Low Contact Angle Hysteresis Research Articles

Self- replenishing superhydrophobic durable polymeric nanocomposite coatings for heat exchanger channels in thermal management applications

We developed two-coat polymeric nanocomposite coatings for the protection of steel structures from the thermal and corrosive environment. To protect the steel structures, the coating selectivity is crucial; we have chosen the coating such way, the coated surface has low surface energy polymer which has low surface polarity. The polymer nanocomposite coatings were prepared using polydimethylsiloxane with an optimal amount of zirconia nanoparticle. The incorporation of zirconia nanoparticle has extended the thermal stability and efficient corrosion protection of steel substrates. The formulated coatings exhibited a superhydrophobic characteristic with CA-165° and low contact angle hysteresis (<5°). The present study focuses more on the robust superhydrophobic behaviour, were studied using the effectiveness of underwater stability over 30 days in static immersion and water impingement analysis with a velocity of 7.302 m/s at continuous exposure. PDMS-ZrO2 composite films showed excellent resistance to different pH stress without loss of coating. The chemical composition and microstructure of the coatings were studied using EDX and SEM, respectively. The predictive life of the coating performance has been further evaluated by pool boiling test, Thermal shock test, and Electrochemical Impedance Spectroscopy.

Adaptive Wetting of Polydimethylsiloxane.

To better understand the wetting of cross-linked polydimethylsiloxane (PDMS), we measured advancing and receding contact angles of sessile water drops on cross-linked PDMS as a function of contact line velocity (up to 100 μm/s). Three types of samples were investigated: pristine PDMS, PDMS where oligomers were removed by toluene treatment, and PDMS with an enriched concentration of oligomers. Depending on the velocity of advancing contact lines and the contact time with water, different modes of wetting were observed: one with a relatively low contact angle hysteresis (Δθ ≈ 10°) and one with a larger hysteresis. We attribute the low hysteresis state, called the lubricated state, to the enrichment of free oligomers at the water–PDMS interface. The enrichment of oligomers is induced by drop contact. The kinetics of the transition to the lubricated state can be described by adaptation theory. PDMS adapts to the presence of water by an enrichment of free oligomers at the interface and a correlated reduction in interfacial tension.

Anti-Icing Properties of Vertically Aligned TiO2 Nanopillars.

Inspired by the superhydrophobic and anti-icing properties of Berberis thunbergii leaves for the first time, a surface with vertically aligned TiO2 nanopillars (VATiO2NPIs) has been developed, which shows excellent anti-icing properties even at -25 °C. The height and diameter of TiO2NPIs arrays were 200 and 70 nm, respectively, and were fabricated using TiO2 atomic-layer deposition in an aqueous solution on an anodized aluminum oxide substrate and modified with stearic acid. The water contact angle measurements for VATiO2NPI-based aluminum surfaces indicated high contact angle (170.2°), low contact angle hysteresis (5°), excellent delay time (385 s), appropriate robustness (>20 cycles of freeze-melt), and low adhesion strength (<95 kPa). The VATiO2NPI surface analyses were done using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction spectroscopy (XRD), and Fourier transform infrared spectroscopy (FT-IR) methods, and they showed that the stearic acid stably absorbed on VATiO2NPIs. Our results indicated that the long delay time of the heterogeneous nucleation and freezing process could be obtained by the desired structural match between the VATiO2NPI apices and water molecules in the initiation of the heterogeneous nucleation. In this study, smooth apices of the VATiO2NPIs with highly potent morphology presented significant anti-icing properties because of the strong phonon scattering between nanopillars that can interrupt the heat transfer process. Such results are not achievable by conventional anti-icing methods and give new insight into the mechanism of the anti-icing process.

Intermediate States of Wetting on Hierarchical Superhydrophobic Surfaces.

Wetting transition on superhydrophobic surfaces is commonly described as an abrupt jump between two stable states-either from Cassie to Wenzel for nonhierarchical surfaces or from Cassie to nano-Cassie on hierarchical surfaces. We here experimentally study the electrowetting of hierarchical superhydrophobic surfaces composed of multiple length scales by imaging the light reflections from the gas-liquid interface. We present the existence of a continuous set of intermediate states of wetting through which the gas-liquid interface transitions under a continuously increasing external forcing. This transition is partially reversible and is limited only by localized Cassie to Wenzel transitions at nanodefects in the structure. In addition, we show that even a surface containing many localized wetted regions can still exhibit extremely low contact angle hysteresis, thus remaining useful for many heat transfer and self-cleaning applications. Expanding the classical definition of the Cassie state in the context of hierarchical surfaces, from a single state to a continuum of metastable states ranging from the centimeter to the nanometer scale, is important for a better description of the slip properties of superhydrophobic surfaces and provides new considerations for their effective design.

Non-Fluorinated Omniphobic Paper with Ultralow Contact Angle Hysteresis.

Existing methods for fabricating oil-repellent paper rely on highly fluorinated and therefore toxic chemicals. Non-fluorinated omniphobic paper with low contact angle hysteresis (CAH) has not been demonstrated. We report a facile method to prepare omniphobic paper through the vapor-phase deposition of chlorosilane molecules to create "liquid-like" polymer brushes on commercially available release liners. Compared to polymer brushes grafted from solution, this solvent-free method avoided physical deformation of the paper, such as curling or wrinkling. The obtained paper displayed low CAH (<6°) and roll-off angles for liquids exhibiting a broad range of surface tension, from 72.8 to 22.4 mN m-1. A hexadecane droplet (15 μL, 27.5 mN m-1) slid off the paper at a tilt angle less than 4°. The effects of surface roughness, composition, and the presence of particle additives on the wetting properties were investigated. The utility of the omniphobic paper was demonstrated in microfluidic, oil funnel, microtiter plate, and food packaging container applications.

Durability of microporous hybrid silica coatings: Optical and wetting properties

Bifunctional hybrid silica sol-gel coatings were synthesized, which showed both increased light transmittance and water-repellence. The as-prepared coatings of variable thickness (100–175 nm) were found to have low refractive index (1.250–1.275), high porosity (36%–42%), and maximum light transmittance up to 99.8%. Water-repellence of the surface was due to high water contact angle (90°) and low contact angle hysteresis (1–2°). Ellipsometric porosimetry and transmission electron microscopy measurements confirmed the microporous structure of the coatings. A systematic investigation for studying the durability and the deterioration of optical and surface properties was studied during aqueous soaking tests at different pH. During these experiments atomic force microscopy showed that the morphology of the surface did not change. The changes in the wetting properties were also confirmed by surface free energy measurements and an increase of the polar component was observed. The optical properties could be regenerated completely or partially after the soaking via heat treatment at 400 °C. The light transmittance of the samples remained high after the soaking, most likely due to the swelling of the solid matrix. The light transmittance and the water-repellent properties of the coatings showed long-term stability up to a year of storage.

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars.

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (θo < 90°) renders them even more wetting (θr < θo < 90°), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica θo ≈ 40° for the water/air system, and θo ≈ 20° for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are θr > 90°. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (θr ≈ 150°-160°) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within <1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

Wetting Properties of Graphene Aerogels

Graphene hydrophobic coatings paved the way towards a new generation of optoelectronic and fluidic devices. Nevertheless, such hydrophobic thin films rely only on graphene non-polar surface, rather than taking advantage of its surface roughness. Furthermore, graphene is typically not self-standing. Differently, carbon aerogels have high porosity, large effective surface area due to their surface roughness, and very low mass density, which make them a promising candidate as a super-hydrophobic material for novel technological applications. However, despite a few works reporting the general super-hydrophobic and lipophilic behavior of the carbon aerogels, a detailed characterization of their wetting properties is still missing, to date. Here, the wetting properties of graphene aerogels are demonstrated in detail. Without any chemical functionalization or patterning of their surface, the samples exhibit a super-lipophilic state and a stationary super-hydrophobic state with a contact angle up to 150 ± 15° and low contact angle hysteresis ≈ 15°, owing to the fakir effect. In addition, the adhesion force of the graphene aerogels in contact with the water droplets and their surface tension are evaluated. For instance, the unique wettability and enhanced liquid absorption of the graphene aerogels can be exploited for reducing contamination from oil spills and chemical leakage accidents.

Engineered superhydrophilicity and superhydrophobicity of graphene-nanoplatelet coatings via thermal treatment

Superwettable materials are destined for a wide range of multifunctional applications. An ingenious, cost-effective yet simple method is proposed to control the superwettability of graphene through a thermal curing process at 300 °C under atmospheric pressure. By functionalizing graphene nanoplatelets (GNPs) with oxygenated groups, superhydrophilic surface with a water/air contact angle of 0° manifesting the ultrafast water permeation property is synthesized. Oppositely, superhydrophobic surface with antiadhesive property to water is obtained by functionalizing GNPs with siloxane groups under the same conditions. The superhydrophobicity is justified with a static contact angle of 163° ± 2.8° with a low contact angle hysteresis of 1.53° ± 0.86°. The chemical structures of the superwettable graphene are elucidated using various spectroscopy techniques. Theoretically, by using molecular dynamics simulations, we have further examined and testified the tunable superwettability phenomena of graphene. This study provides important insights into a facile approach in synthesizing advanced materials with superwettability for multifunctional high-performance applications.

Dropwise condensation on solid hydrophilic surfaces.

Droplet nucleation and condensation are ubiquitous phenomena in nature and industry. Over the past century, research has shown dropwise condensation heat transfer on nonwetting surfaces to be an order of magnitude higher than filmwise condensation heat transfer on wetting substrates. However, the necessity for nonwetting to achieve dropwise condensation is unclear. This article reports stable dropwise condensation on a smooth, solid, hydrophilic surface (θa = 38°) having low contact angle hysteresis (<3°). We show that the distribution of nano- to micro- to macroscale droplet sizes (about 100 nm to 1 mm) for coalescing droplets agrees well with the classical distribution on hydrophobic surfaces and elucidate that the wettability-governed dropwise-to-filmwise transition is mediated by the departing droplet Bond number. Our findings demonstrate that achieving stable dropwise condensation is not governed by surface intrinsic wettability, as assumed for the past eight decades, but rather, it is dictated by contact angle hysteresis.

The Possibility of Changing the Wettability of Material Surface by Adjusting Gravity.

The contact angle, as a vital measured parameter of wettability of material surface, has long been in dispute whether it is affected by gravity. Herein, we measured the advancing and receding contact angles on extremely low contact angle hysteresis surfaces under different gravities (1-8G) and found that both of them decrease with the increase of the gravity. The underlying mechanism is revealed to be the contact angle hysteresis and the deformation of the liquid-vapor interface away from the solid surface caused by gradient distribution of the hydrostatic pressure. The real contact angle is not affected by gravity and cannot measured by an optical method. The measured apparent contact angles are angles of inclination of the liquid-vapor interface away from the solid surface. Furthermore, a new equation is proposed based on the balance of forces acting on the three-phase contact region, which quantitatively reveals the relation of the apparent contact angle with the interfacial tensions and gravity. This finding can provide new horizons for solving the debate on whether gravity affects the contact angle and may be useful for the accurate measurement of the contact angle and the development of a new contact angle measurement system.

Approximately symmetric electrowetting on an oil-lubricated surface.

As the most widely used insulator materials in the electrowetting (EW) systems, amorphous fluoropolymers (AFs) provide excellent hydrophobicity, dielectric properties and chemical inertness; however, they suffer from charge trapping during electrowetting with water and the consequent asymmetric phenomenon. In this study, an ultra-thin oil-lubricated AF surface was proposed to release the charge trapping in the dielectric layer and further suppress the polarity-dependent asymmetry during electrowetting. The negative spontaneously trapped charges gathering on the dielectric/water interface with aging time were characterized by various measurements and calculations, which explained the polarity dependence of the asymmetric electrowetting. Approximately symmetric EW curves withstanding water aging were obtained for the oil-lubricated AF surface, confirming the blocking effect on charge trapping induced by the lubricated surface. The improved reversibility of EW with low contact angle hysteresis brought by the oil-lubricated surface was also demonstrated. This study reveals the mechanism behind the asymmetric EW phenomenon and offers an attractive oil-lubricated EW material system for suppressing the charge trapping on the dielectric/water interface, which can significantly improve the manipulation of the EW devices.

Tunable and Magnetic Thiol-ene Micropillar Arrays.

Tunable and responsive surfaces offer routes to multiple functionalities ranging from superhydrophobic surfaces to controlled adhesion. Inspired by cilia structure in the respiratory pathway, magnetically responsive periodic arrays of flexible and magnetic thiol-ene micropillars are fabricated. Omnidirectional collective bending of the pillar array in magnetic field is shown. Local non-contact actuation of a single pillar is achieved using an electromagnetic needle to probe the responsiveness and the elastic properties of the pillars by comparing the effect of thiol-ene crosslinking density to pillar bending. The suitable thiol-ene components for flexible and stiff magnetic micropillars and the workable range of thiol-to-allyl ratio are identified. The wettability of the magnetic pillars can be tailored by chemical and topography modification of the pillar surface. Low-surface-energy self-assembled monolayers are grafted by UV-assisted surface activation, which is also used for surface topography modification by covalent bonding of micro- and nanoparticles to the pillar surface. The modified thiol-ene micopillars are resistant to capillarity-driven collapse and they exhibit low contact angle hysteresis, allowing water droplet motion driven by repeated bending and recovery of the magnetic pillars in an external magnetic field. Transport of polyethylene microspheres is also demonstrated.

A correlation of metallic surface roughness with its hydrophobicity for dropwise condensation

Abstract The surface morphology alike lotus leaf has high contact angle and low contact angle hysteresis with respect to water droplet because of nanoscale roughness superimposed on micro order roughness in well-organized manner. The major application of superhydrophobic metallic substrate is in the condensing unit of energy and water harvesting devices. In this work, nanoscale roughness is superimposed on micro-order roughness of Aluminum substrate. The chemical texturing method is used to create micro-scale roughness onto Al surface. However, 0.6 keV Ar ions are irradiated for various time span to fabricate nano order roughness on chemically etched substrate. The roughness of samples is measured by Atomic Force Microscopy and static contact angles and their hysteresis are measured by lab customize Goniometer. It is found that the static contact angle and contact angle hysteresis increases after irradiation on chemical etched surfaces. Multi-scale roughness is observed on irradiated substrate. Results also shows that contact angle increases with increasing time of irradiation. However, maximum change in static contact angle is observed for samples irradiated for 45 s. Eventually, the static contact angle is correlated with its surface roughness. This correlation is vital for optimizing parameter affecting condensation rate in dropwise condensation. Finally, the moist air condensation experiments are conducted to know the condensation efficacy of the irradiated substrate. Also, it is found that the drop sliding frequency on vertical irradiated Al substrate is higher than the chemical etched Al substrate.

Moist Air Condensation on Inclined Hydrophobic Metallic Surfaces: Simulation & Experiments

Atmospheric Water Generators (AWGs) are popularly used for harvesting portable water from atmospheric air of hot, humid and arid regions of the world. Hydrophobic metallic surfaces are preferred in AWGs systems because condensation of moist air on/underneath these surfaces have high efficacy. There are several issues to fabricate hydrophobicity on metallic surfaces. Although with the advent of Nanotechnology and thin film coating technologies, the fabrication of hydrophobicity on metallic surfaces has become easier and realizable in the recent era. In this manuscript, a comprehensive mathematical model is developed for simulating the moist air condensation in the form of droplets on various substrates and in different environmental conditions. The experiment is carried out for validation of the present model. Post validation, the effect of surface hydrophobicity, relative humidity and degree of sub-cooling on the condensation rate are addressed. The simulation results show that vertically orientated metallic surfaces having a high contact angle along with low contact angle hysteresis are efficient for condensing unit. Larger condensation rates are observed at higher relative humidity and a high degree of sub-cooling. This research is helpful for designing efficient and effective AWGs for the hot and humid region.

Fabrication of transparent icephobic surfaces with self-reparability: Effect of structuring and thickness of the lubricant-elastomer layer

Biomimetic lubricant-infused slippery surface (LISS) has attracted extensive attention as an alternative to a superhydrophobic surface (SHS) for icephobic applications. In this work, the impact of the material parameters of LISS with a lubricant-elastomer layer on icephobic performance is reported. A layer of polydimethylsiloxane elastomer and silicone oil on smooth and structured polypropylene (PP) surfaces was spin-coated by a simple one-step procedure. Results demonstrate that the ultra-smooth lubricant-elastomer layer (LEL) has an extremely low water contact angle hysteresis. Compared to SHS and smooth surface, the LISSs reduced ice adhesion by an order of magnitude due to its molecular homogeneity, low contact area, and strain mismatch. The LISSs exhibited excellent dynamic mobility at −10 °C, shear stability, evaporation resistance, mechanical robustness, and transparency. Physically damaged LISS can self-repair at 80 °C. The change in structures and LEL thickness of structured PP has little effect on ice adhesion. As the LEL thickness on a smooth PP increases, the ice adhesion decreases. The smooth PP with a thick LEL has very low ice adhesion, indicating that the roughness is not necessary for maintaining the slippery properties. Transparent, stable, anti-dusting, and self-repairing LELs on smooth materials have potential for icephobic applications.

Hydrodynamics and mechanism of hydrophobic foam column tray: Contact angle hysteresis effect

AbstractInterfacial wettability adjustment is a new method for intensifying vapor–liquid mass transfer process. Contact angle effect has been well investigated but not complete due to interfacial wettability consisting of both static behavior (contact angle) and dynamic behavior (contact angle hysteresis). Here, methods of adjusting contact angle hysteresis (CAH) were proposed, and then, the CAH effect on the hydrodynamics was investigated. A multiscale analysis of CAH effect, from interfacial force and wettability to single‐bubble and gas–liquid two‐phase flow inside the foam to bubble swarm hydrodynamics, was conducted, and thus, the hydrodynamic performance criteria were derived. The interfaces had similar contact angles, whereas a significant difference in the CAH was prepared by using the developed sol dip‐coating and spray coating methods. Subsequent experiments revealed that lower CAH can decrease the pressure drop, homogenize the gas distribution, and increase the weeping rate, which are consistent with the derived criteria.

Nanotextured Aluminum-Based Surfaces with Icephobic Properties

Undesirable water freezing (icing) usually occurs in cold environments and may have lethal consequences. Preventing icing usually requires the installation of active thermal systems which consume energy and increase costs. Nanoengineered superhydrophobic surfaces can delay freezing passively; however, when exposed to sub-zero temperatures, they can get covered by frost, which promotes ice formation and impairs their icephobicity. In addition, high thermal conductivity of the surfaces can reduce the frost formation rate. Thus, we chose aluminum as our working material for its good thermal conductivity and widespread industrial usage. We employed electrochemical anodization process to control and tune the surface morphology. Crucially, we demonstrate the feasibility of morphology control at the nanoscale and tunability of the surface solid fraction in the range of 0.1–0.25, while using safer polishing electrolytes and etchants compared to existing practice, i.e., our approach is environmentally friendlier. Surface functionalization and morphology control were used to render the surfaces (super) hydrophobic, with low contact angle hysteresis. The best performing surfaces demonstrate ice nucleation temperatures as low as –19 °C and resist liquid impalement—tested via drop impact velocity up to 3 m/s (Weber number > 300)—demonstrating a clear potential for their exploitation as icephobic surfaces.

Hierarchical Morphology of Poly(ether ether ketone) Aerogels.

The phase diagram for the thermoreversible gelation of poly(ether ether ketone) (PEEK) in 4-chlorophenol (4CP) was constructed over broad temperature and concentration ranges, revealing that PEEK is capable of dissolving and forming gels in both 4CP and dichloroacetic acid (DCA) up to a concentration of 25 wt %. Highly porous aerogels of PEEK were prepared through simple solvent exchange followed by one of two drying methods of solvent removal from the wet gel: freeze-drying or supercritical CO2 fluid extraction (SC-drying). The field-emission scanning electron microscopy analysis showed that gelation of PEEK in 4CP, followed by SC-drying, produced aerogels with well-defined lamellar aggregates as compared to less ordered aggregates formed from DCA. Mechanical properties (in compression) were shown to improve with increasing density, resulting in equivalent compressive moduli at comparable density, regardless of the preparation method (gelation solvent selection, concentration variation, or drying method). Nitrogen adsorption-desorption isotherms indicate that PEEK aerogels are comprised of mesopores (2-50 nm diameter pores) formed from stacked crystalline lamella. PEEK aerogels prepared using SC-drying exhibit higher Brunauer-Emmett-Teller surface areas than freeze-dried aerogels of comparable density. The ultra-small-angle X-ray scattering/small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering analysis revealed a hierarchical morphology of the PEEK aerogels with structural features from PEEK crystallites to agglomerates of stacked lamella that spanned a wide range of length scales. SANS contrast-matching confirmed that the morphological origin of the principle scattering feature in PEEK aerogels is stacked crystalline lamella. Nitrogen sorption measurements of porosity and the specific surface area of the PEEK aerogels were correlated with the SAXS analysis to reveal a remarkably high surface area attributed to the platelet-like, lamellar morphology. Contact angle and contact angle hysteresis (CAH) revealed that low-density PEEK aerogels (ρ < 0.15 g/cm3) have water contact angles above the superhydrophobicity cutoff angle (>150°) and a very low CAH near 1°.

Optimizing Hydrophobicity and Photocatalytic Activity of PDMS-Coated Titanium Dioxide.

Polydimethylsiloxane (PDMS) can be linked to the surface of metal-oxide photocatalysts by immersion and UV illumination. The surfaces become hydrophobic and keep their hydrophobicity even under extended UV exposure. Titanium dioxide (TiO2) is a prominent example of a metal-oxide photocatalyst. Here, we studied the influence of a grafted PDMS layer on the photocatalytic activity and wetting properties of TiO2. By varying the molecular weight of PDMS, we controlled the thickness of the polymer layer from 0.6 to 5.5 nm. We recommend a PDMS molecular weight of 6.0 kDa. It leads to a grafted PDMS layer thickness of 2.2 nm, a receding contact angle of 94°, a low contact angle hysteresis of 9°, and the layer is still photocatalytically active.