Contact Angle Hysteresis Research Articles

A surface-engineered contact lens for tear fluid biomolecule sensing.

The eyes provide rich physiological information and offer diagnostic potential as a sensing site, and probing tear constituents via the wearable contact lens could be explored for healthcare monitoring. Herein, we propose a novel adhesive contrast contact lens platform that can split tear film by natural means of tear secretion and blinking. The adhesive contrast is realized by selective grafting of a lubricant onto a polydimethylsiloxane (PDMS)-based contact lens, leading to high pinning zones on a non-adhesive background. The difference in contact angle hysteresis facilitates the liquid splitting. Further, the method offers control over the droplet volume by controlling the zone dimension. The adhesive contrast contact lens is coupled with fluorescent spectroscopic as well as colorimetric techniques to realize its potential as a diagnostic platform. The adhesive contrast contact lens is exploited to detect the level of lactoferrin in tear by sensitizing split droplets with Tb3+ ions. The adhesive contrast contact lens integrated with a fluorescence spectrometer was able to detect the lactoferrin level up to a concentration of 0.25 mg mL-1. Additionally, a colorimetric detection based on the fluorescence of the lactoferrin-terbium complex is demonstrated for the measurement of lactoferrin, with a limit of detection in the physiological range up to 0.5 mg mL-1.

Morphology and stability of droplets sliding on soft viscoelastic substrates.

We show that energy dissipation partition between a liquid and a solid controls the shape and stability of droplets sliding on viscoelastic gels. When both phases dissipate energy equally, droplet dynamics is similar to that on rigid solids. When the solid is the major contributor to dissipation, we observe an apparent contact angle hysteresis of viscoelastic origin. We find excellent agreement between our data and a non-linear model of the wetting of gels of our own that also indicates the presence of significant slip. Our work opens general questions on the dynamics of curved contact lines on compliant substrates.

Coexisting superhydrophobicity and superadhesion features of Ziziphus mauritiana abaxial leaf surface with possibility of biomimicking using electrospun microfibers

The coexistence of superhydrophobicity and superadhesion features is being reported for the abaxial leaf of Indian jujube (Ziziphus mauritiana) possessing hairy, matted surface texture with fiber dia ranging ∼ 5.6–7.1 μm. Very high-water contact angle (WCA > 143°) and high contact angle hysteresis (CAH ∼ 30°–46°) were observed and compared for the tender, mature, and senescent leaf states. The tender leaf exhibits a maximal superhydrophobicity with WCA as high as ∼151° and an increased roll-off angle from ∼21° to 33°. Conversely, next two leaf states are characterized by excellent adhesion even up to a base tilting of 90° without any tendency to roll-off. An attempt has also been made for biomimicking the leaf's hairy fiber microstructure with polyvinylidene fluoride (PVDF) by employing an electrospinning setup, with adjustable control parameters. The fabricated ∼4.3-μm-dia PVDF-based nonwoven fibers were seen to be replicated at par with excellent superhydrophobicity and high adhesion features. The WCA and CAH of artificially grown fibers were estimated to be ∼145.7° and 49.4°. The nonwoven, yarn-like surface construct of microfibers fits well in worm-like chain model, which considers a normal distribution of segments described through discrete jointed length, persistent length, and bending angle between successive segments.

Surface-Wetting Characteristics of DLP-Based 3D Printing Outcomes under Various Printing Conditions for Microfluidic Device Fabrication.

In recent times, the utilization of three-dimensional (3D) printing technology, particularly a variant using digital light processing (DLP), has gained increasing fascination in the realm of microfluidic research because it has proven advantageous and expedient for constructing microscale 3D structures. The surface wetting characteristics (e.g., contact angle and contact angle hysteresis) of 3D-printed microstructures are crucial factors influencing the operational effectiveness of 3D-printed microfluidic devices. Therefore, this study systematically examines the surface wetting characteristics of DLP-based 3D printing objects, focusing on various printing conditions such as lamination (or layer) thickness and direction. We preferentially examine the impact of lamination thickness on the surface roughness of 3D-printed structures through a quantitative assessment using a confocal laser scanning microscope. The influence of lamination thicknesses and lamination direction on the contact angle and contact angle hysteresis of both aqueous and oil droplets on the surfaces of 3D-printed outputs is then quantified. Finally, the performance of a DLP 3D-printed microfluidic device under various printing conditions is assessed. Current research indicates a connection between printing parameters, surface roughness, wetting properties, and capillary movement in 3D-printed microchannels. This correlation will greatly aid in the progress of microfluidic devices produced using DLP-based 3D printing technology.

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

HypothesisUnderstanding 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. ExperimentsWe 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°). FindingsA 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.

Forced wetting of hydrophobic flocked pile fabrics

The hydrophobicity of a material is enhanced by introducing a multi-scaled roughness on its surface. When a liquid droplet deposited on such a surface is submitted to a stress, a wetting transition is observed. This transition is abrupt for model rigid surfaces. The case of textile materials is very different since they are highly deformable and the liquid may penetrate the structure. In this paper, we focus on hydrophobic pile textile fabrics. Firstly, their wetting properties are characterized by the contact angle, the contact angle hysteresis and the roll-off angle. Then, we study the influence of an external pressure applied to a drop deposited on the surface. The contact angle after squeezing is studied vs. the pressure and vs. the lateral deformation of the fabrics. The results are discussed according to the geometrical parameters of the piles and the microscopic organization of the structure.

Wettability and evaporation of sodium dodecyl sulfate aqueous droplets on microhole-patterned polydimethylsiloxane surfaces under a direct current electric field

Wettability and evaporation of sodium dodecyl sulfate (SDS) aqueous droplets on microhole-patterned polydimethylsiloxane (PDMS) surfaces under a direct current electric field were experimentally studied. When a direct current electric field was present, the contact angle hysteresis for SDS aqueous droplets decreased with increasing applied voltage and solid fraction had an obvious influence on contact angle hysteresis. For electrowetting of SDS aqueous droplets on microhole-patterned PDMS surfaces, both actuation voltage and saturation voltage decreased with increasing SDS concentration (below ∼4.10 mM) and increasing solid fraction. For SDS aqueous droplets evaporating on these microhole-patterned PDMS surfaces with the application of a direct current electric field, the duration of constant contact radius stage was found to become shorter with increasing solid fraction, which can be attributed to the weaker adhesion between the droplet and the surface with a larger solid fraction. SDS concentration and applied voltage were also found to have an obvious influence on the evaporation characteristics.

Joint Experimental and Theoretical Study on Deposition Morphologies in Polymer Sessile Droplets.

Although past experimental and theoretical research has made substantial progress in understanding evaporation behaviors in various suspensions, the fundamental mechanism for polymer sessile droplets is still lacking. One critical effect is the molecular weight on the evaporation behaviors. Here, systematic experiments are carried out to investigate the evaporation behavior of polymer droplets under the effects of polymer concentration, evaporation rate, and especially molecular weight. We obtain polymer films with various morphologies with molecular weights ranging from 2 orders of magnitude to 4 orders of magnitude and polymer concentration across 4 orders of magnitude. We further develop a theoretical model based on the Onsager principle to explain the evaporation mechanism from a dynamic perspective. Analysis indicates that increasing molecular weight or polymer concentration enhances the contact angle hysteresis and slows down the evaporation, resulting in the transition from multiring to coffee ring and eventually to uniform films. The findings offer a guideline for achieving the desired deposition patterns via droplet processing techniques.

Superhydrophobic tin oxide nanostructures coated on copper plates: Wettability tuning by the etching medium

AbstractIn the current work, the chemical etching process and hydrothermal method were used to create bio‐inspired superhydrophobic fluorine‐free tin oxide nanostructures on copper plates. Taguchi's experimental design was also used in investigating coating wettability characteristics based on fabrication parameters. Statistical analysis results demonstrated that the optimal superhydrophobic sample with a water contact angle of 163.40° ± 1.42° and contact angle hysteresis of 3.00° ± 0.80° could be fabricated under the following conditions: sodium hydroxide (2.00 M) as an etching solution, urea‐to‐tin chloride ratio 2.5:1, and 2.00 h reaction times. The effects of each synthesis parameter on the obtained sample's superhydrophobicity were evaluated through multiple one‐variable‐at‐a‐time experiments. Testing the chemical stability of the optimal sample revealed that it was more resistant to deterioration in an alkaline environment (8–10) than in an acidic environment (4–6). The resulting superhydrophobic sample was analyzed by a delay test for water droplets freezing on its surface to determine whether ice formed and accumulated. The created bio‐inspired honeycomb‐like nanocoating demonstrated exceptional mechanical robustness and good anti‐icing performance after 10 consecutive icing cycles. These results indicated that superhydrophobic coatings could be easily and economically produced without using fluoropolymers and silanes on aluminum substrates.

Biofouling resistant materials based on micro‐structured surfaces with liquid‐repellent properties

AbstractAdhesion of contaminants on various polymer‐based devices during fluid‐substrate interactions is a common problem that can cause biofouling and corrosion. In this study, hierarchical structures with submicron features on polypropylene (PP), high‐density polyethylene (HDPE), and polycarbonate (PC) are fabricated by femtosecond laser ablation. The effect of the hierarchical structures on surface wettability, droplet impact, and bacterial attachment has been examined. Our results demonstrate that the structured polymeric substrates facilitate large contact angles and minimal interfacial adhesion, allowing droplets to roll off at a low angle of inclination below 5°. Further, rendering the hierarchical structures with a low‐surface‐energy coating can enable the surfaces to exhibit superamphiphobic properties. The low interfacial adhesion properties, as accounted by the large contact angles and small contact angle hysteresis, of such surfaces prevent bacterial attachment and biofilm formation. The findings provide a design principle for creating affordable biofouling resistant surfaces with a submicron topography that can be used for engineering and biomedical devices.

Hysteresis Phenomena in a Liquid Sessile Drop on a Stretchable Inclined Elastic Substrate

The authors studied states of liquid sessile drops on a stretchable inclined elastic substrate to find out peculiarities of the contact angles hysteresis by digital optical microscopes. The drop elongation and contact angles were recorded on the stages of the band stretching and contraction. An anomalous behavior of the drop was found on the first step of the contraction during which the measured drop parameters remained unchanged. Finally, a new hydrophysical effect was found. It is hysteresis of the drop states, on the background of which the contact angle hysteresis exists.

A seed and bridge layer method for inkjet printing of narrow traces on receding ink-substrate combinations

Inkjet printing of electronic materials is of interest for digital printing of flexible electronics and sensors, but the width of the inkjet-printed lines is still large, limiting device size and performance. Decreasing the drop volume, increasing the drop spacing, and increasing the ink-substrate contact angle are all approaches by which the line width can be lowered, however these approaches are limited by the nozzle geometry, ink coalescence and bead instabilities, and contact angle hysteresis, respectively. Here we demonstrate a novel approach for stable inkjet printing of very narrow lines on ink-substrate combinations with a high contact angle, utilizing the de-wetting of the ink due to the decreased contact angle hysteresis. After printing and drying an initial layer of disconnected seed drops of silver nanoparticle ink, we print an additional layer of bridging drops of the same ink in between the dried seed drops. The bridging drops expand to touch the dried seed drops and then retract into a line, due to the pinning of the wet ink on the dried seed ink but not on the substrate, forming a continuous silver trace. The trace width is decreased from 60 μm with a traditional printing approach down to 12.6 μm with this seed-bridge approach. The electrical conductivity of the silver trace is similar to that of a conventionally printed trace. Due to poor adhesion on the print substrate, the trace was transferred to a separate polymer substrate with a simple hot-pressing procedure, which preserves the electrical conductivity of the trace.

The concept of the contact angle in the process of oil film formation in internal combustion piston engines

In internal combustion piston engines, the process of oil film formation differs from that in industrial machines. The adhesive strength of the molecules at the interface between the coating of engine parts and the lubricating oil affects the load carrying capacity and the ability to form a lubricated film. The geometry of the lubricating wedge between the surfaces of the piston rings and the cylinder wall is determined by the thickness of the oil film and the amount of oil coverage of the ring. This state is modified by many parameters describing the operation of the engine and the physical and chemical parameters of the coatings of the cooperating pairs. For lubricating molecules reaching energies greater than the energy barrier of adhesion at the boundary, sliding occurs. Therefore, the value of the contact angle of the liquid on the surface of the coating depends on the value of the intermolecular force of attraction. According to the author, there is a strong correlation between the contact angle and the lubrication effect. Research indicates that the potential barrier is a function of the contact angle and the contact angle hysteresis (CAH). The innovation of the work consists in the study of the contact angle and CAH in the conditions of thin layers of lubricating oil in cooperation with hydrophilic and hydrophobic coatings. The thickness of the lubricating film was measured under various speed and load conditions using optical interferometry. The study proves that CAH is a better interface parameter to correlate with the effect of hydrodynamic lubrication.

Influence of temperature on wettability of micro hierarchical rough surfaces: A three-dimensional thermodynamic model

Inspired by the micrometer-scale hierarchical microstructures on the surface of cicada wings, this paper presents a three-dimensional thermodynamic analytical model to comprehensively analyze the superhydrophobicity of rough surfaces with micro-hierarchical structures. Emphasis is placed on the variation of normalized free energy and free energy barrier as a function of apparent contact angle for different wettability states. The effect of temperature-dependent intrinsic contact angle on the intertransition between different wettability states is explored. Additionally, the model allows obtaining the equilibrium contact angle and contact angle hysteresis through corresponding calculations. Importantly, the present model elucidates the influence of the height of the micropillar of the paraboloid on the intertransition between different wettability states at a specific temperature. The geometrical parameters of a droplet in a certain environment, when it is in the final stable wettability state, can be designed based on this model. By conducting a thorough investigation into the effect of temperature on the contact angle of rough surfaces with micro-hierarchical structures, this study offers a fresh perspective for understanding the wettability behavior of droplets at the micrometer scale. Moreover, it provides a theoretical basis for designing rough surfaces with micro-hierarchical structures for application in various environments.

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Surface modification is a widely utilized technique for enhancing condensation heat transfer. Two main properties of surfaces in manipulation of condensation heat transfer are the contact angle and contact angle hysteresis. This study focuses on the influence of contact angle (CA) and contact angle hysteresis (CAH) on humid air condensation. For this, hydrophilic and hydrophobic surfaces with varying CA and CAH values were fabricated and tested in a humidity-controlled climate chamber at different relative humidity levels. Three hydrophilic surfaces samples with a contact angle of approximately 70° and CAH values of 10°, 20°, and 42° were tested. Two hydrophobic surfaces with a contact angle of approximately 110° and CAH values of 21° and 39°were also prepared as well as a hydrophobic surface with a contact angle of 96° and a CAH of 43°. The role of CA and CAH in different stages of condensation cycle was investigated. Our findings show that while CA plays the main role in droplet nucleation, CAH has a significant impact on droplet coalescence and departure. Increasing CAH while keeping CA constant has a negative effect on condensation heat transfer in all wettability levels. However, the relationship between changes in CA while keeping CAH constant does not have the same trend in the condensation heat transfer performance for every case. Changing CAH for lower CAH values led to a greater impact on enhancing condensation heat transfer than higher values. Moreover, increasing CAH on hydrophobic surfaces had a more significant effect than on hydrophilic surfaces. Additionally, decreasing CAH had a more pronounced effect on improving condensation heat transfer than increasing CA. The findings emphasize on the importance of considering both the contact angle (CA) and contact angle hysteresis (CAH) in the surface design to have the optimum condensation heat transfer performance.

Heat and Mass Transfer Processes and Evaporation of a Liquid Droplet on a Structured Surface

The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated.

Combing of picogram level DNA equivalent to genomic DNA present in single human cell by self propelled droplet motion over a stable gradient surface

DNA combing is a powerful technique for studying replication profile, fork-directionality and fork velocity. At present, there is requirement of a methodology to comb DNA present in a single human cell for studying replication dynamics at early embryonic stage. In our study, a surface having dual characteristics i.e., affinity towards negatively charged single DNA molecules and a hydrophobic gradient for self propelled droplet motion of combing solution was developed. The surface was made by coating of TCOS (trichloro-octylsilane) by vapor diffusion on APTES (Aminopropyl-triethoxysilane) coated glass slides. A gradient surface having high deposition efficiency (DE) was developed on which 5 picogram DNA equivalent to genomic DNA present in one single human cell can be combed. The gradient surface was thermostable in nature having the ability to sustain boiling temperature for two hours and sustain anisotropy in 70 % ethanol for 80 h. Applicability for multiple runs was enhanced such that the surface can be used for 13–14 times. Factors associated with gradient surface are unidirectional movement of combing solution droplet over the gradient surface for combing straight DNA molecules and a longer gradient surface of more than 1 cm such that long size DNA molecules can be combed. Ellipsometry and contact angle hysteresis confirmed the presence of hydrophobic gradient. XPS (X-ray photoelectron spectroscopy) and FTIR (Fourier Transform Infrared Spectroscopy) confirmed the presence of characteristic affinity towards negatively charged DNA molecules on the gradient surface. Combing solution was optimized for increasing deposition efficiency and for increasing the applicability of gradient surface for multiple runs. High temperature of combing solution was found to increase Deposition Efficiency. Combing solution was also optimized for combing single DNA molecules over the gradient surface. Single DNA molecules were combed by reducing pH and lowering concentration of triton-X in the combing solution. Dye: bp ratio was optimized for high fluorescent intensity and low surface background.

High-efficient and scalable fabrication of robust superhydrophobic microarrays through maskless electrochemical machining

Superhydrophobic microarrays have shown great promise across various domains such as droplet manipulation, anti-icing, oil-water separation, and drag reduction. Nevertheless, the efficient fabrication of robust superhydrophobic microarrays on metallic substrates remains a significant challenge. In this study, we designed a robust micro-pit array with excellent mechanical properties and developed an efficient method for creating the designed array on 304 stainless steel surfaces through the utilization of maskless electrochemical machining (ECM) technology. The resulting microarrays without the need for chemical modification exhibited excellent superhydrophobicity with a contact angle of 155.2° and a contact angle hysteresis of 2.5°, and this superhydrophobic performance possessed good stability. The microstructure and microhardness of the microarrays was subsequently investigated, and the results showed that the maskless ECM treatment significantly improved the mechanical robustness of the designed microarrays. In addition, the simulation and experiments indicated that the morphology and wettability of the fabricated surface could be well controlled by adjusting voltage parameters. The approach outlined in this work is efficient and scalable, and will promote the industrial production and practical application of superhydrophobic metallic surfaces with robust microarrays.

Unraveling the interplay of leaf structure and wettability: A comparative study on superhydrophobic leaves of Cassia tora, Adiantum capillus-veneris, and Bauhinia variegata

In this article, superhydrophobic leaves of Cassia tora, Adiantum capillus-veneris (ACV), and Bauhinia variegata are reported for the first time, and the wettability of these leaf's surfaces was correlated with their surface morphology at micro- and nanoscale. Field Emission Scanning Electron Microscopy (FESEM) images of the surfaces were used to get surface morphological information at the micro-nanoscale structures. A special drying method was implemented to ensure the minimal structural collapse of these surfaces under the high vacuum of FESEM. FESEM images of Cassia tora leaves showed widely spaced, low aspect ratio nanopetals distributed on bumpy blunt microfeatures, responsible for high contact angle hysteresis, and high roll angle measured on the Cassia tora leaves. ACV leaves showed the presence of micrometer-scale spherical morphology made of nanoscale hair-like features. These hierarchical re-entrant surface features generated a very high contact angle and low roll-off angle. Leaves of Bauhinia variegata showed similar superhydrophobic and self-cleaning properties. However, surface features were different, which consisted of a higher aspect ratio and closely spaced nanopetals uniformly distributed over flat surfaces consisting of micro-scale ridges. Our comprehensive investigation covers a detailed analysis of droplet impact studies, shedding light on the intricate dynamics governing droplet behavior on these superhydrophobic surfaces. Furthermore, we extended our analysis to encompass droplet impact on macrostructures to assess their influence on droplet receding and rebound phases. Notably, it was observed that only the microstructure of Cassia tora had a discernible impact on the receding and rebound phases of droplets. Additionally, our experiments examining maximum spreading diameter demonstrated good agreement with established models, further strengthening the scientific basis of our findings. These findings not only contribute to the advancement of our understanding of surface wetting phenomena but also bear practical implications for the development of water-repellent and self-cleaning materials.

Effects of contact angle hysteresis on frosting and defrosting characteristics on vertical superhydrophobic surfaces

Contact angle hysteresis has a significant influence on dynamic frosting and defrosting of vertical superhydrophobic surface, however, the characteristics under the effect of contact angle hysteresis have not been clarified. Therefore, superhydrophobic surfaces with similar contact angles but different contact angle hysteresis were prepared in this paper, and visualization experiment was carried out to discover the effects of contact angle hysteresis on frosting and defrosting characteristics of vertical superhydrophobic surface. Experimental results show that the bounce behavior, size, distribution density and freezing time of condensate droplets are obviously different in the early stage of frosting, and reducing the hysteresis of superhydrophobic surface reduces the distribution of condensate droplets and effectively delay droplet freezing. The anti-frosting performance of the superhydrophobic surface is improved with the decrease of contact angle hysteresis. The frost layer thickness of the surface with the smallest contact angle hysteresis is 50% lower than that of the surface with the smallest. The frost layer peels off from the superhydrophobic surface with smallest contact angle hysteresis at the beginning of defrosting and falls away under the action of gravity. This can effectively improve the defrosting efficiency by reducing defrosting time and retained defrosting water.