Phase transition, shear-induced

A. Shear-induced phase transitions in confined fluids... 

Besides shear-induced phase transitions, Uquid-gas equilibria in confined phases have been extensively studied in recent years, both experimentally [149-155] and theoretically [156-163]. For example, using a volumetric technique, Thommes et al. [149,150] have measured the excess coverage T of SF in controlled pore glasses (CPG) as a function of T along subcritical isochoric paths in bulk SF. The experimental apparatus, fully described in Ref. 149, consists of a reference cell filled with pure SF and a sorption cell containing the adsorbent in thermodynamic equilibrium with bulk SF gas at a given initial temperature T,- of the fluid in both cells. The pressure P in the reference cell and the pressure difference AP between sorption and reference cell are measured. The density of (pure) SF at T, is calculated from P via an equation of state. 

Escalante J, Gradzielski M, Mortensen K, Hoffmann H (2000) The shear induced transition of an originally undisturbed lamellar phase to a vesicle phase. Langmuir 16(23) 8653... 

The existence of a second class of complex phases, the modulated and perforated layer structures, has largely been explored by Bates and co-workers (Forster et al. 1994 Hamley et al. 1993, 1994 Khandpur et al. 1995 Schulz et al. 1996), who used SANS and TEM to investigate shear oriented structures. The thermally-induced phase transition from the lam to the hex phase in polyolefin diblocks was studied in detail by Hamley et al. (1993, 1994) using SANS, TEM and rheology. Intermediate hexagonal modulated lamellar (HML) and hexagonal perforated layer (HPL) structures were observed on heating PEP-PEE, PE-PEP and PE-PEE diblocks, where PEP is poly(ethylene-propylene), PEE is... 

V. M. Sadtler, M. Guely, P. Marchal, and L. Choplin, Shear-induced phase transitions in sucrose ester surfactant,./ Colloid Interface Sci., 270 (2004) 270-275. 

One sees in Figure 15.18 also that this spontaneous snap-off only occurs at lower flow rates. At higher flow rates, the continuous phase does not have the time to replace the oil. The droplets now become larger, and will be dragged away by the cross-flow. Also here, one sees the transition from interfacial tension-induced spontaneous snap-off towards shear-induced snap-off by the cross-flow over the membrane. 

Within the fluctuation-dominated region of the disordered phase of a diblock near Todt shearing can apparently induce a transition to the ordered state (Koppi et al. 1993). Cates and Milner (1989) predicted such a phenomenon, based on a shear-induced suppression of... 

Two types of structure are induced in a fluid adjacent to a flat solid surface layering normal to the surfiice and epitaxial order in the plane of the surface. When a fluid film is confined between two solid surfaces at small separation, both types of order may span the entire film. Ejqjerimental consequences are oscillations in the normal force with film thickness, and phase transitions to solid states that resist shear forces. These two types of ordering give rise to changes in the dynamic response of films to an imposed shear velocity. 

Polymorphic transitions are known to be induced by an external field such as tensile stress, hydrostatic pressure or shearing stress, high electric field and others. Crystalline phase transitions under tensile stress are discussed in Chapter 6 in relation with the observed changes in microhardness. 

Qi and Wang also observed a transient undulated HEX state during the melting of the HEX phase to the disordered state. This is consistent with the experimental observation in the shear cessation experiments of Bates and co-workers (Bates et al., 1994a Almdal et al., 1996). In these experiments, an initially disordered phase of the asymmetric poly(ethylenepropylene-b-ethylethylene) (PEP-PEE) diblock copolymer close to the order-disorder boundary is subjected to a steady shear which induces a transition to the HEX phase. The shear is then suddenly stopped and a transient modulated state is observed as the system relaxes back to the DIS. Insofar as a HEX... 

The behavior and characteristics of confined fluids is more complex than that of bulk liquids or of simple solvated systems described in the chapters mentioned above. One must consider the complexities of the interface between confining walls and the fluid along with confinement-induced phase transitions, critical points, the stratification of the fluid near the confining walls, the idea that confined fluids may sustain certain shear stress without exhibiting structural features normally associated with solid-like phases, and... 

Small molecular mass liquid crystals do not respond to extension and shear stress. Liquid crystalline polymers may exhibit a high elastic state at some temperature due to the entanglements. However, the liquid crystalline network itself is an elastomer, showing rubber elasticity. In the presence of external stress, liquid crystalline networks deform remarkably and then relax back after the release of stress. The elasticity of liquid crystalline networks is more complicated than the conventional network, such as the stress induced phase transition, the discontinuous stress-strain relationship and the non-linear stress optical effect, etc. 

The response of heterogeneous systems to a stress field allows them to be placed in two categories (i) those in which stress induces irreversible changes e.g., precipitation, denaturation of protein, crystallization, etc.) and (ii) those in which the changes are reversible. The classification is not perfect, as the type and magnitude of stress field can be cmcial, but it provides a guide in most cases, miscibility in systems (i) is reduced by stress, while in systems (ii) it is increased. In other words, if a system can be irreversibly modified by rheological means, its solubility will be reduced. An excellent review on phase transition in shear flow was recently published [Onuki, 1997]. 

Rehage, H., Hoffmann, H., and Wunderlich, L, A rheological switch shear induced phase transitions in aqueous surfactant solutions, Ber. Bunsenges., 90, 1071-1075 (1986). 

Mendes, E. Narayanan, J. Oda, R. Kem. F. Candau, S.J. Manohar, C. Shear-induced vesicle-to-wormlike micelle transition. J. Phys. Chem.,B 1997,101, 2256-2258. Gustafsson. J. Oradd, G. Nyden, M. Hansson, P. Almgren, M. Defective lamellar phases and micellar polymorphism in mixtures of glycerol monooleate and CTAB in aqueous solution. Langmuir 1998, 14. 4987-4996, and references therein. 

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