Wenzel Transition Research Articles

Understanding wetting behaviors on rough PTFE surfaces towards n-octane/water mixture separation by molecular dynamics simulation

The wetting behaviors of pure n-octane and water nanodroplets on rectangular pitted, grooved, and rectangular pillared polytetrafluoroethylene (PTFE) surfaces were examined by molecular dynamics simulation. The results showed that the three rough surfaces could achieve superlipophilicity and near superhydrophobicity by changing the rough morphology of surface. The n-octane droplets always presented the Wenzel state on the rough surfaces, and the water droplets were the Wenzel state, Transition state and Cassie state under different phase area fractions and/or roughness factors. Based on completely opposite wetting properties of the two droplets on the same surface, separation performances of mixture nanodroplets were further studied on the three rough PTFE surfaces. The results showed that the n-octane/water separation efficiency of pillared surface was above 98.4 %, significantly higher than those of pitted and grooved surfaces. The rough PTFE surface with special wettability enabled efficient separation of oil/water mixture. This work is expected to pave the way for the rational design and industrial application of oil/water separation materials.

Cold gelation of whey protein isolate with sugars in an ultrasound environment

Dispersions of whey protein isolate (9.5% protein) with 1.5% of sugar (glucose, mannose or xylose) were preheated, and ions-induced gelation was performed in an ultrasound environment at room temperature. Depending on the used sugar, sonication increased, decreased or did not change the rheological properties of gels (storage modulus, hardness and ultrasound viscosity). That was contrary to the formation of Maillard reaction compounds, which probably by occupation of active sides of proteins, induced the formation of a weaker three-dimensional gel network. For most samples, sonication increased the value of the contact angle, switching their surface character from hydrophilic to hydrophobic one. The surface free energy of the obtained gels decreased after sonication, and such surfaces were less susceptible to wetting. The increase in the value of the contact angles corresponded to the increase in the roughness of the gels after sonication. The relationship between the surface roughness and the wettability can be attributed to the surface wetting state (Wenzel state, Cassie-Baxter state, and transition state). For the gels surfaces with lower roughness, the liquid was more susceptible to penetration to the surface due to large adhesive forces (Wenzel model) and for higher roughness, the liquid was more susceptible to resting on the “air micro cushions” (Cassie-Baxter model). The ultrasound-induced cold-gelation for the preparation of natural protein/sugar gels can offer a potential approach to develop new functional products with tailor-made attributes.

Droplet Formation and Impingement Dynamics of Low-Boiling Refrigerant on Solid Surfaces with Different Roughness under Atmospheric Pressure

The dynamic behavior of droplet impingement is one of the most important processes of spray cooling. Although refrigerants with a low boiling point have been widely used in spray cooling, their high volatility makes it difficult to generate a stable droplet under atmospheric pressure, and thus the dynamic behavior of droplet impingement is rarely reported. Therefore, it is of great significance to study the behavior of refrigerant droplet impingement to fill the relevant research gaps. In this paper, an experimental system for single refrigerant droplet generation and impingement at atmospheric pressure has been established. By means of high-speed photography technology, the morphology and dynamics of R1336mzz(Z) droplet impingement on grooved carbon steel walls have been studied. Phenomena such as a truncated sphere, boiling, and finger-shaped disturbance were observed, and the reasons responsible for them were analyzed. The effects of Weber number (We) and surface roughness (Ra) on droplet spreading factor (β) were investigated quantitatively. Higher We always causes a larger βmax, while Ra has a different influence on βmax. The Cassie–Wenzel transition occurs when Ra increases from 1.6 μm to 3.2 μm, leading to a rapid decrease in βmax. An empirical formula has been proposed to predict βmax under different conditions.

Electrostatic incitation on fiber surface for enhancing mechanical properties of fiber-reinforced composite

The excellent wettability contributes to the formation of mechanical interlocking morphologies at the interface as well as to the reduction of the internal defects in the composite for enhancing the mechanical properties of the fiber-reinforced composites. Herein, a facile and effective electrostatic method is proposed, where a small proportion of negative charges are merely sprayed on the fiber surface to modulate wettability. The results revealed that negatively charged fibers exhibit superior wettability for resin. Furthermore, the interfacial shear strength (IFSS) of the electrostatically treated T800H carbon fiber (T800H-ES)-based composite is 29.9% higher than that of the untreated-fiber composites. The improvement in interfacial properties is mainly ascribed to the formation of mechanical interlocking induced by surface charge-triggered Cassie-Baxter (CB) to Wenzel (WZ) transition of a liquid resin on the fiber surface, as verified by molecular dynamics (MD) simulations of carbon fiber–resin interfacial wetting. In addition, T800H-ES based composite has a 41.2% increase in tensile strength, a 22.5% increase in interlaminar shear strength (ILSS), and a 100.5% increase in fracture energy compared to as-received composite. These increases were mainly attributed to the improved interfacial adhesion and the reduction in internal composite defects. Consequently, this study reveals that the electrostatic charge-assisted wetting method is a green, cost-effective, and promising method for preparing composites with outstanding interfacial characteristics and mechanical properties.

The Effect of Topographic Defects on the Superhydrophobic Properties of Coatings Based on ZnO

The results of a study of the hydrophobic properties of a ZnO coating with different morphologies of the surface are presented. It is shown that the Wenzel state is implemented for the case of samples of ZnO with nanoscale surface roughness, while the Cassie state is implemented for microstructured samples. A superhydrophobic state with a contact angle of 151° and work of adhesion of 8.82 mJ/m2 is achieved for a sample with multimodal surface roughness. It is demonstrated that the presence of microscale defects on the surface of the superhydrophobic samples leads to the instability of the Cassie state, while the Cassie–Wenzel transition is fulfilled in the case of increasing the radius of water drops.

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.

Cassie-Wenzel transition of a binary liquid mixture on a nanosculptured surface.

The Cassie-Wenzel transition of a symmetric binary liquid mixture in contact with a nano-corrugated wall is studied. The corrugation consists of a periodic array of nanopits with square cross sections. The substrate potential is the sum over Lennard-Jones interactions, describing the pairwise interaction between the wall particles C and the fluid particles. The liquid is composed of two species of particles, A and B, which have the same size and equal A-A and B-B interactions. The liquid particles interact between each other also via A-B Lennard-Jones potentials. We have employed classical density functional theory to determine the equilibrium structure of binary liquid mixtures in contact with the nano-corrugated surface. Liquid intrusion into the pits is studied as a function of various system parameters such as the composition of the liquid, the strengths of various interparticle interactions, and the geometric parameters of the pits. The binary liquid mixture is taken to be at its mixed-liquid-vapor coexistence. For various sets of parameters the results obtained for the Cassie-Wenzel transition, as well as for the metastability of the two corresponding thermodynamic states, are compared with macroscopic predictions in order to check the range of validity of the macroscopic theories for systems exposed to nanoscopic confinements. Distinct from the macroscopic theory, it is found that the Cassie-Wenzel transition cannot be predicted based on the knowledge of a single parameter, such as the contact angle within the macroscopic theory.

Optimization of Patterned Surfaces for Improved Superhydrophobicity through Cost-Effective Large-Scale Computations.

The pattern design of superhydrophobic surfaces can be significantly aided by computations that predict the Cassie-Baxter (CB) to Wenzel (W) transition, which is responsible for the break-down of superhydrophobic behavior. We present a computational framework for the optimization of patterned surfaces based on the energy barriers of the CB-W transitions which comprises the following elements: (a) design of structured surface patterns, for example, arrays of pillars, with parameterized geometric features such as size, pitch, slope, and roundness. (b) Computation of the wetting states with a modified Young-Laplace equation that facilitates the introduction of solid/liquid interactions for complex surface patterns and has significantly lower computational cost than other commonly used methods, such as the volume-of-fluid, phase-field, and so forth. (c) Incorporation of the modified Young-Laplace in the simplified string method, allowing the calculation of the minimum energy paths of wetting transitions which, apart from the energy barriers, also reveal the transition mechanisms (CB failure modes). (d) Accommodation of large-scale problems with good parallel performance and scalability on multicore-distributed memory systems using fast iterative solvers and the Message Passing Interface communication protocol. We demonstrate the computational framework with a shape optimization study of inverted conical frustum pillars. The optimization objective function is the resistance to the CB-W transition, which is quantified by the energy barrier-a relatively large energy barrier suggests improved superhydrophobicity. We also report the parallel performance, in terms of parallel speedup for problems ranging from three hundred thousands to 12 million degrees of freedom, solved using up to 40 processing cores.

Simultaneous Detection and Repair of Wetting Defects in Superhydrophobic Coatings via Cassie–Wenzel Transitions of Liquid Marbles

AbstractSuperhydrophobic materials that prevent unwanted liquid adhesion can easily lose this property because of limited mechanical durability despite topological/chemical control and/or robust material selection. Here, long‐lasting superhydrophobic coatings with a system to effectively detect and repair damaged areas with “liquid marble,” a droplet covered with hydrophobic nanoparticles, are reported. The particles prevent direct contact between the droplet and the substrate (Cassie state). However, they can adhere to the non‐superhydrophobic damaged area in response to the substrate wettability via an external force or an increase in liquid volume via penetration of the outer nanoparticle layer (Wenzel state). This Cassie–Wenzel transition thus induces self‐assembly of the nanoparticles onto the non‐superhydrophobic area in response to the wettability, restoring superhydrophobicity.

Revisiting the Critical Condition for the Cassie-Wenzel Transition on Micropillar-Structured Surfaces.

Biological and engineering applications of superhydrophobic surfaces are limited by the stability of the wetting state determined by the transition from the Cassie-Baxter state to the Wenzel state (C-W transition). In this paper, we performed water droplet squeeze tests to investigate the critical conditions for the C-W transition for solid surfaces with periodic micropillar arrays. The experimental results indicate that the critical transition pressures for the samples with varying micropillar dimensions are all significantly higher than the theoretical predictions. Through independent measurements, we attributed the disparity to the incorrect assessment of the contact angle on the sidewall surfaces of the micropillars. We also showed that the theoretical models are still applicable when the correct contact angle of the sidewall surfaces is adopted. Our work directly validates and improves the theoretical models regarding the C-W transition and suggests a potential route of tuning superhydrophobicity using finer scale surface features.

Line tension effects on the wetting of nanostructures: an energy method

The superhydrophobicity and self-cleaning property of micro/nano-structured solid surfaces require a stable Cassie–Baxter (CB) wetting state at the liquid–solid interface. We present an energy method to investigate how the three-phase line tension affects the CB wetting state on nanostructured materials. For some nanostructures, the line tension may engender a distinct energy barrier, which restricts the position of the three-phase contact line and affects the stability of the CB wetting state. We ascertain the upper and lower limits of the critical pressure at the CB–Wenzel transition. Our results suggest that superhydrophobicity on nanostructures can be modulated by tailoring the line tension and harnessing the curvature effect. This study also provides new insights into the sinking phenomena observed in the nanoparticle-floating experiment.

Water droplet evaporation from sticky superhydrophobic surfaces

The evaporation dynamics of water from sticky superhydrophobic surfaces was investigated using a quartz crystal microresonator and an optical microscope. Anodic aluminum oxide (AAO) layers with different pore sizes were directly fabricated onto quartz crystal substrates and hydrophobized via chemical modification. The resulting AAO layers exhibited hydrophobic or superhydrophobic characteristics with strong adhesion to water due to the presence of sealed air pockets inside the nanopores. After placing a water droplet on the AAO membranes, variations in the resonance frequency and Q-factor were measured throughout the evaporation process, which were related to changes in mass and viscous damping, respectively. It was found that droplet evaporation from a sticky superhydrophobic surface followed a constant contact radius (CCR) mode in the early stage of evaporation and a combination of CCR and constant contact angle modes without a Cassie–Wenzel transition in the final stage. Furthermore, AAO membranes with larger pore sizes exhibited longer evaporation times, which were attributed to evaporative cooling at the droplet interface.

Well-defined porous membranes for robust omniphobic surfaces via microfluidic emulsion templating

Durability is a long-standing challenge in designing liquid-repellent surfaces. A high-performance omniphobic surface must robustly repel liquids, while maintaining mechanical/chemical stability. However, liquid repellency and mechanical durability are generally mutually exclusive properties for many omniphobic surfaces—improving one performance inevitably results in decreased performance in another. Here we report well-defined porous membranes for durable omniphobic surfaces inspired by the springtail cuticle. The omniphobicity is shown via an amphiphilic material micro-textured with re-entrant surface morphology; the mechanical durability arises from the interconnected microstructures. The innovative fabrication method—termed microfluidic emulsion templating—is facile, cost-effective, scalable and can precisely engineer the structural topographies. The robust omniphobic surface is expected to open up new avenues for diverse applications due to its mechanical and chemical robustness, transparency, reversible Cassie–Wenzel transition, transferability, flexibility and stretchability.

Conformal and non-conformal surface modification of honeycomb-patterned porous films via tunable Cassie–Wenzel transition

We describe here a facile and robust approach to conformal and non-conformal surface modification by tuning the wetting transition between the Wenzel state and the Cassie state.

In situ experiments to reveal the role of surface feature sidewalls in the Cassie-Wenzel transition.

Waterproof and self-cleaning surfaces continue to attract much attention as they can be instrumental in various different technologies. Such surfaces are typically rough, allowing liquids to contact only the outermost tops of their asperities, with air being entrapped underneath. The formed solid–liquid–air interface is metastable and, hence, can be forced into a completely wetted solid surface. A detailed understanding of the wetting barrier and the dynamics of this transition is critically important for the practical use of the related surfaces. Toward this aim, wetting transitions were studied in situ at a set of patterned perfluoropolyether dimethacrylate (PFPEdma) polymer surfaces exhibiting surface features with different types of sidewall profiles. PFPEdma is intrinsically hydrophobic and exhibits a refractive index very similar to water. Upon immersion of the patterned surfaces into water, incident light was differently scattered at the solid–liquid–air and solid–liquid interface, which allows for distinguishing between both wetting states by dark-field microscopy. The wetting transition observed with this methodology was found to be determined by the sidewall profiles of the patterned structures. Partial recovery of the wetting was demonstrated to be induced by abrupt and continuous pressure reductions. A theoretical model based on Laplace’s law was developed and applied, allowing for the analytical calculation of the transition barrier and the potential to revert the wetting upon pressure reduction.

Robust Cassie State of Wetting in Transparent Superhydrophobic Coatings

This paper investigates the stability of the Cassie state of wetting in transparent superhydrophobic coatings by comparing a single-layer microporous coating with a double-layer micro/nanoporous coating. Increasing pressure resistance of superhydrophobic coatings is of interest for practical use because high external pressures may be exerted on surfaces during operation. The Cassie state stability against the external pressure of coatings was investigated by squeezing droplets sitting on surfaces with a hydrophobic plate. Droplets on the single-layer coating transformed to the Wenzel state and pinned to the surface after squeezing, whereas droplets on the double-layer micro/nanoporous coating preserved the Cassie state and rolled off the surface easily. In addition, the contact angle and contact-line diameter of water droplets during evaporation from surfaces were in situ investigated to further understand the stability of coatings against Wenzel transition. A droplet on a microporous coating gradually transformed to the Wenzel state and lost its spherical shape as the droplet volume decreased (i.e., the internal pressure of the droplet increased). The contact line of the droplet during evaporation remained almost unchanged. In contrast, a water droplet on a double-layer surface preserved its spherical shape even at the last stages of the evaporation process, where pressure differences as high as a few thousand pascals were generated. For this case, the droplet contact line retracted during evaporation and the droplet recovered the initial water contact angle. The demonstrated method for the preparation of robust transparent superhydrophobic coatings is promising for outdoor applications such as self-cleaning cover glasses for solar cells and nonwetting windows.

Enhancement of dynamic wetting properties by direct fabrication on robust micro–micro hierarchical polymer surfaces

Understanding evaporation phenomena on hierarchical surfaces is of crucial importance for the design of robust superhydrophobic polymer structures for various applications. This fabrication method enables precise control of the dimensions to elucidate the dynamic wetting behavior affected by geometric parameters. That behavior exhibits three distinct evaporation modes: a constant contact line (CCL), a constant contact angle (CCA), and mixed mode during the droplet evaporation. The droplet evaporation results show that the sticky CCL mode and the Cassie–Wenzel transition can be prevented by engineering hierarchy integration. Moreover, the CCL–CCA transition point time scale exhibits remarkable dependence on surface dimensions such as the area fraction and solid–liquid contact line. Finally, the fabricated hierarchical structures indicate remarkable superhydrophobic properties, static contact angle above 160° and low sliding angle under 10°, with good durability in terms of aging effect and mechanical robustness for 2 months.

Tunable nano-replication to explore the omniphobic characteristics of springtail skin

Springtails (Collembola) are wingless arthropods adapted to cutaneous respiration in temporarily rain-flooded and microbially contaminated habitats by a non-wetting and antiadhesive skin surface that is mechanically rather stable. Recapitulating the robust and effectively repellent surface characteristics of springtail skin in engineered materials may offer exciting opportunities for demanding applications, but it requires a detailed understanding of the underlying design principles. Towards this aim and based on our recent analysis of the structural features of springtail skin, we developed a tunable polymer replication process to dissect the contributions of different structural elements and surface chemistry to the omniphobic performance of the cuticle. The Cassie–Wenzel transition at elevated pressures was explored by in situ plastron collapse experiments and by numerical FEM simulations. The results obtained unravel the decisive role of nanoscopic cuticle structures for the protection of springtails against wetting, and explain how the evolved nanotopography enables the production of omniphobic surfaces even from intrinsically hydrophilic polymer materials. The wetting behavior of a material's surface is a property of fundamental interest to material scientists. Springtails, also known as collembola, are soil-dwelling arthropods that typically respire through their skin. To avoid suffocating in wet conditions, springtails have evolved a complex, hierarchically nanostructured skin surface that repels water with remarkable efficiency. Carsten Werner at the Leibniz Institute of Polymer Research Dresden, Germany, and his colleagues have carried out numerical simulations and made accurate polymer-based replicas of this skin surface in order to understand better its anti-wetting behavior. Their results show that tiny overhangs in the nanostructure help to trap air against the surface in the wet, providing an effective barrier against wetting. The researchers further showed that even intrinsically hydrophilic materials will repel water when structured in this way. An improved understanding of springtail skin behaviour provides valuable insights that will aid scientists to design engineered materials with improved anti-wetting properties. Springtails, wingless arthropods, are adapted to cutaneous respiration in temporarily rain-flooded habitats by a hierarchically structured skin surface. A tunable polymer replication process was applied to dissect the contributions of different structural elements and surface chemistry to the omniphobic performance of the skin.

Metastable Wetting on Superhydrophobic Surfaces: Continuum and Atomistic Views of the Cassie-Baxter–Wenzel Transition

In this Letter, we develop a continuum theory for the Cassie-Baxter-Wenzel (CB-W) transition. The proposed model accounts for the metastabilities in the wetting of rough hydrophobic surfaces, allows us to reconstruct the transition mechanism, and identifies the free energy barriers separating the CB and W states as a function of the liquid pressure. This information is crucial in the context of superhydrophobic surfaces, where there is interest in extending the duration of the metastable superhydrophobic CB state. The model is validated against free energy atomistic simulations.

Particle deposition after droplet evaporation on ultra-hydrophobic micro-textured surfaces

We study the size and shape of the final deposit obtained when a drop with colloidal particles has dried on a ultra-hydrophobic surface made of micro-posts. As expected, most of the particles lie inside a circular area, whose radius roughly corresponds to the Laplace pressure threshold for liquid impalement inside the structure (Cassie–Wenzel transition), inducing a coffee-stain deposit due to contact-line pinning. Less expected is the observation of tiny deposits on top of posts in the area external to the main ring, despite the low macroscopic liquid/solid friction. Experiments are carried out varying the concentration in particles and initial volume of drops, in order to determine the influence of these parameters on the size distribution of deposits. A microscopic insight of the tiny deposits is proposed, based on recent experiments of non-volatile liquid sliding drops.