High Contact Angle Hysteresis Research Articles

Chemical and structural heterogeneity of olive leaves and their trichomes

Many biological surfaces have hairs, known as trichomes in plants. Here, the wettability and macro- and micro-scale features of olive leaves are analyzed. The upper leaf side has few trichomes, while the lower side has a high trichome density. By combining different techniques including electron and atomic force microscopy, trichome surfaces are found to be chemically (hydrophilic-hydrophobic) heterogeneous at the nano-scale. Both olive leaf surfaces are wettable by water, having a high water contact angle hysteresis and great drop adhesion. The ultra-structural pattern observed for epidermal pavement cells differs from the reticulate cuticle structure of trichomes which shows that leaf surface areas may be substantially different despite being located nearby. Our study provides evidence for the nano-scale chemical heterogeneity of a trichome which may influence the functional properties of biological surfaces, such as water and solute permeability or water capture as discussed here for plants.

Wetting, droplet evaporation and corrosion behavior of various composite and textured materials

Experimental studies were conducted on the evaporation rate of a droplet located on the surfaces of various composite and textured materials with different wettability at varying surface temperatures. The maximum droplet evaporation rate was recorded on a textured Cu–SiC composite material with superhydrophilic properties. The minimum droplet evaporation rate corresponded to a superhydrophobic AlMg3 substrate obtained using laser texturing with subsequent hydrophobization procedure. An increase in surface temperature led to a significant deterioration of hydrophobicity for a superhydrophobic substrate and a deterioration of hydrophilicity for a superhydrophilic substrate. As a result, with increasing temperature, the influence of the droplet geometry determined by surface wettability on the droplet evaporation rate for these substrates sharply decreases. Advancing and receding dynamic contact angles, as well as contact angle hysteresis were obtained on all studied substrates. It is generally accepted that a hydrophobic surface exhibits less contact angle hysteresis than a hydrophilic one. However, Cu-SiС composite substrate with a textured hydrophobic surface exhibited the opposite behavior. The contact angle hysteresis on the hydrophobic textured surface was found to be four times higher than that on the hydrophilic polished one. A mechanism was proposed that explains the high contact angle hysteresis and four hysteresis time modes. The minimum hysteresis value corresponded to the superhydrophobic AlMg3 substrate. Anti-corrosion properties were studied for all substrates under consideration. The minimum corrosion current corresponded to a copper substrate with a graphene coating, as well as a superhydrophobic AlMg3 substrate. For a given substrate, there are two corrosion modes with a three-fold difference in the corrosion current. In addition, the wettability behavior of different surfaces was examined using molecular dynamics simulations. The simulation results were found to be consistent with the experimental data.

Icephobic Gradient Polymer Coatings Deposited via iCVD: A Novel Approach for Icing Control and Mitigation.

Materials against ice formation and accretion are highly desirable for different industrial applications and daily activities affected by icing. Although several concepts have been proposed, no material has so far shown wide-ranging icephobic features, enabling durability and manufacturing on large scales. Herein, we present gradient polymers made of 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4D4) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) deposited in one step via initiated chemical vapor deposition (iCVD) as an effective coating to mitigate ice accretion and reduce ice adhesion. The gradient structures easily overcome adhesion, stability, and durability issues of traditional fluorinated coatings. The coatings show promising icephobic performance by reducing ice adhesion, depressing the freezing point, delaying drop freezing, and inhibiting ice nucleation and frost propagation. Icephobicity correlates with surface energy discontinuities at the surface plane resulting from the random orientation of the fluorinated groups of PFDA, as confirmed by grazing-incidence X-ray diffraction measurements. The icephobicity could be further improved by tuning the surface crystallinity rather than surface wetting, as samples with random crystal orientation show the lowest ice adhesion despite high contact angle hysteresis. The iCVD-manufactured coatings show promising results, indicating the potential for ice control on larger scales and various applications.

Wettability-driven synergistic resistance of scale and oil on robust superamphiphobic coating

Disgusting deposits (e.g., scale and crude oil) in daily life and industrial production are always serious problems, posing great threats to the safety and economic development. However, most of developed coatings can only conquer one part of these deposits such as superhydrophobic coatings possess anti-scaling capacity but would adhere crude oil. To integrate scale resistance with oil repellence, we herein report a robust superamphiphobic (SAB) coating simultaneously reducing pollution of scale and oil for extended period of time (two weeks with over 98% reduction). Compared with single role of superhydrophobic and amphiphilic surfaces, the SAB coating can not only inhibit interfacial nucleation of scale but also reduce the adhesion of formed scale and polluted oil. The durability of the SAB coating is evaluated via mechanical tests (sandpaper abrasion, tape stripping and sand falling) and chemical corrosion (corrosive liquid immersing), revealed by sustainable high contact angles and low contact angle hysteresis of water and oil. The universality of this strategy can be further confirmed by adding different particles like kaolin, Al2O3, and SiO2, resisting multiple types of scale (i.e., CaSO4, BaSO4 and MgCO3) and oil (i.e., glycerol, glycol, and mineral oil). Therefore, this study provides an ideal avenue for resisting scale and oil, which may be used for conquering the complexity of application environments (e.g., oil production and transportation).

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.

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.

Relaxation phase during droplet impact on superhydrophobic surfaces with high contact angle hysteresis

The droplet impact process on a solid surface is divided into a spreading phase where the droplet reaches the maximum deformation followed by a retracting phase. However, in the case of surfaces with high contact angle hysteresis, these two phases are connected by a relaxation phase where the contact angle changes from the advancing to the receding contact angle almost without motion of the contact line. Although the relaxation time can represent a significant part of the total droplet contact time, this relaxation regime has been less explored, especially for superhydrophobic surfaces due to the challenge of designing such surfaces with controlled wetting properties. Here, we show that for superhydrophobic surfaces with large contact angle hysteresis, the relaxation time can be comparable to the spreading and retracting time. Our results indicate that both the contact angle hysteresis and the capillary forces play a major role in defining the relaxation time and that relaxation time scales with the inertial–capillary time when using the droplet relative deformation as the characteristic length scale for this relaxation regime.

Freezing Characteristics of a Water Droplet on a Multiscale Superhydrophobic Surface.

Superhydrophobic surfaces have the potential to retard ice formation owing to their super water-repellant nature arising from high static contact angle and low contact angle hysteresis. Most of the previous studies have focused on patterned surfaces with mono-scaled prismatic structures. In contrast, the freezing behavior on multiscaled rough superhydrophobic surfaces that are of practical significance is relatively little studied. This article presents, for the first time, the freezing dynamics of a water droplet interacting with multiscale fractal superhydrophobic surfaces which validates well with experimental measurements. It is shown that the dual effects of increased contact angle and poor interfacial conduction due to trapped air cavities within the roughness features of the superhydrophobic surface lead to increasing freezing time with increasing surface hydrophobicity, determined as a function of the fractal surface parameters. A comparison of the freezing dynamics of sessile droplets of identical contact angle on a smooth versus a rough superhydrophobic surface shows that interfacial asperity thermal resistance contributes to over 14% increase in the freeze time. It is further shown that by tailoring the multiscale characteristics, the freeze time may be increased by up to 7-fold compared to freezing on a smooth surface. The application of the numerical model to studying ice formation on several practical superhydrophobic surfaces of a range of metallic materials and fabrication methods is also discussed, which offers guidelines for the design of anti-icing surfaces in practice.

Drop impact on a sticky porous surface with gas discharge: transformation of drops into bubbles

The impact of drops on a porous surface with high contact-angle hysteresis and gas discharge is studied. Four different impact modes, ranging from complete repulsion to fast immobilization of a drop on the surface, are identified and mapped in a space spanned by the pressure difference of the gas across the porous surface and the impact Weber number of the drop. The most remarkable aspect of the dynamics is the transformation of a drop into a bubble, which occurs when a drop just overcomes the repulsion by the gas flow and wets the surface. The transition to the regime in which a drop is transformed to a bubble is well described by a simple scaling relationship based on a balance between inertia and the repulsive force due to the gas flow.

Scanning Drop Friction Force Microscopy.

Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine the wetting dynamics. Still, existing techniques (e.g., contact angle goniometry) are not sufficient to localize inhomogeneities and image wetting variations. We overcome these limitations through scanning drop friction force microscopy (sDoFFI). In sDoFFI, a 15 μL drop of Milli-Q water is raster-scanned over a surface. The friction force (lateral adhesion force) acting on the moving contact line is plotted against the drop position. Using sDoFFI, we obtained 2D wetting maps of the samples having sizes in the order of several square centimeters. We mapped areas with distinct wetting properties such as those present on a natural surface (e.g., a rose petal), a technically relevant superhydrophobic surface (e.g., Glaco paint), and an in-house prepared model of inhomogeneous surfaces featuring defined areas with low and high contact angle hysteresis. sDoFFI detects features that are smaller than 0.5 mm in size. Furthermore, we quantified the sliding behavior of drops across the boundary separating areas with different contact angles on the model sample. The sliding of a drop across this transition line follows a characteristic stick-slip motion. We use the variation in force signals, advancing and receding contact line velocities, and advancing and receding contact angles to identify zones of stick and slip. When scanning the drop from low to high contact angle hysteresis, the drop undergoes a stick-slip-stick-slip motion at the interline. Sliding from high to low contact angle hysteresis is characterized by the slip-stick-slip motion. The sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

Effectiveness and durability assessment, under extreme environmental conditions, of a superhydrophobic coating applied onto sandstone from Carteia roman archaeological site

Superhydrophobic coatings have been often used to protect stone surfaces against water. The durability of these coatings has been rarely tested for monumental heritage exposed to outdoor conditions. In this work, a highly effective superhydrophobic nanomaterial has been applied in the archaeological site of Carteia, which is located next to the sea and an industrial area. The effectiveness and durability of this nanomaterial have been evaluated in the laboratory and after one year of in situ exposure and compared against two specific commercial products. The results revealed that the product prepared in our laboratory exhibited high consolidant and superhydrophobic effectiveness, high static contact angle and low contact angle hysteresis. After one year, our product effectiveness was maintained due to the uniform coating with 4 mm depth penetration. The consolidant and hydrophobic commercial products presented reduced protection, with cracked coatings and poor penetration into the sandstone structure.

Reactive silver inks for antiviral, repellent medical textiles with ultrasonic bleach washing durability compared to silver nanoparticles.

Medical textiles are subject to particularly harsh disinfection procedures in healthcare settings where exposure risks are high. This work demonstrates a fabric treatment consisting of a reactive silver ink and low surface energy PDMS polymer that provides for superhydrophobicity and antiviral properties against enveloped herpes simplex virus stocks even after extended ultrasonic bleach washing. The antiviral properties of reactive silver ink has not been previously reported or compared with silver nanoparticles. The fabric treatment exhibits high static contact angles and low contact angle hysteresis with water, even after 300 minutes of ultrasonic bleach washing. Similarly, after this bleach washing treatment, the fabric treatment shows reductions of infectious virus quantities by about 2 logs compared to controls for enveloped viruses. The use of silver ink provides for better antiviral efficacy and durability compared to silver nanoparticles due to the use of reactive ionic silver, which demonstrates more conformal coverage of fabric microfibers and better adhesion. This study provides insights for improving the wash durability of antiviral silver fabric treatments and demonstrates a bleach wash durable, repellent antiviral treatment for reusable, functional personal protective equipment applications.

High Hysteresis Suspended Wetting State: A Wetting Regime for Controlled Trapping of Drops on Micro‐Trench Covered Surfaces

AbstractWetting of simple liquids on rough surfaces is usually described either by the Cassie–Baxter model where drops are suspended over the roughness features with air trapped below, or by the Wenzel model where the drops are impaled on the roughness features. According to the Wenzel model, drops can be trapped on the surfaces, whereas in the Cassie–Baxter model, the drops can be easily released upon adding a small tilt. Surfaces with very regular surface pattern, such as closely spaced micro‐trenches, can exhibit the so‐called High Hysteresis Suspended State (HySS). In this state the drops assume a spherical shape with air trapped below them, a characteristic feature of the Cassie–Baxter state, but show a very high contact angle hysteresis (up to >70°), typical for the Wenzel regime. The HySS regime is studied as a function of the surface geometry, applied pressure and the mass of the drop. Through roll‐off measurements it is shown that surfaces with controlled trapping conditions can be generated, where drops can be captured, however, upon addition of a small amount of energy (e.g., induced by tilt) can be released. The parameters that can be used to control the “depth” of the trap are elucidated.

A novel biomimetic nanocomposite exhibiting petal wetting phenomenon: fabrication and experimental investigations

A functional composite material that simultaneously exhibits hydrophobicity and water droplet adhesion has monumental potential in controlling fluid flow, studying phase separation, and biological research. This article reports the fabrication of a petal wetting biomimetic Boron Nitride Nanotubes (BNNTs) -Polydimethylsiloxane (PDMS) nanocomposite achieved by drop casting. The petal effect was investigated by non-destructive techniques. The nanotubes were synthesized by chemical vapor deposition at 1150 °C and were characterized by x-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The mean diameter of the nanotubes was found to be 70 nm. The nanocomposites had BNNT fillers ranging from 0.5 wt% to 2 wt%. Water contact angles for pure PDMS polymer was 94.7° and for the 2 wt% BNNT-PDMS nanocomposite was 132.4°. The petal wetting nanocomposite displayed a characteristic trait of high contact angle hysteresis. The surface roughness parameters of the nanocomposites were determined by atomic force microscopy. Laser scanning confocal microscopy aided in analyzing the droplet penetration and in observing the trapped air between the water droplet and the nanocomposite surface. Based on surface observations, roughness parameters, and the extent of droplet penetration by the surface, we shed light on the Cassie impregnating wetting regime followed by the biomimetic nanocomposite. Such a surface would be beneficial in the study of the embryogenesis of cells and aid in moisture collection.

Evaluating a transparent coating on a face shield for repelling airborne respiratory droplets.

A face shield is an important personal protective equipment to avoid the airborne transmission of COVID-19. We assess a transparent coating on a face shield that repels airborne respiratory droplets to mitigate the spread of COVID-19. The surface of the available face shield is hydrophilic and exhibits high contact angle hysteresis. The impacting droplets stick on it, resulting in an enhanced risk of fomite transmission of the disease. Further, it may get wetted in the rain, and moisture may condense on it in the presence of large humidity, which may blur the user's vision. Therefore, the present study aims to improve the effectiveness of a face shield. Our measurements demonstrate that the face shield, coated by silica nanoparticles solution, becomes superhydrophobic and results in a nominal hysteresis to the underlying surface. We employ high-speed visualization to record the impact dynamics of microliter droplets with a varying impact velocity and angle of attack on coated and non-coated surfaces. While the droplet on non-coated surface sticks to it, in the coated surface the droplets bounce off and roll down the surface, for a wide range of Weber number. We develop an analytical model and present a regime map of the bouncing and non-bouncing events, parametrized with respect to the wettability, hysteresis of the surface, and the Weber number. The present measurements provide the fundamental insights of the bouncing droplet impact dynamics and show that the coated face shield is potentially more effective in suppressing the airborne and fomite transmission.

To evaluate and compare the mechanical properties and wetting ability of heat polymerized polymethyl methacrylate resin after reinforcement with halloysite nanotubes and titania nanoparticles

To evaluate and compare the mechanical properties and wetting ability of heat polymerized polymethyl methacrylate resin after reinforcement with halloysite nanotubes and titania nanoparticles. A conventional heat-cured resin (DPI heat cure material) will be used as the matrix component while halloysite (group A) and titania (group B) nanoparticles as a reinforcing agent. Group C specimens will be fabricated with trevalon (Trevalon – HI). The specimens will be stored in distilled water overnight to simulate oral environment before performing the tests for determining the flexural strength (FS) and contact angle (CA). The contact angle data states that Halloysite reinforced PMMA is more hydrophilic. UTM analysis states that Flexure strength of trevalon highest amongst all the three group. Resin reinforced with halloysite had higher contact angle hysteresis hence greater retentive value and wetting ability while travelon had higher flexure strength.

Dropwise condensation of low surface tension fluids on lubricant-infused surfaces: Droplet size distribution and heat transfer

The use of lubricant-infused surfaces and slippery liquid-infused porous surfaces to promote dropwise condensation of low surface tension fluids has received increasing attention in recent years due to the high condensate droplet mobility and low droplet contact angle hysteresis enabled by these coatings. Though multiple studies have focused on developing models to predict heat transfer during water vapor condensation on hydrophobic lubricant-infused surfaces, a lack of understanding exists for condensation of low surface tension fluids, where the surface becomes wettable to the condensate. In this study, we develop a theoretical model to predict the heat transfer during dropwise condensation of low surface tension fluids (ethanol and hexane) on lubricant-infused surfaces. Using numerical analysis, we develop a relationship between droplet Nusselt number and Biot number to accurately predict individual droplet heat transfer. The droplet size distribution of ethanol and hexane were experimentally determined, with result showing that the size distribution density depends on both the working fluid properties and droplet contact angle behavior. To validate our model, experiments were conducted by condensing pure ethanol and hexane vapor on different diameter tubes coated with lubricant-infused surfaces. From the experimental measurements, two correlations which characterized the distribution density of ethanol and hexane were developed. Using the droplet distribution correlations as input parameters, our developed model predicts the heat transfer coefficients for low surface tension fluid condensation more accurately when compared to the existing models, where the maximum deviation between the prediction and experimental results is less than 15%.

Macroporous Superhydrophobic Coatings with Switchable Wettability Enabled by Smart Shape Memory Polymers

AbstractHere, a facile bottom‐up nanofabrication technology for making smart superhydrophobic coatings with switchable wettability and tunable optical properties by integrating shape memory polymers (SMPs) with templated macroporous photonic crystals is reported. Thermoresponsive shape memory efforts exhibited by a polyurethane‐based shape memory copolymer enable reversible microstructural transitions between a “memorized” permanent configuration composing of highly ordered arrays of macropores and a temporary deformed structure with collapsed macropores. The unique macroporous SMP photonic crystal arrays with large surface roughness can entrap a large portion of air in the interconnecting macropores, resulting in superior superhydrophobic properties including high apparent water contact angle (CA > 160°) and low CA hysteresis. Importantly, the shape memory‐enabled microstructural transitions lead to drastic wettability switching from superhydrophobic (CA > 160°) to hydrophobic (CA ≈ 110°) or from oleophobic (CA ≈139° for hexadecane) to oleophilic (CA ≈ 80°). The large CA tunability (>50°) coupled with the simultaneous color change from shining iridescence to colorless associated with the same microstructural transition can not only provide a noninvasive means for visually indicating the surface wetting states, this favorable coupling between the wetting and optical behaviors of macroporous SMP photonic crystal membranes can also pave the way for developing new tunable nanooptical devices.

Remarkable bactericidal traits of a metal-ceramic composite coating elated by hierarchically structured surface.

SummaryA ceramic-based coating with a hierarchical surface structure was synthesized via solid-state reaction enabled by a double cathode glow discharge technique. This innovative coating comprises two distinct layers, specifically an outer layer with a well-aligned micro-pillar array and a dense inner layer. Both are composed of a face-centered cubic Cu(Co,Ni,Fe) solid solution phase together with a spinel-type Fe(Al,Cr)2O4 oxide. This coating exhibits superhydrophobicity and, yet, a very strong adhesion to water, i.e., the so-called “rose petal effect”. This coating also exhibits highly efficient antibacterial ability against both Staphylococcus aureus and Escherichia coli bacteria under both dark and visible light conditions. The excellent antibacterial property originates from the synergistic effects through the release of Cu ions coupled with photothermal activity upon light activation.

Tailoring the Icephobic Performance of Slippery Liquid-Infused Porous Surfaces through the LbL Method.

There has been increasing interest in recent years in identifying an ice-removal procedure that is low cost and scalable and consumes a negligible amount of energy in order to prevent catastrophic failures in outdoor structures. One of the potential solutions to the structural problems caused by frigid and icy conditions is the use of slippery liquid-infused porous surfaces (SLIPS) to effect passive ice removal using easy, economical, and energy-free means. This work takes advantage of the highly flexible layer-by-layer (LbL) technology to customize and design surfaces that have a high degree of roughness using negatively and positively charged polyelectrolytes and negatively charged silica nanoparticles (NPs). SEM (scanning electron microscopy) images represent the silica nanoparticles deposition on the surface of the thin film. The roughness of these thin films has been demonstrated by AFM (atomic force microscopy) investigation. The main characteristics of these surfaces are their high contact angle and low water contact angle hysteresis, which is achieved by the fluorinated lubricant that is infused in the pores of the films. The ice adhesion strength of the thin films was measured using a home-built normal mode tensile test in an environmental chamber, which confirmed the icephobicity of the surface as having an adhesion strength of less than 5 kPa, implying that this surface is an excellent candidate for passive removal of ice. The thin films were aged for up to 100 days, and the results showed that the thin film could reduce the ice adhesion strength by 65%, even after this period. The ice adhesion strength of the thin film after icing/deicing cycles showed that 80% of the icephobicity of the thin film had been preserved even after 50 cycles.