Biphilic Surfaces Research Articles

Biphilic Functional Surfaces for Frost Prevention and Efficient Active Defrosting

AbstractThe present work addresses the systematic accurate fabrication and design of biphilic surfaces having superhydrophobic circular islands surrounded by a hydrophilic background by investigating their condensation frosting and defrosting behavior. A significant delay in frost formation is observed on samples with higher superhydrophobicity ratio A*, defined as superhydrophobic area to total area ratio. As the superhydrophobic island diameter D increases from D = 500 µm to D = 700 µm (A* from 19.62% to 38.46%), a 50% improvement/delay is observed in terms of frost formation or densification. Besides delaying icing/frosting, the presence of superhydrophobic areas empowers the formation of porous and nonuniform frost structure, which facilitates ice removal during the defrosting process. To this end, as the surface is recovered the ambient temperature, almost complete passive cleaning performance within only 23 s is observed on the biphilic design having superhydrophobic islands with the diameter of D = 500 µm, that is, a superhydrophobicity ratio A* of 19.62%. This work concludes on the optimum biphilic ratio, which is not only effective as a passive method by hindering frosting but also leads to a slush/water free surface after defrosting eased by the Laplace pressure gradient which is imposed by the different biphilic wettability patterns.

Heat transfer enhancement and increase in critical heat flux during boiling on a biphilic silicon surface

Heat transfer enhancement and increase in critical heat flux during boiling on a biphilic silicon surface

Experimental study of the effect of biphilic surfaces on subcooled flow boiling in a horizontal rectangular duct

Experimental study of the effect of biphilic surfaces on subcooled flow boiling in a horizontal rectangular duct

Active Surface Area-Dependent Water Harvesting of Desert Beetle-Inspired Hybrid Wetting Surfaces.

The increasing frequency of water scarcity is an acute worldwide problem. Nature-inspired water harvesting from fog is an important method to obtain freshwater in arid areas. Existing literature reports varied and diversified results in water harvesting capacity by employing a biphilic surface with control over hydrophilic and hydrophobic patterns. In this study, we first demonstrate a facile and scalable method to fabricate a biphilic surface using a simple electroless etching and desilanization technique. Considering the nucleation, growth, and transport of condensate, biphilic surfaces with controlled active surface area of hydrophilic spots were given special attention. We studied the water collection performance of pattern shape with its associated active surface area and further evaluated the critical surface area beyond which the water collection efficiency decreases. A high water collection capacity of 2050 mg cm-2 h-1 was achieved, and the hydrophilic active area-engineered surface retained its efficiency even after 50 test cycles. We further demonstrate high collection efficiency with a square pattern compared to a triangular path-like-patterned surface. The observations and surface engineering strategies reported in this study can provide insights into efficient and sustainable water harvesting devices.

Boiling mechanism of biphilic surfaces based on Helmholtz instability and Taylor instability

This study demonstrated that combination hydrophilic copper and hydrophobic nano-silver surfaces simultaneously enhanced the critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling heat transfer (BHT) with deionized water as the employed working fluid. To further investigate the influence of geometric factors on heat transfer, this study designed three patterns denoted as squares/stripes/networks, ranging from 50 μm to 3000 μm. The effects of pattern size d, spacing p, and pitch ratio p/d on heat transfer results were obtained. Based on bubble visualization, the study further analyzed the reasons for inflection points in the boiling curve and the mechanisms caused by porous hydrophobic coating. Finally, the study proposed a numerical model suitable for predicting CHF of biphilic surfaces based on Helmholtz instability and Taylor instability. The model began with vapor columns escaping toward far-field and the fluid supplied to the heating surface. Concrete model derivation and adjustments are categorized according to the relationship between pattern size and bubble detachment diameter. For surfaces where the bubble detachment diameter exceeds the pattern size, this study originated from the perspective of Helmholtz instability on the vapor column surface. The resulting model is anticipated to quantitatively solve for the former and modulate the wavelength of Taylor instability on biphilic surfaces. Regarding surfaces with bubble detachment diameters smaller than the pattern size, this research begins with the existence time of the micro-liquid layer. The derived model validates the predominant role of p/d in the CHF values on micron-scale biphilic surfaces.

Why do metals become superhydrophilic during nanosecond laser processing? Design of superhydrophilic, anisotropic and biphilic surfaces

This research investigates the impact of the surrounding environment on the characteristics of copper, nickel and tin during nanosecond laser processing. By comparing the results of processes conducted in vacuum and air, we conclude that changes in wetting properties cannot be solely attributed to development of surface microstructure. To elucidate this phenomenon, we conducted model experiments using a tin target. Laser processing parameters for the tin target were established, involving deep cavity melting without ablation, resulting in highly evolved material morphology. Under these conditions, laser processing in air did not lead to metal hydrophilization. Therefore, our study demonstrates that the primary cause of changes in material wetting properties during nanosecond laser processing is the re-deposition of ablation products onto the material surface. These products form a nanoporous layer that enhances wicking capabilities and subsequently serves as a sorbent for various impurities, gradually leading to the hydrophobization of most commonly used materials. The proposed mechanism opens the possibility for the development of new methods to create materials with biphilic and anisotropic wetting properties. The key insight lies in the control of the thickness of the nanoporous layer, as it emerges as a pivotal factor dictating wettability properties and wicking capability.

Saturated Boiling Enhancement of Novec-7100 on Microgrooved Surfaces with Groove-Induced Anisotropic Properties

The effects of the anisotropic properties (wettability and roughness) of microgrooved surfaces on heat transfer were experimentally investigated during pool boiling using Novec-7100 as a working fluid. The idea for introducing the concept of anisotropic wettability in boiling experiments draws inspiration from biphilic surfaces. The investigation is also motivated by two-phase immersion cooling, which involves phase-change heat transfer, using a dielectric liquid as a working fluid. Very few studies have focused on the effects of surfaces with anisotropic properties on boiling performance. Thus, this study aims to examine the pool-boiling heat transfer performance on surfaces with microgroove-induced anisotropic properties under the saturation condition. A femtosecond-laser texturing method was employed to create microgrooved surfaces with different groove spacings. The results indicated that anisotropic properties affected the heat transfer coefficient and critical heat flux. Relative to the plain surface, microgrooved surfaces enhanced the heat transfer performance due to the increased number of bubble nucleation sites and higher bubble detachment frequency. An analysis of bubble dynamics under different surface conditions was conducted with the assistance of high-speed images. The microgrooved surface with a groove spacing of 100 μm maximally increased the BHTC by 37% compared with that of the plain surface. Finally, the CHF results derived from experiments were compared with related empirical correlations. Good agreement was achieved between the results and the prediction correlation.

Etching of a fluoropolymer coating synthesized by the hot wire chemical vapor deposition method in a low-frequency induction discharge plasma

The process of plasma etching for the formation of a biphilic pattern in a continuous homogeneous fluoropolymer coating on a copper substrate is studied. Argon or oxygen plasma of low frequency ferromagnetic amplified induction discharge is used to etch a fluoropolymer coating. Plasma etching was carried out through a mask with parallel slits. The etching rate in argon plasma was 10 nm/min, the etching rate in oxygen plasma was 60 nm/min. Biphilic surfaces were obtained, consisting of fluoropolymer strips on a copper surface. It has been shown that when using both argon plasma and oxygen plasma, it is possible to create biphilic surfaces by etching a continuous homogeneous fluoropolymer coating through a mask. Moreover, oxygen plasma is better suited for this because it has a higher etching rate and weakly changes the wettability of the surface.

Boiling Heat Transfer On Biphilic Surfaces With Arrays Of Grooves

Experiments were carried out on biphilic surfaces made using technology consisted in the deposition (105℃) of fluorinated methoxysilane on a copper surface with arrays of grooves preliminarily created by laser ablation, as a result of which deposition the grooves became superhydrophobic. Grooves were two types: parallel grooves and groove lattice. Size of grooves was 30-43 um, pitch for parallel grooves was 1-1,5 mm and for grove lattice – 1 mm. The experiments were carried out on distilled water at atmospheric pressure. For all surfaces, a significant intensification of heat transfer has been achieved, compared with a smooth copper surface. It was shown, that boiling heat transfer depend only from pitch between grooves for both types of grooves. Dependence of heat transfer coefficient from size of grooves was not observed. Most efficient type of surfaces was groove lattice with smallest pitch.

Drainage during Condensation on Microgrooved Biphilic Surfaces.

In this work, we investigate and compare the condensation behavior of hydrophilic, hydrophobic, and biphilic microgrooved silicon samples etched by reactive ion etching. The microgrooves were 25 mm long and 17-19 μm deep with different topologies depending on the etching process. Anisotropically etched samples had 30 μm wide rectangular microgrooves and silicon ridges between them. They were either left hydrophilic or covered with a hydrophobic fluorocarbon or photoresist layer. Anisotropically etched samples consisted of 48 μm wide semicircular shaped microgrooves, 12 μm wide silicon ridges between them, and a 30 μm wide photoresist stripe centered on the ridges. The lateral dimensions were chosen to be much smaller than the capillary length of water to support drainage of droplets by coalescence rather than droplet sliding. Furthermore, to achieve a low thermal resistance of the periodic surface structure consisting of water-filled grooves and silicon ridges, the trench depth was also kept small. The dripped-off total amount of condensate (AoC) was measured for each sample for 12 h under the same boundary conditions (chamber temperature 30 °C, cooling temperature 6 °C, and relative humidity 60%). The maximum increase in AoC of 15.9% (9.6%) against the hydrophilic (hydrophobic) reference sample was obtained for the biphilic samples. In order to elucidate their unique condensation behavior, in situ optical imaging was performed at normal incidence. It shows that the drainage of droplets from the stripe's surface into the microgrooves as well as occasional droplet sliding events are the dominant processes to clear the surface. To rationalize this behavior, the Hough Circle Transform algorithm was implemented for image processing to receive additional information about the transient droplet size and number distribution. Postprocessing of these data allows calculation of the transient water load on the stripe's surface, which shows an oscillatory behavior not previously reported in the literature.

Numerical study of compound drop mobility over a surface having wettability difference

Three-dimensional numerical simulations have been performed to analyze the effect of wettability difference on a compound droplet through biphilic and wettability gradient surfaces. Three distinct drop–drop configuration regimes during translation are found in both types of surfaces, as the contact angle difference on the surface is varied. Along with these advancing, rear, and central locations of core drop regimes, lens like drop–drop configuration is also realized over the biphilic surface by considering hydrophobic contact angle variation. On the biphilic surface, drop–drop configurations are also reported from simulation with different core-to-shell volume ratios and inversion of core and shell liquids.

Interplay of Drop Shedding Mechanisms on High Wettability Contrast Biphilic Stripe-Patterned Surfaces.

To improve the rate of DWC, numerous studies have adjusted the distribution of drops through biphilic surface patterning and wettability gradients to control the nucleation and drop shedding rates on the condensing surface, yet the connection between drop shedding mechanisms and surface wettability patterning remains unclear. Moreover, wettability patterning places geometric bounds on the governing forces (i.e., gravity, capillary, and inertia), which drive the droplet shedding mechanisms. Thus, the subsequent influence of droplet distribution along the DWC regions on the shedding mechanisms may not be known a priori. In this study, the area fraction, ADWC, of the DWC and also the DWC region width, LN, were varied between 10 and 50% and 0.5-1.5 mm, respectively, to probe the dominant droplet shedding mechanisms on a high wettability contrast surface (i.e., the contact angle on the DWC was 159 ± 3.4° and the hysteresis 9 ± 3.6°, whereas the FWC was nearly perfectly wetting). Humid air was introduced inside a custom-built chamber with the upright steady-state condensation imaged by both real-time and high-speed imaging techniques. We found that the droplet shedding mechanisms changed with increasing LN where the sliding drop radii are reduced with LN while the jumping drop radii remained unchanged with LN. The maximum drop size for shedding also decreased by 13%, which we attribute to the secondary droplet inertia, which helps gravity overcome the capillary retention force. Lastly, although many studies have probed DWC enhancements via surface wettability patterning, an optimal combination of ADWC and LN provided in this study significantly aids in the improvement of future DWC-based condensers and water collector applications.

Numerical Investigations on Enhancement of Pool Boiling Heat Transfer on a Mixed Wettability Surface Employing Lattice Boltzmann Method

Abstract In this paper, a systematic numerical study of pool boiling heat transfer on a mixed wettability heated surface is done using the lattice Boltzmann method (LBM) with a multiple relaxation time (MRT)-based collision operator. The effect of the design parameters, viz, size of the hydrophobic patch (D), spacing between hydrophobic patches (L), number of hydrophobic patches (N), and uneven-sized patches, on pool boiling was studied and results are explained through detailed analysis of bubble nucleation, growth, coalescence, and departure from the heated surface. The results show that mixed wettability surfaces with strategically sized and positioned hydrophobic patches on a hydrophilic surface can result in high heat flux for pool boiling across the entire range of surface superheat or Jacob number (Ja) by combining the advantages of hydrophobic surface in nucleate boiling and hydrophilic surface in transition and film boiling. Further, the mixed wettability surface can delay the onset of film boiling compared to a pure or superhydrophilic surface thereby resulting in higher critical heat flux (CHF). A hydrophobic to total surface area ratio of 30–40% was found to be optimal for all ranges of surface superheat or Jacob number (Ja), which agrees well with the experimental result of 38.46% reported by Motezakker et al. (2019, “Optimum Ratio of Hydrophobic to Hydrophilic Areas of Biphilic Surfaces in Thermal Fluid Systems Involving Boiling,” Int. J. Heat Mass Transfer, 135, pp. 164–174).

Droplet Self-Propulsion on Slippery Liquid-Infused Surfaces with Dual-Lubricant Wedge-Shaped Wettability Patterns.

Young's equation is fundamental to the concept of the wettability of a solid surface. It defines the contact angle for a droplet on a solid surface through a local equilibrium at the three-phase contact line. Recently, the concept of a liquid Young's law contact angle has been developed to describe the wettability of slippery liquid-infused porous surfaces (SLIPS) by droplets of an immiscible liquid. In this work, we present a new method to fabricate biphilic SLIP surfaces and show how the wettability of the composite SLIPS can be exploited with a macroscopic wedge-shaped pattern of two distinct lubricant liquids. In particular, we report the development of composite liquid surfaces on silicon substrates based on lithographically patterning a Teflon AF1600 coating and a superhydrophobic coating (Glaco Mirror Coat Zero), where the latter selectively dewets from the former. This creates a patterned base surface with preferential wetting to matched liquids: the fluoropolymer PTFE with a perfluorinated oil Krytox and the hydrophobic silica-based GLACO with olive oil (or other mineral oils or silicone oil). This allows us to successively imbibe our patterned solid substrates with two distinct oils and produce a composite liquid lubricant surface with the oils segregated as thin films into separate domains defined by the patterning. We illustrate that macroscopic wedge-shaped patterned SLIP surfaces enable low-friction droplet self-propulsion. Finally, we formulate an analytical model that captures the dependence of the droplet motion as a function of the wettability of the two liquid lubricant domains and the opening angle of the wedge. This allows us to derive scaling relationships between various physical and geometrical parameters. This work introduces a new approach to creating patterned liquid lubricant surfaces, demonstrates long-distance droplet self-propulsion on such surfaces, and sheds light on the interactions between liquid droplets and liquid surfaces.

Multiscale simulation of nanodrop over surfaces with varying hydrophilicity

Macroscale solvers primarily depend on empirical constitutive relations, which curbs their applicability and reliability. On the other hand, though the microscale solver is much more accurate, it cannot be used for engineering scale domains because of the high computational cost. As an alternative, in the present study, a new adiabatic two-phase multiscale solver has been developed by coupling Finite volume and molecular dynamics solvers with the help of the domain decomposition method. Due to the finiteness of the molecular subdomain, molecules near the boundary miss forces from the missing molecules. An effort has been made to develop a new boundary force model which mimics the effect of those missing molecules. The spreading of a drop has been simulated over surfaces with varying hydrophilicity using the solver and compared with the result of full molecular dynamics simulation. Further, the computational time has been compared with the full molecular dynamics simulation for different drop sizes to show the usefulness of the developed model. Drop translation over biphilic surfaces has also been simulated with the new solver.

Effect of mixed wettability surfaces on flow boiling heat transfer at subatmospheric pressures

Subatmospheric flow boiling heat transfer is a promising method for electronics cooling due to lower saturation temperatures. However, pressure is a crucial parameter that affects surface tension and vapor density. In this study, the effect of surface mixed wettability configuration on bubble dynamics and flow boiling was investigated under atmospheric and subatmospheric pressure conditions. Superhydrophilic, superhydrophobic, and mixed-wettability surfaces were prepared and tested at various heat fluxes and three system pressures of 48 kPa, 68 kPa, and 101 kPa. The channel dimensions were 50 mm × 15 mm, and the channel had a depth of 1 mm. The results showed that biphilic surfaces enhanced the performance up to 28% compared to superhydrophilic surfaces at high heat fluxes for subatmospheric boiling. Flow visualization efforts reveal that mixed-wettability surfaces improve heat transfer by extending the efficient slug regime to higher heat fluxes by preventing dried spot formation. These surfaces benefit from high density nucleation sites at low and medium heat fluxes, resulting in a noticeable performance improvement compared to the superhydrophilic surface. The obtained experimental data in this study will be helpful for the development of thermal-fluid systems operating under subatmospheric conditions.

Pool boiling on a biphilic surface where bubbles can move horizontally on the surface

In pool boiling on a horizontal surface, the flow of vapor is mainly the departure of vapor bubbles from the boiling surface. If the surface is composed of specially laid-out areas with varied wettabilities, vapor can move on the surface and out of the boiling region. In the present work, a silicon surface with such a function is fabricated using laser ablation and silanization grafting methods, and pool boiling on the surface is experimentally studied. On the biphilic surface, the boiling region is super-hydrophilic (SHI), and two opposite sides of the SHI region each is in connection to a wedge-shaped SHO (super-hydrophobic) track. The two sides of the SHO track are bounded by HO (hydrophobic) areas. Tests with air bubbles underwater show that the SHO tracks enable the directional spontaneous movement of the air bubbles on the surface, and the driving force for the natural motion is analyzed. Boiling heat transfer on the surface is compared with a SHI surface without the SHO tracks. The biphilic surface shows noticeable enhancement of heat transfer for low heat fluxes (< ∼150W/cm2) but reduces heat transfer for high heat fluxes in nucleate boiling. In addition, the biphilic surface extends the heat flux and surface temperature ranges of nucleate boiling, showing ∼14% increase of critical heat flux. The effects on boiling heat transfer are related to the observations of vapor transport by the SHO tracks.

Study of flow boiling in a vertical mini-channel with surface structuring: Heat transfer analysis using inverse method

Using a boiling fluid to cool components is a very efficient process, which is often used in micro/mini channel flows. A proposed application is the direct contact liquid cooling of lithium-ion battery cells, where a dielectric fluid is required and where the effects a cell thermal runaway could be mitigated by a convective boiling flow between the cells. For such applications, better designs require both to understand confined convective boiling with coupled flow phenomena and to quantify the local heat transfer accurately. The present paper introduces new experimental results of flow boiling heat transfer of a dielectric fluid, the HFE-7100, in a vertical rectangular 1 mm deep, 30 mm wide and 120 mm long mini-channel. The test section is an aluminum block instrumented with three lines of five K-type thermocouples located all along the flow. The experiment mainly aims at determining the heat transfer coefficient and CHF and at characterizing both the flow regimes and the dry-out phenomenon. Measurements are carried out from start of boiling to dry-out, at different mass fluxes (140, 390 and 648 kg/(m2.s)) and at a pressure of 1.1 bar. These tests were performed on a reference unmodified surface and compared to two biphilic surfaces including coating techniques. The visualization of the flow is achieved through a glass pane with a high-speed camera to identify different flow patterns. The local heat transfer coefficient is calculated by a resolution of a 2D inverse heat conduction method. This approach uses a numerical Finite Difference Method (FDM) for the spatial discretization and a Tikhonov's method for regularization. The inverse 2D method permits to follow the evolution of the heat transfer coefficient along the channel with the visualized flow pattern. The experimental results show a small sensitivity of the mass flux on the heat transfer coefficient and a predominance of nucleate boiling. On the other hand, there is a significant sensitivity of the mass flux on the dry-out occurrence. Finally, the sensitivity of two techniques of biphilic surface coating on boiling and on the dry-out occurrence is quantified and discussed.

Effect of the Size of the Superhydrophobic Regions of Biphilic Surfaces on the Bubble Dynamics

The current work aims to experimentally evaluate the effect of the size of circular superhydrophobic regions of biphilic surfaces on the bubble dynamics under pool boiling conditions. Biphilic surfaces are structured surfaces with tunable wettability, presenting an array of hydrophobic small spots in a hydrophilic surface or vice versa. The factors that affect the bubble dynamics are of geometric nature such as the diameters of the bubbles, their volume, and the height of the centroid, and of more complex nature such as the departure frequency of the bubbles and the rate of evaporation mass transfer. In this study, the bubble dynamics and boiling performance were evaluated by adjusting the diameter of the single circular superhydrophobic regions. A stainless steel AISI 304 foil was used as the base hydrophilic region, and the superhydrophobic regions were made by spray coating the NeverWet® superhydrophobic solution over well-defined masks. The main conclusion was that the bubble dynamics are clearly affected by the diameter of the superhydrophobic spots. The smaller spots favored the generation of more uniform and stable bubbles, mainly due to the border surface tension forces’ dominance. With the increase in the diameter of the bubbles, the surface tension acting at the border with the much larger hydrophilic region impacts the process less. Thus, the smaller superhydrophobic regions had higher evaporation mass transfer rates. The region with the best pool boiling performance along with improved bubble dynamics was the superhydrophobic region with an 0.8 mm diameter, corresponding to a superhydrophobic area to total area ratio of 0.11%. Moreover, this experimental work confirmed that the bubble dynamics’ impacting factors such as the diameter at the various stages of development of the bubbles can be modulated according to the final objectives of the design and fabrication of the biphilic surfaces. The research significance and novelty of this work come from the comprehensive study of the geometrical pattern of the heat transfer surface in pool boiling conditions and its impact on the bubble dynamics and heat transfer capability. We also suggest further studies considering nanoscale superhydrophobic spot arrangements and the future usage of different working fluids such as nanofluids.

The temperature dependent dynamics and periodicity of dropwise condensation on surfaces with wetting heterogeneities

HypothesisBiphilic surfaces, namely surfaces comprising hydrophilic areas with a (super)hydrophobic background, are used in nature and engineering for controlled dropwise condensation and liquid transport. These, however, are highly dependent on the surface temperature and subcooling. ExperimentsHere, biphilic surfaces were cooled inside a rotatable environmental chamber under controlled humidity. The condensation dynamics on the surface was quantified, depending on the subcooling, and compared to uniform superhydrophobic (USH) surfaces. Rates of condensation and transport were analyzed in terms of droplet number and size, covered area and fluid volume over several length scales. Specifically, from microscale condensation to macroscale droplet roll-off. FindingsFour phases of condensation were identified: a) initial nucleation, b) droplets on single patches, c) droplets covering adjacent patches and d) multi-patch droplets. Only the latter become mobile and roll off the surface. Cooling the surface to temperatures between T = 2–16 °C shows that lowering the temperature shortens some of the condensation parameters linearly, while others follow a power law, as expected from the theory of condensation. The temperature dependent condensation dynamics on (super)biphilic surfaces is faster in comparison to uniform superhydrophobic surfaces. Nevertheless, within time intervals of a few hours, droplets are mostly immobile. This sets guiding lines for using biphilic surfaces in applications such as water collection, heat transfer and separation processes. Generally, biphilic surfaces are suitable for applications in which fluids should be collected, concentrated and immobilized in specific areas.