IntroductionThe behavior of lamellar phases under shear is a subject that has been receiving a lot of attention in the recent years [1][2][3][4]. In fact, many different shear effects have been reported: transformation from lamellar phases to Multi-lamellar vesicles, MLVs (or ''onions''), in different amphiphilic systems and followed by different techniques [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]; formation of MLVs in presence of submicron-size particles (''stuffed onions'') [21]; changes in lamellar orientation [17,22,23]; formation of intermediates with cylindrical structure between a lamellar and MLV phases [24][25][26]; reduction in lamellar spacing [27]; transitions from MLVs to unilamellar vesicles [28] and ''layering'' effects on onions [8,29]. As a practical application, these MLVs can be used, for instance, to encapsulate chemicals leading to a new kind of controlled micro-reactor [30] or as carriers for oligonucleotide delivery [31].The size distribution of shear-induced multi-lamellar surfactant vesicles depends on the applied shear rate. In general, the MLV systems are shear thinning and the average MLV radius, R, decreases as the shear rate increases [5,26,32]. For higher surfactant concentrations, the inter-bilayer spacing is independent of the MLV size [33]. Hence, a decrease in the MLV size is accompanied by an increase in the MLV number density, N/V, where N is the number of vesicles and V the total volume. The MLVs at higher surfactant concentrations are polyhedral. They fill space with a total volume fraction which is essentially unity [8,34] and N/V is approximately proportional to R )3 . Abstract We have investigated the reversibility in the shear-induced multi-lamellar vesicle (MLV) size during stepwise cycling of the shear rate by employing common rheometry, polarized light microscopy and rheo-optic techniques. We thus address the question whether there is a true MLV steady state, irrespective of history. The system studied, was the nonionic surfactant triethylene glycol decyl ether (C 10 E 3 ) with a concentration of 40 wt.% in D 2 O and a constant temperature of 25°C. It was found that the MLV size varies reversibly with varying shear rate, and hence there exists a true steady state in the presence of shear flow. The experimental observations of reversibility are however restricted to higher shear rates. Because the transformation of the size results from the shear strain, the process is very slow at lower shear rates, where the steady state cannot be reached within a reasonable experimental time.

PACS 47.57.Qk-Complex fluids and colloidal systems: Rheological aspects PACS 64.70.mj-Experimental studies of liquid crystal transitions PACS 61.30.Jf-Defects in liquid crystals Abstract-We study the rheological properties of a thermotropic liquid crystal, 8CB, in the smectic phase close to the smectic-nematic (Sm-N) transition temperature. Three different regimes were identified in the flow curves at different temperatures: i) appearance of the yield stress at low stresses, ii) power law behavior at intermediate shear stresses, and iii) Newtonian at higher shear stresses. The vanishing of the yield stress at the Sm-N transition temperature is correlated with a rapid growth of focal conic domains. The constructed dynamic phase diagram exhibits the two different smectic phases together with the flow-induced Sm-N transition.

We studied viscoelastic properties and scaling behavior of multilamellar vesicles (MLVs) confined between two parallel plates as a function of the shear rate and sample thickness (gap size between parallel plates). The rheological properties are classified into two regimes; the shear-thinning regime at high shear rates and the shear-thickening regime at low shear rates. In the former, the MLV radius results from the mechanical balance between the effective surface tension sigma(eff) and viscous stress force. The MLV radius is independent of the gap size. sigmaeff estimated by van der Linden model is 2.1+/-0.15x10(-4) Nm-1 corresponding to the same value obtained by SANS measurement. Power law exponents for the steady state viscosity and yield stress against pre-shear rate ([see text], [see text]) well agree with prediction based on the layering of membranes. Therefore, viscoelastic properties in this regime could be modeled by assuming that the dynamics of MLVs are driven by layering of MLV polydomains, which could be accompanied by the viscous dissipation, i.e., the stress relaxation on the MLV, induced by continuous sequence of yields of MLVs. The flow curve is empirically explained by the assumption of a relaxation time for the MLV shape. In the latter, however, scaling laws observed in the shear-thinning regime break down. The MLV radius increases when the gap size is reduced below the threshold value and MLV is no longer formed at very small gap sizes. Different dynamics from the shear-thinning regime seem to dominate the viscoelasticity.

We study the shear-induced lamellar/onion transformation of the polymer-grafted lamellar phase composed of nonionic surfactant and amphiphilic triblock copolymers. Increase in the mole fraction of polymer X P and the degrees of polymerization of hydrophilic chain N EO remarkably affects the shearinduced onion phase formation behavior. From the viewpoint of the defect-mediated rheology, the shear modulus G 0 is attributed to the oily streak network density. Linear increase in G 0 with preshear rate prior to the onion formation indicates that the increase in the oily streak network density is essential as a pretransition. The oily streak is composed of the focal conic domains (FCDs), which locally have inhomogeneous layer spacing. The critical shear stress s c is well scaled by the effective increment of the bending modulus theoretically predicted for the polymer-grafted membranes, Dk $ X P N 1.27 EO . Experimental results suggest that the shear-induced onion phase is achieved by reorganizing the layer orientation of FCDs with the negative Gaussian curvature in order to eliminate the inhomogeneous layer spacing.

The influence of a triblock copolymer, poly(ethylene oxide)20-b-poly(propylene oxide)70-b-poly(ethylene oxide)20 (Pluronic P123) on the phase behavior and on the shear-induced multilamellar vesicle (MLV, also called Onion) formation in the lyotropic lamellar phase of the nonionic surfactant C10E3 was investigated by means of rheology, small-angle neutron scattering (SANS), and microscopy. Added triblock copolymer shifted the Lalpha-L3 phase transition to lower temperatures. In the presence of triblock copolymer, MLV structure was not stable and easily transformed back into the lamellar phase with increasing polymer concentration and temperature. In the study of the shear-induced MLV formation, we found an increase of the critical shear rate for the onset of the shear-thickening, which also indicates the instability of MLV in the presence of the triblock copolymer. No MLV formation was observed at high polymer concentration. Suppression of the shear-induced MLV formation might be attributed to the enhancement of the effective surface tension originating from the excluded volume effect between polymers adsorbed onto the membranes.