Influence of Shear on Lyotropic Lamellar Phases with Different Membrane Defects

Small-angle X-ray and neutron scattering applied to lipid-based nanoparticles: Recent advancements across different length scales

Lipid-based nanoparticles (LNPs), ranging from nanovesicles to non-lamellar assemblies, have gained significant attention in recent years, as versatile carriers for delivering drugs, vaccines, and nutrients. Small-angle scattering methods, employing X-rays (SAXS) or neutrons (SANS), represent unique tools to unveil structure, dynamics, and interactions of such particles on different length scales, spanning from the nano to the molecular scale. This review explores the state-of-the-art on scattering methods applied to unveil the structure of lipid-based nanoparticles and their interactions with drugs and bioactive molecules, to inform their rational design and formulation for medical applications. We will focus on complementary information accessible with X-rays or neutrons, ranging from insights on the structure and colloidal processes at a nanoscale level (SAXS) to details on the lipid organization and molecular interactions of LNPs (SANS). In addition, we will review new opportunities offered by Time-resolved (TR)-SAXS and -SANS for the investigation of dynamic processes involving LNPs. These span from real-time monitoring of LNPs structural evolution in response to endogenous or external stimuli (TR-SANS), to the investigation of the kinetics of lipid diffusion and exchange upon interaction with biomolecules (TR-SANS). Finally, we will spotlight novel combinations of SAXS and SANS with complementary on-line techniques, recently enabled at Large Scale Facilities for X-rays and neutrons. This emerging technology enables synchronized multi-method investigation, offering exciting opportunities for the simultaneous characterization of the structure and chemical or mechanical properties of LNPs.

Shear-Induced Ordering of Nanopores and Instabilities in Concentrated Surfactant Mesh Phases

Mixed surfactant systems with strongly bound counterions show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nanopores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with a high degree of coherence between nanopores in three dimensions. In situ small-angle synchrotron X-ray study under different shear stress conditions shows sharp Bragg peaks in the X-ray diffraction, successfully indexed to the rhombohedral lattice with R3̅m space group symmetry. The ordered mesh phase shows isomorphic twinning and buckling at higher shear stress. Our experimental studies bring out rich non-equilibrium phase transitions in concentrated cationic surfactant systems with strongly bound counterions hitherto not well explored and provide motivation for a quantitative understanding.

Hydrophobe Containing Polypeptoids Complex with Lipids and Induce Fusogenesis of Lipid Vesicles

The hydrophobic effect of alkyl group insertion into phospholipid bilayers is exploited in modifying and modulating vesicle structure. We show that amphiphilic polypeptoids (peptide mimics) with n-decyl side chains, which we term as hydrophobe-containing polypeptoids (HCPs), can insert the alkyl hydrophobes into the membrane bilayer of phospholipid-based vesicles. Such insertion leads to disruption of the liposomes and the formation of HCP-lipid complexes that are colloidally stable in aqueous solution. Interestingly, when these complexes are added to fresh liposomes, remnant uncomplexed hydrophobes (the n-decyl groups) bridge liposomes and fuse them. The fusion leads to the engulfing of liposomes and the formation of multilayered vesicles. The morphology of the liposome system can be changed from stopping fusion and forming clustered vesicles to the continued formation of multilayered liposomes simply by controlling the amount of the HCP-lipid complex added. The entire procedure occurs in aqueous systems without the addition of any other solvents. There are several implications to these observations including the biological relevance of mimicking fusogenic proteins such as the SNARE proteins and the development of new drug delivery technologies to impact delivery to cell organelles.

Multilamellar Vesicle Formation Probed by Rheo-NMR and Rheo-SALS under Large Amplitude Oscillatory Shear

The formation of multilamellar vesicles (MLVs) in the lyotropic lamellar phase of the system triethylene glycol mono n-decyl ether (C₁₀E₃)/water is investigated under large amplitude oscillatory shear (LAOS) using spatially resolved rheo-NMR spectroscopy and a combination of rheo-small angle light scattering (rheo-SALS) and conventional rheology. Recent advances in rheo-NMR hardware development facilitated the application of LAOS deformations in high-field NMR magnets. For the range of investigated strain amplitudes (10-50) and frequencies (1 and 2 rad s^-1), MLV formation is observed in all NMR and most SALS experiments. It is found that the MLV size depends on the applied frequency in contrast to previous steady shear experiments where the shear rate is the controlling parameter. The onset of MLV formation, however, is found to vary with the shear amplitude. The LAOS measurements bear no indication of the intermediate structures resembling aligned multilamellar cylinders observed in steady shear experiments. Lissajous curves of stress vs strain reveal a transition from a viscoelastic solid material to a pseudoplastic material.

Multilamellar vesicle formation from a planar lamellar phase under shear flow

The formation of multilamellar vesicles (MLVs) from the lamellar phase of nonionic surfactant system C12E5/D2O under shear flow is studied by time-resolved small angle neutron and light scattering during shear flow. A novel small angle neutron scattering sample environment enables the tracking of the lamellae alignment in the velocity-velocity gradient (1-2) plane during MLV formation, which was tracked independently using flow small angle light scattering commensurate with rheology. During the lamellar-to-multilamellar vesicle transition, the primary Bragg peak from the lamellar ordering was observed to tilt, and this gradually increased with time, leading to an anisotropic pattern with a primary axis oriented at ∼25° relative to the flow direction. This distorted pattern persists under flow after MLV formation. A critical strain and critical capillary number based on the MLV viscosity are demonstrated for MLV formation, which is shown to be robust for other systems as well. These novel measurements provide fundamentally new information about the flow orientation of lamellae in the plane of flow that cannot be anticipated from the large body of previous literature showing nearly isotropic orientation in the 2,3 and 1,3 planes of flow. These observations are consistent with models for buckling-induced MLV formation but suggest that the instability is three-dimensional, thereby identifying the mechanism of MLV formation in simple shear flow.

Large Amplitude Oscillatory Shear Induces Crystal Chain Orientation in Velocity Gradient Direction

Imposition of large amplitude oscillatory shear (LAOS) on crystallizing polymer melts results in lamellar orientation in the shear gradient direction, in contrast to the flow-orientation observed for steady shear. LAOS enhances the formation of plate-like nuclei and orients their normals in the gradient direction. An Arrhenius temperature dependence (with activation energy ≈ 226 kJ/mol) characterizes the relaxation of crystal orientation with annealing.

Self-assembled structures of amphiphiles regulated via implanting external stimuli

This review article has summarized recent achievements of manipulating amphiphilic molecules and their self-assembled structures via different external stimuli.

Re-entrant lamellar/onion transition with varying temperature under shear flow

We have found for the first time the reentrant lamellar/onion (lamellar-onion-lamellar) transition with varying temperature under constant shear rate by using simultaneous measurements of shear stress and small-angle X-ray scattering (Rheo-SAXS) for a nonionic surfactant (C(14)E(5))/water system, which exhibits the lamellar phase in a wide temperature range from 15-75 °C. The onion state exists in a closed region in the temperature-concentration diagram at a constant shear rate. Temperature dependence of the lamellar repeat distance (d) at rest has also been measured at several concentrations. It is shown that the increase of d with increasing temperature is necessary for the existence of the lower transition. We have investigated the change in the lamellar orientation in the lamellar-to-onion and onion-to-lamellar transition processes near the upper and lower transition temperatures. For all four kinds of transition processes, the following change in the lamellar orientation is observed; lamellar state (oriented to the velocity gradient direction) ↔ further enhancement of the orientation to the velocity gradient direction ↔ enhancement of the orientation to the neutral direction ↔ onion state.

Structural transitions induced by shear flow and temperature variation in a nonionic surfactant/water system

In this study, we investigate structural transitions of tetraethylene glycol monohexadecyl ether (C(16)E(4)) in D(2)O as a function of shear flow and temperature. Via a combination of rheology, rheo-small-angle neutron scattering and rheo-small-angle light scattering, we probe the structural evolution of the system with respect to shear and temperature. Multi-lamellar vesicles, planar lamellae, and a sponge phase were found to compete as a function of shear rate and temperature, with the sponge phase involving the formation of a new transient lamellar phase with a larger spacing, coexisting with the preceding lamellar phase within a narrow temperature-time range. The shear flow behavior of C(16)E(4) is also found to deviate from other nonionic surfactants with shorter alkyl chains (C(10)E(3) and C(12)E(4)), resembling to the C(16)E(7) case, of longer chain.

Self-assembled aggregates originated from the balance of hydrogen-bonding, electrostatic, and hydrophobic interactions

Rich phase behavior was observed in salt-free cationic and anionic (catanionic) mixtures of a double-tailed surfactant, di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA), and tetradecyldimethylamine oxide (C(14)DMAO) in water. At a fixed C(14)DMAO concentration, phase transition from L(1) phase to L(α) phase occurs with increasing amounts of DEHPA. Moreover, in the L(α) phase, with the increase in DEHPA concentration, a gradual transition process from vesicle phase (L(αv)) to stacked lamellar phase (L(αl)) was determined by cryo- and FF-TEM observations combining with (2)H NMR measurements. The rheological data show that the viscosity increases with DEHPA amounts for L(αv) phase samples because of the increase in vesicle density. At a certain molar ratio of DEHPA to C(14)DMAO, i.e., 80:250, the samples are with the highest viscoelasticity, indicating the existence of densely packed vesicles. While for L(αl) phase samples, with increasing DEHPA amount, a decrease of bilayer curvature was induced, leading to a decrease of viscosity obviously. Compared with general catanionic surfactant mxitures, in addition to the electrostatic interaction of ion pairs, the transition of the microstructures is also ascribed to the formation of the hydrogen bonding (-N(+)-O-H···O-N-) between C(14)DMAO molecules and protonated C(14)DMAOH(+), which induces the growth of aggregates and the decrease of aggregate curvatures.

Shear-induced defect formation in a nonionic lamellar phase

(2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta approximately gamma(n), with n approximately -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.

Transition from vesicle phase to lamellar phase in salt-free catanionic surfactant solution

A salt-free cationic and anionic (catanionic) surfactant system was formed by mixing a double-tailed di-(2-ethylhexyl) phosphoric acid (DEHPA, commercial name P204), which is an excellent extractant of rare earth metal ions, with a single-tailed cationic trimethyltetradecylammonium hydroxide (TTAOH) in water. With the mole ratio (r) of DEHPA to TTAOH varying from 0.9 to 1, the phase transition occurred from a densely stacked vesicle phase (Lalphav) to a lamellar phase (Lalphal). Macroscopic properties, such as polarization and rheology, were measured and changed greatly during the course of the phase transition. When r was 0.96 or 0.98, the steady state shear curves exhibited two yield stress values, indicating the coexistence of the Lalphav phase and the Lalphal phase. The Lalphal phase formed in the salt-free and zero-charged system (r=1.0) is defective and undulating, which was confirmed by cryogenic transmission electron microscopy (cryo-TEM). The deuterium nuclear magnetic resonance spectra (2H NMR) showed that a single peak (singlet) split into two symmetric peaks (doublet) gradually, indicating the phase transition from the Lalphav phase to the Lalphal phase. Correspondingly, phosphorus nuclear magnetic resonance spectra (31P NMR) presented changes in both the chemical shift and the peak width, indicating that these two types of bilayer structures are of different anisotropy degrees and different viscosities. When the Lalphal phase is subjected to a certain shear force, it can be reversed to a Lalphav phase again, which was proved by rheological, 2H NMR, and 31P NMR measurements. Furthermore, a theoretical consideration about the formation of the defective and undulating Lalphal phase was taken into account from a viewpoint of energy.

Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformation

The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.

Highly ordered patterns of parabolic focal conics in lamellar lyotropic systems

In this paper, we report the experimental observation of the formation of highly ordered parabolic focal conical patterns in lamellar surfactant solutions. Predominantly, we investigated mixtures of sodium dodecyl sulfate, water, hexanol, and decane, located in the immediate vicinity of the region of the L3 and Lalpha phase coexistence. The experimental studies of the formation of the patterns and of their temporal development are described. We give a simple model picture for the underlying structure, corroborated by the experimental results. There appears to be only one independent length scale that controls the appearance of the whole pattern.

Flow-induced effects in mixed surfactant mesophases

Spatially resolved small-angle neutron scattering, SANS, has been used to investigate the response of the mixed microstructure of the dialkyl chain cationic and nonionic surfactant mixtures of (2,3-diheptadecyl ester ethoxy-n-propyl-1), 1,1,1-trimethyl ammonium chloride/octadecyl monododecyl ether, DHTAC/C18EO10, and DHTAC/dodecyl monododecadecyl ether, Coco20, over the velocity flow pattern of a crossed-slot elongational flow cell. The two different surfactant mixtures have different relative amounts of lamellar and micellar components, and this results in some differences in the flow-induced response. For the DHTAC/C18EO10, which is predominantly in the form of lamellar fragments, a complex pattern of orientational ordering is observed which reflects the competition between or demixing of the two principal flow directions in the cell.

Nonequilibrium molecular dynamics simulation study on the orientation transition in the amphiphilic lamellar phase under shear flow

By the extensive large-scale nonequilibrium molecular dynamics simulation on an effective generic model-A2B2 tetramer for amphiphiles, we investigate the shear-induced parallel to perpendicular orientation transition in the lamellar phase as a function of segregation degree and shear rate. Under low rate shear flow the evolution of parallel lamellar configurations at different segregation strengths shows a similar kinetic pathway independent of the segregation degree. While under high rate shear flow in which the lifetime of undulation instability exceeds the characteristic time of the applied shear flow, the kinetic pathway of the shear-induced parallel-to-perpendicular orientation transition in lamellar systems is the segregation degree dependent. Comparing the temporal mesoscopic domain morphology, the microscopic chain conformation, and macroscopic observable-viscosity changes with the experimentally proposed mechanisms, we find that the undulation instability, partial breakup of monodomain, grain rotation, and recombination combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase in or near the intermediate segregation limit, and that the undulation instability, domain dissolution, and reformation along the preferred direction combined with defect migration and annihilation are the kinetic pathway for the parallel-to-perpendicular orientation transition in the lamellar phase close to the order-to-disorder phase transition point. A detailed underlying microscopic picture of the alignment process illustrates that the orientation transition is driven by the alignment of molecules with shear flow. The orientation diagram that characterizes the steady-state orientations as a function of shear rate and attractive potential depth is built, in which the attractive potential depth takes the role of an inverse temperature, somewhat like the Flory-Huggins interaction parameter. The microscopic mechanism of the critical orientation transition condition is discussed.

Vorticity banding during the lamellar-to-onion transition in a lyotropic surfactant solution in shear flow

We report on the rheology of a lyotropic lamellar surfactant solution (SDS/dodecane/pentanol/ water), and identify a discontinuous transition between two shear thinning regimes which correspond to the low-stress lamellar phase and the more viscous shear-induced multilamellar vesicle, or "onion" phase. We study in detail the flow curve, stress as a function of shear rate, during the transition region, and present evidence that the region consists of a shear-banded phase where the material has macroscopically separated into bands of lamellae and onions stacked in the vorticity direction. We infer very slow and irregular transformations from lamellae to onions as the stress is increased through the two-phase region, and identify distinct events consistent with the nucleation of small fractions of onions that coexist with sheared lamellae.

Elongational flow induced ordering in surfactant micelles and mesophases

We have used small angle neutron scattering, SANS, to investigate the elongational flow induced ordering in surfactant micelles and mesophases. Spatially resolved SANS measurements have been used to determine the distribution of orientational ordering over the flow velocity pattern in an elongational flow cell, and comparison with the effects of shear flow are made. Two different surfactant systems have been studied, the charged wormlike mixed micelles of hexaethylene monododecyl ether, C16E6/hexadecyl trimethylammonium bromide, C16TAB (3% C16E(6)/5 mol% C16TAB), and the Lalpha lamellar phase of C16E6 (50.6 wt% C16E6 at 55 degrees C), and a substantially different response is observed. The orientational distribution of the Lalpha lamellar phase of C16E6 reflects the flow velocity pattern distribution within the cell, whereas for the wormlike mixed micelles of C16E6/C16TAB this is not the case, and this is associated with the shear thinning behavior of that system.

Shear-induced parallel-to-perpendicular orientation transition in the amphiphilic lamellar phase: A nonequilibrium molecular-dynamics simulation study

The present work is devoted to a study of the shear-induced parallel-to-perpendicular orientation transition in the lamellar system by the large-scale nonequilibrium molecular-dynamics (NEMD) simulation. An effective generic model-A2B2 tetramer for amphiphilies is used. The NEMD simulation produces unambiguous evidence that undulation instability along the vorticity direction sets in well above a critical shear rate and grows in magnitude as the shear rate is further increased. At a certain high shear rate, the coherent undulation instability grows so large that defects are nucleated and the global lamellar monodomain breaks into several aligned lamellar domains. Subsequently layers in these domains rotate into the perpendicular orientation with the rotation of chains towards the y direction, merge into a global perpendicular-aligned lamellar monodomain, and organize into a perfect well-aligned perpendicular lamellar phase by the migration and annihilation of edge dislocations and disclinations. The macroscopic observable viscosity as a function of time or shear rate is correlated with the structural response such as the mesoscopic domain morphology and the microscopic chain conformation. The onset of undulation instability concurs with the start-up of shear-thinning behavior. During the orientation transformation at the high shear rate, the complex time-dependent thixotropic behavior is observed. The smaller viscosity in the perpendicular lamellar phase gives an energetic reason for the shear-induced orientation transition.

Size and viscoelasticity of spatially confined multilamellar vesicles

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.

Shear-induced discontinuous and continuous topological transitions in a hyperswollen membrane system

Here we demonstrate that both discontinuous and continuous transition between the sponge and lamellar phase can be induced by steady shear flow for a hyperswollen membrane system. The discontinuous nature of the transition is revealed by a distinct hysteresis in the rheological behavior between shear-rate increasing and decreasing measurements at a constant temperature. This discontinuity becomes weaker with an increase in the shear rate and temperature, and the transition eventually becomes a continuous one without any hysteresis. We also found another shear-induced transition in a one-phase lamellar region. The dynamic phase diagram in a nonequilibrium steady state under shear is constructed for the sponge-lamellar transition as well as another transition in a stable lamellar phase. Possible physical mechanisms for these shear-induced transitions are discussed.

Shear-induced collapse in a lyotropic lamellar phase

An entropically stabilized cetylpyridinium chloride, hexanol, and heavy brine lyotropic lamellar phase subjected to shear flow has been observed here by small angle neutron scattering to undergo collapse of smectic order above a threshold shear rate. The results are compared with theories predicting that such a lamellar phase sheared above a critical rate should lose its stability by a loss of resistance to compression due to the suppression of membrane fluctuations.

31P and 1h NMR as probes of domain alignment in a rigid crystalline surfactant mesophase

A viscous reverse hexagonal surfactant mesophase containing bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and alpha-phosphatidylcholine (lecithin), with comparable volume fractions of isooctane and water, was characterized by Fourier transform (31)P and (1)H NMR spectroscopy. Shear alignment was reflected through both (31)P NMR and (1)H NMR spectra. A complicated (31)P spectrum was observed as a result of superposition of chemical shifts according to the distribution of crystalline domains prior to shear. The initially disordered samples with polydomain structures become macroscopically aligned after Couette shear. (31)P NMR chemical shift anisotropy characteristics are used to elucidate orientation of the hexagonal phase. Interestingly, (1)H NMR spectra exhibit spectral changes upon shear alignment closely corresponding with that of (31)P NMR spectra. These observations complement the findings of mesophase alignment obtained using SANS and imply that (31)P and (1)H NMR spectroscopy can be used as probes to define microstructure and monitor orientation changes in this binary surfactant system. This is especially beneficial if these mesophases are used as templates for materials synthesis.

Shear-induced orientation of a rigid surfactant mesophase

An optically clear, crystalline, gel-like mesophase is formed by the addition of water to a micellar solution consisting of a mixture of 0.85 M anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and a 0.42 M zwitterionic surfactant phosphatidylcholine (lecithin) in isooctane. At 25 degrees C and water to AOT molar ratio of 70, the system has a columnar hexagonal microstructure with randomly oriented domains. The shear-induced orientation and subsequent relaxation of this structure were investigated by rheological characterization and small-angle neutron scattering (SANS). The rheological response implies that the domains align under shear, and remain aligned for several hours after cessation of shear. Shear-SANS confirms this picture. The sheared gel mesophase retains its alignment as the temperature is increased to 57 degrees C, indicating the potential to conduct templated polymer and polymer-ceramic composite materials synthesis in aligned systems.

Anomalous decrease in lamellar spacing by shear flow in a nonionic surfactant/water system

We study the effects of shear flow on the structure of a lamellar phase in a C16E7 [hepta(oxyethylene glycol)-n-hexadecyl ether]/water system (40-55 wt % of C16E7) at 70 degrees C using small-angle neutron scattering in the range of shear rate of 10(-3)-30 s(-1). At the shear rate 0.1-1 s(-1), the repeat distance (d) is decreased significantly (down to about 40% of d at rest in the most significant case) and discontinuously with increasing shear rate. With the further increase in the shear rate, d increases through a sharp minimum (referred to as d*). Such a shear rate dependence of d is obtained for all the principal orientations of lamellae. As the concentration of C16E7 decreases from 55 to 40 wt %, d increases from 6.5 to 8.5 nm at rest whereas d* remains almost constant (approximately equal to 5 nm). Moreover, d* is found to be almost equal to the thickness of bilayers obtained from the line shape analysis of small-angle X-ray scattering at rest. The results strongly suggest that the water layer is excluded by shear flow and that the lamellar phase segregates into surfactant-rich and water-rich regions, although these regions do not reach macroscopic size.

Shear-induced formation of vesicles in membrane phases: kinetics and size selection mechanisms, elasticity versus surface tension

Multilamellar vesicles can be formed upon shearing lamellar phases (L(alpha)) and phase-separated lamellar-sponge (L(alpha)/L(3)) mixtures. In the first case, the vesicle volume fraction is always 100% and the vesicle size is monitored by elasticity ("onion textures"). In the second system the vesicle volume fraction can be tuned from 0 to 100% and the mean size results from a balance between capillary and viscous forces ("Taylor droplets"). However, despite these differences, in both systems we show that the formation of vesicles is a strain-controlled process monitored by a universal primary buckling instability of the lamellae.

Dynamical coarse-graining of highly fluctuating membranes under shear flow

The effect of strong shear flow on highly fluctuating lamellar systems stabilized by intermembrane collisions via the Helfrich interaction is studied. Advection enters the microscopic equation of motion for a single membrane via a nonlinear coupling. Upon coarse-graining the theory for a single bilayer up to the length scale of the collision length, at which a hydrodynamic description applies, an additional dynamical coupling is generated which is of the form of a wave-vector-dependent tension that is nonlinear in the applied shear rate. This new term has consequences for the effects of strong flow on the stability and dynamics of lamellar surfactant phases.

Scaling of shear-induced transformations in membrane phases

Surfactant sponges are complex-fluid phases made up of convolutions of bilayer sheets. Although isotropic and free flowing they exhibit transient birefringence when stirred, reminiscent of the birefringence of lamellar phases. Previous attempts to understand this effect have led to confusing and often conflicting results. We have used a novel approach to designing the chemical system that gives us control over the relevant parameters needed to study microstructural and macroscopic responses of these phases to shear. We find a remarkable universal scaling behavior for both sponge and shear-induced lamellar states, which resolves a number of long-standing questions about these systems.