WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

A new method for the investigation of porous structures using mercury porosimetry

The influence of wetting angle hysteresis on volume hysteresis and retention of mercury in porous solids was considered. Although different explanations are possible (as explained in the paper), the resulting effect for cylindrical pores in the examined sample of active carbon was that the advancing angle θ_A = 162°, and the effective value of the receding angle θ_R = 90°. The rise of the mercury volume (V_m) intruded in the second run after the preceding reduction of pressure (p) to zero corresponds with a formation of spherical caps at the orifice of pores. The dependence of V_m on p in such a case has been called local hysteresis and it has been used to evaluate the true θ_A, the number of pores with a radius corresponding to the pressure of intrusion and their mean length.     

Wettability studies of reactive brazing alloys on CVD diamond plates

Wettability of both the diamond and the insert surfaces by the filler metal in CVD diamond brazed-on cutting tools is a key condition for good brazing strength. The brazing process of CVD diamond thick plates still has to be improved, namely on the influence of the brazing alloy composition and of the substrate surface finishing quality in wettability. In this study, contact angle measurements were performed in a dedicated high vacuum furnace coupled with a video recording system. Diamond films with different thickness (75<t<300 μm), and thus having distinct grain sizes and roughness, were grown with fixed conditions by the MPCVD technique on Si substrates and chemically detached for wettability experiments. Roughness parameters were evaluated by profilometry and AFM, which was used to observe the grown diamond surfaces of the self-standing films. The reactive Ag–Cu–Ti brazing system was investigated. Results showed a very good wettability in the temperature range 800–850°C, namely for the diamond surface where a minimal contact angle of 10° was reached. A Ti-rich thin reaction layer (0.5–0.8 μm) was detected at the drop side of the substrate/brazing alloy interface in both substrate materials, proving the affinity of Ti to carbon. The influence of the diamond roughness on the contact angle θ_R is notable, obeying a linear dependence of the type cosθ_R=cosθ₀+k cosθ₀·(R_a/G)², where R_a and G, the grain size, are related to asperity height and width, respectively. This relationship is based on the well-known Wenzel equation that correlates the real contact angle to the surface area increasing with roughness.     

The combined effect of roughness and heterogeneity on contact angles: the case of polymer coating for stone protection

The individual effects of heterogeneity and roughness on contact angles have been repeatedly analysed in the literature, but the application of the accepted models to practical situations is often not correctly performed. In the present paper the combined effects of roughness and heterogeneity on the contact angles of water on stone surfaces protected by a hydrophobic polymer coating are considered. Two different kinds of calcareous stone with different surface roughnesses and porosities were protected against the effect of water absorption by two different polymer coatings. The contact angles of water on the protected stone surfaces were measured by the Wilhelmy and the sessile drop techniques. A comparison of the results obtained shows not only the limits of the static sessile drop technique, but also the combined effect of roughness and heterogeneity. Some considerations are developed on the application of commonly accepted models to surfaces with a combination of roughness and heterogeneity. Some other results obtained with techniques such as roughness measurements, mercury porosimetry, energy dispersive X-ray spectroscopy (EDXS), thermogravimetric analysis (TGA), water absorption by capillarity experiments (WAC), all able to show the structure and properties of the obtained films, are also compared with those obtained from contact angle measurements. It is concluded that the static contact angle is not well correlated with the degree of protection; on the contrary, the receding contact angles are well correlated with the degree of protection actually obtained. An ideal protecting agent should have a receding contact angle greater than 90°.     

Laser-Patterned Super-Hydrophobic Pure Metallic Substrates: Cassie to Wenzel Wetting Transitions

A femtosecond laser was used to create microstructures on very pure metal surfaces. The irradiated samples initially showed super-hydrophilic behavior. With time and exposure to ambient air the contact angle increased to about 160° with very low hysteresis. The surfaces supported the Cassie and Wenzel wetting states, depending on the technique used to deposit the water droplets. The created surface morphologies were idealized with a geometric model that is an assembly of densely packed cylindrical pillars with semispherical caps. Using this geometric model for calculation of the surface roughness, a theoretical Young contact angle of about 99° was calculated for all samples from the Wenzel and Cassie–Baxter equations. While the value of 99° significantly differs from the measured hydrophilic contact angles on the polished pure metallic samples, it indicates that a laser-induced surface reaction must be responsible for the evolution of contact angles to super-hydrophobic ones and that this phenomenon is independent of the type of metal.     

Fabrication of hydrophobic Ti3SiC2 surface with micro-grooved structures by wire electrical discharge machining

The near-superhydrophobic Ti₃SiC₂ surfaces with regular and controllable micro-grooved structures were fabricated by wire electrical discharge machining (WEDM). The surface topographies and chemical compositions of smooth and micro-grooved Ti₃SiC₂ surfaces were characterized. The micro removal mechanism of Ti₃SiC₂ in the process of electrical discharge machining was also analyzed. The wetting mechanism of micro-grooved Ti₃SiC₂ surface was discussed along with the static contact angle, anisotropic wettability and contact angle evolution versus time. The relationships between parallel and perpendicular contact angles, depth-width ratio of micro-grooved structures and surface roughness of textured surface were investigated. The experimental results show that the parallel contact angle on the textured Ti₃SiC₂ surface increased by about 164% compared with the one on the smooth surface, and near-superhydrophobic surface with obvious anisotropy was roughly achieved. The experimental parallel contact angles were very close to theoretical contact angles calculated by Cassie-Baxter formula. It is confirmed that the depth-width ratio may be used to predict the parallel contact angle with the average prediction error of 2.4%. The perpendicular contact angles had a good correlation with the depth-width ratio and surface roughness.     

Surface properties of perfluoroalkylethyl acrylate/n-alkyl acrylate copolymers

Wetting behavior of perfluoroalkylethyl acrylate (FA)/n-alkyl acrylate (AA) copolymers with the various length of side chains of the AAs is discussed from a standpoint of surface molecular mobility. The copolymerization reactivity ratio indicates that these polymers are random copolymers. The surface properties were studied by measuring dynamic contact angle, static contact angle and freeze-dried X-ray photoelectron spectroscopy, and the bulk properties by wide-angle X-ray diffraction and differential scanning calorimetry. The advancing contact angles for water were independent of side-chain length of AAs and were almost constant at 120°. We have attributed this phenomenon to the orientation of perfluoroalkyl groups (R_f groups, C_xF_2x+1) in air, which is independent of side-chain length of AAs. On the contrary, the receding contact angles showed small values of about 45° when n numbers below 8 and increased when n numbers above 12. This can be explained as follows. High wettability during the receding process at the n numbers below 8 results from regression of R_f groups at the water–solid interface caused by minimization of the interface free energy. The low wettability during the receding process at the n numbers above 12 shows that R_f groups cannot regress due to its crystallization. This mechanism is also supported by other measurements. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1741–1749, 1999     

Hydrophobic recovery of repeatedly plasma-treated silicone rubber. Part 1. Storage in air

Silicone rubber is used for a wide variety of biomedical and industrial applications due to its good mechanical properties, combined with a hydrophobic surface. Frequently, however, it is desirable to alter the surface hydrophobicity of silicone rubber. Often this is done by plasma treatments but the effects are usually transient. In this study, surfaces of medical grade silicone rubber have been repeatedly modified by means of oxygen, argon, carbon dioxide, and ammonia RF plasma treatments with a 24 h time interval in between treatments. Treated samples were stored in air prior to surface characterization by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), streaming potential measurements, and profilometry for surface roughness. The carbon percentage of the surfaces decreased after plasma treatment, while the silicon and oxygen percentages increased irrespective of the plasma used. The formation of Si-O-Si bridges between siloxane chains after plasma treatment was demonstrated by the appearance of a new component in the Si_2p peak but the degree to which this occurred differed per gas. Streaming potential measurements in a 10 mM potassium phosphate buffer indicated a more negatively charged surface for treated samples compared to untreated samples (-23.3 mV at pH 7.0). Surface roughness increased slightly for repeatedly plasma-treated samples from R_A = 0.35 μm to R_A = 0.46 μm, while scanning electron microscopy showed the presence of several 'cracks' spanning the surface after repeated treatment. Argon, carbon dioxide, and ammonia plasmas significantly reduced the advancing water contact angle from 115° to 58°, 72°, and 85°, respectively, on a more permanent basis (especially when the treatments were repeated after recovery). Oxygen plasma effects on water contact angles generally disappeared within 5 h, also after repeated treatment.     

Hysteresis in porosimetry

On a smooth surface of SiO₂ in air, hysteresis of the wetting angel of mercury was found. In an evacuated system of controlled pore glass this hysteresis is even greater and is the cause of volume hysteresis in mercury intrusion porosimetry. Pores with diverging radius along their length also cause volume hysteresis. In this case the slope of the walls in the conical parts of the pore is of great importance. This is illustrated for selected models. The procedure for the evaluation of the volume of ink-bottle pores, previously suggested in the literature, cannot be considered a correct, therefore. The values of the surface tension of mercury, λ_L, of the advancing wetting angle θ_A, and of the receding angle, θ_R, depend on the radii of the pores. Their values have been evaluated. Under the condition of mercury extrusion the line tension τ plays a role: its value, τ = 236 × 10^?9 mN, has been determined.     

Preparation and characterization of superhydrophobic FEP-Teflon surfaces

Abstraet-Supcrhydrophohic FEP-Teflon was prepared by argon ion etching followed by oxygen glow discharge treatment of commercially available FEP-Teflon sheet material. This combined treatment yielded an increase in water contact angle from 109° to > 140°. Ion etching alone caused a small increase in surface roughness and a loss of fluorine from the surface, but the water contact angles increased only to 120°. Scanning electron micrographs of ion-etched surfaces showed stalky protrusions with a diameter of approximatcly 40 nm. Glow discharge treatment of ion-etched surfaces reduced the length of these protrusions and therewith the microscopic surface roughness. However, in all cases the macroscopic surface roughness was less than 1 um (R_A value). X-Ray photoelectron spectroscopy indicated major changes in elemental surface composition as a result of the treatments. These modifications did not influence the infrared absorption spectra (attenuated total reflection) of the modified surfaces, indicating that the chemical changes brought about are really superficial. It is concluded that the superhydrophobicity created is mainly due to changes in the specific, microscopic surface topography resulting from ion etching, but also partly due to the role of the glow discharge treatment, restoring a high surface concentration of fluorine after ion etching.     

The wettability of industrial surfaces: Contact angle measurements and thermodynamic analysis

The water wettability of surfaces, whose surface conditions are comparable to those used in heat and mass transfer equipment, has been investigated experimentally and theoretically.In the first part, results of contact angle measurements for water on metal and non-metal surfaces are reported. With hydrophobic non-metal surfaces (e.g. Teflon) water forms large advancing and receding contact angles, and the contact angle hysteresis is small. Surface contamination is of minor influence. Hydrophilic metal surfaces (copper, nickel) are completely wetted by water only if the surfaces are extremely clean. Surface contamination reduces the wettability drastically. Under most industrial conditions advancing contact angles between 40° and 80°, and receding contact angles smaller than 20° can be expected, and the contact angle hysteresis is large. Corrosion can enhance the water wettability.In the second part, a thermodynamic analysis of the wetting of heterogeneous surfaces is presented. Equilibrium considerations for a model surface consisting of two components of different wettability provide the advancing and receding contact angles for a heterogeneous surface as a function of the equilibrium contact angles, surface fractions, and the distribution function of the two components. The advancing and receding contact angles as well as all the intermediate contact angles indicate metastable states of equilibrium of the system. The results of the model calculations give a physically based explanation for the characteristic wetting behaviour of industrial surfaces found experimentally.     

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.     

Hydrophobic and superhydrophobic surfaces fabricated by plasma polymerization of perfluorohexane,perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene)

Fluoropolymer films were deposited on silicon (1 0 0) wafers, glass, epoxy, and hierarchical dual-sized filler epoxy composite surfaces by plasma polymerization of perfluorohexane, perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene). The procedure involved continuous wave plasma-enhanced deposition, followed by a discharge-off period, with the monomer gas feed maintained. Silanization of silicon wafers and glass surfaces with triethoxyvinylsilane was employed to improve plasma fluoropolymer bonding to these substrates. The presence of double bonds in perfluoro(2-methylpent-2-ene) and perfluoro(4-methylpent-2-ene) was found to influence fluoropolymer coating topography, thereby increasing surface roughness in modified glass and epoxy substrates. All fluorocarbons provided a similar level of hydrophobization of flat substrates, exhibited by water contact angles (WCA) of about 110°. Hydrophobization of nanocomposite hierarchical surfaces by plasma polymerization provided superhydrophobic surfaces, with WCA of 160° and contact angle hysteresis below 8°.     

Instability of the three-phase contact region and its effect on contact angle relaxation

Contact angle relaxation was measured for captive air bubbles placed on solid surfaces of varying degrees of heterogeneity, roughness, and stability, in water. The experimental results indicate that both advancing and receding contact angles undergo slow relaxation in these water-air-solid systems, due to instabilities of the three-phase contact line region. It is shown that the advancing contact angle decreases and the receding contact angle increases for many systems over a period of a few hours. Also, examples of reverse progressions are reported. Additionally, in extreme cases, the contact angle oscillates down and up, over and over again, preventing the system from stabilization/equilibration. Four different mechanisms are proposed to explain the contact angle relaxation. These include (i) pinning of the three-phase contact line and its slow evolution; (ii) the formation of microdroplets on the solid surface and their coalescence with the base of the gas bubble, which causes dynamic behavior of the three-phase contact line; (iii) deformation of the solid surface and its effect on the apparent contact angle; and (iv) chemical instability of the solid.     

The Porous Phase Barrier and Crystallization

Abstract

A porous body can be used as a phase barrier, or to safely store or transmit a metastable fluid phase (B) under certain definable conditions even when a stable phase (A) outside the porous body is in intimate contact with the metastable phase inside the body. The condition to be met can be expressed by

R < 2σV _A [(S _B -S _A ) cos θ _A δT]

where R is effective (“cylindrical”) radius of the pores; δT is degrees of supercooling (or superheating); σ is specific surface free energy of the phase boundary; θ _A is the contact anglen of Phase A with the material of the porous body; V and S are partial molal volume and entropy of the indicated phases, respectively. Phase B, the “metastable” phase by conventional test, is found to be the stable phase so long as it remains confined within sufficiently small pores. If the “metastable” phase (B) is a supercooled liquid, strongly adsorbed by the porous material (θ _A > 90°), Phase A can be crystalline, as demonstrated by the natural process of frost heaving of soil. This implies new methods of managing crystallization processes, including one whereby saline water is purified by an “ice sandwich” that sustains reverse osmosis and another whereby components of a binary eutectic mixture may be completely separated.     

Contact Angle Hysteresis on Polymer Surfaces: An Experimental Study

In order to characterize a solid surface, the commonly used approach is to measure the advancing and receding contact angles, i.e., the contact angle hysteresis. However, often an estimate of the average wettability of the solid–liquid system is required, which involves both the dry and wetted states of the surface. In this work, we measured advancing and receding contact angles on six polymer surfaces (polystyrene, poly(ethylene terephthalate), poly(methyl methacrylate), polycarbonate, unplasticized poly(vinyl chloride), and poly(tetrafluoroethylene)) with water, ethylene glycol and formamide using the sessile drop and captive bubble methods. We observed a general disagreement between these two methods in the advancing and receding contact angles values and the average contact angle determined separately by each method, although the contact angle hysteresis range mostly agreed. Surface mobility, swelling or liquid penetration might explain this behaviour. However, we found that the 'cross' averages of the advancing and receding angles coincided. This finding suggests that the cross-averaged angle might be a meaningful contact angle for polymer–liquid systems. Hence, we recommend using both the sessile drop and captive bubble methods.     

Microsphere tensiometry to measure advancing and receding contact angles on individual particles

In this paper, a method to measure the advancing and receding contact angles on individual colloidal spheres is described. For this purpose, the microspheres were attached to atomic force microscope cantilevers. Then the distance to which the microsphere jumps into its equilibrium position at the air-liquid interface of a drop or an air bubble was measured. From these distances the contact angles were calculated. To test the method, experiments were done with silanized silica spheres (4.1 μm in diameter). From the experiments with drops, an advancing contact angle of 101 ± 4° was determined. A receding contact angle of 101 ± 2° was calculated from the jump-in distance into a bubble. Both experimental techniques gave the same contact angle. In contrast, on similarly prepared planar silica surfaces, a clear hysteresis was measured with the sessile drop method; contact angles of 104.5 ± 1° and 93.8 ± 1° were determined for the advancing and receding contact angles, respectively.     

Comparative analysis of the dynamic contact angles for two types of superhydrophobic layered double hydroxide film surfaces

The wetting properties of two types of superhydrophobic layered double hydroxide (LDH) films modified by treatment with laurate (La) anions, film and ZnAl-La-LDH film, have been investigated using the sessile drop and Wilhelmy plate methods. The spreading kinetics of water droplets on ZnAl-La-LDH film surfaces was fitted by a power equation, and an effective lubrication coefficient was calculated. Unlike one-cycle measurements reported in a number of earlier studies, cycling contact angle measurements using the Wilhelmy plate method provide more information about the mechanism of contact angle hysteresis. Both the advancing contact angles (θ_a) and the receding contact angles (θ_r) of film—in which the La anions are adsorbed on the surface of the LDH—obtained in different cycles were found to be independent of cycle number. In contrast, for the ZnAl-La-LDH film—in which the La anions are intercalated into the interlayer galleries of the LDH—the advancing contact angles (θ_a) and the receding contact angles (θ_r) obtained in different cycles were found to vary. It was found that both θ_a and θ_r decreased with increasing number of cycles. The different contact angle hysteresis behaviors were qualitatively and quantitatively investigated by means of dynamic contact angle analysis. The results suggest that contact angle hysteresis of and ZnAl-La-LDH films can be ascribed to dynamic hysteresis and intrinsic hysteresis, respectively.     

Plasma surface treatments and biodegradation of poly(butylene succinate) sheets

The sheets prepared by the extrusion of the melt of poly(butylene succinate) were treated with inorganic gas plasmas. Bionolle, the commercially available polyester, was also used, and the treatment effects were compared. Plasma susceptibility by the continuous plasma of 13.56 MHz and by pulsed plasmas was evaluated by the weight loss rates by etching. Advancing and receding contact angles of water (θ_a , θ_r ) on the plasma‐treated sheets were obtained by the Wilhelmy method. Decay of hydrophilicity was considerable in θ_a , but θ_r was less changed. The biodegradation was examined by the preliminary soil‐burial tests. The polymer sheets were biologically degraded, and the characteristic morphology appeared on the surface according to the SEM observation. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1121–1129, 2000