Fluid Rheological Effects on the Flow of Polymer Solutions in a Contraction-Expansion Microchannel

Flow of wormlike micellar solutions over concavities

We present a comprehensive investigation combining numerical simulations with experimental validation, focusing on the creeping flow behavior of a shear-banding, viscoelastic wormlike micellar (WLM) solution over concavities with various depths (D) and lengths (L). The fluid is modeled using the diffusive Giesekus model, with model parameters set to quantitatively describe the shear rheology of a 100 : 60 mM cetylpyridinium chloride:sodium salicylate aqueous WLM solution used for the experimental validation. We observe a transition from "cavity flow" to "expansion-contraction flow" as the length L exceeds the sum of depth D and channel width W. This transition is manifested by a change of vortical structures within the concavity. For L ≤ D + W, "cavity flow" is characterized by large scale recirculations spanning the concavity length. For L > D + W, the recirculations observed in "expansion-contraction flow" are confined to the salient corners downstream of the expansion plane and upstream of the contraction plane. Using the numerical dataset, we construct phase diagrams in L-D at various fixed Weissenberg numbers Wi, characterizing the transitions and describing the evolution of vortical structures influenced by viscoelastic effects.

Elasto-inertial particle focusing in sinusoidal microfluidic channels

Dean flow existing in sinusoidal channels could enhance the throughput and efficiency for elasto-inertial particle focusing. However, the fundamental mechanisms of elasto-inertial focusing in sinusoidal channels are still unclear. This work employs four microfluidic devices with symmetric and asymmetric sinusoidal channels to explore the elasto-inertial focusing mechanisms over a wide range of flow rates. The effects of rheological property, flow rate, sinusoidal channel curvature, particle size, and asymmetric geometry on particle focusing performance are investigated. It is intriguing to find that the Dean flow makes a substantial contribution to the particle elasto-inertial focusing. The results illustrate that a better particle focusing performance and a faster focusing process are obtained in the sinusoidal channel with a small curvature radius due to stronger Dean flow. In addition, the particle focusing performance is also related to particle diameter and rheological properties, the larger particles show a better focusing performance than smaller particles, and the smaller flow rate is required for particles to achieve stable focusing at the outlet in the higher concentration of polyvinylpyrrolidone solutions. Our work offers an increased knowledge of the mechanisms of elasto-inertial focusing in sinusoidal channels. Ultimately, these results provide supportive guidelines into the design and development of sinusoidal elasto-inertial microfluidic devices for high-performance focusing.

Effects of Tween 20 addition on electrokinetic transport in a polydimethylsiloxane microchannel

Tween 20 is frequently added to particle suspensions for reducing the particle-wall adhesion and particle-particle aggregation in microfluidic devices. However, the influences of Tween 20 on the fluid and particle behaviors have been largely ignored. We present in this work the first experimental study of the effects of Tween 20 addition on the electrokinetic transport of fluids and particles in a polydimethylsiloxane microchannel. We find that adding 0.1% v/v Tween 20 to a buffer solution can significantly reduce the electroosmotic mobility as well as the electrokinetic and electrophoretic mobilities of polystyrene particles and yeast cells. Further increasing the Tween 20 concentration within the range typically used in microfluidic applications continues reducing these mobility values, but at a smaller rate. Our finding suggests that Tween 20 should be used with care in electrokinetic microdevices when the flow rate or particle/cell throughput is an important parameter.

Numerical Mechanistic Modelling of Drug Release from Solvent-Removal Zein-Based In Situ Gel

The development of effective drug delivery systems remains a focus of extensive research to enhance therapeutic outcomes. Among these, in situ forming gels (ISG) have emerged as a promising avenue for controlled drug release. This research focuses on the mathematical modeling of levofloxacin HCl (Lv) release from zein-based ISG using the cup method, aiming to mimic the environment of a periodontal pocket. The drug release behavior of the ISGs was investigated through experimental observations and numerical simulations employing forward and central difference formula. Notably, the experimental data for drug release from the 20% w/w zein-based ISG formulations closely aligned with the simulations obtained from numerical mechanistic modeling. In summary, 20% w/w zein-based ISG formulations demonstrated nearly complete drug release with the maximum drug concentration at the edge of the matrix phase values consistently around 100-105%, while 25% w/w zein-based ISG formulations exhibited somewhat lower drug release extents, with values ranging from 70-90%. Additionally, the rate of drug transport from the polymer matrix to the external phase influenced initial release rates, resulting in a slower release. The utilization of glycerol formal as a solvent extended drug release further than dimethyl sulfoxide, thanks to denser matrices formed by high-loading polymers that acted as robust barriers to solvent removal and drug diffusion. Furthermore, UV-vis imaging was utilized to visualize the matrix formation process and solvent diffusion within the ISGs. The imaging results offered valuable insights into the matrix formation kinetics, controlled drug release mechanisms, and the influence of solvent properties on drug diffusion. The combination of mathematical modeling and experimental visualization provides a comprehensive understanding of drug release from zein-based ISGs and offers a foundation for tailored drug delivery strategies.

Microfluidic Study of Enhanced Oil Recovery during Flooding with Polyacrylamide Polymer Solutions

A series of experiments have been carried out on the flooding of microfluidic chips simulating a homogeneous porous structure with various displacement fluids. Water and polyacrylamide polymer solutions were used as displacement fluids. Three different polyacrylamides with different properties are considered. The results of a microfluidic study of polymer flooding showed that the displacement efficiency increases significantly with increasing polymer concentration. Thus, when using a 0.1% polymer solution of polyacrylamide grade 2540, a 23% increase in the oil displacement efficiency was obtained compared to water. The study of the effect of various polymers on the efficiency of oil displacement showed that the maximum efficiency of oil displacement, other things being equal, can be achieved using polyacrylamide grade 2540, which has the highest charge density among those considered. Thus, when using polymer 2515 with a charge density of 10%, the oil displacement efficiency increased by 12.5% compared to water, while when using polymer 2540 with a charge density of 30%, the oil displacement efficiency increased by 23.6%.

An experimental study of the merging flow of polymer solutions in a T-shaped microchannel

The merging flow through a T-junction is relevant to sample mixing and particle manipulation in microfluidic devices. It has been extensively studied for Newtonian fluids, particularly in the high inertial regime where flow bifurcation takes place for enhanced mixing. However, the effects of fluid rheological properties on the merging flow have remained largely unexplored. We investigate here the flow of five types of polymer solutions along with water in a planar T-shaped microchannel over a wide range of flow rates for a systematic understanding of the effects of fluid shear thinning and elasticity. It is found that the merging flow near the stagnation point of the T-junction can either be vortex dominated or have unsteady streamlines, depending on the strength of elasticity and shear thinning present in the fluid. Moreover, the shear thinning effect is found to induce a symmetric unsteady flow in comparison to the asymmetric unsteady flow in the viscoelastic fluids, the latter of which exhibits greater interfacial fluctuations.

Levofloxacin HCl-Incorporated Zein-Based Solvent Removal Phase Inversion In Situ Forming Gel for Periodontitis Treatment

Zein is composed of nonpolar amino acids and is a water-insoluble protein used as the matrix-forming agent of localized in situ forming gel (ISG). Therefore, this study prepared solvent removal phase inversion zein-based ISG formulations to load levofloxacin HCl (Lv) for periodontitis treatment using dimethyl sulfoxide (DMSO) and glycerol formal (GF) as the solvents. Their physicochemical properties were determined, including viscosity, injectability, gel formation, and drug release. The topography of dried remnants after drug release was revealed using a scanning electron microscope and X-ray computed microtomography (μCT) to investigate their 3D structure and % porosity. The antimicrobial activities were tested against Staphylococcus aureus (ATCC 6538), Escherichia coli ATCC 8739, Candida albicans ATCC 10231, and Porphyromonas gingivalis ATCC 33277 with agar cup diffusion. Increasing zein concentration or using GF as the solvent notably enhanced the apparent viscosity and injection force of the zein ISG. However, its gel formation slowed due to the dense zein matrix barrier's solvent exchange: the higher loaded zein or utilization of GF as an ISG solvent prolonged Lv release. The SEM and μCT images revealed the scaffold of dried ISG in that their % porosity corresponded with their phase transformation and drug release behavior. In addition, the sustainability of drug diffusion promoted a smaller antimicrobial inhibition clear zone. Drug release from all formulations was attained with minimum inhibitory concentrations against pathogen microbes and exhibited a controlled release over 7 days. Lv-loaded 20% zein ISG using GF as a solvent exhibited appropriate viscosity, Newtonian flow, acceptable gel formation and injectability, and prolonged Lv release over 7 days with efficient antimicrobial activities against various test microbes; thus, it is the potential ISG formulation for periodontitis treatment. Consequently, the Lv-loaded solvent removal zein-based ISGs proposed in this investigation offer promising potential as an efficacious drug delivery system for periodontitis treatment by local injection.

Effects of vertical confinement on the flow of polymer solutions in planar constriction microchannels

The flow of polymer solutions under extensional conditions is frequently encountered in numerous engineering fields. Planar contraction and/or expansion microchannels have been a subject of interest for many studies in that regard, which, however, have mostly focused on shallow channel structures. We investigate here the effect of changing the depth of contraction-expansion microchannels on the flow responses of three types of polymer solutions and water. The flow of viscoelastic polyethylene oxide (PEO) solution is found to become more stable with suppressed vortex formation and growth in the contraction part while being less stable in the expansion part with the increase of the channel depth. These opposing trends in the contraction and expansion flows are noted to have similarities with our recent findings of constriction length-dependent instabilities in the same PEO solution (M. K. Raihan, S. Wu, Y. Song and X. Xuan, Soft Matter, 2021, 17, 9198-9209), where the contraction flow gets stabilized while the expansion flow becomes destabilized with the increase of the constriction length. In contrast, the entire flow becomes less stable in deeper channels for the shear-thinning xanthan gum (XG) solution as well as the shear thinning and viscoelastic polyacrylamide (PAA) solution. This observation aligns with that of water flow, which is attributed to the reduced top/bottom wall stabilizing effects.

Vortex evolution patterns for flow of dilute polymer solutions in confined microfluidic cavities

Flow instability in confined cavities has attracted extensive interest due to its significance in many natural and engineering processes. It also has applications in microfluidic devices for biomedical applications including flow mixing, nanoparticle synthesis, and cell manipulation. The recirculating vortex that characterizes the flow instability is regulated by the fluid rheological properties, cavity geometrical characteristics, and flow conditions, but there is a lack of quantitative understanding of how the vortex evolves as these factors change. Herein, we experimentally study the flow of dilute polymer solutions in confined microfluidic cavities and focus on a quantitative characterization of the vortex evolution. Three typical patterns of vortex evolution are identified in the cavity flow of dilute polymer solutions over a wide range of flow conditions. The geometrical characteristics of the cavity are found to have little effect on the patterns of vortex evolution. The geometry-independent patterns of vortex evolution provide us an intuitive paradigm, from which the interaction and competition among inertial, elastic and shear-thinning effects in these cavity-induced flow instabilities are clarified. These results extend our understanding of the flow instability of complex fluids in confined cavities, and provide useful guidelines for the design of cavity-structured microfluidic devices and their applications.

Constriction length dependent instabilities in the microfluidic entry flow of polymer solutions

Transport phenomena of fluids and particles through contraction and/or expansion geometries have relevance in many applications. Polymer solutions are often the transporter in these processes, giving rise to flow complexities. The separation distance between a contraction and a following expansion in microfluidic entry flow can affect the interplay between the shear and extension force dominated flow regimes, but the process is still little understood. We investigate the rheological responses of such constriction length dependent instabilities with three different polymer solutions and water in planar contraction-expansion microchannels differing only in the constriction length. The viscoelastic polyethylene oxide (PEO) solution is found to exhibit strong constriction length-dependent instabilities in both the contraction and expansion flows. Such a dependence is, however, completely absent from the flow of shear-thinning xanthan gum (XG) solution and Newtonian water. Interestingly, it is only present in the expansion flow of the both shear thinning and viscoelastic polyacrylamide (PAA) solution.

Flow of Non-Newtonian Fluids in a Single-Cavity Microchannel

Having a basic understanding of non-Newtonian fluid flow through porous media, which usually consist of series of expansions and contractions, is of importance for enhanced oil recovery, groundwater remediation, microfluidic particle manipulation, etc. The flow in contraction and/or expansion microchannel is unbounded in the primary direction and has been widely studied before. In contrast, there has been very little work on the understanding of such flow in an expansion-contraction microchannel with a confined cavity. We investigate the flow of five types of non-Newtonian fluids with distinct rheological properties and water through a planar single-cavity microchannel. All fluids are tested in a similarly wide range of flow rates, from which the observed flow regimes and vortex development are summarized in the same dimensionless parameter spaces for a unified understanding of the effects of fluid inertia, shear thinning, and elasticity as well as confinement. Our results indicate that fluid inertia is responsible for developing vortices in the expansion flow, which is trivially affected by the confinement. Fluid shear thinning causes flow separations on the contraction walls, and the interplay between the effects of shear thinning and inertia is dictated by the confinement. Fluid elasticity introduces instability and asymmetry to the contraction flow of polymers with long chains while suppressing the fluid inertia-induced expansion flow vortices. However, the formation and fluctuation of such elasto-inertial fluid vortices exhibit strong digressions from the unconfined flow pattern in a contraction-expansion microchannel of similar dimensions.

Liquid-Liquid Flows with Non-Newtonian Dispersed Phase in a T-Junction Microchannel

Immiscible liquid–liquid flows in microchannels are used extensively in various chemical and biological lab-on-a-chip systems when it is very important to predict the expected flow pattern for a variety of fluids and channel geometries. Commonly, biological and other complex liquids express non-Newtonian properties in a dispersed phase. Features and behavior of such systems are not clear to date. In this paper, immiscible liquid–liquid flow in a T-shaped microchannel was studied by means of high-speed visualization, with an aim to reveal the shear-thinning effect on the flow patterns and slug-flow features. Three shear-thinning and three Newtonian fluids were used as dispersed phases, while Newtonian castor oil was a continuous phase. For the first time, the influence of the non-Newtonian dispersed phase on the transition from segmented to continuous flow is shown and quantitatively described. Flow-pattern maps were constructed using nondimensional complex We^0.4·Oh^0.6 depicting similarity in the continuous-to-segmented flow transition line. Using available experimental data, the proposed nondimensional complex is shown to be effectively applied for flow-pattern map construction when the continuous phase exhibits non-Newtonian properties as well. The models to evaluate an effective dynamic viscosity of a shear-thinning fluid are discussed. The most appropriate model of average-shear-rate estimation based on bulk velocity was chosen and applied to evaluate an effective dynamic viscosity of a shear-thinning fluid. For a slug flow, it was found that in the case of shear-thinning dispersed phase at low flow rates of both phases, a jetting regime of slug formation was established, leading to a dramatic increase in slug length.