| Colloidal particles have a larger size compared with molecules (several nanometer toseveral micron scale) and the interaction between the particles can be adjusted in a wide range,so it is widely used as a model experimental system to study the fundamental problems incondensed matter physics, such as gelation and glass transition, and so on. In the present work,three colloidal model systems with a rather clearly structure were used, including hectoriteclay-filled water-in-oil emulsion, temperature-sensitive Poly(N-isopropylacrylamide)(PNIPAm) microgel suspension and aqueous dispersion of hectorite clay. The basic idea of theresearch work were: firstly, we selected the appropriate method to prepare three colloidalmodel system. Later, a series of parameters, such as the mass percentage of colloidal particles,temperature, the mole concentration of inorganic electrolytes NaCl and the mass percentageof polyethylene glycol (poly (ethylene glycol), PEG) were modified to modulate theinteraction U(h) between colloidal particles. Lastly, the glass transition and physical gelationprocess in water-in-oil emulsion, PNIPAm microgel suspension and aqueous dispersion ofhectorite clay with different composition were studied by oscillatory shear rheology, in orderto relate the linear and nonlinear viscoelasticity behavior with the microscopic structure of thesample. The main works and results were as following:1. Two types of water-in-oil emulsion: E (pure water as droplet phase) and E-L2S6(asuspension of hectorite clay as droplet phase, L2S6) with different water-to-oil phase volumeratio were prepared and the effect of droplet elasticity on the linear and nonlinearviscoelasticity of water-in-oil emulsions was investigated by small amplitude oscillatory shear(SAOS) and large amplitude oscillatory shear (LAOS) rheological methods. The microscopicmorphology of the emulsions was observed with an optical microscope. The emulsiondroplets changed from isolated dispersed state to the jammed state with increasingwater-to-oil phase volume ratio was observed. SAOS frequency sweep experiments found thateffect of droplet elasticity was most significant in jammed state, where the clay-filledemulsion E-L2S6showed a much higher dynamic storage modulus G’ than the clay-free one.Water-in-oil emulsion in jammed state is a typical yield stress fluid, which showed a typicallyrectangular Lissajous curve under LAOS flow field. The nonlinear viscoelastic understationary flow state was measured by In/1and quantitative Lissajous curve parameters GM, GLand GK.The droplet elasticity was found to contribute to the nonlinear viscoelasticity ofwater-in-oil emulsions in jammed state.2. A series of PNIPAm microgel suspensions with different particle mass percentage wwere prepared by surfactant-free emulsion polymerization. The jamming and unjammingtransition in thermo-sensitive PNIPAm microgel suspensions was observed by rheology with three independent variables of temperature T, microgel weight fraction w, and the shear stressσ. The effective volume fraction φeff, depending on both T and w, was adopted to interpret thistransition. In the jamming state, the plateau modulus G’pmeasured by SAOS frequencysweep tests behaved the same power law to φeffwith varying T and w, indicating that thechange of interaction potential with temperature was insignificant. SAOS temperatures testsshowed that the critical temperature Tcincreased with increasing w but the critical volumefraction cdecreased with increasing w at the jamming transition because higherconcentration of particles lead to strong repulsive interaction. The shear-unjamming transitionoccurred as the shear stress exceeded the yield stress σy. A jamming phase diagram for themicrogel suspensions was established in a (1/w, σ, T) coordinate system.3. The aqueous dispersion of cw=2.9~3.5wt%hectorite clay were prepared. Theconductivity and pH values tests at different temperatures combined with calculationdemonstrated that the nature of aging aqueous dispersion of hectorite clay was RLCAcolloidal gel. With constant preshear condition, the higher the temperature of the sample, thefaster the rate of gelation was found. The G’, G’’ versus twcurves from samples with differenttemperature could be shifted into a mastercurve (G’, G’’ versus tw/at), the shift factor reflectingthe effect of temperature on the gelation rate. Therefore, increase in temperature couldaccelerate the physical gelation process, but couldn’t change the kinetics route of gelation.Considering the van der Waals attraction and electric double layer repulsion interactionbetween Laponite particle, a quantitative relationship between the shift factor aTand theinterparticle interaction potential U(h) was found with the help of the colloidal coagulationkinetic theory. We investigated the effect of temperature on the linear and nonlinearviscoelastic of hectorite clay gel samples by SAOS frequency sweep and LAOS strain sweepteasts. The results showed that the higher the temperature, the larger the linear elastic modulusG’, the higher the strength of “overshoot peak of strain” under yielding and the larger theyield stress value.4. Aqueous dispersion of hectorite clay with different hectorite clay mass percentageand NaCl concentration were prepared. G’, G’’ versus frequency ω results of the same sampleat different observation time during the gelation process was obtained by SAOS time-resolvedviscoelastic spectrum technique. Results of different times can be shifted along the frequencyaxis into a mastercurve (G’, G’’ versus at at). The shifting factor atreflects the dependence ofrelaxation time τ of the sample on observation time twin the gelation process. For sampleL1Scs, the G’, G’’ versus twcurves from samples of different NaCl concentration could beshifted into a mastercurve (G’, G’’ versus tw/at), that is,“aging time-salt concentrationsuperposition” was established, the shift factor reflecting the effect of salt contration on the gelation rate. Therefore, increase in NaCl concentration could accelerate the gelation process,but couldn’t change the kinetics route of gelation. For sample L3.5, results of different agingtimes can be shifted along the frequency axis into a mastercurve. It shows a two-step agingfeature, indicating that the structural relaxation dynamics for aqueous dispersion of hectoriteclay gradually slower with increasing tw, and the physical gelation process could be dividedinto two stages: In the first step, a colloidal gel was formed and the relaxation time increaseexponentially with observation time; in the second step, the gel network structure graduallyimproved and the relaxation time increase with observation time in a power-law form.5. The aqueous dispersion of3.0wt%hectorite clay with different PEG masspercentages were prepared. The effect of mass percentage of PEG on the linear and nonlinearviscoelasticity for hectorite clay/PEG dispersion was investigated by steady state and dynamicshear rheological methods. The re-entrant gelation behavior due to depletion force from freePEG chains was firstly observed in hectorite clay/PEG dispersion system by SAOS frequencysweep tests. Steady shear viscosity and LAOS dynamic moduli results suggested that PEGmass percentage had a similar effect on the yield behavior of hectorite clay/PEG dispersionsystem. |
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