Wenzel State Research Articles

Influence of surface roughness on contact angle hysteresis and spreading work

Surface roughness is an important factor that affects dynamic wetting behavior, which can improve the surface hydrophobicity, so it is of great significance to obtain a better understanding of roughness effect from both theoretical and practical perspectives. In this paper, we studied the influence of macro-size surface roughness on contact angle hysteresis and spreading work and analyzed the relationship between contact angle hysteresis and spreading work. Results showed that as the surface roughness increased, both the advancing contact angle and the receding contact angle continued to increase until their maximum values were reached, and then started to decrease within the range of surface roughness studied, while the contact angle hysteresis presented the opposite trend. In addition, with the increase of surface roughness the spreading work initially increased to a certain maximum value, then continuously decreased to the minimum value, and then began to increase within the range of the surface roughness studied. These trends could be attributed to the surface wetting state (Wenzel state, Cassie state, and transition state) changing with the change of surface roughness. These findings can provide guidance for the preparation of wetted surfaces with specific functions, especially when it is required to change the wettability without changing the surface chemical properties.

Effect of initial wetting state on plastron recovery through heating

Applicability of superhydrophobic surfaces depends on maintaining the trapped gas pockets in the micro/nano roughness valleys of the surface. It is well known that besides efforts of scientists to construct robust surfaces against wetting transition, it is also desirable to develop strategies to restore the lost plastron underwater. The current paper systematically investigates the effect of initial wetting state on the possibility of plastron recovery. It is shown that different temperature for water and surface as well as different air solubility in water can result in different wetting state. It is also shown that despite the Cassie–Baxter state which skips nucleate boiling and leads to film boiling, in Wenzel state, heating results in nucleate boiling and nucleate bubbles easily detach from the surface due to strong contact line pinning. Interestingly, when surface has parahydrophobic behavior (water wets the micro structures but the nano features remain dry), the plastron is revivable through existing air in the water; but if we degas the water the plastron can be recovered by applying heat flux.

Wetting Dynamics of a Water Droplet on Micropillar Surfaces with Radially Varying Pitches

The wetting dynamics of a sessile droplet on square micropillar substrates with radially varying pitches, prepared on silicon wafers using a photolithography technique, is investigated experimentally. Two configurations are considered, namely, substrates with radially increasing pitch and radially decreasing pitch. The droplet initially placed at the center experiences a wettability gradient because of the variation in pitch of the micropillar substrate leading to complex wetting dynamics. We observed that the droplet remains in the Cassie-Baxter state in the case of a radially increasing pitch and exhibits a higher contact angle than that on a smooth surface during its spreading stage. In contrast, the droplet experiences the Wenzel condition in the case of a radially decreasing pitch and assumes a lower contact angle relative to that observed on a smooth surface. The wetted diameter of the droplet in the radially decreasing pitch configuration is found to be smaller than that observed in the radially increasing pitch configuration. Our study also reveals that increasing the size of the pillars increases the wetted diameter of the droplet in both configurations. Theoretical models developed using the Cassie-Baxter and Wenzel states for the radially increasing and radially decreasing pitches satisfactorily predict the experimental behaviors.

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces.

We experimentally and theoretically examine the influence of a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on superhydrophobic (SH) surfaces made of hydrophobic microcylinders. We use two types of micropatterns of different surface roughness, r, and packing fraction, ϕ, and vary nine dimensionless surfactant concentrations (CS), normalized by the critical micelle concentration (CMC), in the experiments. At low CS, some of the surfactant-laden droplets are in a gas-trapping, Cassie-Baxter (CB) state on the high-roughness microstructures. In contrast, some droplets are in a complete-wetting Wenzel (W) state on the low-roughness microtextures. We found that the contact angle of CB drops can be well predicted using a thermodynamic model considering surfactant adsorption at the liquid-vapor (LV) and solid-liquid (SL) interfaces. At high CS, however, all of the DDAB drops wet in a Wenzel mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of superhydrophobic surfaces used, the Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low CS due to a thermodynamic energy barrier. The metastable CB wetting state becomes more stable on the SH microtextures with greater ϕ and r, in agreement with our experimental observations. Finally, we generalize this surface-energy analysis to provide useful designs of surface parameters for a DDAB-laden surfactant droplet on the SH surface with a stable and robust CB state.

CO2 wetting on pillar-nanostructured substrates

CO2 capture by dropwise CO2 condensation on cold solid surfaces is a promising technology. Understanding the role of the nanoscale surface and topographical features of CO2 droplet wetting characteristics is of importance for CO2 capture by this technology, but this remains unexplored as of yet. Here, using large-scale molecular dynamics (MD) simulations, the contact angle and wetting behaviors of CO2 droplets on pillar-structured Cu-like surfaces are investigated for the first time. Dynamic wetting simulations show that, by changing the strength of the solid–liquid attraction a smooth Cu-like surface offers a transition from CO2-philic to CO2-phobic. By periodically pillared roughening of the Cu-like surfaces, however, a higher contact angle and a smaller spreading exponent of a liquid CO2 droplet are realized. Particularly, a critical crossover of CO2-philic to CO2-phobic can appear. The wetting of the pillared surfaces by a liquid CO2 droplet proceeds non-uniformly. A liquid CO2 droplet is capable of exhibiting a transition from the Cassie state to the Wenzel state with increasing increasing inter-pillar distance, and increasing pillar width. The wetting morphologies of the metastable Wenzel state of a CO2 droplet are very different from each other. The findings will inform the ongoing design of CO2-phobic solid surfaces for practical dropwise condensation-based CO2 capture applications.

Water‐Based Robust Transparent Superamphiphobic Coatings for Resistance to Condensation, Frosting, Icing, and Fouling

AbstractDue to the considerable demand for improving energy conservation and emission reduction, superhydrophobic coatings mainly derived from organic solvents have attracted significant interest, based on their potential role in enhancing heat transfer. Although water‐based coatings are relatively safe and economical, and contain low volatile organic compounds (VOCs), the realization of coatings with excellent anti‐condensation, anti‐frosting, anti‐icing, and passive self‐cleaning properties remains challenging, owing to the Wenzel state of small‐sized condensate microdrops. Herein, a water‐based transparent superamphiphobic coating is developed through a single‐step spraying method performed at ambient temperature using water and liquid rheological additives to disperse fluorinated SiO2 nanochains. The coating exhibits resistance to damage induced by low‐ and high‐temperature processing, chemical solution attacks, outdoor exposure, and freeze–thaw cycles. Most importantly, the porous structure consisting of nanopores and sub‐micropores, results in the suppression of condensation, frosting, and icing as well as facile removal of condensate micro‐drops, sparse frost, and ice. Furthermore, fouling can be eliminated via condensation or a cycle of frosting and defrosting utilizing the moisture in the air. The findings of this study may represent a significant advance in long‐term, efficient, and low‐VOC superamphiphobic technologies for large‐scale applications.

Influence of adhesives on the construction of a robust superamphiphobic surface

Abstract A hierarchical structure is a prerequisite for spray-coating of a superamphiphobic surface because of its capability to enhance surface roughness, and retain large numbers of fine air-pockets. However, utilization of adhesives always compromises the performance of the super-repellent coating. Therefore, it is of great significance and extremely challenging to fabricate a superamphiphobic surface. In the present work, a commercial inorganic adhesive aluminum dihydrogen phosphate (ADP) was applied to a hierarchically mesoporous TiO2 suspension to fabricate superamphiphobic coating. N2 adsorption-desorption isotherms were used to characterize the air-pockets inside the coating and to correlate the wettability of the coating with varied ADP contents. The results indicated that with the increase of ADP, it covered TiO2 building blocks and filled the space, which destroyed the hierarchical structure, resulting in a change from Cassie to Wenzel state. When the addition amount of ADP was less than 8%, superamphiphobic surface with excellent mechanical robustness could be achieved.

Effect of gravity in the Cassie-to-Wenzel transition on a micropatterned surface

When the Cassie–Baxter and Wenzel states coexist for a liquid droplet on a micropatterned surface, the Cassie-to-Wenzel transition takes place if the energy barrier is overcome. Although multiple metastable states coexist due to the micropattern, this paper presents a simple Cassie-to-Wenzel transition of a 2 µL water droplet on a particular micropillared surface: When the droplet is gently deposited above the surface, it equalizes to the Cassie state at zero gravity; however, it transitions to the Wenzel state at the terrestrial gravity, in which the gravitational potential energy overcomes the energy barrier between the Cassie and Wenzel states.

Importance of moderate size of pillars and dual-scale structures for stable superhydrophobic surfaces: A molecular dynamics simulation study

In this work, we first create three-dimensional molecular-dynamics (MD) model to study the effect of structure parameters including pillar size, pillar height, pillar groove, and intrinsic contact angle (θe, or Young’s angle) on wetting behavior of drops on the one-scale nanopillared surface. We find that drops tend to exhibit the Cassie state on the nanopillared surface with small pillar size and large θe. Considering comprehensively the stability of the Cassie state, excellent hydrophobicity, and mechanical durability of microstructures, the pillar height and the pillar groove should be moderate. Then, only considering the wettability, we could see the dual-scale structure as the one-scale structure with large θe* that results from the small-scale structure for increasing the θe and cannot be achieved on the smooth surface in experiments. Importantly, we observe that the dual-scale structure, corresponding to large θe* resulting from the small-scale structure for increasing the θe, are important for the formation and the stability of the Cassie state. Moreover, the dual-scale structure can prevent the transition of drops from the Cassie state to the Wenzel state. Interestingly, fabricating the small-scale structure on top of the large-scale structure (corresponding to large θe* of the one-scale structure) can facilitate the transformation of the Wenzel state of drops on the large-scale structure to the Cassie state, and the Cassie state is stable. This study provides an inspiration for the fabrication of stable artificial superhydrophobic surfaces.

Enabling highly effective underwater oxygen-consuming reaction at solid-liquid-air triphasic interface

Electrode surfaces with superhydrophobic and conductive properties exhibit a unique energy-mass transfer behavior in electrolytes. Therefore, the rational design of electrode surface structures, conductivity, and infiltration performance is expected to yield more efficient electrochemical reactions and solve the gas-deficit problem that hinders many underwater gas-consuming reaction systems. In this paper, hydrogenated carbon nano-onions displaying superhydrophobicity and conductivity were prepared by microwave plasma-enhanced chemical vapor deposition, which exhibited remarkable transferability and designability. The fabricated carbon nano-onions were used to modify the surfaces of various materials and structures toward their application to self-cleaning, oil-water separation, and enabling highly effective underwater oxygen-consuming reactions. Systematic analyses of the different wetting states of the superhydrophobic electrodes coated with the fabricated carbon nano-onions indicated that the different wetting states have important effects on their behaviors for underwater nucleation reactions, thus changing the efficiency of underwater oxygen-consuming reaction systems. The results reveal that designing a superhydrophobic electrode with a Wenzel-Cassie coexistent underwater wetting state, rather than a Cassie state or Wenzel state, is the key to enabling highly effective underwater oxygen-consuming reactions.

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.

Study of droplet self-migration on silicon surface with radial micro-fin structures

Bidirectional wetting gradient is built on the silicon surface with radial micro-fin structures by mask-assisted lithography process. Through theoretical analysis, the control of the migration speed and direction of the droplets can be achieved by adjusting the wetting state of the surface and the droplet dropping position. The wetting model is the essence of controlling the direction of droplet movement. An experimental study is carried out on the micro-fin surface, and the critical position of the wetting model switching Rr is found. A droplet (with a volume of 0.05 ml) can move spontaneously toward the center when put on the dry, dense-fin area (where the droplet is in the Cassie state and the proportion of solid is more than 0.306, namely f1r = 0.306). Dripping continuously at the same position, the resistance force is reduced because the previous droplet wets the micro-fin surface, so the droplets can gather to the center at a faster speed. On the contrary, when the micro-fins get sparse (where the proportion of solid P is less than 0.306) or the grooves between the fins are completely wetted, the droplet, in Wenzel state, tends to flow to the outer edge area.

Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof

We investigate wetting and water repellency characteristics of Colocasia esculenta (taro) leaf and an engineered surface, bioinspired by the morphology of the surface of the leaf. Scanning electron microscopic images of the leaf surface reveal a two-tier honeycomb-like microstructures, as compared to previously-reported two-tier micropillars on a Nelumbo nucifera (lotus) leaf. We measured static, advancing, and receding angle on the taro leaf and these values are around 10% lesser than those for the lotus leaf. Using standard photolithography techniques, we manufactured bioinspired surfaces with hexagonal cavities of different sizes. The ratio of inner to the outer radius of the circumscribed circle to the hexagon (b/a) was varied. We found that the measured static contact angle on the bioinspired surface varies with b/a and this variation is consistent with a free-energy based model for a droplet in Cassie-Baxter state. The static contact angle on the bioinspired surface is closer to that for the leaf for b/a ≈ 1. However, the contact angle hysteresis is much larger on these surfaces as compared to that on the leaf and the droplet sticks to the surfaces. We explain this behavior using a first-order model based on force balance on the contact line. Finally, the droplet impact dynamics was recorded on the leaf and different bioinspired surfaces. The droplets bounce on the leaf beyond a critical Weber number (We ~ 1.1), exhibiting remarkable water-repellency characteristics. However, the droplet sticks to the bioinspired surfaces in all cases of We. At larger We, we recorded droplet breakup on the surface with larger b/a and droplet assumes full or partial Wenzel state. The breakup is found to be a function of We and b/a and the measured angles in full Wenzel state are closer to the predictions of the free-energy based model. The sticky bioinspired surfaces are potentially useful in applications such as water-harvesting.

Superrepellency of underwater hierarchical structures on Salvinia leaf

Biomimetic superhydrophobic surfaces display many excellent underwater functionalities, which attribute to the slippery air mattress trapped in the structures on the surface. However, the air mattress is easy to collapse due to various disturbances, leading to the fully wetted Wenzel state, while the water filling the microstructures is difficult to be repelled to completely recover the air mattress even on superhydrophobic surfaces like lotus leaves. Beyond superhydrophobicity, here we find that the floating fern, Salvinia molesta, has the superrepellent capability to efficiently replace the water in the microstructures with air and robustly recover the continuous air mattress. The hierarchical structures on the leaf surface are demonstrated to be crucial to the recovery. The interconnected wedge-shaped grooves between epidermal cells are key to the spontaneous spreading of air over the entire leaf governed by a gas wicking effect to form a thin air film, which provides a base for the later growth of the air mattress in thickness synchronously along the hairy structures. Inspired by nature, biomimetic artificial Salvinia surfaces are fabricated using 3D printing technology, which successfully achieves a complete recovery of a continuous air mattress to exactly imitate the superrepellent capability of Salvinia leaves. This finding will benefit the design principles of water-repellent materials and expand their underwater applications, especially in extreme environments.

Влияние топографических дефектов на супергидрофобные свойства покрытий на основе ZnO

Results of investigation of hydrophobic properties of ZnO coating with different surface morphology are given. It was shown that for the case of ZnO samples with surface roughness in the nanometer range, the Wenzel state is realized, and for microstructured samples - the Сassie state. For a sample with multimodal surface roughness, a superhydrophobic state was achieved with an edge angle of 151 ° and an adhesion operation of 8.82 mJ/m2. It has been demonstrated that the presence of micrometer defects on the surface of superhydrophobic samples leads to instability of the Сassie state, and as the radius of water drops increases, the Сassie-Wenzel transition is realized.

Objective quantification of surface roughness parameters affecting superhydrophobicity.

This study proposes new optical roughness parameters that can be objectively quantified using image processing techniques, and presents an analysis of how these parameters are correlated with the degree of superhydrophobicity. To this end, photolithography and dry etching processes were used to form regular square pillars with different heights and spacings with a length of tens of micro-meters on silicon wafers. Optical roughness parameters of the specimens were obtained using image processing, and surface wettability was characterized using static contact angle and sliding angle measurements for water droplets of volume VD = 3.5 μl or 12 μl. As a result, seven optical roughness parameters were derived to describe the surface roughness topography in a multi-faceted way. Between the Cassie–Baxter state and the Wenzel state, two distinct wetting states intermediate state I, and intermediate state II were observed. Multiple linear regression of optical roughness parameters and superhydrophobicity demonstrated that in the stable Cassie–Baxter state, the contact angle can be increased or sliding angle decreased more effectively by adjusting the spacing between pillars than by just tuning the solid area fraction. However, in the metastable state where the Cassie–Baxter state can be changed to intermediate state I and vice versa by adjusting VD or surface geometry, reducing the solid area fraction is a priority to ensure a stable Cassie–Baxter state. Horizontal-perspective roughness parameters had a great effect on dynamic wettability in the Cassie–Baxter state. The results confirmed that the proposed optical roughness parameters may be useful for quantitative analysis of the complex effects of roughness on superhydrophobic surfaces.

Comparative study of the tribological behaviour of emulsions and demoulding oils at the concrete/formwork interface

To ensure a performing demoulding, a release agent must resist the friction of concrete on the formwork. Emulsions are interesting alternatives that leads to a reduced consumption of oil on construction sites. The behaviours of two commercial emulsions are compared with two conventional demoulding oils. The adhesion energy calculated were favourable to the emulsions. The water evaporation kinetics leads to the measurements of film thickness. Tests with the tribometer have shown that the films produced from the emulsions generate a friction reduction of 30% and 40% at the concrete/formwork interface. Such performance can be explained by Wenzel's state of wettability, which facilitates the film fit into the formwork roughness.

Gibbsian Thermodynamics of Wenzel Wetting (Was Wenzel Wrong? Revisited).

When a drop is in contact with a rough surface, it can rest on top of the rough features (the Cassie-Baxter state) or it can completely fill the rough structure (the Wenzel state). The contact angle (θ) of a drop in these states is commonly predicted by the Cassie-Baxter or Wenzel equations, respectively, but the accuracy of these equations has been debated. Previously, we used fundamental Gibbsian composite-system thermodynamics to rigorously derive the Cassie-Baxter equation, and we found that the contact line determined the macroscopic contact angle, not the contact area that was originally proposed. Herein, to address the various perspectives on the Wenzel equation, we apply Gibbsian composite-system thermodynamics to derive the complete set of equilibrium conditions (thermal, chemical, and mechanical) for a liquid drop resting on a homogeneous rough solid substrate in the Wenzel mode of wetting. Through this derivation, we show that the roughness must be evaluated at the contact line, not over the whole interfacial area, and we propose a new Wenzel equation for a surface with pillars of equal height. We define a new dimensionless number H = h(1 - λsolid)/R to quantify when the drop's radius of curvature (R) is large enough compared to the size of the pillars for the new Wenzel equation to be simplified (h is the pillar height; λsolid is the line fraction of the spherical cap's circumference that is on the pillars). Our new line-roughness Wenzel equation can be simplified to cos θW = ρ cos θY when H ≪ 1, where ρ is the line roughness. We also perform a thermodynamic free-energy analysis to determine the stability of the equilibrium states that are predicted by our new Wenzel equation.

Study of hemiwicking with lattice Boltzmann simulations: A wetting state is dynamically trapped by pinning of imbibition front

Hemiwicking has been introduced to describe the wetting state in which a liquid film surrounds a drop. To fully understand this special wetting state, we performed energy analysis and systematic lattice Boltzmann (LB) simulations on the wetting state through spreading liquid droplets on pillared hydrophilic substrates. Although the energy analysis shows that the hemiwicking is energetically unfavorable, droplets in stable hemiwicking are indeed observed in our LB simulations. This observation led us to conclude that we have obtained a result that is the same as the result obtained in the published experiment and theory: hemiwicking is dynamically trapped by the pinning of the imbibition front during invasion of the substrate texture by the liquid film. Our simulations show that the special wetting state is always found to emerge near the phase boundary between the liquid film and the Wenzel state. For the morphology of the droplet, strong deviation of the apparent contact angle from hemiwicking is observed when the contact line of the liquid imbibition film is close to the spherical caplike droplet. We also show that there exist at least two different kinetic pathways for the formation of hemiwicking, including spreading and evaporation.

How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures.

Surfactants, as amphiphilic molecules, adsorb easily at interfaces and can detrimentally destroy the useful, gas-trapping wetting state (Cassie-Baxter, CB) of a drop on superhydrophobic surfaces. Here, we provide a quantitative understanding of how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different roughness (r) and solid fraction (ϕ). Experimentally, at low surfactant concentrations (C), some drops attain a homogeneous wetting state (Wenzel, W), while others attain the CB state whose large contact angles can be predicted by a thermodynamic model. In contrast, all of our high-C drops attain the Wenzel state. To explain this observed transition, we consider the free energy and find that, theoretically, for our surfaces the W state is always preferred, while the CB state is metastable at low C, consistent with experimental results. Furthermore, we provide a beneficial blueprint for stable CB states for applications exploiting superhydrophobicity.