Dynamics Of Colloidal Suspensions Under External Fields By Particle Tracking

Dynamic heterogeneities exist in complex fluids,as well as in colloidal suspensions.In industrial applications,colloidal suspensions often bear external fields,which makes the observation and characterazation of dynamic heterogeneities more difficult.As a tool with high resolution both on space and time,particle tracking microrheology has capacities to detect dynamic heterogeneities,so far it is still in development.Microrheology measures rheological properties based on generalized Stokes–Einstein relationship,which breaks down in nonequilibrium,inhomogeneous systems.Evaporating sessile colloidal suspension droplet is a typical dynamic system with spherical particles inside,?-FeOOH nanorods suspensions with laser-thermo induced flow field is a typical system with rod-like particles inside.In this thesis,we discussed the effect of external fields on colloidal suspensions dynamic behavior and explored the rules in dynamic microstructure of colloidal suspensions under external fields.The present work was divided as following parts.1.We set up observables and analytical methods to study the dynamic behavior in colloidal suspensions bear external fields.Observables include forward-moving factor F and reduced mean square radius of gyration g.Analytical methods include polar coordinate tranforming and returning all particles polar angles to zero,multi-mode Gaussian fitting.2.We used the fluorescent particles as probe particles in particle tracking experiments to study drying sessile droplets.Evaporating suspensions sessile drop were observed on glasses under microscope,probe particles moved freely and few particles was motionless in salts-free suspension,oppositely,numbers of motionless particles were found in suspensions with salts.After proposed forward-moving factor F,we divided the particles behaviors into two parts: F > 0.25 particles moved more and more directional in salts-free suspensions;But particles with F < 0.25 became more and more in suspensions with salts.Further analysis indicated that the introduction of salts compress double electical layer of particles and substrates,the particles with F < 0.25 were actually static.3.At low salt concentrations,the velocity of mobile particles increses with the evaporation time to a peak and then decreases.The velocity is lower for higher salt concentrations.The reason for the salt-dependence of the mobile particles may be the joint effects of both the flow field and the substrate.Considering the dimensionless mean square radius of gyration (?) with F,there are five cases for the behavior of particles.The corresponding position of all particle trajectories on the F–(?) coordinates distinguish separately the effects of the flow field and the substrate friction forces.Then we found most mobile particles were free from substrate friction.4.As most mobile particles were free from substrate friction,the results of present study suggest that the effects of added salts reducing the electro-kinetic effect and hence the radial velocity of the mobile particles.5.?-FeOOH nanorods were prepared at room temperature and characterized by TEM,XRD,polarized light microscope and physical property measurement system.In particle tracking experiments,?-FeOOH suspensions volume fractions 10 vol%,9 vol% in concentrated regime and 8 vol%,7 vol% in semidilute regime were chosen with respect to Doi–Edwards theory.The probe particles moved radially in ROI due to the exciting light induced thermal convection flow field.We found the MSD of particles increased with lower volume fraction of ?-FeOOH suspensions but decreased in different level according to different volume fraction when magnetic field was applied.We suggest that probe particles were confined by ?-FeOOH nanorods surrounded.6.We transformed the particle motions from radial motion to one dimensional random walk with directional motion superposition.After calculating the forward-moving factors and van Hove correlation of particles,we divided the particles motion into static particles and mobile particles.These results indicated the confinement of probe particles came from the decrease of rotational diffusive coefficient induced by higher ?-FeOOH nanorods volume fraction or applied magnetic field.Moreover,we used double-mode normal distribution fit the one dimension displacement probability density distribution of probe particle and extracted the flow field velocities and viscocities of ?-FeOOH nanorods suspensions.