Session X8: Focus Session: Wormlike Micellar Fluids and Vesicles

Rheo-NMR of shear banded flow in wormlike micelles

Rheo-NMR gives access to detailed information about the flow field generated by the device used to induce deformational flow. It also provides information about colloidal or molecular organisation and dynamics, under conditions of flow. In particular, NMR offers the possibility of measuring nuclear spin relaxation times and molecular self-diffusion coefficients, sensitive respectively to molecular brownian motions and their restrictions due to local structure. Furthermore, through the use of orientation-dependent terms in the spin interactions, such as the nuclear quadrupole or dipolar interactions, NMR permits the measurement of molecular order parameters. When combined with imaging methods, NMR in principle allows such measurements to be spatially localized, often with resolution down to a few 10s of microns In the study of shear banding phenomena in wormlike micelles, Rheo-NMR has proven of especial value, not only indicating the clear existence of shear bands, but also that they are associated with fluctuations, and sometimes, with molecular alignment. The subtlety of the correspondence (or lack of correspondence) between birefringence effects and shear banded flow has also been revealed. Recent measurements of shear-banded flow under Couette flow of the micellar system 10\% w/v cetylpyridinium chloride and sodium salicylate (CPyCl/NaSal) molar ratio 2:1 in 0.5 M NaCl in $^1$H$_2$O, indicate that shear banding fluctuations are consistent with the shear stress fluctuations observed in rheological measurements. Furthermore we find a coupling between flow fluctuations in the gradient and vorticity directions. Using $^2$H NMR spectroscopy on a deuterated probe molecule (n-decane) in the wormlike micellar interior, direct measurement of the shear-induced nematic phase transition is reported. More recently we have used Rheo-NMR to investigate the flow and alignment properties of worm-like micelles formed by a 5\% w/w solution of the BASF difunctional block copolymer non-ionic surfactant, Pluronic P105 in water along with 4.3\% w/v 1-phenylethanol-d5. A variety of shear-banding and alignment behaviours are observed, along with both stable and fluctuating flows.   

Modeling the Inhomogeneous Response of Steady and Transient Flows of Entangled Micellar Solutions

Surfactant molecules can self-assemble in solution into long flexible structures known as wormlike micelles. These structures entangle, forming a viscoelastic network similar to those in entangled polymer melts and solutions. However, in contrast to `inert' polymeric networks, wormlike micelles continuously break and reform leading to an additional relaxation mechanism and the name `living polymers'. Observations in both classes of entangled fluids have shown that steady and transient shearing flows of these solutions exhibit spatial inhomogeneities such as `shear-bands' at sufficiently large applied strains. In the present work, we investigate the dynamical response of a class of two-species elastic network models which can capture, in a self-consistent manner, the creation and destruction of elastically-active network segments, as well as diffusive coupling between the microstructural conformations and the local state of stress in regions with large spatial gradients of local deformation. These models incorporate a discrete version of the micellar breakage and reforming dynamics originally proposed by Cates and capture, at least qualitatively, non-affine tube deformation and chain disentanglement. The `flow curves' of stress and apparent shear rate resulting from an assumption of homogeneous deformation is non-monotonic and linear stability analysis shows that the region of non-monotonic response is unstable. Calculation of the full inhomogeneous flow field results in localized shear bands that grow linearly in extent across the gap as the apparent shear rate increases. Time-dependent calculations in step strain, large amplitude oscillatory shear (LAOS) and in start up of steady shear flow show that the velocity profile in the gap and the total stress measured at the bounding surfaces are coupled and evolve in a complex non-monotonic manner as the shear bands develop and propagate.   

Rheology and Acoustics of Highly Concentrated Wormlike Micellar Fluids

Wormlike micellar fluids have long been studied as a model non-Newtonian fluid. The dynamic microstructure of the fluid gives rise to a rich diversity of hydrodynamic phenomenon. Generally, these fluids are studied in a low concentration regime (0-20 mM of surfactant). In this talk, we will discuss recent rheological and acoustic measurements of highly concentrated wormlike micellar fluids with concentrations of 50 - 400 mM cetyltrimethylammonium bromide (CTAB) and 30 - 240 mM (respectively) sodium salicylate (NaSAL) in the temperature range of 22 - 45 Celsius. In 200 mM CTAB fluids, the activation energy, derived from stress relaxation time measurements, exhibit a discontinuous shift from about 60 kT below 33 celsius to about 40 kT above. Speed of sound and acoustic attenuation measurements were obtained by a broadband Fourier spectroscopic method using ultrasonic waves between 2 and 8 MHz. Each of the concentrations measured exhibits an peak in the acoustic attenuation between 33 and 38 Celsius. The speed of sound increases monotonically over 22 - 40 Celsius, very similar to pure water in both magnitude and temperature dependence. Attenuation measurements as a function of acoustic power using high intensity focused ultrasound will also be discussed.   

The flow of wormlike micelles in microchannels: a micro-PIV study of shear-banding, interfacial instabilites and tracers migration

We characerize by Particle Image Velocimetry the Poiseuille flow a semi-dilute solution of wormlike micelles (a CTAB and sodium nitrate aqueous solution) in pressure resistant microchannels. Thanks to the high aspect ratio of our channels, we can measure the local rheology of the solution, independantly from the slippage at the wall, according to a method already validated on non-newtonian polymer solutions. As the pressure driving the flow is increased, the velocity profiles reveal first a newtonian phase, then apparition of a dramatically lower viscosity second phase at the walls, which is the so called shear banding regime. First we deduce the local rheology of the solution from these velocity profiles, in agreement with the macroscopic rheology obtained in Couette rheometers. Then we study the development of an instability at the interface between the two phases, with a wavevector in the vorticity direction and a wavelength corresponding to smallest dimension of the channel. Finally we discuss the hypothesis of passive tracers : depending on their size, we observe a tracer depletion in the high-shear phase, which may be to high normal forces.   

Reversible and irreversible flow-induced phase transition in micellar solutions

It is well known that wormlike micelles form shear-induced structures (SIS). SIS formation is typically accompanied by the appearance of a gel-like phase. While both configurational dynamics of the micelles in flow and electrostatics are recognized as the key factors that influence such phase transitions, there are no universally applicable criteria for the onset strain rate as function of salt concentration. In this work, first, we examine the effect of salt concentration on the critical strain rate for CTAB/NaSal solutions and show that a ``self-similar'' phase transition regime exists. Second, we show that under strong (elongational) flow conditions, the phase transitions are irreversible, leading to the formation of gels that are stable even after the flow is stopped. Results obtained from atomic force microscopy studies of the structure of such gels will be presented.   

Investigating the structures and phase behavior of anionic perfluorinated surfactant using SANS

We have examined the structures formed by mixtures of tetraethylammonium perfluorooctylsulfonate (TEAFOS) and lithium perfluorooctylsulfonate (LiFOS) in water using small angle neutron scattering (SANS). SANS is an ideal method to characterize the morphology of such soft materials because the wavelength of the cold neutrons is comparable to the characteristic length scale of the surfactant structures and the possibility to apply the contrast variation technique. Results were obtained for mixtures at a constant surfactant concentration of 100 mM and different TEA/Li ratios at a temperature of 30\r{ }C. SANS curves were obtained either from the fluorinated micellar core or from the hydrogenated counterion atmosphere surrounding the micelles applying contrast matching. A transitional change in shape from spherical to prolate micelles was observed for TEA fractions greater than 30 mM. For TEA fractions greater than 55mM, threadlike micelle structures are present. From the SANS data we are able to correlate the counterion binding of the two different species to the shape and size of the micellar structure and confirm the role that the counterion environment plays in macroscopic rheological properties.   

Fabrication of phospholipid vesicles from double emulsions in microfluidics

Phospholipids self-assemble into lipid vesicles also known as liposomes. The formation of liposomes via conventional techniques such as electroformation has been studied extensively. However, the liposomes formed through electroformation are polydisperse and have low encapsulation efficiency. We present a new method to fabricate monodisperse phospholipid vesicles with high encapsulation efficiency from water-in-oil-in-water double emulsions. We generate phospholipid stabilized monodisperse double emulsions using a glass microcapillary device. This process allows efficient encapsulation within the inner aqueous drop. The middle oil phase is a volatile organic solvent in which phospholipids are dissolved. As the organic solvent evaporates, phospholipids self-organize into vesicles. This technique is versatile in the choice of phospholipids and we have generated vesicles from different types of phospholipids.   

Concentration dependence of dynamics of a droplet microemulsion

We will present a concentration dependence of dynamics of a spherical droplet microemulsion, consisting of aerosol-OT (AOT), as a surfactant, water, and decane. This mixture forms spherical microemulsion in a wide range of concentration. With keeping water to surfactant ratio constant, concentration of water plus surfactant was changed. The static structure of this system has been determined by small-angle neutron scattering (SANS) using the relative form factor method in the droplet concentration range from 5 to 75 {\%} [1]. Dynamics of droplet microemulsions have been determined using neutron spin echo technique, which is suitable to measure dynamics of systems in nanometer and nanosecond scales. We measured dynamics of the system at 5, 30, and 60 {\%} of droplet concentration. Using analogous data reduction procedure to SANS, contribution of shape fluctuations is decoupled from structure fluctuations. Concentration dependence of shape fluctuations and structure fluctuations will be discussed. [1] M. Nagao et al., Phys. Rev. E 75, 061401 (2007).   

Dielectrophoresis of Functional Phospholipid Vesicles

Recently, there has been an emerging interest in using AC-dielectrophoresis (DEP) to transport and assemble phospholipid vesicles (liposomes) and nanoparticles to form functional bio-assemblies where the underlying charge polarization mechanism of colloids in AC fields strongly depends on nano-scaled surface charge. In this work, we study liposomes segregation and aggregation in the presence of nanocolloids and salts in which the biological functionality of liposomes is augmented by the physical functionality of inorganic coating and particles. Liposomes, synthesized by sonication with 1,2-Dioleoyl-sn-Glycero-3-Phosphate (DOPA), are manipulated at varied AC-field frequencies across fabricated micro-electrodes in a quadrapole configuration on glass. We observe the co-assembly of liposome and opposite-charged nanocolloids by confocal microscopy and SEM, where the smaller nanocolloids are captured in between liposome junctions to form stabilized composite vesicles at several distinct frequencies. We observe a strong dependence of the liposome DEP mobility on the number of nanoparticles present in suspension and propose a new mechanism based on charge segregation and charged nanocolloid entrainment in the double layer.   

High-throughput Microsphere Encapsulation in Emulsion Droplets by Electrospray

Colloidal clusters generated through emulsion encapsulation and evaporation open up the possibilities for assembly of complex crystal structures. Encapsulation in monodisperse emulsion droplets facilitates higher yield of identical clusters as building blocks. We utilize electrospray in an oil-in-water co-flow fluidic device to generate uniform emulsion droplets in micron size and at the rate of ten thousand droplets per second. We investigate the effect of applied voltage, flow rate and the conductivity of liquid on the droplet formation. We further show that incorporation of microspheres into the inner oil fluid enables the encapsulation and formation of clusters.