutions in free space were carried out.The microrheological properties of PEO solutions with mass concentrations of 0.4wt%,0.6wt%,0.8wt% and 1.0wt% at different temperatures such as 25 oC,35oC and 45 oC were measured respectively.On the basis of tracking the Brownian motions of the probe particles,the MSDs of the particles in PEO solutions at different concentrations and temperatures were calculated.The frequency domain distributions of viscoelastic modulus of PEO solutions under different conditions were obtained by using generalized Stokes-Einstein relationship(GSER)and Fourier transform.The results show that the confined trend of Brownian motions of the probe particles increases with the increase of the concentration of the PEO solution,and the Brownian motions speed increase with the increase of the temperature.Taking the 1.0wt% PEO solution as an example,we can see that the MSD of the probe particles at 25? is only 2.17 ?m2,which is the minimum value under all operating conditions,indicating that the particle movement is the most confined by the network structure formed by PEO chain molecules under this operating condition.When the temperature rises to 45?,the MSD of the probe particles reaches 21.23 ?m2,and the Brownian motion is significantly enhanced.The calculation results of viscoelastic modulus show that the viscoelastic modulus G"(?)is dominant while the elastic modulus G'(?)is weak under the existing concentration conditions.Both of them show an increasing law with the increase of frequency,and a decreasing trend with the increase of temperature,and the decrease rate of elastic modulus is greater than that of viscous modulus.Still taking the concentration of1.0wt% PEO solution as an example,we can see that the maximum average value of the viscosity modulus of the solution at 25? is 0.5499 Pa,and that of the elastic modulus is 0.1582 Pa.Although the viscous modulus is still dominant,the elastic modulus and the viscous modulus values are at the same level,and the microscopic elastic characteristics of the solution begin to be significant;when the temperature is increased to 45?,the maximum average value of its viscous modulus and elastic modulus are reduced to 0.3055 Pa and 0.0746 Pa,respectively,and the microscopic elastic characteristics are weakened.The standard deviation analysis of MSD shows that the measurement error at the same concentration increases with the increase of the particle tracking time,and also shows an increasing trend with the increase of temperature.Finally,the passive microrheological experiments of PEO solutions in the confined space of porous media were carried out.Three kinds of quantitative filter papers with different pore sizes were selected as porous media materials.Based on the single particle tracking technique,the microrheological properties of PEO solutions with mass concentrations of 0.4wt% and 0.6wt% in filter paper porous media were measured and the MSDs were calculated.Accordingly,the abnormal diffusion laws and the microscopic viscoelastic properties were studied.The experimental results show that the Brownian motions of the probe particles in this confined spaces have obvious directionality,and the directionality weakens as the filter paper pore size decreases;The power exponents of particle Brownian diffusions in the filter paper experiments are 0 <? <1,showing the anomalous diffusion characteristics similar to the behavior of super-diffusion.The calculation results of the viscoelastic modulus show that the PEO solutions in the confined space are still dominated by the viscous modulus;at the same concentration,the microscopic elastic modulus G'(?)of the PEO solution increases with the decrease of the pore size,and the pore size has little effect on the value of its viscosity modulus G"(?).Taking 0.6wt% PEO solution as an example,we can see that the maximum average modulus of elasticity in the experiments of fast,medium and slow pore size filter paper are 0.0272 Pa,0.0821 Pa and 0.1567 Pa,respectively,showing a clearly increasing trend,while the maximum average value of its viscosity modulus is0.2976 Pa,0.2694 Pa and 0.2989 Pa,respectively,without clear trend.In addition,it is found that the presence of paper fibers enhances the local microelasticity of the PEO solution,but has no significant effect on the viscous modulus by comparing with the free space experimental results. |
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