Divergent Microchannels Research Articles

Solar Steam Generator with Divergent Microchannels for Highly Efficient and Salt‐Resistant Brine Evaporation (Adv. Mater. Technol. 18/2024)

Solar Steam Generator with Divergent Microchannels for Highly Efficient and Salt‐Resistant Brine Evaporation (Adv. Mater. Technol. 18/2024)

Solar Steam Generator with Divergent Microchannels for Highly Efficient and Salt‐Resistant Brine Evaporation

AbstractA novel solar steam generator featuring divergent microchannels (SSG‐DM) is developed via directional freezing of a polypyrrole (PPy)–cellulose nanofiber (CNF) suspension in an inverted frustum mold. Contrary to the conventional 3D SSGs characterized by random micropores (SSG‐RM), SSGs incorporating vertical microchannels (SSG‐VM), fabricated through directional freezing procedure, demonstrate increased evaporation performance due to enhanced water transport to the top surface. However, SSG‐VM manifests a limitation in the form of reduced salt resistance on its side surfaces where water transport is restricted. This issue is addressed by directional freezing of a PPy–CNF suspension in an inverted frustum mold, followed by freeze‐thawing gelation in an AlCl3 solution. The fabricated SSG features divergent microchannels that spanned from the bottom surface to the top and side surfaces, with a gradual increase in diameter. The evaporation rate of SSG‐DM is recorded at 2.42 kg m−2h−1 under 1‐sun illumination; this rate further increases to 15.73 kg m−2h−1 in the presence of a 4 m s−1 wind. Notably, SSG‐DM demonstrates excellent salt resistance in highly saline brine (10 wt%) under windy conditions (4 m s−1), whereas SSG‐RM and SSG‐VM are ineffective under the same conditions.

COMPUTATIONAL ANALYSIS OF GRAVITY AND PRESSURE-DRIVEN OBLIQUE FLOW OF MULTIPHASE FLUID THROUGH THE STEEP DIVERGENT MICROCHANNEL

In this computational study, we investigate the gravity and pressure-driven electroosmotic multiphase flow of third-grade fluid through steep microchannel by using the semi-analytical technique. A two-dimensional mathematical model is proposed here for the supercritical bi-phase flow with the suspension of the nanometallic particles. A special type of non-Newtonian fluid (third-grade) is selected as a base fluid. The governing equations for the current flow problem are obtained through Poisson–Boltzmann equations, continuity equations, and Cauchy’s momentum equations. The Poisson–Boltzmann equation is simplified by using the Debye–Hückel approximation. The symbolic software MATHEMATICATM is used to obtain the closed-form expression of electric potential, fluid and particle velocities, pressure gradient, and volumetric flow rate, respectively. The computational results are discussed qualitatively under the impact of the physical parameters embedded in the study. The graphical plots of fluid and particle-phase velocities are generated against the different parameters used in this study by adopting the same software as discussed earlier. The electrokinetic forces are responsible for electroosmotic flow which is modeled through the Laplace and Poisson–Boltzmann equations. The externally applied potential and electrical double-layer potential is governed by the Laplace and Poisson–Boltzmann equations, respectively.

Bifurcated divergent microchannel heat sinks for enhanced micro-electronic cooling

The thermal performance of microchannel heat sinks (MHSs) can be enhanced by utilizing extended surfaces such as pin fins, ribs, and bifurcation plates. Extended surfaces, however, adversely affect the hydraulic performance of the MHSs. Divergent microchannels, on the other hand, offer higher hydraulic performances but lower thermal performances than parallel microchannels. Here, bifurcated divergent MHSs are studied to examine the opposing effects of bifurcation and divergence on the thermo-hydraulic performance. Bifurcation plates with constant cross-section, variable width, and variable height are considered. Results showed that the pressure drop penalty and average heat transfer coefficient of bifurcated divergent MHSs are lower than the parallel MHSs. Considering the performance index (PI) defined as the ratio of heat transfer coefficient to pressure drop penalty, the bifurcated divergent microchannels outperform the parallel MHSs. Results also revealed that increasing the width of the bifurcation plate in the flow direction decreases the overall performance of the bifurcated divergent microchannels. The divergent bifurcation microchannels with a variable height bifurcation plate outperform the ones with a constant cross-section bifurcation plate. This study confirms the advantage of bifurcated divergent microchannels for increasing the cooling performance of the MHSs.

Investigating simultaneous effects of temperature, surface heterogeneity and geometry on fluid mixing in electroosmotic flow considering temperature dependent properties by Nernst–Planck Poisson method

Numerical study of the effect of temperature and heat transfer as well as surface heterogeneity effects on mixing and electroosmotic flow in microchannels with convergent/divergent and straight mixing chamber is carried out. The fluid properties and surface zeta potentials are assumed to be temperature dependent. Based on the Nernst–Planck Poisson model, the governing equations of this problem include the Laplace equation, Poisson equation, Navier–Stokes and the propagation equation as well as the energy equation, respectively for the distribution of external electric field, distribution of internal electric field due to the charge of ions, fluid motion, concentration field and temperature distribution, which are solved by the finite element method. Flow rate, the mixing efficiency and a new parameter called mixing capacity, which is achieved by multiplying flow rate by mixing efficiency, are obtained for convergent/divergent and straight microchannels in a constant external electric field distributed under different wall temperatures, and performance of the geometries in flow transfer and mixing development is investigated. For electroosmotic flow, heterogeneous microchannels create vortices which improve mixing. Different arrangements of heterogeneous pieces of the surface create different patterns of vortices, each of which has different effects in the various geometries on fluid flow and mixing. Also, the dependence of the molecular diffusion on temperature causes increasing the mixing of species with the temperature increment. The results show that divergent microchannels have the maximum flow rate and the minimum mixing efficiency, but this efficiency can be increased by increasing the wall temperature. Convergent microchannels also have high mixing efficiency due to low flow rates and sufficient time for mixing even at low and near ambient temperatures, although their mixing capacity is less than the divergent microchannels.

Manipulation of Particle/Cell Based on Compressibility in a Divergent Microchannel by Surface Acoustic Wave.

The mechanical properties (compressibility or deformability) of cells are closely related to their death, migration, and differentiation. Accurate separation and manipulation of bioparticles based on these mechanical properties are still a challenging in the field of acoustofluidics. In this work, based on surface acoustic waves (SAW) and divergent microchannels, we developed a new method for separating and detecting particles or cells with different compressibility. The difference in acoustic radiation force (Fr) caused by compressibility are gradually amplified and accumulated by decreasing the flow velocity, and they are finally reflected in the particle migration distance. During the transverse migration process, the alternating dominance of the acoustic radiation force and the Stokes resistance force (Fs) drives the particles to create three typical migration patterns: intermittent migration, compound migration, and near-wall migration. In the present tilted SAW device, a 91% separation success rate of ∼10 μm polystyrene (PS) and polydimethylsiloxane (PDMS) particles can be achieved by optimizing the acoustic field input power and the fluid velocity. The application potential of the present divergent microchannel is validated by separating the myelogenous leukemia cell K562 and the natural killer cell NK92 that have similar densities and sizes (∼15 μm) but different compressibility. The results of this work are expected to provide valuable insights into the acoustofluidics separation and detection of the cells that are with different compressibility.

Transition Routes of Electrokinetic Flow in a Divergent Microchannel with Bending Walls.

Electrokinetic flow can be generated as a highly coupled phenomenon among velocity fields, electric conductivity fields, and electric fields. It can exhibit different responses to AC electric fields in different frequency regimes, according to different instability/receptivity mechanisms. In this investigation, by both flow visualization and single-point laser-induced fluorescence (LIF) method, the response of AC electrokinetic flow and the transition routes towards chaos and turbulence have been experimentally investigated. It is found, when the AC frequency ff>30 Hz, the interface responds at both the neutral frequency of the basic flow and the AC frequency. However, when ff≥30 Hz, the interface responds only at the neutral frequency of the basic flow. Both periodic doubling and subcritical bifurcations have been observed in the transition of AC electrokinetic flow. We hope the current investigation can promote our current understanding of the ultrafast transition process of electrokinetic flow from laminar state to turbulence.

Electro-osmotically driven generalized Newtonian blood flow in a divergent micro-channel

A theoretical analysis of electro-osmosis peristaltic flow of generalized Newtonian fluid bounded in a complex wavy divergent channel is performed. There are many complex rheological generalized Newtonian fluids, out of which Ellis fluid model is considered to approximate the fluid present within the channel. The flow equations are modelled using balances of mass and momentum while the body force term is introduced using well known Poisson equation. An analytical solution to the governing flow equations is presented and the closed form expressions for longitudinal velocity, flow rate and stream function are developed. The numerical solution of pressure rise is obtained via NIntegrate build in command of Mathematica 12.3. Influence of diverse rheological and electrical parameters on flow characteristics are analysed via plots.

Pressure-Driven Nitrogen Flow in Divergent Microchannels with Isothermal Walls

Gas flow and heat transfer in confined geometries at micro-and nanoscales differ considerably from those at macro-scales, mainly due to nonequilibrium effects such as velocity slip and temperature jump. Nonequilibrium effects increase with a decrease in the characteristic length-scale of the fluid flow or the gas density, leading to the failure of the standard Navier–Stokes–Fourier (NSF) equations in predicting thermal and fluid flow fields. The direct simulation Monte Carlo (DSMC) method is employed in the present work to investigate pressure-driven nitrogen flow in divergent microchannels with various divergence angles and isothermal walls. The thermal fields obtained from numerical simulations are analysed for different inlet-to-outlet pressure ratios (1.5≤Π≤2.5), tangential momentum accommodation coefficients, and Knudsen numbers (0.05≤Kn≤12.5), covering slip to free-molecular rarefaction regimes. The thermal field in the microchannel is predicted, heat-lines are visualised, and the physics of heat transfer in the microchannel is discussed. Due to the rarefaction effects, the direction of heat flow is largely opposite to that of the mass flow. However, the interplay between thermal and pressure gradients, which are affected by geometrical configurations of the microchannel and the applied boundary conditions, determines the net heat flow direction. Additionally, the occurrence of thermal separation and cold-to-hot heat transfer (also known as anti-Fourier heat transfer) in divergent microchannels is explained.

Hybrid Transition Metal Dichalcogenide/Graphene Microspheres for Hydrogen Evolution Reaction.

A peculiar 3D graphene-based architecture, i.e., partial reduced-Graphene Oxide Aerogel Microspheres (prGOAM), having a dandelion-like morphology with divergent microchannels to implement innovative electrocatalysts for the hydrogen evolution reaction (HER) is investigated in this paper. prGOAM was used as a scaffold to incorporate exfoliated transition metals dichalcogenide (TMDC) nanosheets, and the final hybrid materials have been tested for HER and photo-enhanced HER. The aim was to create a hybrid material where electronic contacts among the two pristine materials are established in a 3D architecture, which might increase the final HER activity while maintaining accessible the TMDC catalytic sites. The adopted bottom-up approach, based on combining electrospraying with freeze-casting techniques, successfully provides a route to prepare TMDC/prGOAM hybrid systems where the dandelion-like morphology is retained. Interestingly, the microspherical morphology is also maintained in the tested electrode and after the electrocatalytic experiments, as demonstrated by scanning electron microscopy images. Comparing the HER activity of the TMDC/prGOAM hybrid systems with that of TMDC/partially reduced-Graphene Oxide (prGO) and TMDC/Vulcan was evidenced in the role of the divergent microchannels present in the 3D architecture. HER photoelectron catalytic (PEC) tests have been carried out and demonstrated an interesting increase in HER performance.

DSMC investigation of rarefied gas flow through diverging micro- and nanochannels

Direct simulation Monte Carlo (DSMC) method with simplified Bernoulli-trials (SBT) collision scheme has been used to study the rarefied pressure-driven nitrogen flow through diverging microchannels. The fluid behaviours flowing between two plates with different divergence angles ranging between 0$^{\circ}$ to 17$^{\circ}$ are described at different pressure ratios (1.5${\le}{\prod}{\le}$2.5) and Knudsen numbers (0.03${\le}$Kn${\le}$12.7). The primary flow field properties, including pressure, velocity, and temperature, are presented for divergent microchannels and are compared with those of a microchannel with a uniform cross-section. The variations of the flow field properties in divergent microchannels, which are influenced by the area change, the channel pressure ratio and the rarefication are discussed. The results show no flow separation in divergent microchannels for all the range of simulation parameters studied in the present work. It has been found that a divergent channel can carry higher amounts of mass in comparison with an equivalent straight channel geometry. A correlation between the mass flow rate through microchannels, the divergence angle, the pressure ratio, and the Knudsen number has been suggested. The present numerical findings prove the occurrence of Knudsen minimum phenomenon in micro- and Nano- channels with non-uniform cross-sections.

Convergent–divergent micromixer coupled with pulsatile flow

An efficient mixing system for a microfluidic platform that uses periodic sinusoidal characteristics in space and time is proposed in this work. A convergent–divergent channel with sinusoidal walls (space-periodic) represented the most effective coupling with pulsatile (time-periodic) flow among the five tested geometries. The effects of sinusoidal pulsing on mixing performance in the convergent–divergent microchannel were examined using unsteady Navier–Stokes equations. The mixing performance was analyzed using the Reynolds and Strouhal numbers, and the ratio of pulsing amplitude to steady flow velocity. The micromixer shows interesting mixing behaviors with these parameters in terms of instantaneous mixing patterns as well as overall mixing performance. The micromixer with pulsing inputs achieved mixing index, 0.92 within two periods of the sinusoidal walls.

Effects of microchannel geometry on preconcentration intensity in microfluidic chips with straight or convergent–divergent microchannels

Preconcentration microfluidic devices are fabricated incorporating straight or convergent-divergent microchannels and hydrogel or Nafion membranes. Sample preconcentration is achieved utilizing concentration-polarization effects. The effects of the microchannel geometry on the preconcentration intensity are systematically examined. It is shown that for the preconcentrator with the straight microchannel, the time required to achieve a satisfactory preconcentration intensity increases with an increasing channel depth. For the convergent-divergent microchannel, the preconcentration intensity increases with a reducing convergent channel width. Comparing the preconcentration performance of the two different microchannel configurations, it is found that for an equivalent width of the main microchannel, the concentration effect in the convergent-divergent microchannel is faster than that in the straight microchannel.

Transient flow of microcapsules through convergent–divergent microchannels

The study deals with a microfluidic method to investigate the transient behavior of microcapsules in flow. The technique consists of investigating ovalbumin microcapsules passing through a convergent–divergent microchannel made of PolyDiMethylSiloxane. We work with three types of square microchannel with, respectively, cross section values of h × h = 30 × 30, 50 × 50 and 70 × 70 μm. The microchannels length is L = 3h. We analyze the kinetics of deformation of the microcapsules in the microchannels for velocity ranging from 2 to 5 cm/s and for microcapsule size ratio d/h ranging from 0.9 to 2.5. The relaxation process at the pore outlet is modeled using an exponential relaxation law. We show that that the relaxation time at the divergent outlet depends on the microcapsule size ratio d/h. Thanks to the analytical expression of the relaxation, we extract a shear modulus of the membrane equal to 0.04 N/m. This value is consistent with the value of 0.07 N/m that we found using the steady state analysis performed in cylindrical glass capillaries. Thus, it is interesting to notice that the microcapsule behavior based on a simple analytical model can be successfully described despite the complex flow situation consisting of deformable microcapsule in confined square microchannels.

Microfluidic analysis of CO2 bubble dynamics using thermal lattice-Boltzmann method

By means of microfluidic analysis with a thermal lattice-Boltzmann method, we investigated the hydrophilic, thermal and geometric effects on the dynamics of CO2 bubbles at anode microchannels (e.g., porous layers and flow channels) of a micro-direct methanol fuel cell. The simulation results show that a more hydrophilic wall provides an additional attractive force to the aqueous methanol in the flow direction and that moves the CO2 bubble more easily. The bubble propagates quicker in the microchannel with a positive temperature gradient imposed from the inlet to the exit, mainly due to the Marangoni effect. Regarding the geometric effect of the microchannel, the bubble moves more rapidly in a divergent microchannel than in a straight or convergent channel. On the basis of the quantitative evaluation of hydrophilic, thermal and geometric effects, we are able to design the bubble-removal technique in micro fuel cells.

Estimation of zeta potential by electrokinetic analysis of ionic fluid flows through a divergent microchannel

The streaming potential is generated by the electrokinetic flow effect within the electrical double layer of a charged solid surface. Surface charge properties are commonly quantified in terms of the zeta potential obtained by computation with the Helmholtz–Smoluchowski (H–S) equation following experimental measurement of streaming potential. In order to estimate a rigorous zeta potential for cone-shaped microchannel, the correct H–S equation is derived by applying the Debye–Hückel approximation and the fluid velocity of diverging flow on the specified position. The present computation provides a correction ratio relative to the H–S equation for straight cylindrical channel and enables us to interpret the effects of the channel geometry and the electrostatic interaction. The correction ratio decreases with increasing of diverging angle, which implies that smaller zeta potential is generated for larger diverging angle. The increase of Debye length also reduces the correction ratio due to the overlapping of the Debye length inside of the channel. It is evident that as the diverging angle of the channel goes to nearly zero, the correction ratio converges to the previous results for straight cylindrical channel.