High Contact Angle Hysteresis Research Articles

Surface Properties of Superhydrophobic Coatings for Stone Protection

Superhydrophobic films are produced by a simple and low cost method. Silica (SiO2) nanoparticles are dispersed in solutions of Rhodorsil 224, a commercial poly(alkyl siloxane) which is used for the protection of outdoor cultural heritage objects, and the suspensions are sprayed on glass surfaces. It is shown that the siloxane-nanoparticle composite films prepared from dispersions of high particle concentrations (≥ 0.5% w/v) exhibit superydrophobic properties (high static contact angle and small hysteresis) which can be rationalized by the Cassie-Baxter model, according to quantitative measurements obtained by SEM images. Siloxane-nanoparticle films are then deposited (sprayed) on “Opuka”, a fine-grained argillite which was used for the restoration of the castle of Prague. It is shown that the treated stone surfaces exhibit superydrophobic properties, similar to the treated glass surfaces. The efficacy of the superhydrophobic films to protect Opuka is evaluated by performing water contact angle, water capillary absorption, water vapor permeability and colorimetric measurements. It is shown that the use of nanoparticles in the protective coating has a positive effect on the results of the aforementioned tests, except for the colorimetric measurements.

High contact angle hysteresis of superhydrophobic surfaces: Hydrophobic defects

A typical superhydrophobic surface is essentially nonadhesive and exhibits very low water contact angle (CA) hysteresis, so-called Lotus effect. However, leaves of some plants such as scallion and garlic with an advancing angle exceeding 150° show very serious CA hysteresis. Although surface roughness and epicuticular wax can explain the very high advancing CA, our analysis indicates that the unusual hydrophobic defect, diallyl disulfide, is the key element responsible for contact line pinning on allium leaves. After smearing diallyl disulfide on an extended polytetrafluoroethylene (PTFE) film, which is originally absent of CA hysteresis, the surface remains superhydrophobic but becomes highly adhesive.

Dual hierarchical biomimic superhydrophobic surface with three energy states

A low hysteresis surface prepared by two-length-scaled hierarchical textures to mimic the Lotus effect is proposed. The fabricated textures incorporate self-masked nanorods on microextrusions. A high static contact angle (160°) and low hysteresis (∼2.7°) are obtained and comparable to the surface properties of a natural lotus leaf. The stability of hydrophobicity is described with respect to three energy states (nonwetting, microwetting, and nanowetting) based on dynamic contact angle analysis by droplet impinging onto the surface. The estimated texture-induced energy barrier based on the principle of energy conservation is in good agreement to those estimated from Laplace’s law.

Micro-, nano- and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion

Superhydrophobic surfaces exhibit extreme water-repellent properties. These surfaces with high contact angle and low contact angle hysteresis also exhibit a self-cleaning effect and low drag for fluid flow. Certain plant leaves, such as lotus leaves, are known to be superhydrophobic and self-cleaning due to the hierarchical roughness of their leaf surfaces. The self-cleaning phenomenon is widely known as the 'lotus effect'. Superhydrophobic and self-cleaning surfaces can be produced by using roughness combined with hydrophobic coatings. In this paper, the effect of micro- and nanopatterned polymers on hydrophobicity is reviewed. Silicon surfaces patterned with pillars and deposited with a hydrophobic coating were studied to demonstrate how the effects of pitch value, droplet size and impact velocity influence the transition from a composite state to a wetted state. In order to fabricate hierarchical structures, a low-cost and flexible technique that involves replication of microstructures and self-assembly of hydrophobic waxes is described. The influence of micro-, nano- and hierarchical structures on superhydrophobicity is discussed by the investigation of static contact angle, contact angle hysteresis, droplet evaporation and propensity for air pocket formation. In addition, their influence on adhesive force as well as efficiency of self-cleaning is discussed.

Ultralow contact angle hysteresis and no-aging effects in superhydrophobic tangled nanofiber structures generated by controlling the pore size of a 99.5% aluminum foil

Superhydrophobic surfaces designed to improve hydrophobicity have high advancing contact angles corresponding to the Cassie state, but these surfaces also exhibit high contact angle hysteresis. We report here a simple and inexpensive method for fabricating superhydrophobic tangled nanofiber structures with ultralow contact angle hysteresis and no-aging degradation, based on a widening process. The resulting nanostructures are suitable for diverse applications including microfluidic devices for biological studies and industrial self-cleaning products for automobiles, ships and houses.

Tunability of the Adhesion of Water Drops on a Superhydrophobic Paper Surface via Selective Plasma Etching

We report the fabrication of a sticky superhydrophobic paper surface with extremely high contact angle hysteresis: advancing contact angle ∼150° (superhydrophobic) and receding contact angle ∼10° (superhydrophilic). In addition, we report the controlled tunability of the contact angle hysteresis from 149.8 ± 5.8° to 3.5 ± 1.1°, while maintaining superhydrophobicity, as defined through an advancing contact angle above 150°. The hysteresis was tuned through the controlled fabrication of nano-scale features on the paper fibers via selective plasma etching. The variations in contact angle hysteresis are attributed to a transition of the liquid–surface interaction from a Wenzel state to a Cassie state on the nano-scale, while maintaining a Cassie state on the micro-scale. Superhydrophobic cellulosic surfaces with tunable stickiness or adhesion have potential applications in the control of aqueous drop mobility and the transfer of drops on inexpensive, renewable substrates.

Reversible electrowetting on silanized silicon nitride

Electrowetting is a phenomenon that has received increasing attention in recent years. It offers much promise for applications in lab-on-a-chip devices, liquid lenses, and optical displays. The contact angle can be controlled electrically facilitating the transport of liquid droplets by electrocapillary forces. This paper focuses on the requirements of the dielectric layers in electrowetting on dielectric; firstly, to provide the required dielectric qualities, that is, sufficient dielectric strength and high capacitance and, secondly, to satisfy the surface wetting characteristics, that is, sufficiently high hydrophobicity and low contact angle hysteresis. We present experimental work that, firstly, compares the dielectric characteristics of several insulating layers and secondly, proposes silanization as a way to form hydrophobic layers for electrowetting. The relation between electrowetting and dielectric breakdown was tested by means of measuring the contact angle of a water droplet placed between two electrodes on an insulated surface. The leakage current and resistance between these electrodes were measured as a function of applied voltage for the following dielectric coatings: silicon dioxide, silicon nitride and Parylene C. We compare the contact angle response of electrowetting on octadecyltrichlorosilane (OTS) monolayers and plasma-enhanced chemical vapour deposition Teflon-like fluoropolymer layers. The contact angle hysteresis of the OTS monolayers is clearly lower and this results in reversible electrowetting, that is, the hydrophobicity is sufficient to avoid contact line pinning and dewetting occurs on removal of an applied potential.

Energy transitions in superhydrophobicity: low adhesion, easy flow and bouncing

The concept of superhydrophobicity was introduced in the 1990s as a result of theinvestigation of the microstructure of extremely water-repellent plant leaves. Since thattime, artificial superhydrophobic surfaces have been developed and implemented,stimulated by advances in nanotechnology, and giving one of the most successful examplesof a bio-inspired technology transferred into engineering applications. Superhydrophobicityis usually defined as the ability of a surface to have (i) a very high water contact angle (CA)and (ii) low CA hysteresis. Here we argue that the ability of a water droplet to bounce off asurface constitutes a third property that is crucial for applications. Furthermore,this property is naturally related to the first two properties, since the energybarriers separating the ‘sticky’ and ‘non-sticky’ states needed for bouncing dropletshave the same origin as those needed for high CA and for low CA hysteresis.

Facile preparation of superhydrophobic coatings by sol–gel processes

Different organic/inorganic compositions and deposition methods were used to prepare superhydrophobic surfaces using metal alkoxides and the sol–gel process. Both surface roughness and composition had to be adjusted in order to obtain very high contact angles and low contact angle hysteresis as a necessary requirement for superhydrophobicity. Multilayer samples with a fluorinated organic–inorganic top layer showed water contact angles of about 157° with low hysteresis (2°). Water drops rolled easily off their surface at a tilt angle as low as 4°.

Effect of Contact Angle Hysteresis on the Measurement of Capillary Forces

We conduct experimental investigations of macroscopic capillary forces between two flat rigid substrates characterized by their advancing and receding contact angles with water. Our results exhibit excellent agreement with theoretical predictions obtained by the numerical solution of the capillary equation. On the basis of this comparison, we use the measurements of the capillary force to investigate the phenomenon of contact angle hysteresis. We present examples of force measurements for surfaces that display low, moderate, and high contact angle hysteresis and compare results for a larger variety of substrates. Finally, we show that for the case of water, the role of viscosity is insignificant within the range of force and velocity measured in the present work.

Understanding of sliding and contact angle results in tilted plate experiments

Tilted plate experimental methodology is studied in order to address misinterpretations and omissions that exist in the literature. The effect of methodology on subsequent sliding angle measurements is quantitatively assessed in two parts. The first part is a comparison of drop placement techniques in the conventional tilted plate method (drops placed on a level surface, then inclined). The second part is a comparison between the conventional tilted plate method and a modified method, in which the drop is placed on a surface that is already inclined. The first part of the study involved water drops placed on alkyl ketene dimmer (AKD) surfaces using a needle from above as well as below the surface (through a hole). For drops placed from below, three different drop locations (with respect to the hole in the surface) are included in the study. It was found that the drop placement technique had a statistically significant impact on sliding angle measurements, and the most consistent and unbiased measurements resulted when the drop was placed from below, with the hole leading (downhill from) the drop. The second part of the study involved water drops on both AKD (high contact angle hysteresis system) and fluorinated silicon (low contact angle hysteresis) surfaces. For either system, it was shown that sliding angles were lower when drops were placed on surfaces that were already inclined. Since the maximum and minimum contact angles were identical between the two experimental methods, the difference in adhesion was due to variation in contact line shape. The results of this study show that errors of 50–60% would result from using conventional tilted plate measurements to predict droplet mobility on fixed, inclined surfaces, such as those found in condensers. The important distinction between repellency (measured by the advancing contact angle) and drop mobility is also discussed. Further, it was observed that the advancing and receding contact angles do not always correspond to the maximum and minimum contact angles observed in a tilted plate experiment.

PREPARATION AND CHARACTERIZATIONS OF PTFE GRADIENT NANOSTRUCTURE ON SILK FABRIC

Superhydrophobic materials have been extensively studied because of their wonderful array of properties and applications. In this study, normal and superhydrophobic surface of silk fabric have been prepared via deposition of different shapes of PTFE nanostructure using magnetron sputter coating. The effects of PTFE sputter coating on surface morphology and surface chemical properties were characterized using atomic force microscopy (AFM) and ATR-FTIR (attenuated total reflection-Fourier transform infrared spectroscopy). The wettability of the fabric was characterized through measuring the surface contact angle by drop shape analysis apparatus and dynamic contact angle by Wilhelmy technique. As evaluated by water contact angle measurements, all the treatments resulted in a significant enhancement in the hydrophobicity of silk fabric, while larger sputtering pressures brought bigger PTFE nanoparticles, which led to higher contact angles. The results have also revealed that alternant working pressures, could bring gradient nanostructures which generated both high contact angle and less contact angle hysteresis.

Preparation of hybrid film with superhydrophobic surfaces based on irregularly structure by emulsion polymerization

A superhydrophobic surface originated from quincunx-shape composite particles was obtained by utilizing the encapsulation and graft of silica particles to control the surface chemistry and morphology of the hybrid film. The composite particles make the surface of film form a composite interface with irregular binary structure to trap air between the substrate surface and the liquid droplets which plays an essential role in obtaining high water contact angle and low water contact angle hysteresis. The water contact angle on the hybrid film is determined to be 154 ± 2° and the contact angle hysteresis is less than 5°. This is expected to be a simple and practical method for preparing self-cleaning hydrophobic surfaces on large area.

Multiscale Roughness and Stability of Superhydrophobic Biomimetic Interfaces

The stability of a composite interface of roughness-induced superhydrophobic surfaces is studied. To have high contact angle and low contact angle hysteresis, superhydrophobic surfaces should be able to form a composite interface with air pockets in the valleys between asperities (pillars). However, the composite interface may be unstable and can be irreversibly transformed into a homogeneous interface. We formulate a stability criterion and analyze the stability of the composite interface for several typical roughness profiles. To resist destabilizing mechanisms, multiscale (hierarchical) roughness is required. Such multiscale roughness is found in natural and artificial superhydrophobic surfaces.

Emergence of superhydrophobic behavior on vertically aligned nanocone arrays

The authors investigate the properties of water drops on uniform arrays of vertically aligned nanocones. A simple model predicts a variety of behaviors depending on the nanocone aspect ratio. At low aspect ratios, surface tension pulls the water into the nanocone array, resulting in a wetted drop with high contact angle hysteresis. At high aspect ratios, the effect of the surface tension is reversed, resulting in little liquid-solid contact and drops which easily slide off the surface. Experiments show that the transition occurs as a gradual change in drop contact angles, but an abrupt change in sliding angles.

Adhesion and Wetting Hysteresis of a Metal (Mercury) on an Oxide Glass in Air and Nitrogen

In general, oxygen can considerably change the wetting behavior of oxides by molten metals. This work is a basic illustration of the oxidation effect on the wetting behavior of glass by a liquid metal. Taking mercury as a model of a metal, the importance of the metal oxidation in the glass/metal interaction has been observed by measuring wetting contact angles of mercury on glass and the ability of calibrated mercury drops to slide down, under gravitation, on an inclined glass plate in air or in nitrogen. It is believed that the highest force of detachment by sliding of the mercury drop in air results from the metal oxidation which can be interpreted by a higher contact angle hysteresis when the metal is exposed to air.

Surfactant induced Marangoni motion of a droplet into an external liquid medium

We report in this paper a particular case of the Marangoni effect—the migration of a squalane droplet in the external water medium, driven by the asymmetric adsorption of the surfactant at one side of the droplet. The specific features of this experiment compared to similar reports on drop motion lies in both the presence of the external liquid and the existence of two “discrete” profile and velocity regimes during the migration of the drop. The amazing dynamic profile of the droplet (cometlike) observed in the earlier stage of the motion was related to the strong transient pinning of the rear side of the drop where adsorption sets in. This strong retention preceding the drop motion was related to the external hydrostatic pressure of the environmental fluid which, under the particular geometry of nonsymmetric diffusion, results in a higher contact angle hysteresis (compared to that in air), before the drop can move. As shown by these results, the whole macroscopic behavior of this drop migration, including the pinning strength, the magnitude and velocity of the motion is governed by the viscosity, the external hydrostatic pressure and the pressure within the thin wetting film on the rear side of the drop. Using a simplified hydrodynamic approach, an estimated average velocity of ∼1.5 mm/s was found, which compares well to measured ones, especially in the earlier stage of the motion where the drop retains the comet like profile. In addition, some preliminary results are briefly presented, which cleary show how both the velocity and amplitude of the migration, as well as the characteristics of the thin film on the rear side depend on the drop size and viscosity.

Fluid shear induced endothelial cell detachment from modified polystyrene substrata

In this study, human umbilical vein endothelial cells were seeded for 3 and 24 h on polystyrene (PS), tissue culture polystyrene (TCPS) and polystyrene modified by oxygen plasma treatment (PtPS) in order to investigate their detachment behaviour during exposure to fluid shear. All three materials have smooth surfaces at the submicron level. Equilibrium water contact angles were higher on PS (88°) than on TCPS (78°) and PtPS (79°). Furthermore, contact angle hysteresis, i.e. the difference between advancing and receding angles, was much larger on TCPS (39°) and PtPS (41°) than on PS (14°), indicating either a large surface heterogeneity or a possible reorientation of functional groups on TCPS and PtPS. X-ray photoelectron spectroscopy data revealed that both TCPS and PtPS had oxygen incorporated at their surfaces (18.6% and 9.9%, respectively), whereas only TCPS had a small amount of nitrogen (0.9%) at its surface. Cells seeded on these materials were exposed to various shear stresses in the range 88–352 dyn cm −2 in order to gain more insight into the influence of the specific physico-chemical surface properties of these polystyrene surfaces on cell retention, cell morphology and migration. The retention of cells having adhered during 24 h on TCPS and PtPS was not different from the retention of cells having adhered during 3 h, but retention on TCPS and PtPS was higher than on PS. At shear stresses of 88 and 176 dyn cm −2, however, differences in the morphology of cells adhered to TCPS during 3 and 24 h were observed, indicating that between 3 and 24 h changes in cell-substratum interactions occurred. Migration on TCPS and PtPS was accompanied by the presence of fibrillike structures left behind on the surface. Summarizing, this study shows that cell retention is higher on modified polystyrenes than on polystyrene, the only major difference between the polystyrenes being the higher contact angle hysteresis and oxygen content of the modified polystyrene surfaces.