Joshua J. Cardiel

Microstructure and rheology of a flow-induced structured phase in wormlike micellar solutions

Surfactant molecules can self-assemble into various morphologies under proper combinations of ionic strength, temperature, and flow conditions. At equilibrium, wormlike micelles can transition from entangled to branched and multiconnected structures with increasing salt concentration. Under certain flow conditions, micellar structural transitions follow different trajectories. In this work, we consider the flow of two semidilute wormlike micellar solutions through microposts, focusing on their microstructural and rheological evolutions. Both solutions contain cetyltrimethylammonium bromide and sodium salicylate. One is weakly viscoelastic and shear thickening, whereas the other is strongly viscoelastic and shear thinning. When subjected to strain rates of ∼103 s−1 and strains of ∼103, we observe the formation of a stable flow-induced structured phase (FISP), with entangled, branched, and multiconnected micellar bundles, as evidenced by electron microscopy. The high stretching and flow alignment in the microposts enhance the flexibility and lower the bending modulus of the wormlike micelles. As flexible micelles flow through the microposts, it becomes energetically favorable to minimize the number of end caps while concurrently promoting the formation of cross-links. The presence of spatial confinement and extensional flow also enhances entropic fluctuations, lowering the energy barrier between states, thus increasing transition frequencies between states and enabling FISP formation. Whereas the rheological properties (zero-shear viscosity, plateau modulus, and stress relaxation time) of the shear-thickening precursor are smaller than those of the FISP, those of the shear-thinning precursor are several times larger than those of the FISP. This rheological property variation stems from differences in the structural evolution from the precursor to the FISP.

A stable flow-induced structured phase in wormlike micellar solutions

A stable flow-induced transition of a wormlike micellar solution to a structured solution phase is studied. This transition occurs when the solution is flowed through a specially designed microfluidic device. Rheological properties of the structured phase are presented along with a simple model for structure formation. We model the solution and structure formation using a network scission model that contains two species of interacting, elastically-active micellar chains. The model permits transition to the structured state when the chains experience a high degree of elongation primarily due to extensional strains that are present in the microchannel flow but absent in conventional shear flows.

Nanoporous Scaffold with Immobilized Enzymes during Flow‐Induced Gelation for Sensitive H2O2 Biosensing

www.MaterialsViews.com C O M M U Nanoporous Scaffold with Immobilized Enzymes during Flow-Induced Gelation for Sensitive H 2 O 2 Biosensing N IC By Donglai Lu , Joshua Cardiel , Guozhong Cao , and Amy Q. Shen * A IO N Biosensors play indispensible roles in disease diagnosis, drug screening, and forensic applications, while nanoporous scaffolds hold an enormous potential in improving the performance of biosensors. [ 1 ] One important area of biosensor research is the immobilization of enzymes with retained or enhanced activities and lifetimes as it is critical to enhance biosensor performance. [ 2–10 ] In spite of signifi cant advances made recently for enzyme immobilization such as covalent binding, [ 3–5 ] direct adsorption, [ 6 , 7 ] and entrapment in different substrate materials, [ 8–10 ] the design of a simple, in-situ, and cost-effective process that can be reliably deployed in immobilization of enzymes while retaining the enzyme’s native stabilities and reactivities remains a signifi cant challenge. [ 11 , 12 ] For example, the standard fabrication process for enzyme immobilization is usually very complicated and costly; [ 3 , 4 , 7 , 10 ] the immobilized enzyme molecules tend to have non-uniform distribution in the host matrix, easily denature, leach, and lose activities over time. Further, the host matrices are often not highly biocompatible and bristle in nature. Here, we present a novel and versatile fl ow-induced gelation method to immobilize enzymes inside nanoporous scaffolds to meet these challenges. The biosensor designed by the nanoporous scaffold shows high sensitivity, stability, selectivity, and good precision. The novel enzyme immobilization method introduced here is based on our recent work of forming stable nanoporous scaffolds with proper hydrodynamic conditions for a given self-assembly precursor (see Figure 1 and the Supporting Information, SI). [ 13 ] When subject to fl ow, fl ow-induced structure formation of self-assembled surfactant micelles occurs in a narrow range of concentrations of specifi c ionic surfactant solutions with added salts. Existing work on phase transitions in macroscopic geometry, has been reported to be reversible under shear fl ow conditions. [ 14 ] However, the major challenge of utilizing shear-induced structures as nanotemplates is the structure breakdown and disintegration once the fl ow is stopped. We are able to obtain irreversible nanoporous scaffolds by using specially designed microfl uidic devices (see Figure 1 ). The irreversible gel formation results from the large shear and extension strain rates and total strain generated by the fl ow through the device, under mixed extensional and shear

Flow-induced structured phase in nonionic micellar solutions.

In this work, we consider the flow of a nonionic micellar solution (precursor) through an array of microposts, with focus on its microstructural and rheological evolution. The precursor contains polyoxyethylene(20) sorbitan monooleate (Tween-80) and cosurfactant monolaurin (ML). An irreversible flow-induced structured phase (NI-FISP) emerges after the nonionic precursor flows through the hexagonal micropost arrays, when subjected to strain rates ~10(4) s(-1) and strain ~10(3). NI-FISP consists of close-looped micellar bundles and multiconnected micellar networks as evidenced by transmission electron microscopy (TEM) and cryo-electron microscopy (cryo-EM). We also conduct small-angle neutron scattering (SANS) measurements in both precursor and NI-FISP to illustrate the structural transition. We propose a potential mechanism for the NI-FISP formation that relies on the micropost arrays and the flow kinematics in the microdevice to induce entropic fluctuations in the micellar solution. Finally, we show that the rheological variation from a viscous precursor solution to a viscoelastic micellar structured phase is associated with the structural evolution from the precursor to NI-FISP.

Formation of crystal-like structures and branched networks from nonionic spherical micelles.

Crystal-like structures at nano and micron scales have promise for purification and confined reactions, and as starting points for fabricating highly ordered crystals for protein engineering and drug discovery applications. However, developing controlled crystallization techniques from batch processes remain challenging. We show that neutrally charged nanoscale spherical micelles from biocompatible nonionic surfactant solutions can evolve into nano- and micro-sized branched networks and crystal-like structures. This occurs under simple combinations of temperature and flow conditions. Our findings not only suggest new opportunities for developing controlled universal crystallization and encapsulation procedures that are sensitive to ionic environments and high temperatures, but also open up new pathways for accelerating drug discovery processes, which are of tremendous interest to pharmaceutical and biotechnological industries.

Worming their way into shape: toroidal formations in micellar solutions.

We report the formation of nanostructured toroidal micellar bundles (nTMB) from a semidilute wormlike micellar solution, evidenced by both cryogenic-electron microscopy and transmission electron microscopy images. Our strategy for creating nTMB involves a two-step protocol consisting of a simple prestraining process followed by flow through a microfluidic device containing an array of microposts, producing strain rates in the wormlike micelles on the order of 10(5) s(-1). In combination with microfluidic confinement, these unusually large strain rates allow for the formation of stable nTMB. Electron microscopy images reveal a variety of nTMB morphologies and provide the size distribution of the nTMB. Small-angle neutron scattering indicates the underlying microstructural transition from wormlike micelles to nTMB. We also show that other flow-induced approaches such as sonication can induce and control the emergence of onion-like and nTMB structures, which may provide a useful tool for nanotemplating.

Formation and flow behavior of micellar membranes in a T-shaped microchannel

Understanding the formation and instability behavior of membranes is of fundamental interest and practical relevance to various biotechnological applications and self-assembly systems. Surfactant micellar membranes serve as a simple model system when surfactant molecules self-assemble into micellar structures under flow, but observing such process in real time is a major challenge due to limitations in spatiotemporal resolutions. We use a simple T-shaped microchannel to capture the formation and flow behavior of an ionic surfactant micro-micellar-membrane (μMM) when an aqueous stream of organic salt sodium salicylate (NaSal) meets a stream of cationic surfactant cetyltrimethylammonium bromide (CTAB). The μMM is shown to grow and become unstable depending on the flow rate, as characterized using micro-particle image velocimetry, fluorescence microscopy, flow birefringence, and bulk rheometry. We propose a simple model that accounts for the flow, elasticity and inertia of the μMM to analyze its flow behavior. Our experimental protocol can be easily replicated in conventional laboratories without the need of utilizing sophisticated equipment such as synchrotron small angle X-ray scattering and micro-electronics circuits. Our combined experimental and modeling results can be extrapolated to provide new insights to study the flow behavior and thermodynamic phases of lipid membranes, membrane proteins, and biological membranes.