Drops on microstructured surfaces coated with hydrophilic polymers: Wenzel's model and beyond

A feasibility study on femtosecond laser texturing of sprayed nanocellulose coatings

Nanocelluloses are emerging as natural materials with favourable properties for coating industry and can be applied by state-of-the-art spraying technology. While additional functionalities are commonly introduced through chemical modification, the surface microstructuring of nanocellulose coatings with high throughput methods remains unexplored. Here, a femtosecond laser is used for texturing spray-coated coatings made of cellulose nanofibrils (CNF) or cellulose nanocrystals (CNC). For coating thickness of 1.5 to 8 μm, processing limits were determined with maximum ablation energy linearly increasing with coating thickness and minimum ablation energy decreasing or increasing depending on the apparent coating density. Within applicable processing window of pulse rate and power setting, the operational ranges were determined for creating one-dimensional and two-dimensional surface patterns, requiring a higher laser energy for CNC compared to CNF coatings and yielding thinnest possible resolved patterns of 17 μm as determined by the laser spot diameter. The laser ablation under low energy corresponds to an increase in surface roughness and intensifies surface hydrophilicity, while the line patterns are able to pin water droplets with rising water contact angles up to 90°. Present feasibility study opens future possibilities for managing surface properties of nanocellulose coatings in applications where tuning of surface hydrophilicity is required.

Energy Dissipation during Wenzel Wetting via Roughness Scale Interface Dynamics

A numerical method is proposed to simulate the roughness scale interface dynamics of a slow-moving fluid interface as it advances over a chemically homogeneous rough surface. Analysis of the governing augmented Navier-Stokes and Young's boundary condition equations shows how the local interface behavior can be represented via a series of incrementally advanced equilibrium interfacial morphologies. Combined with a roughness scale mechanical energy balance [Harvie, D. J. E. Contact-angle hysteresis on rough surfaces: mechanical energy balance framework. J. Fluid Mech. 2024, 986, A17], the simulations are used to calculate the energy dissipation associated with a surface decorated with a periodic array of round-edge square pillars. This dissipation is used to predict static contact angle hysteresis (CAH) from knowledge of just the surface roughness topography and equilibrium contact angle. We show that the energy dissipated varies approximately as ϕln ϕ (with ϕ being the area fraction), becoming zero as ϕ → 0. The CAH predicted by our method is in good agreement with the experimental results of Forsberg et al. [Forsberg, P. S.; Priest, C.; Brinkmann, M.; Sedev, R.; Ralston, J. Contact line pinning on microstructured surfaces for liquids in the Wenzel state. Langmuir 2010, 26, 860-865], thereby demonstrating that our numerical method of simulating interfacial dynamics adequately captures the real interface motion, as well as illustrating how far-field contact angle and energy dissipation approaches are consistent for this surface. We also compute CAH for an interface moving at 45° to the surface periodicity direction to show that the experimental measurements are bracketed by the 0° and 45° advance direction results. The proposed method opens up the field to quantitative analysis, surface functionalization, and design for different specific applications.

Energy dissipation during homogeneous wetting of surfaces with randomly and periodically distributed cylindrical pillars

HYPOTHESIS

Understanding contact angle hysteresis on rough surfaces is important as most industrially relevant and naturally occurring surfaces possess some form of random or structured roughness. We hypothesise that hysteresis can be described by the dilute defect model of Joanny & de Gennes [1] and that the energy dissipation occurring during the stick-slip motion of the contact line is key to developing a predictive equation for hysteresis.

EXPERIMENTS

We measured hysteresis on surfaces with randomly distributed and periodically arranged microscopic cylindrical pillars for a variety of different liquids in air. The inherent (flat surface) contact angles tested range from lyophilic (θe=33.8°) to lyophobic (θe=112.0°).

FINDINGS

A methodology for averaging the measured advancing and receding contact angles on random surfaces is presented. Based on these results correlations for roughness-induced energy dissipation are derived, and an equation for predicting the advancing and receding contact angles during homogeneous (Wenzel) wetting on random surfaces is presented. Equations that predict the onset of the alternate wetting conditions of hemiwicking, split-advancing, split-receding and heterogeneous (Cassie) wetting are also derived, thus defining the range of validity for the homogeneous wetting equation. A 'cluster' concept is proposed to explain the measurably higher hysteresis exhibited by structured surfaces compared to random surfaces.

π-Conjugated Small Organic Molecule-Modified 2D MoS2 with a Charge-Localization Effect Enabling Direct and Sensitive SERS Detection

Organic semiconductors have been discovered to exhibit impressive surface-enhanced Raman scattering (SERS) activity recently. However, owing to the underdeveloped candidate materials and relatively low SERS sensitivity, practical application of SERS detection based on organic materials is still a challenge. Herein, we explored ways to further enhance the SERS sensitivity of π-conjugated fluorinated 7,7,8,8-tetracyanoquinodimethane derivatives (F_nTCNQ, n = 2, 4) by utilizing the charge-localization effect induced by two-dimensional (2D) MoS₂ flakes. A strong Raman signal enhancement in SERS has been realized via an organic/2D heterostructure constructed by F_nTCNQ nanostructures grown on a 2D MoS₂ flake. Moreover, F₂TCNQ and F₄TCNQ show different SESR sensitivities due to different numbers of cyano groups leading to different charge transfer (CT) directions. The SERS enhancement factor (EF) of methylene blue (MB) molecules on the optimal F₄TCNQ/MoS₂ nanocomposite substrate can reach as high as 2.531 × 10⁶, and the concentration of the limit of detection (LOD) is as low as 10^-10 M. The SERS results for MB, rhodamine 6G (R6G), and 4-aminothiophenol (4-ATP) molecules demonstrate that high versatility, low cost, good stability, and easy preparation will make the F_nTCNQ/MoS₂ SERS platform promising for the detection of trace molecules. The studies on the complex microscopic interaction of organic/2D composite nanomaterials will provide some novel insights into improved SERS performance and mechanisms.

Surface Modification of Backsheets Using Coupling Agents for Roll-To-Roll Processed Thin-Film Solar Photovoltaic (PV) Module Packaging Application

For many flexible electronic and photonic devices, moisture stability is one of the most important factors that affects its short- and long-term performance. To maintain the performance, the device should be packaged in such a way that it hermetically blocks moisture from the device; however, in practice, it is rather difficult to achieve. The more practical solution is to impede the moisture ingress to the device. In optoelectronic devices that will be outdoors like solar cells, the interfacial adhesion strength between the encapsulant layer (adhesive) and a moisture barrier layer is also a critical parameter. This paper presents surface modifications of poly(ethylene terephthalate) (PET) carrier films, one of the layers in the trilayer barrier film that directly adheres to an encapsulant, using chemical, UV/ozone, and both treatments to improve adhesion with the thermoset encapsulant polymer material. Whereas previous studies also utilized treatment methods to increase the wettability characteristics, in this paper, we not only present the results of the adhesion strength upon various techniques to achieve good adhesion but also screen their behavior upon exposure to a damp-heat (60 °C, 90% RH) environment. We found that the combined treatment method increases the adhesion by up to 12.1-fold and demonstrates up to a 200% increase in adhesion strength even upon our severe damp-heat environmental condition.

Laser Inscription of Microfluidic Devices for Biological Assays

A rapid and direct CO₂ laser ablation method was developed to create superhydrophilic surfaces and arrays of hydrophobic-superhydrophilic patterns for application in bioassays. Here, a combination of superhydrophilic and hemiwicking wetting characteristics was exploited to create microfluidic slides that were used as biological assays that prevented cell aggregation. This feature allowed microscopic analyses to be carried out at the individual cell level. This bioassay enabled control of cell population in localized areas (15 cells cm^-2). The device had 84% transparency, allowing direct fluorescence microscopy measurements in transmission mode. High adhesion of aqueous fluids on superhydrophilic areas surrounded by superhydrophobic boundaries provided selective retention and confinement. The adhered droplets maintained retention under 180° substrate tilt. These architectures provided rapid self-partitioning of the liquid into an array of droplets. The hydrophobic-superhydrophilic patterned arrays may have applications in microfluidic bioassays, high-throughput screening, and medical diagnostics.

Contact angle hysteresis on doubly periodic smooth rough surfaces in Wenzel's regime: The role of the contact line depinning mechanism

We report here on the contact angle hysteresis, appearing when a liquid meniscus is in contact with doubly sinusoidal wavelike patterned surfaces in Wenzel's wetting regime. Using the full capillary model we obtain numerically the contact angle hysteresis as a function of the surface roughness factor and the equilibrium contact angle for a block case and a kink case contact line depinning mechanism. We find that the dependencies of the contact angle hysteresis on the surface roughness factor are different for the different contact line depinning mechanisms. These dependencies are different also for the two types of rough surfaces we studied. The relations between advancing, receding, and equilibrium contact angles are investigated. A comparison with the existing asymptotical, numerical, and experimental results is carried out.

Photoactive PANI/TiO2/Si composite coatings with 3D bio-inspired structures

We showed that 3D compound-eye coatings with excellent photo-activity could be obtained by anisotropic wet etching, hydrothermal synthesis and chemical oxidation.

Titanium Oxide/Silicon Moth-Eye Structures with Antireflection, p-n Heterojunctions, and Superhydrophilicity

By employing KOH etching of silicon and hydrothermal growth of titanium oxide (TiO₂), TiO₂ nanorods assembled on the silicon micropyramids to form biomimetic composite coating, similar to moth-eye structures. The biomimetic composite coating possessed not only the micro-nano hierarchical structures but also the p-n heterojunctions, resulting in a decrease in the reflection of incident light and an increase in the separation efficiency of photogenerated carriers. Meanwhile, the structures showed excellent superhydrophilicity, making for the self-cleaning of the material surface. We further demonstrate that by exploiting the advantages of this method, the application of such structures in the photocatalysis field is thus straightforward.

Wetting against the nap - how asperity inclination determines unidirectional spreading

We have carried out wetting experiments on textured surfaces with high aspect ratio asperities in the Wenzel state. When inclination is imparted to the asperities, we observe a strictly unidirectional spreading opposite to the direction in which the asperities point. The advancing contact angle decreases markedly as inclination increases. A crude numerical analysis successfully accounts for this behaviour, highlighting the interplay between Gibbs pinning at the top of the structures and imbibition along the valleys between them. In Gibbs pinning non-linearities play a major role and we find that simple line averaging - i.e. a rule of mixture - cannot account for this evolution except for weak surface perturbations, i.e. large inclinations.

Hybrid Cassie-Wenzel model for droplets on surfaces with nanoscale roughness

Several models have been developed to predict the contact angle of a droplet sitting on a roughened surface; however, no such model has been developed for substrates with nanoscale surface structures. In this paper we propose a hybrid Cassie-Wenzel model, which considers two factors attributed to the breakdown of macroscopic predictions, including the width of the wall-fluid depletion region and the coexistence of Cassie and Wenzel states in cases where the wall-fluid interface presents nanoscale structures. At the molecular scale, the parameter of surface roughness can be corrected by treating the wall-fluid interface as a hybrid Cassie-Wenzel state in which the fraction in the Wenzel state depends on fluid density within the cavities. A more general model developed using data fitted to fluid density is able to account for deviating tendencies induced by nanoscale surface features. A comparison of predicted results obtained in this study with those from previous works demonstrates that the proposed hybrid Cassie-Wenzel model is applicable to the evaluation of wettability in a wide range of substrates with nanoscale surface structures, corresponding to a Cassie state, a Wenzel state, and a mixed state. More importantly, the present work provides a quantitative approach to the estimation of wettability even amidst nanoscale effects, which can have a significant influence in cases with surface features at the molecular scale.

Uptake of phosphorus from surfactant solutions by wheat leaves: spreading kinetics, wetted area, and drying time

The delivery and uptake of nutrients at the surface of plant leaves is an important physicochemical phenomenon that depends on leaf surface morphology and chemistry, fertilizer formulation chemistry (including adjuvant and associated surfactants), wetting dynamics, and many other physical, chemical and biological factors. In this study, the role of spreading dynamics in determining uptake of the macronutrient phosphorus from phosphoric acid fertilizer solution in combination with three different adjuvants was measured in the absence of droplet run-off and splashing. When run-off and splashing losses were zero, spreading and drying rates had a small to negligible effect on the uptake efficiency. The results suggest that uptake may be much less sensitive to the specific choice of adjuvant and long time-scale spreading behaviour than one might intuitively expect.

Droplet on a regularly patterned solid. Wenzel's regime and meso-scale roughness

The applicability of Wenzel's equation to describe a liquid droplet settled on the solid surface regularly patterned with rectangular prisms was examined by means of simulations of the droplet/surface system morphology and energetics. The droplet deposited on the meso-scale surface roughness (i.e. the droplet size was larger than the size of heterogeneities by about an order of magnitude) was considered. Several different approaches to the estimation of the contact angle were employed. The discrepancies between the results of simulated experimental measurements and the predictions based on the Wenzel equation were analyzed and discussed. The influence of three-phase contact line effects on the droplet morphology and the existence of metastable states was shown.

Noncircular stable displacement patterns in a meshed porous layer

We report noncircular, stable liquid propagation patterns in a displacement process in a confined thin patterned porous layer. For constant fluid injection rates, the average front location of the interface r(t) exhibits a power-law behavior r ∝ t(1/2); however, when surface tension effects become important, the interface displays noncircular shapes, e.g., square, rectangular, or octagonal, and maintains the same shape during most of the injection process. The interface shape is controlled by the value of a dimensionless group representing the strength of surface tension stresses relative to stresses accompanying injection. Furthermore, we show that the propagation patterns of the interface can be controlled by the relative orientation of the different porous layers.

Multiscale effect of hierarchical self-assembled nanostructures on superhydrophobic surface

In this work, we describe self-assembled surfaces with a peculiar multiscale organization, from the nanoscale to the microscale, exhibiting the Cassie-Baxter wetting regime with extremely low water adhesion: floating drops regime with roll-off angles < 5°. These surfaces comprise bundles of hierarchical, quasi-one-dimensional (1D) TiO2 nanostructures functionalized with a fluorinated molecule (PFNA). While the hierarchical nanostructures are the result of a gas-phase self-assembly process, their bundles are the result of the capillary forces acting between them when the PFNA solvent evaporates. Nanometric features are found to influence the hydrophobic behavior of the surface, which is enhanced by the micrometric structures up to the achievement of the superhydrophobic Cassie-Baxter state (contact angle (CA) ≫ 150°). Thanks to their high total and diffuse transmittance and their self-cleaning properties, these surfaces could be interesting for several applications such as smart windows and photovoltaics where light management and surface cleanliness play a crucial role. Moreover, the multiscale analysis performed in this work contributes to the understanding of the basic mechanisms behind extreme wetting behaviors.

Bioinspired Directional Surfaces for Adhesion, Wetting and Transport

In Nature, directional surfaces on insect cuticle, animal fur, bird feathers, and plant leaves are comprised of dual micro-nanoscale features that tune roughness and surface energy. This feature article summarizes experimental and theoretical approaches for the design, synthesis and characterization of new bioinspired surfaces demonstrating unidirectional surface properties. The experimental approaches focus on bottom-up and top-down synthesis methods of unidirectional micro- and nanoscale films to explore and characterize their anomalous features. The theoretical component of the review focuses on computational tools to predict the physicochemical properties of unidirectional surfaces.

Theory and simulation of angular hysteresis on planar surfaces

A simple model is proposed to simulate contact angle hysteresis in drops on a planar surface. The model is based on assuming a friction force acting on the triple contact line in such a way that the contact line keeps fixed for contact angles comprised between the advancing angle and the receding one and is allowed to move in order to avoid angles outside this interval. The model is straightforwardly applied to axisymmetric drops for which a simple solution of the Young-Laplace equation can be obtained. A variation of the method has also been implemented for nonaxisymmetric drops by resorting to the public-domain "Surface Evolver" software. Comparison with experiments shows the excellent performance of the model.

Modeling receding contact lines on superhydrophobic surfaces

We use mesoscale simulations to study the depinning of a receding contact line on a superhydrophobic surface patterned by a regular array of posts. For the simulations to be feasible, we introduce a novel geometry where a column of liquid dewets a capillary bounded by a superhydrophobic plane that faces a smooth hydrophilic wall of variable contact angle. We present results for the dependence of the depinning angle on the shape and spacing of the posts and discuss the form of the meniscus at depinning. We find, in agreement with ref 17 , that the local post concentration is a primary factor in controlling the depinning angle and show that the numerical results agree well with recent experiments. We also present two examples of metastable pinned configurations where the posts are partially wet.

Modification of surface wettability through adsorption of partly fluorinated statistical and block polyelectrolytes from aqueous medium

The wetting properties of electrostatically charged hydrophilic substrates were modified through adsorption of ultrathin layer of amphiphilic block or statistical polyelectrolyte from aqueous medium. The studied polymers were copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2,2,2-trifluoroethyl methacrylate (TFEMA). They were adsorbed on mica from varying pH conditions, either as dissolved unimers or as kinetically trapped aqueous nanoparticles. The structures (by atomic force microscopy) and wetting properties (by dynamic contact angle measurements) of the obtained surface layers were determined. The majority of the surface layers consisted of polymeric nanoparticles with varying surface coverage. Annealing at 150 °C flattened and spread the particles on the surfaces. The surface wettability was found to be significantly influenced by the morphology and chemical composition of the obtained polymeric surface layer. The surfaces with the most homogeneous and smooth polymer layers exhibited the lowest contact angle hysteresis. The advancing/receding contact angles on the most hydrophilic copolymer layer on mica were 47°/<20°, and on the least hydrophilic layer they were 96°/63°. On unmodified mica surface the water contact angle is ∼0°. When those copolymers that provided the highest contact angles on mica were adsorbed on cellulose fiber substrates and annealed at 120 °C, highly hydrophobic surfaces were obtained, with advancing contact angles around 160°.

Contact line pinning on microstructured surfaces for liquids in the Wenzel state

The wettability of surfaces microstructured with square pillars was studied, where the static advancing contact angle on the planar surface was 72 degrees. We observed elevated advancing angles (up to 140 degrees) on these structures for droplets in the Wenzel state. No air was trapped in the structured surfaces beneath the liquid, ruling out the well-known Lotus leaf effect. Instead, we show that the apparent hydrophobicity is related to contact line pinning at the pillar edges, giving a strong dependence of wetting hysteresis on the fraction of the contact line pinned on pillars. Simulating the contact line pinning on these surfaces showed similar behavior to our measurements, revealing both strong pinning at the edges of the pillars as well as mechanistic details.

Gecko adhesion pad: a smart surface?

Recently, it has been shown that humidity can increase the adhesion of the spatula pads that form the outermost (adhesive) surface of the tokay gecko feet by 50% relative to the main adhesion mechanism (i.e. van der Waals adhesive forces), although the mechanism by which the enhancement is realized is still not well understood. A change in the surface hydrophobicity of a gecko setal array is observed when the array, which supports the spatulae, is exposed to a water drop for more than 20 min, suggesting a change in the hydrophilic-lyophilic balance (HLB), and therefore of the conformation of the surface proteins. A surface force apparatus (SFA) was used to quantify these changes, i.e. in the adhesion and friction forces, while shearing the setal array against a silica surface under (i) dry conditions, (ii) 100% humidity and (iii) when fully immersed in water. The adhesion increased in the humid environment but greatly diminished in water. Although the adhesion forces changed significantly, the friction forces remained unaffected, indicating that the friction between these highly textured surfaces is 'load-controlled' rather than 'adhesion-controlled'. These results demonstrate that the gecko adhesive pads have the ability to exploit environmental conditions to maximize their adhesion and stabilize their friction forces. Future designs of synthetic dry adhesives inspired by the gecko can potentially include similar 'smart' surfaces that adapt to their environment.

When Wenzel and Cassie are right: reconciling local and global considerations

The condition under which the Wenzel or Cassie equation correctly estimates the most stable contact angle is reiterated and demonstrated: these equations do hold when the drop size is sufficiently large compared with the wavelength of roughness or chemical heterogeneity. The numerical demonstrations somewhat mimic recent experiments that seemingly refuted the Wenzel and Cassie equations and show that these experiments were performed only for drops of sizes similar in order of magnitude to the wavelength of roughness or chemical heterogeneity. Under such conditions, the Wenzel and Cassie equations are a priori not expected to be valid. It is also explained that both the local equilibrium condition at the contact line and the global equilibrium condition involving the wetted area within the contact line are necessary and complementary.

Teflon is hydrophilic. Comments on definitions of hydrophobic, shear versus tensile hydrophobicity, and wettability characterization

Comments are made concerning the recent use of adjectives to describe solid surfaces that exhibit anomalously high water contact angle values. We suggest that the meaning of the word hydrophobic be resolved before it is modified, for example, to superhydrophobic and further modified, for example, to sticky superhydrophobic and before the definitions of these new words become issues of contention. The case is made that the first statement in the title is appropriate with experiments that demonstrate significant attractive interaction between liquid water and the surface of solid Teflon. Four types of experiments are described: the interaction of a silicon-supported covalently attached perfluoroalkyl monolayer (a model Teflon surface) with a sessile water drop (1) and with a thin film of water on a clean silicon wafer surface (2), the interaction of 1 and 12 microm diameter solid Teflon particles with a water droplet surface (3), and the interaction of a thin (<5 microm) Teflon film with a water droplet (4). The concepts of shear and tensile hydrophobicity are introduced, and the recommendation that two numbers, advancing and receding contact angle values, should be considered necessary data to characterize the wettability of a surface. That the words hydrophobic, hydrophilic, and their derivatives can and should only be considered qualitative or relative terms is emphasized.