Superhydrophobic Channels Research Articles

Heat transfer enhancement and drag reduction in transverse groove-bounded microchannels with offset

The shrinking physical scale of microfluidic chips brings significant benefits like less expense of chemicals, shorter diagnosing time and highly parallel testing, but at the same time presents big challenges in drag reduction and thermal management. The ridge/groove structure on the superhydrophobic surface inside microchannels, which entraps air so as to reduce the skin friction, is believed to affect the internal heat transfer. In the current study, the flow and heat transfer between two walls with eccentric transverse microgrooves was studied based on numerical simulations, to investigate its effect on overall drag reduction, heat transfer enhancement and possible mechanisms. The changes in drag reduction capability and convective heat transfer were respectively evaluated in terms of the effective slip length and Nusselt number. The overall thermal performance and efficiency were evaluated using the goodness factor. It was found that the eccentricity of transverse microgooves inside superhydrophobic channel increases the effective slip length but reduces heat transfer slightly, which is especially obvious at low Reynolds number, large pattern length and moderate shear free fraction. Besides drag reduction and heat transfer, the flow fields for different cases were investigated in details, which show that the increase in the effective slip length of superhydrophobic microchannel with offset can be attributed to the increase of average velocity on air-water interface.

A novel CO2 gas removal design for a micro passive direct methanol fuel cell

This paper presents an arrangement of super hydrophobic lateral venting micro channels, which is fabricated around the anode gas diffusion electrode of micro direct methanol fuel cell (μDMFC). Work in this paper is aimed to prove the validity of the lateral venting design. Three types of DMFCs with lateral venting design is tested by contrast with control groups respectively. With the lateral venting configuration, CO2 gas can release directly from the anode diffusion layer of membrane electrode assembly (MEA), which prevents CO2 gas accumulating on the anode and decreasing the anode mass transportation. Results show that the novel structure can prevent the formation of CO2 gas barrier to a great extent, which not only avoids the anode concentration loss but also improves the discharging stability, providing a new way of design and optimization on the DMFC.

Asymptotic formulae for flow in superhydrophobic channels with longitudinal ridges and protruding menisci

This paper presents new asymptotic formulae for flow in a channel with one or both walls patterned with a longitudinal array of ridges and arbitrarily protruding menisci. Derived from a matched asymptotic expansion, they extend results by Crowdy (J. Fluid Mech., vol. 791, 2016, R7) for shear flow, and thus make no restriction on the protrusion into or out of the liquid. The slip length formula is compared against full numerical solutions and, despite the assumption of small ridge period in its derivation, is found to have a very large range of validity; relative errors are small even for periods large enough for the protruding menisci to degrade the flow and touch the opposing wall.

Two-phase flow pressure drop in superhydrophobic channels

Superhydrophobic surfaces have been shown to reduce drag in single-phase channel flow; however, little work has been done to characterize their drag-reducing ability found in two-phase flows. Adiabatic, air-water mixtures were used to explore the influence of hydrophobicity on two-phase flows and the hydrodynamics which might be present in flow condensation environments. Pressure drop measurements in a rectangular channel with one superhydrophobic wall (cross-section approximately 0.37×10mm) and three transparent hydrophilic walls were obtained. Data for air/water mixtures with superficial Reynolds numbers ranging from 22–215 and 55–220, respectively, were obtained for superhydrophobic surfaces with three different cavity fractions. Agreement between experimentally obtained two-phase pressure drop data and correlations in the literature for conventional smooth control surfaces was better than 20 percent, which is within the accuracy of the correlations. The data reveal a reduction in the pressure drop for two-phase flow in a channel with a single superhydrophobic wall compared to a control scenario. The observed reduction is approximately 10 percent greater than the reduction that is observed for single-phase flow (relative to a classical channel).

Enhance performance of micro direct methanol fuel cell by in situ CO 2 removal using novel anode flow field with superhydrophobic degassing channels

Abstract Capillary blocking caused by CO2 bubbles in anode flow field (AFF) is one of the bottlenecks for performance improvement of a micro direct methanol fuel cell (μDMFC). In this work, we present a novel AFF structure with nested layout of hydrophilic fuel channels and superhydrophobic degassing channels which can remove most of CO2 from AFF before it is released to the fuel channels. The new AFFs are fabricated on Ti substrates by using micro photochemical etching combined with anodization and fluorination treatments. Performance of the μDMFCs with and without superhydrophobic degassing channels in their AFF is comparatively studied. Results show that the superhydrophobic degassing channels can significantly speed up the exhaust of CO2 from the AFF. CO2 clogging is not observed in the new AFFs even when their comparison AFFs have been seriously blocked by CO2 slugs under the same operating conditions. 55% and 60% of total CO2 produced in μDMFCs with N-serpentine and N-spiral AFF can be respectively removed by the superhydrophobic degassing channels. The power densities of the μDMFCs equipped with new serpentine and spiral AFFs are respectively improved by 30% and 90% compared with those using conventional AFFs. This means that the new AFFs developed in this work can effectively prevent CO2-induced capillary blocking in the fuel channels, and finally significantly improve the performance of the μDMFCs.

Superhydrophobic contoured surfaces created on metal and polymer using a femtosecond laser

A process to fabricate superhydrophobic surfaces on metals and polymers is presented. Using high speed femtosecond laser pulses, surface structures have been created on metal surfaces and transferred onto Polydimethylsiloxane (PDMS) to achieve superhydrophobic surfaces. Various surface micro and nano structures are presented and their wetting properties are discussed. Water contact angles of over 150° with a roll off angle of less than 5° were achieved on the textured copper and PDMS surfaces. A fast and inexpensive method to create microfluidic devices with textured superhydrophobic inner channel walls is also presented. These channels allow a controllable fluid flow rate through microfluidic devices. A 186% increase in fluid flow rate though PDMS microchannels was achieved through the fabrication of hydrophobic structures on the inner channel walls.

Exact Formulae for the Effective Slip Length of a Symmetric Superhydrophobic Channel with Flat or Weakly Curved Menisci

We derive exact formulae for the effective slip length for the pressure-driven longitudinal flow of a liquid (held in the Cassie--Baxter state) through a channel bounded by two superhydrophobic walls that are both etched with a periodic array of parallel grooves, where these arrays are symmetrically aligned. These formulae allow for either flat or weakly curved menisci of the gas bubbles trapped in the grooves, but in both cases are expressed as contour integrals of functions of a partial derivative of the velocity field of the liquid corresponding to just flat menisci. This field is governed by Poisson's equation with a constant forcing term in the interior of a rectangle, subject to a mixture of homogeneous Dirichlet and homogeneous Neumann boundary conditions. We construct this partial derivative of it in an exact form by using conformal mapping and loxodromic function theory. Also fundamental to our derivation is the use of reciprocity arguments.

Influence of texture on thermal transport in streamwise-aligned superhydrophobic turbulent channels

A series of Direct Numerical Simulations (DNS) is performed for a systematic study of thermal transport in a periodic turbulent channel bounded by superhydrophobic surface (SHS) walls at a friction Reynolds number of Reτ=180. The SHS examined here is comprised of interspersed ridges and cavities aligned along the mean streamwise flow direction. The SHS is modelled as a planar surface consisting of spanwise-alternating regions of free-shear and no-slip boundary conditions. Water is considered as the bulk fluid where the SHS surface is assumed to be in a Cassie-Baxter state with non-wetting cavities containing air. The ridges are maintained at a constant temperature while heat transfer through the air/water interface supported by the ridges is assumed negligible and is modelled as adiabatic. Phase averaged statistics were obtained for a range of relative periodicity widths. Reorganisation of secondary flow structures owing to an increase in SHS feature width and its effect on thermal transport are analyzed with a particular focus on phase-averaged statistics. The relative feature width or periodicity length is found to influence strongly on thermal performance, or the average Nusselt number. A reduction in the turbulent Prandtl number is observed in the buffer region due to an increase in thermal eddy diffusivity relative to the momentum eddy diffusivity. The amount of heat transfer and mixing depend upon the periodicity length, and there is a reduction in the heat transfer along with the drag-reduction. A substantial decrease in the temperature fluctuations is observed in the vicinity of the wall as the production of temperature variance is diminished with an increase in the periodicity length, primarily due to a reduction in the mean temperature gradient in the wall-normal direction.

Magnetohydrodynamic actuation of droplets for millimetric planar fluidic systems

In this work, a magnetohydrodynamic method is proposed for the actuation of droplets in small-scale planar fluidic systems, providing an alternative to commonly used methods such as electrowetting-on-dielectric. Elementary droplet-based operations, including transport, merging, and mixing, are demonstrated. The forces acting on millimetric droplets are carefully investigated, with a primary focus on the magnetic actuation force and on the unbalanced capillary forces that arise due to hysteresis. A super-hydrophobic channel is 3D printed to guide the droplets, with thin wires installed as contact electrodes and permanent magnets providing a static magnetic field. It is shown that droplet motion is enhanced by increasing the droplet size and minimizing the electrode contact surface. The effects of channel geometry on threshold voltage and minimum moveable droplet volume are characterized. Finally, the presence of electrolysis is investigated and mitigating strategies are discussed.

On the mechanism of turbulent drag reduction with super-hydrophobic surfaces

The mechanism of turbulent drag reduction (DR) with super-hydrophobic (SH) surfaces is investigated by direct numerical simulation (DNS) and analysis of the governing equations in channel flow. The DNS studies were performed using lattice Boltzmann methods in channels with ‘idealized’ SH surfaces on both walls, comprised of longitudinal micro-grooves (MG), transverse MG, or micro-posts. DRs of $5\,\%$ to $83\,\%$, $-4\,\%$ to $20\,\%$, and $14\,\%$ to $81\,\%$ were realized in DNS with longitudinal MG, transverse MG, and micro-posts, respectively. By mathematical analysis of the governing equations, it is shown that, in SH channel flows with any periodic SH micro-pattern on the walls, the magnitude of DR can be expressed as $DR=U_{slip}/U_{bulk}+O({\it\varepsilon})$, where the first term represents the DR resulting from the effective slip on the walls, and the second term represents the DR or drag increase (DI) resulting from modifications to the turbulence dynamics and any secondary mean flows established in the SH channel compared to a channel flow with no-slip walls at the same bulk Reynolds number as the SH channel. Comparison of this expression to DNS results shows that, with all SH surface micro-patterns studied, between 80 % and 100 % of the DR in turbulent flow arises from the effective slip on the walls. Modifications to the turbulence dynamics contribute no more than 20 % of the total DR with longitudinal MG or micro-posts of high shear-free fraction (SFF), and a DI with transverse MG or micro-posts of moderate SFF. The effect of the SH surface on the normalized dynamics of turbulence is found to be small in all cases, and confined to additional production of turbulence kinetic energy (TKE) within a thin ‘surface layer’ of thickness of the order of the width of surface micro-indentations. Outside of this ‘surface layer’, the normalized dynamics of turbulence proceeds as in a turbulent channel flow with no-slip walls at the friction Reynolds number of the SH channel flow.

Heterogeneous wetting surfaces with graphitic petal-decorated carbon nanotubes for enhanced flow boiling

Two-phase cooling is widely employed in temperature-sensitive devices for high heat flux dissipation owing to the benefits of latent heat exchange. The objective of this work is to elucidate the subcooled flow boiling characteristics of heterogeneous wetting surfaces using water as the working fluid. Heterogeneous wetting surfaces with alternating parallel stripes of superhydrophobic (SHo) and superhydrophilic (SHi) regions are fabricated with graphitic petal-decorated carbon nanotube (GPCNT) coatings. Graphitic petals are a carbon nanostructure comprising of stacks of graphene layers standing vertically on a substrate. GPCNTs are synthesized on bulk copper substrates by a two-step microwave plasma enhanced chemical vapor deposition technique. A combination of Teflon coating, shadow mask, and oxygen plasma treatment is utilized to create composite wetting surfaces with differing superhydrophilic area fractions and SHo to SHi channel width ratios. Boiling experiments reveal that heterogeneous wetting surfaces with higher superhydrophilic area fraction (0.66 and 0.85) exhibit significant reduction in surface superheat and higher heat transfer coefficients throughout the entire boiling regime as opposed to uniform SHo and SHi boiling surfaces. Flow visualization reveals enhanced active nucleation site density and preferential bubble nucleation in the superhydrophobic channels of composite wetting surfaces. Isolated near-spherical vapor morphologies on surfaces with higher superhydrophilic area fraction promote thin film evaporation and enhanced bubble ebullition cycles, thereby leading to improved two-phase thermal performance.

Flows and mixing in channels with misaligned superhydrophobic walls

Aligned superhydrophobic surfaces with the same texture orientation reduce drag in the channel and generate secondary flows transverse to the direction of the applied pressure gradient. Here we show that a transverse shear can be easily generated by using superhydrophobic channels with misaligned textured surfaces. We propose a general theoretical approach to quantify this transverse flow by introducing the concept of an effective shear tensor. To illustrate its use, we present approximate theoretical solutions and Dissipative Particle Dynamics simulations for striped superhydrophobic channels. Our results demonstrate that the transverse shear leads to complex flow patterns, which provide a new mechanism of a passive vertical mixing at the scale of a texture period. Depending on the value of Reynolds number two different scenarios occur. At relatively low Reynolds number the flow represents a transverse shear superimposed with two corotating vortices. For larger Reynolds number these vortices become isolated, by suppressing fluid transport in the transverse direction.

Novel Flow Field with Superhydrophobic Gas Channels Prepared by One-step Solvent-induced Crystallization for Micro Direct Methanol Fuel Cell

The CO2-induced capillary blocking in anode flow field is one of the key adverse factors to reduce the performance of a micro-direct methanol fuel cell (μDMFC). In order to solve this problem, new polycarbonate (PC) flow field plates with nested arrangement of hydrophilic fuel channels and superhydrophobic gas channels were designed, fabricated, and tested in this work. The gas channels were treated with solvent-induced crystallization using acetone solution. The superhydrophobicity with 160° water contact angle and 2° tilting angle was obtained on the PC substrates. A dummy cell using hydrogen peroxide decomposition reaction and a test loop were separately set up to evaluate the flow fields’ performance. It was found that a 37 % pressure drop decrease can be obtained in the new serpentine flow field compared with that of the conventional one. The benefit of the new flow field to remove gas bubbles was also confirmed by an in situ visualization study on the dummy cell. Results show that the auxiliary superhydrophobic gas channels can speed up the discharge of the gas bubbles from the flow field, which will in turn improve the μDMFC performance.

Principles of transverse flow fractionation of microparticles in superhydrophobic channels.

We propose a concept of fractionation of micron-sized particles in a microfluidic device with a bottom wall decorated by superhydrophobic stripes. The stripes are oriented at an angle α to the direction of a driving force, G, which generally includes an applied pressure gradient and gravity. Separation relies on the initial sedimentation of particles under gravity in the main forward flow, and their subsequent lateral deflection near a superhydrophobic wall due to generation of a secondary flow transverse to G. We provide some theoretical arguments allowing us to quantify the transverse displacement of particles in the microfluidic channel, and confirm the validity of theoretical predictions in test experiments with monodisperse fractions of microparticles. Our results can guide the design of superhydrophobic microfluidic devices for efficient sorting of microparticles with a relatively small difference in size and density.

High Adhesive Superhydrophobicity and Wetting Mechanism of Locust Wing Surface

The complex wettability, chemical composition and microstructure of locust wing surface were investigated by a video-based contact angle (CA) meter, a Fourier transform infrared spectrometer (FT-IR) and a scanning electron microscope (SEM). A model for hydrophobicity of wing surface was established on the basis of Cassie equation. The wetting mechanism was discussed from the perspective of biological coupling. The wing surface is a waxy layer composed mainly of long chain hydrocarbon, tallate and fatty-acid alcohol, possesses multiple-dimensional rough microstructures including primary structure (wing vein grids), secondary structure (regularly arraying micrometric pillar gibbosities), and tertiary structure (nanocorrugations). The diameter, height, and spacing of pillar gibbosity are 3.0~10.2 μm, 3.4~9.2 μm, and 7.5~18.5 μm, respectively. Locust wing surface is of high adhesive superhydrophobicity (CA 150.1~157.3°). The complex wettability of the wing surface ascribes to coupling effect of material element (waxy crystal) and structural element (hierarchical rough microstructure). Locust wing can be potentially used as a biomimetic template for design of special functional surface. This work may bring insights for preparation of micro-controllable superhydrophobic surface and no-loss microfluidic channels.

Effective temperature jump length and influence of axial conduction for thermal transport in superhydrophobic channels

This paper presents a numerical investigation of thermal transport in a parallel-plate channel comprised of superhydrophobic walls. The scenario analyzed is laminar, fully developed, steady flow with constant properties. The superhydrophobic walls considered here have alternating micro-ribs and cavities aligned perpendicular to the flow direction and are made of a highly conductive material. The cavities are assumed to be non-wetting and contain air whereas the bulk liquid is water. The thermal transport through the ribs is considered to have a constant heat flux while the thermal transport through the air/liquid interface over the cavity is considered to be negligible. Numerical results have been obtained for a range a Peclet numbers, cavity fractions, and relative channel widths. A limited number of results were also obtained where the rib was maintained at a constant temperature condition for comparison. In general, the thermal transport is a strong function of all the parameters explored. By comparison to previous analytical work, the influence of axial conduction is found to be significant and is most pronounced at large relative channel widths, low Peclet numbers, and large cavity fractions. Lastly, the ratio of temperature jump length to hydrodynamic slip length is presented in terms of the varied parameters and is compared to previous results where axial conduction is neglected and other work where diffusion is assumed dominant.

ZIF-71 as a potential filler to prepare pervaporation membranes for bio-alcohol recovery

Metal–organic frameworks (MOFs) are an emerging class of materials for potential use in separation. ZIF-71 has super-hydrophobic pore channels and a very flexible pore architecture, which make ZIF-71 a potential candidate for recovering bio-alcohols from fermentation broths. In the present study, mixed matrix membranes (MMMs) based on PDMS and ZIF-71 were prepared for separation of alcohols (methanol, ethanol, isopropanol (IPA) or sec-butanol) from aqueous solutions. Experimental results reveal that the pervaporation performance of the PDMS membrane was improved in both flux and separation factors upon embedment of ZIF-71. The pervaporation separation factors for alcohols of ZIF-71 filled the PDMS membrane (PDMS : ZIF-71 = 10 : 2) nearly double compared to those of unfilled PDMS membranes, suggesting that ZIF-71 is an excellent filler to prepare organophilic pervaporation membranes for bio-alcohol recovery.

Effective three-dimensional superhydrophobic aerogel-coated channel for high efficiency water-droplet transport

Three-dimensional superhydrophobic surfaces have been used effectively to optimize droplet transport efficiency in diverse fluidic systems. However, the fabrication methods for superhydrophobic surfaces applicable to fluidic devices usually involve complicated process. Herein, we report a simple but effective method of fabricating a superhydrophobic surface using organically modified silica aerogel. Superhydrophobic aerogel thin film having highly porous micro/nanostructured surface with methyl groups was realized inside a 3D channel by coating it. To demonstrate that the aerogel-coated surface effectively facilitates movement of water droplets, the droplet-based flow characteristics regarding the triple line were conducted.

Isoflux Nusselt Number and Slip Length Formulae for Superhydrophobic Microchannels

We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.

Hot embossing and thermoforming of biodegradable three-dimensional wood structures

Hot embossing allows for low cost fabrication of nanostructures with highest aspect ratios on large surfaces and is commonly based on a large variety of polymers. The widespread use of nanostructured materials, however, poses environmental challenges. Here, we show that biodegradable wood from renewable resources can be structured on the nano-scale and that surface characteristics can be imprinted and three-dimensionally formed. In particular, we show that so-called liquid wood made out of lignin can be structured with feature sizes down to 100–200 nm and three dimensionally shaped by microthermoforming. We demonstrate optical active surfaces by diffractive elements, the change of hydrophobic to super-hydrophobic behavior and the fabrication of super-hydrophobic microfluidic channels. Based on these results we anticipate new applications in the nano- and micro-range, particularly for use in sensitive environments without posing a risk to nature. The new compound material will allow for a substitution of conventional polymers used as material in one way applications of e.g. the consumer, the food or the disposable medical industry.