• Large amplitude oscillatory shear (LAOS)
• Dynaimcal arrest of LAPONITE® suspensions

Large amplitude oscillatory shear (LAOS)

Among the methods of nonlinear rheological measrument, large amplitude oscillatory shear (LAOS) is unique because it separately controls the time scale (in frequency) and the deformation scale (in strain amplitude). A large number of quantitative parameters for the extent of viscoelastic nonlinearity have been proposed with successful applications on linking the structures and nonlinear rheology of various materials. A review on LAOS is available here. I also wrote a small account of its history.In my research, LAOS is used to investigate the rheological behavior of a variety of complex fluids.

1. LAPONITE® gel

LAPONITE® is a synthetic hectorite. It disperse in water as nano-sized platelets, but undergoes gelation over time. The gels can be weaken by adding poly(ethylene glycol). Under LAOS we found that the LAPONITE® gel exhibit delayed yielding.

Publication: Polymer 2011, 52, 1402-1409

2. Gelatin gels

Gelatin is a water soluble protein obtained by partial hydrolysis of collagen that is often used as substrate for cell culture. When a gelatin solution is cooled below sol-gel transition temperature, the coil molecules start to form inter-chain triple helices, which crosslinks the sample into an elastic hydrogel. We found significant strain-stiffening effect of gelatin gels under LAOS. The extent of strain-stiffening was quantified and linked to the molecular structures with the help of network models.

Publication: in preparation.

3. Jammed emulsion

Concentrated emulsions exhibited a plateau in modulus under small deformation and a yield stress at large deformation. The yielding of emulsions behaves a significant nonlinear viscoelasticity. We modified the elasticity of droplets of a water-in-oil emulsion by filling the water phase with LAPONITE®. We found that the elastic effect is present in both linear and nonlinear viscoelastic regime.

Publication: submitted to Rheol. Acta.

Dynaimcal arrest of LAPONITE® suspensions

The time-dependent evolution of LAPONITE® suspensions results in various phases depending on the clay and ionic concentration. Recently consensus has been made on the phase diagram of the clay suspension waiting for long enough time. However, the route of this ergodic-to-nonergodic evolution is still unclear. We measured the linear mechanical response during the evolution of LAPONITE® suspension with different PEG concentrations. We found two parallel universal scaling relationship in the response and aging time scale, respectively.

Universal response function (the response time scale)

The linear viscoelastic responses of the clay suspensions measured at different instants of the waiting time t_w of evolution follow a universal relaxation spectrum, reminiscent of the time — aging time superposition often found in glassy materials. In addition, although the addition of PEG slows down the evolution, it preserves the universal relaxation spectrum.

The universal master curve of relaxation spectrum is similar in shape to the dynamical mechanical response of many soft glassy materials. However, unlike conventional mechanical spectra, the master curve here reflects the far-from-equilibrium nature of the samples.

Publication: submitted to Phys. Rev. Lett.

Universal route of evolution (the waiting time scale)

The evolution of LAPONITE® suspension along the t_w time scale also follow a universal kinetic route. This universality has been checked by Y. Joshi among a large variety of conditions (Langmuir 2012, 28, 5826-5833). We measured the evolution of the structural relaxation time and showed that the universal kinetics was followed by the whole dynamical response. The kinetics was uniquely dependent on the interparticle interaction, which is modified by varying the concentration of PEG. The relationship between the interaction potential and the rate of aging was established between experiment and calculation.

Publication: J. Colloid Interf. Sci. 2012, 376, 76-82.