Effect Of Surface Wettability Research Articles

Numerical investigation and experimental verification of liquid water dynamic transfer characteristics in the flow field of PEMFC with dead-ended anode during gas purging

Water accumulation in the anode during the operation of a proton exchange membrane fuel cell with the dead-ended anode (DEA) has a significant impact on its performance. In this study, numerical simulation and experimental verification were combined to investigate the water accumulation and water dynamic transfer characteristics in the anode flow field during dead-ended operation and gas purging, respectively. The effects of placement orientation, surface wettability, and purge time on the water dynamic transfer characteristics are investigated. The results show that the placement orientation has significant effects on the water distribution characteristics and drainage for PEMFC. When placed vertically and with a purging time of 50 ms, the liquid water saturation in the channel is 5.52 %, which drains 14.91 % more water than that of horizontal placement. Besides, Water discharge velocity and water removal ratio should be considered comprehensively in the development of the purge strategy, the optimal purging time is 70 ms with the water discharge velocity and water removal rate are 0.39 μl/ms and 97.34 %, respectively. Finally, the transfer process of liquid water in the flow field with DEA mode is observed by visual experiment, and the results of the numerical simulation are verified.

Cavitation suppression and transformation of turbulence structure in the cross flow around a circular cylinder: Surface morphology and wettability effects

Passive methods of flow and cavitation control appear to offer some of the best prospects in the field of hydraulic engineering and marine applications. In this article, we aimed at an experimental examination of the effect of wall roughness/wettability on the occurrence of cavitation and turbulence structure in the cross flow around and in the wake of a circular cylinder in two characteristic regimes. For this, we used three test bodies with different surface morphologies: smooth (reference), micro-scale irregularities (rough) and regular large-scale (of the order of a millimeter) texture (finned). Using high-speed imaging to observe vapor cavities, we revealed that cavitation is noticeably suppressed by both types of roughness. Applying the method of vapor phase detection (Pervunin et al., 2021), this finding was then quantitatively confirmed through an in-depth analysis of an ensemble of instantaneous velocity fields measured by PIV, indicating that modification of wall morphology is an effective method of cavitation control. The procedure of statistical vector filtration (Heinz et al., 2004) allowed us to remove outliers from the velocity fields and, thus, calculate various turbulence characteristics, including higher-order moments (i.e., the coefficients of skewness and excess). Wall irregularities were found to significantly affect the turbulence structure of the wake flow, but the higher-order moments downstream of the modified-surface cylinders turned out to be unexpectedly insensitive to a change in the flow regime, as opposed to the smooth one. Regardless of the type of surface morphology, the influence of roughness on the mechanism of formation of large-scale vortices and their characteristics was weakened. However, it caused overall disorganization of liquid motion in the cylinder wake, thus making local flow conditions highly unsteady. In addition, this process became more chaotic with an increase in the scale of irregularities.

Elucidating the effects of surface wettability on boiling heat transfer using hydrophilic and hydrophobic surfaces with laser-etched microchannels

This study explores the nuanced interplay between surface wettability and morphology in nucleate boiling heat transfer. Surfaces with identical microstructures but varying wettability are created on copper substrates using laser texturing and a hydrophobic agent. The spacing between laser-etched microchannels is adjusted to achieve different contact angles and percentages of laser-textured areas. Boiling performance is assessed with variations in channel spacing and surface coverage, revealing significant differences between hydrophilic and hydrophobic surfaces. The critical heat flux (CHF) enhancement mechanisms are identified, emphasizing either enhanced liquid replenishment or separate boiling and liquid-supply regions. A critical laser line spacing of 100 μm is determined, where hydrophobic and hydrophilic variants exhibit similar CHF. Interestingly, hydrophilic surfaces show a weak correlation between the heat transfer coefficient (HTC) and contact angle, while hydrophobic surfaces demonstrate a strong correlation. This separation of surface morphology and wettability confirms that lower wettability promotes early nucleate boiling and high HTC, whereas hydrophilic variants contribute to enhanced CHF with lower HTC. These distinctions are most prominent at high coverage with laser-textured areas and diminish as coverage decreases. The findings provide valuable insights into optimizing surface properties for efficient boiling heat transfer applications.

Numerical investigation of surface wettability effect on two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT) is promising for heat removal due to its simple structure and high performance in heat transfer. However, investigating the phase change process in TPCT through experiments with current technology is still challenging. To study the various surface wettability effects on TPCT, this is the first systematical volume of fluid (VOF) method-based numerical study. The wettability in terms of contact angles (5°, 40°, 120°, and 175°) is endowed on the surfaces of TPCT. The numerical results of heat transfer coefficients of the TPCT show good agreement compared with the correlations. The simulation results reveal that the hydrophilic evaporator and the hydrophobic condenser benefit heat transfer. Within the input power of 376.14 W, the hydrophilic evaporator can reduce the thermal resistance of the evaporator to 35.3 % and delays the vapor film formation. The hydrophobic condenser decreases the thermal resistance of the condenser at 3.64 % and prevents filmwise condensation. The surface wettability is critical to the temperature distribution along the evaporator, but the main temperature drop is located between the adiabatic section and the condenser. It is also noted that the thermal resistance of the condenser is dominant to the overall thermal resistance. This study provides a detailed physical insight into the surface wettability effect on TPCT and benefits the future design in thermal engineering applications.

Effect of molecular branching and surface wettability on solid-liquid surface tension and line-tension of liquid alkane surface nanodroplets

HypothesisSurface nanodroplets have important technological applications. Previous experiments and simulations have shown that their contact angle deviates from Young's equation. A modified version of Young's equation considering the three-phase line tension (τ) has been widely used in literature, and a wide range of values for τ are reported. We have recently shown that molecular branching affects the liquid–vapour surface tension γlv of liquid alkanes. Therefore, the wetting behaviour of surface nanodroplets should be affected by molecular branching. This study conducted molecular dynamics (MD) simulations to gain insight into the wetting behaviour of linear and branched alkane nanodroplets on oleophilic and oleophobic surfaces. We aim to examine the Young equation's validity and branching's effect on fundamental properties, including solid–liquid surface tension γsl and line tension τ. SimulationsThe simulations were performed on a linear alkane, triacontane (C30H62), as well as four of its branched isomers: 2,6,13,17-tetrapropyloctadecane,2,6,9,10,13,17-hexaethyloctadecane, 2,5,7,8,11,12,15-heptaethylhexadecane and 2,3,6,7,10,11-hexapropyldodecane. Nanodroplets with a diameter of approximately 15 nm were released onto the surfaces, and their contact angles were measured. Additionally, using a novel approach, the solid–liquid surface tension (γsl), the validity of Young's equation and line tension for all alkane and surface combinations are determined. FindingsIt was discovered that the calculated γsl, deviated from the theoretical γsl,Young predicted from Young's equation for all alkanes on oleophilic surfaces. However, this deviation was minimal for branched alkanes on the oleophobic surfaces but more significant for the linear alkane. The findings indicated that γsl < 0 for oleophilic surfaces and γsl > 0 for oleophobic surfaces. Moreover, it was observed that |γsl| was lower for branched molecules and decreased as branching increased. Line tension values were then determined through a novel method, showing τ was positive for oleophilic surfaces ranging from 1.30 × 10-10 to 6.27 × 10-11N. On an oleophobic surface, linear alkane shows a negative line tension of −1.15 × 10-10N and branched alkanes up to two orders of magnitude lower values ranging from −2.09 × 10-12 to 2.43 × 10-11N. Line tension values between −1.15 × 10-10 and + 1.1 × 10-10N are calculated for various linear alkane and surface combinations. These findings show the dependence of line tension on the contact angle and branching, demonstrating that for linear alkanes, τ is significant, whereas, for branched alkanes, line tension is smaller or negligible for large contact angles.

Droplet evaporation on curved surfaces: transients of internal and external transport phenomena

We explore the transient evolution of thermo-fluid-dynamics of evaporating sessile droplets over curved substrates in the liquid and gaseous domains. A computational model using the Arbitrary Lagrangian-Eulerian framework is adopted. The governing equations in both liquid and gaseous domains are solved in a fully coupled manner, considering coupled effects of evaporative cooling and heat advection due to bulk fluid motion. This bulk motion in the liquid domain is caused by natural advection due to thermal actuations such as thermal Marangoni flow and buoyancy-driven convection. For the gaseous domain, the additional effects of solutal convection (due to vapour-concentration variation), Stefan flow and interfacial viscous stresses are also considered. To depict a generalized role of substrate curvature, both concave and convex surfaces with curvatures over a wide range are studied. The surface wettability effects are also explored by varying the true contact angle of the droplets. Computational predictions on evaporation rate and internal flow field are validated against experimental results from literature. The interplay of wetting state and substrate curvature is noted to substantially affect the evaporation process and its thermo-fluidics. The convex curvature significantly augments internal advection while the same is weakened over concave substrates due to altered mass loss rate. Consequently, the duration of multi-vortex Marangoni flows in the development stages of evaporation and the advection in the external gaseous domain is markedly different for different curvatures. Further, on superhydrophobic curved surfaces, the effects of re-distributed evaporative fluxes play a major role. In such cases, the reduced mass flux over a large interfacial area near the periphery expedites the stable state Marangoni and external flow features.

Degradation profiles of the poly(ε-caprolactone)/silk fibroin electrospinning membranes and their potential applications in tissue engineering

Degradation profiles are critical for the optimal application of electrospun polymer nanofibers in tissue regeneration, wound healing, and drug delivery systems. In this study, natural and synthetic polymers and their composites were subjected to in vivo transplantation and in vitro treatment with lipases, macrophages, and acetic acid to evaluate their degradation patterns. The effects of environmental stimulation, surface wettability, and polymer components on the degradation profiles of the electrospinning poly(ε-caprolactone)/silk fibroin (PCL/SF) nanofibers were first evaluated. In vivo degradation study demonstrated that bulk degradation, characterized by the transition from microfibers to nanofibers, and surface erosion, characterized by fusion between the microfibers or direct erosion from both ends of the microfibers, occurred in the electrospun membranes; however, bulk degradation dominated their overall degradation. Furthermore, the degradation rates of the electrospun PCL/SF membranes varied according to the composition, morphology, and surface wettability of the composite membranes. After the incorporation of silk fibroin (SF), the degradation rate of the SF/PCL composite membranes was faster, accompanied by larger values of weight loss and molecular weight (Mw) loss when compared with that of the pure poly(ε-caprolactone) (PCL) membrane, indicating a close relationship between degradation rate and hydrophilicity of the electrospinning membranes. The in vitro experimental results demonstrated that enzymes and oxidation partially resulted in the surface erosion of the PCL/SF microfibers. Consequently, bulk degradation and surface erosion coordinated with each other to enhance the hydrophilicity of the electrospinning membranes and accelerate the in vivo degradation.

Impact dynamics of water droplets on oil-covered dielectrowetting substrate: Effects of oil film thickness and surface wettability

Droplet impacting on solid or liquid film is a ubiquitous phenomenon in relevant processes such as spray cooling and inkjet printing on primer layers. In this study, the dynamics of water droplets impacting a solid substrate covered by a silicon oil film is investigated under the influences of the surface wettability of the dielectrowetting substrate (69.3°-117.4°) and the thickness of the silicon oil film (7.79–21.8 μm) at We=6.81. A phase diagram is established, which reflects the relationships between the impact dynamic behavior, equilibrium contact angle, and the oil film thickness. The maximum spreading factor βmax and maximum out-of-plane height Hmax/D0 for rebound of sub-droplet behaviors was smaller than 13 % and greater than 3 times than those droplet deposition behaviors, respectively. Finally, the retraction dynamics of the droplet after reaching its maximum spreading diameter is analyzed and discussed based on retraction rate γw=vr/Dmax=σw(cosθrmin-cosθeq)/fkDmax through the force balance at the three-phase contact line. Results of the study indicate that the surface wettability of the dielectrowetting substrate and thickness of the silicon oil film significantly affect the impact dynamics of water droplets on a solid substrate covered by the silicone oil film, especially in the retraction stage. This study sheds new light on controlling the dynamic impact of the droplets on silicon oil films, which is of importance to the control or optimization of processes such as inkjet printing on a primer layer.

Wetting and Spreading Behavior of Axisymmetric Compound Droplets on Curved Solid Walls Using Conservative Phase Field Lattice Boltzmann Method.

Compound droplets have received increasing attention due to their applications in many several areas, including medicine and materials. Previous works mostly focused on compound droplets on planar surfaces and, as such, the effects of curved walls have not been studied thoroughly. In this paper, the influence of the properties of curved solid wall (including the shape, curvature, and contact angle) on the wetting behavior of compound droplets is explored. The axisymmetric lattice Boltzmann method, based on the conservative phase field formulation for ternary fluids, was used to numerically study the wetting and spreading of a compound droplet of the Janus type on various curved solid walls at large density ratios, focusing on whether the separation of compound droplets occurs. Several types of wall geometries were considered, including a planar wall, a concave wall with constant curvature, and a convex wall with fixed or variable curvature (specifically, a prolate or oblate spheroid). The effects of surface wettability, interfacial angles, and the density ratio (of droplet to ambient fluid) on the wetting process were also explored. In general, it was found that, under otherwise identical conditions, droplet separation tends to happen more likely on more hydrophilic walls, under larger interfacial angles (measured inside the droplet), and at larger density ratios. On convex walls, a larger radius of curvature of the surface near the droplet was found to be helpful to split the Janus droplet. On concave walls, as the radius of curvature increases from a small value, the possibility to observe droplet separation first increases and then decreases. Several phase diagrams on whether droplet separation occurs during the spreading process were produced for different kinds of walls to illustrate the influences of various factors.

Effect of Surface Wettability on Nanoparticle Deposition during Pool Boiling on Laser-Textured Copper Surfaces

Prior studies have evidenced the potential for enhancing boiling heat transfer through modifications of surface or fluid properties. The deployment of nanofluids in pool boiling systems is challenging due to the deposition of nanoparticles on structured surfaces, which may result in performance deterioration. This study addresses the use of TiO2–water nanofluids (mass concentrations of 0.001 wt.% and 0.1 wt.%) in pool boiling heat transfer and concurrent mitigation of nanoparticle deposition on superhydrophobic laser-textured copper surfaces. Samples, modified through nanosecond laser texturing, were subjected to boiling in an as-prepared superhydrophilic (SHPI) state and in a superhydrophobic state (SHPO) following hydrophobization with a self-assembled monolayer of fluorinated silane. The boiling performance assessment involved five consecutive boiling curve runs under saturated conditions at atmospheric pressure. Results on superhydrophilic surfaces reveal that the use of nanofluids always led to a deterioration of the heat transfer coefficient (up to 90%) compared to pure water due to high nanoparticle deposition. The latter was largely mitigated on superhydrophobic surfaces, yet their performance was still inferior to that of the same surface in water. On the other hand, CHF values of 1209 kW m−2 and 1462 kW m−2 were recorded at 0.1 wt.% concentration on both superhydrophobic and superhydrophilic surfaces, respectively, representing a slight enhancement of 16% and 27% compared to the results obtained on their counterparts investigated in water.

Effects of viscosity ratio and surface wettability on viscous fingering instability in rectangular channel

This paper explores the impact of viscosity ratio and surface wettability on immiscible viscous fingering instability within a rectangular channel. Numerical investigations are conducted across a range of viscosity ratios (VR) from 0.0009 to 0.5 and wall wettability ([Formula: see text]) from 15° to 150°. The volume of fluid (VOF) model is employed to track the development of finger-shaped instability at the fluid interface. Our results indicate that higher viscosity ratios lead to increased displacement efficiency. Additionally, we find the formation of necking at low VR, which diminishes at higher VR values. The finger-shaped pattern splits into two parts at a wettability of 15°; beyond this threshold, no such splitting occurs. Furthermore, a transition from hydrophilic to superhydrophobic wettability abolishes necking, resulting in enhanced displacement efficiency. Notably, as wettability shifts from hydrophilic to super hydrophobic, instability shifts toward the left side. These findings hold relevance for applications in drug delivery, clinical processes and oil recovery.

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

Sliding motion of highly deformed droplets on smooth and rough surfaces: Shape oscillation, chaotic breakage, corner shape, and pearling

The sliding behavior of droplets on smooth and rough surfaces with various surface wettabilities is investigated by many-body dissipative particle dynamics simulations. On a smooth surface, as the driving force (Bo) increases, the droplet shape and velocity (Cac) before breakage can be classified into four distinct regimes: (I) nearly spherical cap with Cac∝Bo; (II) oval shape with negative deviation from the linear relation; (III) elongated shape without a neck, where Cac decreases with increasing Bo; and (IV) oscillation of an elongated shape with fluctuating sliding velocity. On rough surfaces, corner-shaped droplets, which are absent on a smooth surface, can be observed. A further increase in Bo leads to the formation of cusp and pearling. Different from pinching-off on rough surfaces, which produces a cascade of smaller droplets through groove-induced shedding, chaotic breakage of a droplet on a smooth surface is caused by an unsteady flow field. Finally, a universal linear relationship between the sliding velocity based on the surface velocity (Cas) and the modified driving force (Bo**) is derived to take into account the effects of surface wettability and roughness.

Rising of asymmetric bubble through milli-tubes: Experimental, theoretical and numerical analysis

Appearance of Taylor bubble is ubiquitous in two-phase fluid flow through millimeter-sized tubes. Precise control over bubble generation and stability in those tubes are instrumental in unit processes in fine chemical industries. In this article, we systematically study the role of wall surface wettability in the structure and dynamics of the gaseous bubble flowing through water across milli-tubes. Specifically, from imaging experiments, we find that in a non-wettable milli-tube, the bubble loses its axial symmetry to reduce contact of the liquid phase with the tube wall. Moreover, we observe that the bubble speed is significantly higher in hydrophobic-coated tubes in comparison to the tubes without coating. Computational simulation balancing the viscous, gravitational, and surface forces recovers the observed effect of surface wettability on bubble shape and velocity. Finally, we establish the observed quantitative correlation between the bubble velocity and the surface wettability using an analytical model. Altogether, integrating experiments, numerical simulation, and theoretical analysis, we demonstrated the significant influence of wall-surface wettability in the shape and velocity of the Taylor bubble through milli-tubes.

An LBM study of multichannel flow boiling for electronic thermal management coupling flow instability mitigation

The rising heat dissipation requirement on electronic devices urges a more efficient and energy-saving cooling strategy to keep the equipment operating within a safe temperature range. Multichannel flow boiling provides a straightforward solution to this challenge due to enormous latent energy of vapor preventing heat accumulation. However, pressure drop minimization and flow instability mitigation should also be considered for optimizing multichannel design. Hence, a Lattice Boltzmann Method (LBM) study on multichannel flow boiling process is conducted to provide design-based suggestions. The effects of surface wettability, channel number, input heat flux, and inlet velocity on the two-phase flow characteristics, heat transfer coefficient enhancement, dimensionless pressure drop, and flow instability are compared to examine the overall performance. Non-dimensional pressure drop is proposed for comparison through dividing the pressure drop under two-phase flow stage by the pressure drop under single-phase flow stage. Results show that hydrophilic coating prevents the film boiling transition at high input heat flux and reduces the maximum temperature for safer electronic operation. Designing a dense channel array can enhance the overall HTC but also significantly increase flow instability on the inlet, leading to shorter pumping operation life. A hybrid design of multichannel with downstream microgap region is proposed, and results indicate great mitigation ability with the increase of gap length and inlet velocity. These findings offer an invaluable blueprint for multichannel heat sink design and reveal the mechanisms of flow boiling enhancement.

Surface wettability effects on self-cleaning efficacy: Outdoor experimental study

Self-cleaning coatings of photovoltaic (PV) panel and building glass have received a great deal of attention over the last two decades, using both hydrophobic and hydrophilic treatments. Here, we present a comparison study of the outdoor self-cleaning performances of superhydrophilic, hydrophobic and superhydrophobic glass coatings, over 200 days, using different tilt angles. It was found that the average transmission of all the samples linearly decreased over time after rainfall. We have defined relative self-cleaning efficacy (RSCE), using the average transmission difference of coated and uncoated samples, at a given tilt angle. We have found that the RSCE depends on both surface wettability and tilt angle. The superhydrophobic coating was found to have the best RSCE, with the superhydrophilic coating next, although both superhydrophobic and superhydrophilic coating have similar RSCE when cleaned by rainfall. Surprisingly, the RSCE is higher at a lower tilt angle for all the samples, despite a higher transmission at a higher tilt angle, suggesting that the RSCE is higher at lower tilt angles due to weaker interaction between dust particles and the surfaces.

Effect of surface topography and wettability on shear bond strength of Y-TZP ceramic

Zirconia ceramics have been widely used as dental restorations due to their esthetic appearance and high flexural strength. The bonding of zirconia with resin cement should rely on both mechanical and chemical bonds. This study was performed to investigate the effect of zirconia surface topography and its wettability after surface pretreatments on the microshear bond strength (μSBS) of a resin cement. Zirconia slabs were prepared and randomly divided into 5 groups based on the surface treatment as follows: no treatment (control), air abrasion (AB), etching with hydrofluoric acid (F), the mixture of hydrofluoric acid and nitric acid (FN), or the mixture of hydrochloric acid and nitric acid (CN) for 10 min. The specimens were subjected to investigation of surface roughness characteristics [average roughness (Ra), peak-to-valley average distance (Rpv), skewness (Rsk), and kurtosis (Rku)] using atomic force microscopy (AFM) and measurements of surface contact angle (θc) and μSBS of a resin cement. In addition, the area % of the nanoscale surface irregularity (nSI%) was calculated from the AFM images. The effects of nSI%, Ra and θc on the μSBS were analyzed by multiple linear regression analysis (p < 0.05). Multiple regression analysis revealed that the nSI% was the most predominant factor for the μSBS (p < 0.001). A surface with larger nSI%, higher Ra and relatively lower θc was essential for establishing a reliable resin-zirconia bond.

Insight into the influence of surface wettability on flotation properties of solid particles – Critical contact angle in flotation

Bubble-particle attachment is a fundamental element of the flotation process, and its occurrence depends on the stability of the thin liquid film separating these objects. In this study, the effect of surface wettability on the stability of liquid film and subsequent attachment was investigated by flotation of glass microspheres (70–110 μm) with controlled contact angles (9–89°) in a Hallimond tube. Additionally, pH was varied to determine the electrostatic interactions between the bubble and particles. The results showed that for the flotation process to take place, the minimum (critical) value of the contact angle must be exceeded. This value was found to be ca. 25° when the electrostatic interactions were either attractive or weakly repulsive and as high as 62° for strong repulsive electrostatic interactions. These findings highlight the role of attractive hydrophobic interactions, whose importance increases with increasing surface contact angle in the context of stability of thin liquid films.

Effect of rock surface wettability on hydrogen-cushion gas adsorption with application to hydrogen storage in depleted oil and gas reservoirs

In this study, adsorption capacity of Berea sandstones varying wettability states with respect to hydrogen (H2) and hydrogen-methane (H2-CH4) mixture are examined to assess the viability of depleted oil and gas reservoirs for large scale hydrogen storage. We investigate pure hydrogen and hydrogen-methane mixture adsorptions as a function of pressure, temperature, and rock surface wettability. The obtained results indicate that as the CH4 fraction increases from 0 to 20 %, the gas uptake exhibits a more significant rise, going from 0.98 to 1.68 cm3/g for oil-wet sandstone and from 1.04 to 1.2 cm3/g for untreated samples, both at 25 °C. This suggests a pronounced affinity of CH4 for the aged rock. The adsorption capacity of the H2-CH4 mixture increases from 1.2 to 1.68 cm3/g at 80 bar and 25 °C when rock samples are exposed to crude oil compared with the untreated rock samples. Investigating the impact of temperature on adsorption capacity reveals a decrease in gas uptake as the measurement temperature rises from 25 to 60 °C across all pressure levels. All rock samples exhibit a positive hysteresis in adsorption and desorption isotherms at various temperatures. The Freundlich, Redlich-Peterson, and Sips models are found to be more representative in describing the adsorption characteristics, suggesting multilayer adsorption on the rock surface. Our findings provide insights into the impact of natural gas adsorption and desorption on overall hydrogen production. This study can improve the accuracy and efficacy of reservoir simulations and flow models that depict the movement of hydrogen and natural gas through porous media within sandstone reservoirs.

Wettability Effects of Curved Superhydrophobic Surfaces on Drag Reduction in Taylor–Couette Flows of Water and Oil

Abstract This study examines the effects of surface wettability on the drag-reducing performance of three hydrophobic coatings, namely, flouropel coating (FPC-800M), superhydrophobic binary coating (SHBC), and ultra-ever dry (UED)—when applied to curved aluminum surfaces. The wettability and flow characteristics were characterized using three liquids of different viscosities: de-ionized water and silicone oils of 5 and 10 cSt. Static and dynamic contact angles on the surfaces were measured, and the drag reduction was evaluated using a Taylor–Couette flow cell in a rheometer. The static contact angle (SCA) measurements indicated that the coated surfaces were superhydrophobic for water, with a maximum static contact angle of 158 deg, but oleophilic for the 10 cSt silicone oil, with a static contact angle of 13 deg. The rheometer measurements using water showed a maximum drag reduction of 18% for the UED-coated surfaces. Interestingly, the oleophilic surfaces (which have low SCA) showed a maximum drag reduction of 6% and 7% in the silicone oils. The observed drag reduction is due to an increase in the plastron thickness, which is caused by an increase in the Reynolds number and dynamic pressure coupled with a decrease in the static pressure normal to the superhydrophobic wall.