Shear-induced transitions and instabilities in surfactant wormlike micelles

In this review, we report recent developments on the shear-induced transitions and instabilities found in surfactant wormlike micelles. The survey focuses on the non-linear shear rheology and covers a broad range of surfactant concentrations, from the dilute to the liquid-crystalline states and including the semi-dilute and concentrated regimes. Based on a systematic analysis of many surfactant systems, the present approach aims to identify the essential features of the transitions. It is suggested that these features define classes of behaviors. The review describes three types of transitions and/or instabilities : the shear-thickening found in the dilute regime, the shear-banding which is linked in some systems to the isotropic-to-nematic transition, and the flow-aligning and tumbling instabilities characteristic of nematic structures. In these three classes of behaviors, the shear-induced transitions are the result of a coupling between the internal structure of the fluid and the flow, resulting in a new mesoscopic organization under shear. This survey finally highlights the potential use of wormlike micelles as model systems for complex fluids and for applications.Comment: 64 pages, 31 figures, 2 table

STRUCTURE, DYNAMICS AND RHEOLOGY OF SURFACTANT MICELLES AND MICELLE-NANOPARTICLE SOLUTIONS: A MOLECULAR DYNAMICS STUDY

Surfactant micelles are widely used in a number of industrial, commercial and household products and processes. Understanding flow-microstructure coupling in micellar systems can benefit applications ranging from targeted drug delivery and detergency to enhanced oil recovery and hydrofracking. Amongst micellar fluids, wormlike micelles (WLMs) are extremely interesting due to their structural similarity to polymers and their ability to constantly undergo scission and recombination at equilibrium. More recently, much has been generated in studying the effect of adding colloidal particles to WLMs. Colloidal particles can not only add functionality to the fluid but also act as viscosity modifiers. Such solutions can be used to design active nanomaterials for applications in energy harvesting and sensing. While several theories and continuum-level computational models have been developed to study the dynamics and rheology of WLMs, molecular-level explorations of the flow-structure coupling in such solutions is lacking. Further, in the case of mixtures of colloidal particles and WLMs, there are only a handful of attempts to develop theoretical/computational frameworks capable of describing their thermodynamics, self-assembly and phase behavior. The goals of this thesis are to uncover mechanisms by which WLMs interact with colloidal particles and to determine how these interactions affect the macroscopic properties of mixtures of model WLMs and colloidal nanoparticles (NPs) using molecular dynamics (MD) simulations. Coarse-grained (CG) molecular models and corresponding force-fields are employed to describe the NP, cationic cetyltrimethylammonium chloride (CTAC) surfactant, hydrotropic sodium salicylate (NaSal) salt, solvent and the underlying physico-chemical interactions. Results are first presented for the dynamics of a single self-assembled rodlike micellar aggregate under shear flow. The effect of shear rate on the configurational dynamics, e.g. orientation distribution of the end-to-end vector and tumbling frequency are presented and compared to experimental observations as well as predictions from stochastic simulations and mesoscopic theories. Further, a relationship between micelle length and stretching force is presented and compared with experimental estimates of similar forces in biological systems. Finally, a shear rate-independent energy barrier for micelle scission is identified for relatively large shear rates. We also show that the addition of NPs to surfactant solutions can result in the formation of NP-surfactant complexes (NPSCs). The effect of NP charge and surface chemistry on the nature of the self-assembly is discussed. Further, such NPSCs can further interact with WLMs, in the presence of NaSal salt, to form electrostatically stabilized micelle-NP junctions via an end cap attachment mechanism. The dynamics, energetics and stability of such junction formation is also described in detail. These junctions can give rise to unique rheological modifications of WLMs such as significant buildup in viscosity and viscoelasticity. Large-scale equilibrium and non-equilibrium MD simulations consisting of several NPs and WLMs are performed to study the flow-microstructure coupling in such systems. The relationship between the zero-shear viscosity, NP volume fraction and salt concentration at a fixed surfactant concentration is presented. Shear thinning behavior is observed for all of the systems studied. Shear thinning is accompanied by flow-alignment and shear-induced isotropic-to-nematic transitions in micellar systems. Further, the evolution of the first normal stress difference, N1, is presented as a function of time and shear rate, and compared with experimental observations for similar systems. The results of this work provides insight into the mechanisms of self-assembly in WLMs and colloidal NPs and demonstrate that rheological properties of WLMs can be uniquely controlled by the addition of NPs

Slippage and Migration in Taylor-Couette Flow of a Model for Dilute Wormlike Micellar Solutions

Submitted to J. Non-Newt Fluid Mechanics, June 2005In this paper we explore a model, most appropriate for dilute or semi-dilute worm-like micellar solutions, in an axisymmetric circular Taylor-Couette geometry. This study is a natural continuation of earlier work on rectilinear shear flows. The model, based on a bead-spring microstructure with nonaffine motion, reproduces the pronounced plateau in the stress strain-rate flow curve as observed in laboratory measurements of steady shearing flows. We also carry out a linear stability analysis of the computed steady state solutions. The results show shear-banding in the form of sharp changes in velocity gradients, spatial variations in number density, and in alignment or stretching of the micelles. The velocity profiles obtained in numerical solutions show strong qualitative agreement with those of laboratory experiments.NSF Collaborative Research Projec

MOLECULAR DYNAMICS STUDY ON THE STRUCTURE, DYNAMICS AND STRESS RESPONSE OF DILUTE MICELLAR SYSTEMS IN UNIAXIAL EXTENSIONAL DEFORMATION

Micellar structures have been proposed for potential application in hydrotropy, biomimetics, dispersion and emulsification, enhanced oil recovery, detergency, templating, drug delivery, personal care products, drag reduction, nanoscale reaction vessels, therapeutic gene delivery, bio-catalysis and so on. Though several studies exist, there still remains a gap in the current knowledge on structural response of single micelles in solution to uniaxial extensional flow deformation. These knowledge gaps are possibly due to the inability of traditional experimental studies to investigate micellar properties at the time- and length-scale pertinent to self-assembly and micellar dynamics. To this end, this work aims to utilise coarse-grained molecular dynamics simulations to investigate the dynamics and structural response of various infinitely dilute micellar solutions under the influence of uniaxial extensional flow. Spherical vesicles formed from hexacosanoate anion and octyltrimethylammonium cation; rod-like and worm-like micelles formed from hexacosanoate and palmitate anions; and branched worm-like micelles formed from cetyltrimethylammonium cation and sodium salicylate anion have been parametrised according to the Martini force field formalism. These structures were simulated in equilibrium; under uniaxial extensional flow; and in cessation of uniaxialextensional flow. Changes in micellar structure in uniaxial extensional flow and subsequent stress responses are presented for each micellar system at varying deformation rates. It is observed that structural changes and stress response are dependent on micellar stress relaxation ability whilst undergoing uniaxial deformation. The nature and varying influence of stress relaxation as a function of deformation rate is studied for each structure. Deformation of these structures in a direction normal to their principal orientation is also investigated. It is shown that orientation has a short-term effect on the dynamics and structural evolution of non-isotropic micellar structures. Finally, structural and stress responses following cessation of uniaxial extensional flow are presented

Structure, Rheology and Optical Properties of Plasmonic Fluids

Fluids with tunable optical and rheological properties are of fundamental and practical interest. They can be easily processed to manufacture thin films and interfaces for applications such as molecular detection and light trapping in photovoltaics. Cationic surfactants such as cetyl-trimethylammonium bromide have the ability to self assemble with metallic nanoparticles to form a corona or a double-layer vesicular structure. These structures upon further interaction with wormlike micelle fragments are hypothesized to form micelle-nanoparticle elastic networks. In this dissertation, solution phase self-assembly is utilized to uniformly distribute various metallic nanoparticles to produce stable multicomponent plasmonic fluids with remarkable color uniformity. The optical properties of the fluids can be robustly tuned by varying the species, concentration, size and/or shape of the nanoparticles. Multicomponent plasmonic fluids capable of broadband absorption of visible light are produced via the self-assembly route. Small angle X-ray scattering and rheological studies suggest that the nanoparticles are incorporated into the wormlike micelle network to form a more compact double network. These fluids exhibit rich rheological behavior depending on the nanoparticle concentration and the salt to surfactant molar ratio. Specifically, non-monotonic dependence of zero shear viscosity on nanoparticle concentration, rheopexy, shear thickening, shear banding and shear thinning are observed. The fluids exhibit enhanced viscoelasticity upon the addition of more nanoparticles. The mechanical, rheological and optical properties of plasmonic fluids greatly depend upon the temperature due to the structural changes of the micellar solutions. The application of plasmonic fluids to efficient light trapping in photovoltaic cells, plasmon-enhanced microalgal growth and optofluidic devices have been designed and demonstrated in this dissertation

Tailoring the self-assembly, interfacial properties and rheological behaviour of sugar-based surfactants

Surfactants are important ingredients in many formulated products used in everyday life. Many of these surfactants originates from fossil-based materials and degrades slowly in aquatic systems. As society strives towards having a smaller environmental footprint, surfactants that are non-toxic, biodegradable and can be synthesized from renewable raw materials need to be developed. To this end sugar-based surfactants, or alkylglycosides, is a promising class of surfactants that have the properties that are sought after. To utilize these surfactants to their full potential more knowledge is required about their behaviour both at interfaces and in solution.In this thesis the behaviour of the alkylglycoside C16G2, and how it is affected by small changes in its molecular structure, has been studied with scattering techniques and rheometry. The anomeric configuration was found to have a large impact as β-C16G2 allowed for a more efficient packing of the headgroup, compared to α-C16G2. This is apparent both at interfaces, where the adsorption was higher for the β anomer, and in solution, where β-C16G2 forms very elongated worm-like micelles while α-C16G2 forms shorter cylindrical micelles. The difference in self-assembly affects the rheological behaviour of these solutions where highly viscous, shear thinning and viscoelastic properties reflects the long micelles of β-C16G2, while solutions of α-C16G2 are Newtonian with low viscosity. The effect of introducing a double bond in the tailgroup of β-C16G2 was also investigated, where a significant decrease in Krafft point was seen, while the formation of worm-like micelles and viscous solutions was still evident

Crystallization-Driven Self-Assembly of Coil-Comb-Shaped Polypeptoid Block Copolymers: Solution Morphology and Self-Assembly Pathways

Copyright © 2019 American Chemical Society. Crystallization-driven self-assembly (CDSA) of amphiphilic polymers into well-defined nanoscopic structures with different morphologies and functionalities has attracted increasing attention. Here, we investigate the CDSA of coil-comb-shaped diblock copolypeptoids, namely, poly(N-methyl glycine)-b-poly(N-decyl glycine) (PNMG-b-PNDG), in dilute methanol solution using X-ray/neutron solution scattering in conjunction with cryogenic transmission electron microscopy techniques. A series of PNMG-b-PNDGs were synthesized by sequential benzyl amine-initiated ring-opening polymerizations of the corresponding N-substituted N-carboxyanhydrides, in which the degree of polymerization and the length of the blocks were varied. The PNMG-b-PNDG polymers with a lower volume fraction of the crystalline PNDG blocks (fPNDG = 0.44) were found to slowly self-assemble into one-dimensional long wormlike nanofibrils in methanol. The nanofibrils bear an anisotropic crystalline core where the comb-shaped PNDG blocks were stacked in a face-to-face fashion along the long axis of the nanofibrils. Upon increasing fPNDG to 0.61 and 0.68, the final morphology of PNMG-b-PNDG micelles changed from wormlike nanofibrils to rigid short nanorods and then two-dimensional nanosheets. The nanofibrils were formed by a self-seeding growth pathway that involves the initial formation of a few seeded crystals followed by the addition of soluble unimers to the preferred crystal facets resulting in the gradual elongation of the micelles. By contrast, the nanorods were formed by a two-stage process involving the formation of spherical micelles with an amorphous core in the first stage and rapid confined crystallization of the micellar core and their fusion into rodlike nanostructures at the second stage. Understanding the relationship between chemical composition, micellar morphology, and CDSA pathway of coil-comb-shaped diblock copolypeptoids is an important step toward the rational design of anisotropic polymeric nanostructures with tailorable morphology

Spatially resolved quantitative rheo-optics of complex fluids in a microfluidic device

In this study, we use microparticle image velocimetry (μ-PIV) and adapt a commercial birefringence microscopy system for making full-field, quantitative measurements of flow-induced birefringence (FIB) for the purpose of microfluidic, optical rheometry of two wormlike micellar solutions. In combination with conventional rheometric techniques, we use our microfluidic rheometer to study the properties of a shear-banding solution of cetylpyridinium chloride (CPyCl) with sodium salicylate (NaSal) and a nominally shear-thinning system of cetyltrimethylammonium bromide (CTAB) with NaSal across many orders of magnitude of deformation rates (10-2 ≤ math ≤ 104s-1). We use μ-PIV to quantify the local kinematics and use the birefringence microscopy system in order to obtain high-resolution measurements of the changes in molecular orientation in the wormlike fluids under strong deformations in a microchannel. The FIB measurements reveal that the CPyCl system exhibits regions of localized, high optical anisotropy indicative of shear bands near the channel walls, whereas the birefringence in the shear-thinning CTAB system varies more smoothly across the width of the channel as the volumetric flow rate is increased. We compare the experimental results to the predictions of a simple constitutive model, and we document the breakdown in the stress-optical rule as the characteristic rate of deformation is increased.National Science Foundation (U.S.) (Graduate Research Fellowship