Application Of Oscillatory Shear Research Articles

The Formation of a 12‐Hydroxystearic Acid/Vegetable Oil Organogel Under Shear and Thermal Fields

AbstractExternal laminar oscillatory shear applied during crystallization in combination with different temperature fields was used to modify the microstructure and physical properties of edible oil organogels. Crystallization at a high cooling rate (30 °C/min) resulted in a spherulitic microstructure with a higher oil‐binding capacity, lower storage modulus and lower yield stress compared with a material (with a fibrillar microstructure) crystallized at a slow cooling rate (1 °C/min). The application of an oscillatory shear resulted in the formation of novel microstructures depending on the cooling regime used. The application of an oscillatory shear (strain > 500 % and frequency = 1 Hz) resulted in the thickening of fibers observed in the slow‐cooled material and an increased incidence of spherulite nucleation in the rapidly cooled material. Increasing the frequency of the oscillatory shear applied did not change the microstructure for the slow‐cooled gel but further increased the incidence of nucleation for the rapidly cooled gel. The application of controlled‐strain oscillatory shear to the crystallizing gel at either cooling rates resulted in an oily and very soft, paste‐like material. This material had a lower storage modulus and poorer oil‐binding capacity compared with the same gel crystallized statically. Reduction of the oscillatory strain from a maximum of 1500 to 500 % moderately mitigated the loss of mechanical properties and oil‐binding capacity although these properties were in no way comparable to those obtained from static crystallization. The study shows that the application of oscillatory shear and different cooling regimes can be used to tailor a crystalline organogel. However, the application of continuous shear must be done with care as application of excessive shear can result in a complete breakdown in gel structure and large amounts of oil loss.

Ordered Phases in Aqueous Solutions of Diblock Oxyethylene/Oxybutylene Copolymers Investigated by Simultaneous Small-Angle X-ray Scattering and Rheology

The phase behavior of gels of E40B10 in 0.2 mol dm-3 aqueous K2SO4 was studied as a function of temperature and concentration. E40B10 is a diblock copolymer of poly(oxyethylene) (E) and poly(oxybutylene) (B), where the subscripts denote the number of repeats. The phase of the material was characterized by both simultaneous rheology and small-angle X-ray scattering, (SAXS). Depending on polymer volume fraction in the range 23−38% a body-centered cubic (bcc) structure or a face-centered cubic (fcc) structure was observed at low temperature, and at high temperature a hexagonally packed rod structure was formed. The phase transitions were shown to be characterized by discontinuous changes in the values of the dynamic shear moduli. A bcc−fcc transition was observed at high concentration, the corresponding transition temperature increasing with increasing polymer concentration. The effects of reciprocating shear were shown to increase the degree of order, manifested as a sharpening of the diffraction peaks in the SAXS pattern. The dynamic moduli decreased rapidly on the application of oscillatory shear and recovered equally rapidly when the deformation ceased. The decrease in moduli was shown, via the SAXS patterns acquired simultaneously, to be correlated to structural changes within the gel.

Lyotropic Liquid Crystals Under Simple Couette and Oscillatory Shear

Small angle neutron scattering (SANS) has been utilized to investigate the flow properties of the discotic micellar isotropic (I), nematic (N) and lamellar (L α) phases in aqueous solutions of cesium perfluoro-octanate. Under simple shear, no preferred orientation was observed in the I phase. The N phase aligned with the nematic director along the shear gradient velocity, providing the first evidence for a positive value of the viscosity parameter, α2. The L α phase oriented with lamellae parallel to the shear plane. A reorientation of the director near the N-L α transition was witnessed upon cooling from the N phase at a constant shear rate and is attributed, primarily to a change in the value of α2 as a result of increasing lamellar correlations in the nematic phase. The application of oscillatory shear induced significant shifts in the phase boundaries, including nematic-like order some 7 K into the quiescent I phase.

Equilibrium and Shear Induced Nonequilibrium Phase Behavior of PMMA Microgel Spheres

Polymethylmethacrylate (PMMA) spheres suspended in benzyl alcohol are found to swell to more than two times that of their dry radius and have been observed to undergo an equilibrium phase transition from liquid to crystalline structure with increasing concentration. The width of the coexistence region is found to be narrower by nearly half compared to simulation results for hard sphere systems. Comparison of the measured freezing point and fractional density change upon melting with those calculated from soft sphere simulations are consistent with a purely repulsive interparticle potential on the order of 1/r20. Analysis of powder pattern scattering profiles from samples in the crystallized region of the equilibrium phase diagram indicates crystallites made up of a registered random stacking of hexagonal close packed planes, similar to that found in monodisperse suspensions of hard spheres. With the application of oscillatory shear, nonequilibrium microstructures similar to those found in model hard sphere systems have been observed in these suspensions.