Shear-induced Structural Transitions Research Articles

Structural hierarchy of mechanical extensibility in human von Willebrand factor multimers.

The von Willebrand factor (VWF) is a multimeric glycoprotein composed of 80- to 120-nm-long protomeric units and plays a fundamental role in mediating platelet function at high shear. The exact nature of the shear-induced structural transitions have remained elusive; uncovering them requires the high-resolution quantitative analysis of gradually extended VWF. Here, we stretched human blood-plasma-derived VWF with molecular combing and analyzed the axial structure of the elongated multimers with atomic force microscopy. Protomers extended through structural intermediates that could be grouped into seven distinct topographical classes. Protomer extension thus progresses through the uncoiling of the C1-6 domain segment, rearrangements among the N-terminal VWF domains, and unfolding and elastic extension of the A2 domain. The least and most extended protomer conformations were localized at the ends and the middle of the multimer, respectively, revealing an apparent necking phenomenon characteristic of plastic-material behavior. The structural hierarchy uncovered here is likely to provide a spatial control mechanism to the complex functions of VWF.

Shear-induced liquid-crystalline phase transition behaviour of colloidal solutions of hydroxyapatite nanorod composites.

Liquid-crystalline (LC) bio-inspired materials based on colloidal nanoparticles with anisotropic morphologies such as sheets, plates, rods and fibers were used as functional materials. They show stimuli-responsive behaviour under mechanical force and in electric and magnetic fields. Understanding the effects of external stimuli on the structures of anisotropic colloidal particles is important for the development of highly ordered structures. Recently, we have developed stimuli-responsive hydroxyapatite (HAP)-based colloidal LC nanorods that are environmentally-friendly functional materials. In the present study, the ordering behaviour of HAP nanorod dispersions, which show LC states, has been examined using in situ small-angle neutron scattering and rheological measurements (Rheo-SANS) under shearing force. The structural analyses and dynamic viscosity observations provided detailed information about the effects of shear force on the structural changes of HAP nanorods in D2O dispersion. The present Rheo-SANS measurements unraveled three kinds of main effects of the shear force: the enhancement of interactions between the HAP nanorods, the alignment of HAP nanorods to the shear flow direction, and the formation and disruption of HAP nanorod assemblies. Simultaneous analyses of dynamic viscosity and structural changes revealed that the HAP nanorod dispersions exhibited distinctive rheological properties accompanied by their ordered structural changes.

Shear-induced structural and thermodynamic phase transitions in micellar systems.

In this contribution a methodology to compute and classify shear-induced structural and phase transitions in surfactant/water mixtures from rheological measurements is presented. Non-linear rheological experiments, considering variations in surfactant concentration and temperature, are analyzed. In particular, the parameters of the BMP (Bautista-Manero-Puig) model, obtained from the fitting of the shear stress versus shear rate data, which are functions of surfactant concentration and temperature, allow classifying structural and phase transition boundaries. To test this methodology, we consider the analysis of the shear-induced structural and phase transitions of two micellar systems, cetyltrimethylammonium tosylate (CTAT)/water as a function of CTAT concentrations and Pluronics P103/water as a function of temperature. We found that the CTAT/water system presents a first-order phase transition at 30 ° C, and around 31 to 32 wt.% from isotropic to nematic phases, whereas a 20 wt.% Pluronics P103 aqueous micellar solution has two second-order (structural) phase transitions, one from spherical to cylindrical micelles at 33.1 ° C, and another one from cylindrical micelles to a nematic phase at 35.8 ° C and one first-order phase transition around 37.9 ° C at high shear rates near to the cloud point previously reported. The proposed methodology is also able to identify the instability regions where the wormlike micelles are broken, producing the typical shear banding behavior.

Dynamics of Melting and Recrystallization in a Polymeric Micellar Crystal Subjected to Large Amplitude Oscillatory Shear Flow

Shear-induced structural transitions of a micellar cubic phase during large amplitude oscillatory shear flow is studied with time-resolved oscillatory rheological small angle neutron scattering. This technique allows us to resolve the structural changes within a cycle of oscillation. By applying a strain rate near the critical melting shear rate, melting and recrystallization occurs in a cyclic mode. The maximum degree of order is observed when the shear stress reaches a plateau value during the large amplitude oscillatory shear cycle, whereas melting is maximized at the strain rate wave peaks. This structural evolution confirms the cyclic mechanism of sticking and sliding of 2D hexagonal close-packed layers [I. W. Hamley et al., Phys. Rev. E 58, 7620 (1998)].

Direct Imaging of the Orientational Dynamics of Block Copolymer Lamellar Phase Subjected to Shear Flow

An imaging torsional parallel-plate shear cell, producing a well-defined range of shear rates, was used to probe the dynamics of shear-induced structural transitions in complex fluids. The photonic properties of high molecular weight poly(styrene-b-isoprene) symmetric diblock copolymer solutions were exploited to visualize the parallel-to-perpendicular shear-induced orientation transition of the lamellar phase. The online imaging demonstrated that the transition is purely shear rate controlled. Exploiting different shear pulse profiles demonstrates that below a critical value shear flow aligns the lamellar phase in the parallel orientation and in the perpendicular orientation at shear rates above this critical value. The reverse perpendicular-to-parallel shear-induced orientation transition of the lamellar phase was also detected. However, experimental observations suggest that the mechanisms of the forward and the reverse orientation transition are different. It was also found that at low shear rates, when the perpendicular alignment is unstable, the change in lamellae orientation from perpendicular back to parallel is strain controlled. There is a good correlation between results obtained from the imaging technique and by other techniques such as rheology (both steady shear and dynamic measurements), ex situ polarized light imaging, and small-angle X-ray scattering.

Shear-Induced Structural Transition and Recovery in the Salt-Free Catanionic Surfactant Systems Containing Deoxycholic Acid

Shear-induced structural transition and recovery were studied by freeze-fracture transmission electron microscopy, cryogenic transmission electron microscopy (cryo-TEM), rheological measurements, and the variation of birefringent textures in the system of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/deoxycholic acid (DeCA)/H(2)O as a function of the molar fraction of DeCA [x = n(DeCA)/(n(DeCA) + n(LA))]. At x = 0.3, giant vesicles and planar lamellar structures were formed before exerting shearing. Shear thickening and large hysteresis loop are observed under shearing, indicating the occurrence of structural transition. Multilamellar and close-packed vesicles were determined by cryo-TEM observations. Exerting further shearing, the elastic properties of the sample were increased due to the strip off of the outer shell of the vesicles and formation of small vesicles. After shearing was stopped, the sample can slowly relax back to the original state. At x = 0.25, lamellar structures, giant vesicles, and small unilamellar vesicles were observed. Cryo-TEM observations show that the multilamellar vesicles can be formed after exerting shearing, and the sample cannot spontaneously recover to the original state. At x = 0.2, vesicles are dominant in the solution, and the aggregates structures are almost the same before and after shearing. Shearing can increase the elastic properties of the sample, which is ascribed to the strip off of the outer shells of the multilamellar vesicles, forming much smaller vesicles in the solution.

Fluid mechanical shear induces structural transitions in assembly of a peptide–lipid conjugate

Peptide amphiphiles (PA) can self-assemble into both spherical micelles and worm-like micelles. The control of worm-like micelle formation of PA is an area of active research, most often accomplished by modulating the temperature, salt content, or pH of the environment. In this work, we demonstrate the shear-induced formation of worm-like micelles in our designed peptide amphiphile C16-W3K. Before adding shear, the peptide amphiphiles form spherical micelles in solution and exhibit little to no viscoelasticity. As the solution is subjected to simple shear flow, with increasing shear rate, spherical micelles form rapidly into elongated worm-like micelles up to microns in length. Though it has been reported that some dilute surfactant solutions exhibit shear-induced increase in viscosity due to a shear-induced structural transition, unlike this class of surfactants, the PA micelles change their structures from sphere to worm-like irreversibly and the resultant worm-like micelles are highly stable due to the β-sheet formation, i.e. intermolecular hydrogen bonding, in their peptide regions. In our PA system, shear force induced the change not only of the micelle structure but also of the peptide secondary structure simultaneously. Such hierarchical transitions caused by simple shear make this PA system useful for application as an injectable tissue engineering matrix.

二分子膜系ラメラ相における流動誘起多層膜ベシクル構造形成

Shear-induced instability of bilayer membrane systems has attracted much attention. One of the most interesting shear effects is a shear-induced structural transition from the lamellar to the multilamellar vesicle. In this review, we show two topics related to the non-equilibrium structural formation phenomena under shear; the influence of the triblock copolymer on the shear-induced multilamellar vesicle formation, and the shear quench-induced discontinuous size change of the multilamellar vesicle. In the first topic, we study the effect of the triblock copolymer on the shear-induced lamellar-to-multilamellar vesicle formation. Shear-induced multilamellar vesicle formation behavior is quite sensitive to the interaction between grafted polymer chains and surfactant bilayer membranes. Entropy based contribution to the modulus of bilayer controls the shear-induced multilamellar vesicle formation behavior. In the second topic, we study how the kinetic pathway on the size change process of the multilamellar vesicle is decided under shear. These studies will give us a hint how the soft matter systems change their structure under non-equilibrium states.

Shear-induced structural transition in the lamellar phase of the C16E7/D2O system. Time evolution of small-angle neutron scattering at a constant shear rate

The time evolution of small-angle neutron scattering is measured for the lamellar phase of a nonionic surfactant C16H33(OC2H4)7OH (C16E7) in D2O at 48 wt% at 343 K under shear flow. At the shear rates of 0.3, 1 and 3 s−1, a new diffraction peak appears at higher q [where q = (4π/λ)sin θ, and λ and 2θ are the wavelength of the neutron beam and the scattering angle, respectively] about 1–2 h after applying shear flow and coexists with the initial diffraction peak. The coexistence of two peaks continues even after 5 h at 0.3 s−1 whereas at 1 and 3 s−1 the peak at lower q disappears after about 3 h. These results indicate that the repeat distance decreases discontinuously and so suggest some sort of transition. A plot of the repeat distance after 5 h versus shear rate shows a minimum at 1 s−1, which is in good agreement with our previous results obtained by increasing the shear rate stepwise.

Shear-induced structural transition in a lyotropic lamellar phase studied using small angleneutron and light scattering

We study the effects of shear flow on structures of the lamellar phase formed in a nonionic surfactantC16E7 (hepta(oxyethyleneglycol)-n-hexadecylether)/D2O system by using small angle neutron scattering (SANS) andsmall angle light scattering (SALS) in the range of shear rate10−3–30 s−1. As the shear rateincreases from 0.3 to 1 s−1, the lamellar repeat distance for the 48 wt% sample is decreasedsignificantly and discontinuously. With further increase in the shear rate,d increases througha sharp minimum at 1 s−1. The minimum value does not depend on the surfactant concentration verymuch and is nearly equal to the thickness of the bilayers. At this shear rate(1 s−1), the intensity of the polarized SALS is strongly enhanced in the small angleregion in the vorticity direction. These results suggest that water layers areexcluded by shear flow and that large concentration fluctuations on theμm scaleare induced, which corresponds to local segregation into two regions; one of these has concentratedlamellar structures and the other is a water-rich region. As the shear rate increases further (to3 s−1), a broad diffraction peak appears in the vorticity direction in the polarizedSALS whereas a four-lobe pattern is observed in the depolarized SALS.There results, together with the two-dimensional SANS pattern, suggestthe formation of close-packed onions elongated along the flow direction at3 s−1. Comparison is made with the study of Nettesheim et al (2003 Langmuir 19 3618) whoreport the formation of multilamellar cylinders as intermediate structures, between lamellaeand spherical onions.

Shear-induced mesostructure in nanoplatelet-polymer networks

The shear response of a model polymer–clay gel is measured using small-angle neutron and light scattering, optical microscopy, and rheometry. As the flow disrupts the transient network that forms between clay and polymer, coupling between composition and stress leads to the formation of a macroscopic domain pattern, while the clay platelets orient with their surface normal parallel to the direction of vorticity. We discuss similarities with shear-induced structural transitions observed in other complex fluids, and we offer a physical explanation for the orientation of the clay platelets.

Quantifying the Importance of Micellar Microstructure and Electrostatic Interactions on the Shear-Induced Structural Transition of Cylindrical Micelles

Using static light scattering (SLS) and small-angle neutron scattering (SANS) in conjunction with rheological measurements, the significance of electrostatic interactions and micellar microstructure to the formation of shear-induced structures (SIS) in systems of dilute aqueous cylindrical micelles is investigated. This is accomplished through a systematic study of the influence of increasing NaCl concentration on the dilute aqueous micellar system of 11 mM cetyltrimethylammonium p-toluenesulfonate (CTAT). Concurrent with increases in the zero-shear viscosity (ηo), low NaCl concentrations lead to the growth of micelles through increases in the degree of screening of electrostatic interactions. These systems, which exhibit discernible electrostatic or correlation peaks in the SANS results, undergo shear-induced structural transitions, whereby critical shear rates tend to increase with NaCl concentration. With further increases in the NaCl concentration, intermicellar electrostatic interactions are adequately screened, and no SIS is observed. The addition of NaCl to these systems increases the micellar flexibility, as shown by SLS. The ηo of these systems decreases, and the critical shear rates for these shear-thinning systems increases, consistent with expectations for dilute polyelectrolyte systems.

Shear-induced nano-macro structural transition in a polymeric bicontinuous microemulsion.

Bicontinuous microemulsions arise in a narrow concentration range for ternary blends containing two immiscible homopolymers and the corresponding diblock copolymer. Steady shear reveals four distinct regimes of response as a function of shear rate, corresponding to flow-induced transitions in fluid structure. In situ neutron scattering shows flow-induced anisotropy in the nanometer-scale microemulsion structure at moderate shear rates, while higher rates induce bulk phase separation, with micron-size morphology, which is characterized with in situ light scattering and optical microscopy.

Controlling the Shear-Induced Structural Transition of Rodlike Micelles Using Nonionic Polymer

We demonstrate that nonionic polymer can be used to control the shear-induced transition observed in the cationic rodlike micellar system of cetyltrimethylammonium p-toluene sulfonate (CTAT). Hydroxypropyl cellulose (HPC) suppresses this transition, while poly(ethylene oxide) (PEO) enhances the zero-shear viscosity, with little change to the critical rates of the transition. It is proposed that the availability of hydrophobic moieties of HPC allows for its influence on the onset of the transition. This is consistent with binding, rheological, and small-angle neutron scattering (SANS) studies. Furthermore, quiescent SANS shows that both polymers affect intermicellar interactions, solvent−micelle interactions, and/or micellar length. While the onset of the shear-induced transition may be altered with polymer, polymer composition does not affect micellar flexibility and/or interactions in the aligned transitioned state.

Shear-induced structural transitions in newtonian non-newtonian two-phase flow

We show the existence under shear flow of steady states in a two-phase region of a brine-surfactant system in which lyotropic dilute lamellar (non-Newtonian) and sponge (Newtonian) phases are coexisting. At high shear rates and low sponge phase-volume fractions, we report on the existence of a dynamic transition corresponding to the formation of a colloidal crystal of multilamellar vesicles (or "onions") immersed in the sponge matrix. As the sponge phase-volume fraction increases, this transition exhibits a hysteresis loop leading to a structural bistability of the two-phase flow. Contrary to single phase lamellar systems where it is always 100%, the onion volume fraction can be monitored continuously from 0 to 100 %.

Shear and Substrate Dependent Changes in Fibrinogen and Von Willebrand Factor Studied by Atomic Force Microscopy

Abstract Atomic force microscopy (AFM) provides unique opportunities to study cell-surface and molecular scale interactions in three dimensions under aqueous conditions. Two plasma proteins, von Willebrand Factor (vWf) and fibrinogen, play central roles in the regulation of hemostasis and thrombosis by participating in coagulation and facilitating adhesion and aggregation of activated platelets. vWf and fibrinogen are believed to facilitate platelet adhesion under regions of relatively high and low vascular wall shear stress, respectively. Consequently, elucidating vWf and fibrinogen structure-function relations under shear is of considerable importance in developing a comprehensive understanding of the pathophysiology of thrombogenesis. Previously, we reported on molecular level AFM images of human vWf during shear-induced structural transition and human fibrinogen under aqueous conditions when both proteins were adsorbed on a hydrophobic surface. This presentation will report on the shear dependent interactions of dimeric and multimeric vWf with a hydrophobic surface and the substrate-dependent interactions of fibrinogen.

Transient small-angle neutron scattering experiments on micellar solutions with a shear-induced structural transition

Transient small-angle neutron scattering (SANS) experiments on a sheared 5 mM aqueous surfactant solution of trimethyltetradecylammonium salicylate (TTMA-Sal) are reported. The radially symmetric scattering pattern without shear shows a weak ringlike correlation «peak». For steady shear rates above a threshold value of Γ th =50 s -1 , the scattering pattern becomes rapidly anisotropic and the ring is compressed into two sharp, symmetric peaks. These changes are related to a shear-induced structural transition. The analysis of the data reveals the existence of two different types of micelles: Short rodlike micelles (called type I) are always present, and they are only weakly aligned