Perry Cheung

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.

In situ pressure measurement within deformable rectangular polydimethylsiloxane microfluidic devices

In this paper, we present a simple procedure to incorporate commercially available external pressure transducers into existing microfluidic devices, to monitor pressure-drop in real-time, with minimal design modifications to pre-existing channel designs. We focus on the detailed fabrication steps and assembly to make the process straightforward and robust. The work presented here will benefit those interested in adding pressure drop measurements in polydimethylsiloxane (PDMS) based microchannels without having to modify existing channel designs or requiring additional fabrication steps. By using three different devices with varying aspect ratio channels ([Formula: see text], width/depth), we demonstrate that our approach can easily be adapted into existing channel designs inexpensively. Furthermore, our approach can achieve steady state measurements within a matter of minutes (depending on the fluid) and can easily be used to investigate dynamic pressure drops. In order to validate the accuracy of the measured pressure drops within the three different aspect ratio devices, we compared measured pressure drops of de-ionized water and a 50 wt. % glycerol aqueous solution to four different theoretical expressions. Due to the deformability of PDMS, measured pressure drops were smaller than those predicted by the rigid channel theories (plate and rectangular). Modification of the rigid channel theories with a deformability parameter α provided better fits to the measured data. The elastic rectangular expression developed in this paper does not have a geometric restriction and is better suited for microchannels with a wider range of aspect ratios.

Microfluidic flows of wormlike micellar solutions

The widespread use of wormlike micellar solutions is commonly found in household items such as cosmetic products, industrial fluids used in enhanced oil recovery and as drag reducing agents, and in biological applications such as drug delivery and biosensors. Despite their extensive use, there are still many details about the microscopic micellar structure and the mechanisms by which wormlike micelles form under flow that are not clearly understood. Microfluidic devices provide a versatile platform to study wormlike micellar solutions under various flow conditions and confined geometries. A review of recent investigations using microfluidics to study the flow of wormlike micelles is presented here with an emphasis on three different flow types: shear, elongation, and complex flow fields. In particular, we focus on the use of shear flows to study shear banding, elastic instabilities of wormlike micellar solutions in extensional flow (including stagnation and contraction flow field), and the use of contraction geometries to measure the elongational viscosity of wormlike micellar solutions. Finally, we showcase the use of complex flow fields in microfluidics to generate a stable and nanoporous flow-induced structured phase (FISP) from wormlike micellar solutions. This review shows that the influence of spatial confinement and moderate hydrodynamic forces present in the microfluidic device can give rise to a host of possibilities of microstructural rearrangements and interesting flow phenomena.

Elastic instabilities in a microfluidic cross-slot flow of wormlike micellar solutions

We present a series of experiments involving an elastic instability that occurs with the flow of wormlike micellar solutions in a microfluidic cross-slot device. We use four different concentrations of an aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal); two highly viscoelastic and two weakly viscoelastic. Flow in the microfluidic cross-slot device is examined using birefringence and PIV measurements. With all of the solutions we observe the formation of a birefringent band along the outflow axis, the intensity of which increases with the flow rate. In the two highly viscoelastic solutions, as the flow rate increases, the instability results in flow transitioning from a stable symmetric flow to a stable asymmetric flow, to an unsteady asymmetric flow. With the weakly viscoelastic solutions the instability results in the flow transitioning directly from a stable symmetric flow to an unsteady flow. The critical Weissenberg numbers at which the transitions occur, increase with increasing elasticity number (defined as the ratio of the Weissenberg number to the Reynolds number). With the two highly viscoelastic solutions we also observe the formation of lip vortices along the walls of the inlet channels. For one of the highly viscoelastic solutions the symmetry-breaking occurs before the formation of the secondary flow, whereas in another solution the secondary flow develops prior to the symmetry-breaking. This indicates that these instabilities are complex phenomena, whose behavior is likely influenced by a combination of factors such as Weissenberg number, inertial effects, and rheological behavior of the micellar solutions.

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.

Local micelle concentration fluctuations in microfluidic flows and its relation to a flow-induced structured phase (FISP)

Shear-induced structures (SIS) are known to form in flows of wormlike micellar solutions. In simple shear cases these structures (SIS) are temporary and disintegrate upon cessation of the flow; while in certain mixed-flow cases these flow-induced structured phases (FISPs) are stable and long-lived. Here, we compare the flow of a micellar solution in a microfluidic device containing an array of microposts, with that of Poiseuille flow in a microchannel. In the former case a stable permanent FISP can be produced, whereas in the latter no structured phase (SIS or FISP) occurs. Using the fluorescent dye Nile Red, we are able to observe the local micelle concentration of the solutions during flow. We find that in the microfluidic device containing microposts, we can generate local concentration variation on the order of up to 25% that strongly correlates with the FISP, while in the microchannel with Poiseuille flow, no measurable concentration differences are observed. The relevance of these concentration variations for the formation of the FISP are discussed. The use of Nile Red as a probe into the local micelle concentration may also serve as a simple and complementary optical tool (i.e. with birefrigence and light scattering) towards investigation of a variety of flow-induced structures, as opposed to more complicated and expensive scattering techniques that involve neutrons and X-rays.

The freedom of confinement in complex fluid

When it comes to self-assembly of photonic, drug-delivery, and biomimetic materials, big opportunities can be found in small spaces.

Probing liquid distribution in partially saturated porous materials with hydraulic admittance

The distribution of two immiscible fluids in a complex porous material during displacement is often central to understanding its function. Characterization of this distribution is traditionally determined via optically transparent flow cells. However, for opaque or thin porous materials of the order of hundreds of microns, optical visualization proves to be difficult and requires sophisticated imaging techniques that are expensive and difficult to come by. We describe here a bench-top tool that dynamically probes the hydraulic pathways leading to each free-interface within a single capillary and a bundle of seven capillaries at various saturations (i.e., hydraulic path lengths). A small volumetric displacement was applied to each interface such that the interfaces remained pinned at the capillary walls and the resultant oscillatory pressure drop was measured to determine the hydraulic admittance at each applied oscillation frequency. When the magnitude of the hydraulic admittance was plotted vs. applied oscillation frequency, a resonance peak was found for each degenerately filled capillary. The corresponding peaks were represented by a half-loop (100% filled) and full loops (partially filled) in a Nyquist plot. We compared the theoretical and measured admittance curves and found good agreement for both capillary systems at high filled states. The theoretical predictions became worse when the hydraulic path length was comparable to the capillary radius. The analysis for the hydraulic admittance of a bundle of capillaries is developed here and experimentally validated for the first time.

Effects of Air on Enclosure Design of Lead-Zirconate-Titanate (PZT) Thin-Film Diaphragm Microactuators

In this paper, we evaluate performance of two types of piezoelectric diaphragm micro-actuators: open-end design and closed-end design. In the open-end design, the micro-actuator consists of a base silicon diaphragm, a layer of bottom electrode, a layer of lead-zirconate-titanate (PZT) thin film, and a layer of top electrode. The diaphragm is anchored on a silicon substrate by etching the silicon substrate from the back to form a cavity under the diaphragm. In the closed-end design, the bottom of the cavity is sealed with a piece of glass, silicon or PDMS. Experimental results show that the measured displacements from the closed-end design are always 5%–30% lower than those from the open-end design. To explain the experimental results, we hypothesize that the air inside the cavity of the closed-end design behaves like an elastic spring increasing the stiffness of the closed-end design. To confirm the hypothesis, we estimate the stiffness of the air by modeling the air as an ideal gas with a constant temperature. We also model the diaphragm as a lumped spring. Combination of the stiffness from the diaphragm and the air predicts the overall stiffness and displacement of the closed-end design. The predictions agree well with the experimental measurements, indicating that the air in the cavity significantly stiffens the closed-end design.Copyright