Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Polymers for enhanced oil recovery: A paradigm for structure–property relationship in aqueous solution

This paper reports a plug-based, microfluidic method for performing multi-step chemical reactions with millisecond time-control. It builds upon a previously reported method where aqueous reagents were injected into a flow of immiscible fluid (fluorocarbons)
				(H. Song et al., Angew. Chem. Int. Ed., 2003, 42, 768). The aqueous reagents formed plugs – droplets surrounded and transported by the immiscible fluid. Winding channels rapidly mixed the reagents in droplets. This paper shows that further stages of the reaction could be initiated by flowing additional reagent streams directly into the droplets of initial reaction mixture. The conditions necessary for an aqueous stream to merge with aqueous droplets were characterized. The Capillary number could be used to predict the behavior of the two-phase flow at the merging junction. By transporting solid reaction products in droplets, the products were kept from aggregating on the walls of the microchannels. To demonstrate the utility of this microfluidic method it was used to synthesize colloidal CdS and CdS/CdSe core-shell nanoparticles.

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Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system

Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: Prospects and challenges

Shear thickening is a type of non-Newtonian behavior in which the stress required to shear a fluid increases faster than linearly with shear rate. Many concentrated suspensions of particles exhibit an especially dramatic version, known as Discontinuous Shear Thickening (DST), in which the stress suddenly jumps with increasing shear rate and produces solid-like behavior. The best known example of such counter-intuitive response to applied stresses occurs in mixtures of cornstarch in water. Over the last several years, this shear-induced solid-like behavior together with a variety of other unusual fluid phenomena has generated considerable interest in the physics of densely packed suspensions. In this review, we discuss the common physical properties of systems exhibiting shear thickening, and different mechanisms and models proposed to describe it. We then suggest how these mechanisms may be related and generalized, and propose a general phase diagram for shear thickening systems. We also discuss how recent work has related the physics of shear thickening to that of granular materials and jammed systems. Since DST is described by models that require only simple generic interactions between particles, we outline the broader context of other concentrated many-particle systems such as foams and emulsions, and explain why DST is restricted to the parameter regime of hard-particle suspensions. Finally, we discuss some of the outstanding problems and emerging opportunities.

https://iopscience.iop.org/article/10.1088/0034-4885/77/4/046602/pdf

Shear thickening in concentrated suspensions: phenomenology, mechanisms and relations to jamming

The effect of copolymers on the breakup and coalescence of polybutadiene (PB) drops in polydimethylsiloxane (PDMS) is studied using a four-roll mill flow cell. Copolymers are produced at the interface by a reaction between functionalized homopolymers. They reduce the interfacial tension and thus enhance breakup; they also inhibit coalescence of drops. Under the conditions of our experiments, the latter effect is much more significant than the former. For example, the addition of copolymer sufficient to reduce the interfacial tension by only 3% relative to the bare interface value is found to reduce the critical capillary number Cac for coalescence by a factor of 6. The critical capillary number for coalescence in the absence of copolymer is also measured for the first time. It is found to scale with the drop radius a as Cac∼a−0.82±0.03 and with the viscosity ratio λ as Cac∼λ−0.41±0.06.

http://physics.nyu.edu/pine/reprints/2000-03-00-PhysFluids-Hu.pdf

Drop deformation, breakup, and coalescence with compatibilizer

Shear thickening of low-concentration solutions of wormlike micelles is investigated using simultaneous rheological and visualization measurements. Shear-induced structures (SISs) are directly visualized in transparent Couette cells using a laser light scattering technique similar to dark-field microscopy. From these measurements, four different regimes of behavior are identified. In regime I, which occurs below a critical shear stress σc, the shear rate increases monotonically with stress and no shear thickening or SISs are observed. In regime II, which occurs for stresses greater than σc but less than σs, SISs nucleate inhomogeneously and grow from the inner cylinder of the Couette cell. In this regime, the steady state shear rate initially decreases with increasing stress and then increases again as the stress is raised. The steady state in regime II is characterized by two coexisting states separated by a cylindrical interface (concentric with the Couette cylinders). Near the inner cylinder, viscous S...

/pdf/shear-thickening-in-low-concentration-solutions-of-wormlike-50sb6xrt16.pdf

Shear thickening in low-concentration solutions of wormlike micelles. I. Direct visualization of transient behavior and phase transitions

We have studied shear banding in Couette flow using a combination of particle tracking velocimetry (PTV), small angle light scattering, microscopic visualization, and flow birefringence. Time-resolved local shear rate characterization by PTV has enabled a direct study of the kinetics of shear banding. A first stage, which precedes banding, is tilting of the shear rate during which the local shear rate increases towards the inner and decreases towards the outer gap surface. A shear banding stage then proceeds with a low shear band growing away from the outer gap surface. Shear rate tilting is found to be due to a coupling of local shear thinning with the nonzero stress gradient of the flow geometry. The low shear band starts when the local shear rate at the outer surface touches down to a critical reentanglement shear rate. The effective lifetime of the shear bands is the same as the chain reentanglement time. These factors lead us to suggest that both the progression from tilt to shear banding and the int...

Kinetics and mechanism of shear banding in an entangled micellar solution

We observe a shear-flow-induced phase transition in wormlike micelle solutions which depends on whether the applied stress or the applied shear rate is held constant. When the shear stress is held constant, the system macroscopically phase separates into two coexisting phases with a single stable interface between them. By contrast, when the shear rate is held constant, the system still undergoes a phase transition, but the steady states are homogeneous and no coexistence is observed. This leads to dramatically different rheological behavior and unusual long-lived metastable states.

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Observation of bulk phase separation and coexistence in a sheared micellar solution

The rheology of the shear-thickened state is investigated in low-concentration solutions of wormlike micellar solutions using mechanical, optical, and velocity profile measurements. The zero-shear-rate viscosity of the solutions increases by more than a factor of 1000 as the concentration of surfactant is increased from 1 to 10 mM. By contrast, the apparent viscosity of the shear-thickened state of these same solutions is observed to be remarkably independent of concentration over a wide range of shear rates. This is shown to be a consequence of the development of slip layers between the very viscous gellike shear-induced structures (SISs) which form in the bulk of the surfactant solution and on the walls of the Couette devices in which the measurements are made. As the applied shear stress is increased even further, there is evidence that the SIS fractures give rise to a shear-rate-independent stress and an apparent viscosity, which decreases with increasing shear rate. After the SIS fractures, large flu...

http://physics.nyu.edu/pine/reprints/1998-09-00-JOR-Hu-II.pdf

Yuntao Hu

Papers

Drop deformation, breakup, and coalescence with compatibilizer

Shear thickening in low-concentration solutions of wormlike micelles. I. Direct visualization of transient behavior and phase transitions

Kinetics and mechanism of shear banding in an entangled micellar solution

Observation of bulk phase separation and coexistence in a sheared micellar solution

Shear thickening in low-concentration solutions of wormlike micelles. II. Slip, fracture, and stability of the shear-induced phase