Cassie-Baxter Theory Research Articles

Fabrication of superhydrophobic surfaces on Mg alloy substrates via primary cell corrosion and fluoroalkylsilane modification

AbstractThe present work reports a simple and safe two‐step process to render magnesium (Mg) alloy surfaces superhydrophobic via primary cell corrosion and subsequently cover it with a fluoroalkylsilane (FAS) film. The surfaces were characterized by scanning electron microscopy (SEM), optical microscopy, energy‐dispersive X‐ray spectroscopy (EDS), Fourier‐transform infrared spectrophotometry (FTIR), X‐ray diffraction (XRD), and optical contact angle measurements. The power generated via the primary cell corrosion of copper and Mg alloys was also measured using a digital multimeter. The results show that micro/nanometer‐scale binary rough structures and an FAS film with a low surface energy were present on the Mg alloy surfaces, both of which confer good superhydrophobicity with a water contact angle of 162.8° and a tilting angle of 2°. The micro/nanometer‐scale binary rough structures consisted of micrometer‐scale grains, cluster‐like structures composed of nanometer‐scale needles, and network‐like structures composed of nanometer‐scale sheets. Superhydrophobicity was analyzed by the Cassie–Baxter theory. Findings show that only about 6.3% of the water surface was in contact with the Mg alloy substrates, while the remaining 93.7% was in contact with the air cushion. The unique advantage of the proposed method is that power can be generated during the machining process of the superhydrophobic surfaces on the Mg alloy substrates.

Versatile Superhydrophobic Surfaces from a Bioinspired Approach

We report the surface wettability and morphology of conductive polymer films obtained by electrodeposition of poly(3,4-ethylenedioxythiophene) (i.e., PEDOT) derivatives containing an alkyl chain (PEDOTHn with n = 4, 6, 8, 10, and 12) or a phenyl (PEDOTPh) group in the 2-position. Even if the general approach in the literature is the use of highly fluorinated tail to reach superhydrophobic materials, we point out that versatile surfaces (from hydrophilic to superhydrophobic) can be obtained, in one step, without any fluorinated chemistry. Indeed, superhydrophobic surfaces were formed, using Bu4NPF6 and acetonitrile as electrolyte, by electrodeposition of PEDOT derivatives substituted with n-C10H21 and n-C12H25 chains, while the polymer films substituted with n-C8H17 and n-C6H13 were hydrophobic and those substituted with n-C4H9 and phenyl were hydrophilic. In the PEDOTHn series, the polymer films were structured only from n-C8H17, which proves the influence of long alkyl chains on both the surface wettability and the surface morphology due to the increase in polymer insolubility. By changing acetonitrile by dichloromethane, as solvent of electropolymerization, it is possible to produce smooth surfaces and as a consequence to determine the chemical and physical parts of the contact angles as well as roughness factors or air fractions following the Wenzel and Cassie–Baxter theories. By changing the salt for the electrodeposition, it also was possible to reach superhydrophobic surfaces even with short alkyl chains as well as without alkyl chains but using perfluorinated salts. In this work the mechanism to reach structured surfaces is also discussed. This work contributes to the formation of bioinspired superhydrophobic surfaces.

Preparation and characterization of superhydrophobic surfaces based on hexamethyldisilazane-modified nanoporous alumina.

Superhydrophobic nanoporous anodic aluminum oxide (alumina) surfaces were prepared using treatment with vapor-phase hexamethyldisilazane (HMDS). Nanoporous alumina substrates were first made using a two-step anodization process. Subsequently, a repeated modification procedure was employed for efficient incorporation of the terminal methyl groups of HMDS to the alumina surface. Morphology of the surfaces was characterized by scanning electron microscopy, showing hexagonally ordered circular nanopores with approximately 250 nm in diameter and 300 nm of interpore distances. Fourier transform infrared spectroscopy-attenuated total reflectance analysis showed the presence of chemically bound methyl groups on the HMDS-modified nanoporous alumina surfaces. Wetting properties of these surfaces were characterized by measurements of the water contact angle which was found to reach 153.2 ± 2°. The contact angle values on HMDS-modified nanoporous alumina surfaces were found to be significantly larger than the average water contact angle of 82.9 ± 3° on smooth thin film alumina surfaces that underwent the same HMDS modification steps. The difference between the two cases was explained by the Cassie-Baxter theory of rough surface wetting.

Rapid fabrication of superhydrophobic surfaces on copper substrates by electrochemical machining

Hierarchical micrometer-nanometer-scale binary rough structures were fabricated on copper substrates by electrochemical machining in a neutral NaCl electrolyte. The rough structures are composed of the micrometer scale potato-like structures and the nanometer scale cube-like structures. After modified by the fluoroalkylsilane, the copper surfaces reached superhydrophobicity with a water contact angle of 164.3° and a water tilting angle less than 9°. This method has a high processing efficiency which can take just 3 s to fabricate the roughness required by the superhydrophobic surface. The effect of the processing time on wettability of the copper surfaces was investigated in this paper. The possible mechanism of the formation of the hierarchical roughness was also proposed, and the wettability of the copper surfaces was discussed on the basis of the Cassie–Baxter theory.

Wettability versus roughness of engineering surfaces

Wetting of real engineering surfaces occurs in many industrial applications (liquid coating, lubrication, printing, painting, …). Forced and natural wetting can be beneficial in many cases, providing lubrication and therefore reducing friction and wear. However the wettability of surfaces can be strongly affected by surface roughness. This influence can be very significant for static and dynamic wetting [1]. In this paper authors experimentally investigate the roughness influence on contact angle measurements and propose a simple model combining Wenzel and Cassie–Baxter theories with simple 2D roughness profile analysis. The modelling approach is applied to real homogeneous anisotropic surfaces, manufactured on a wide range of engineering materials including aluminium alloy, iron alloy, copper, ceramic, plastic (poly-methylmethacrylate: PMMA) and titanium alloy.

Stroke Asymmetry of Tilted Superhydrophobic Ion Track Textures

The stroke asymmetry of contact angles of water drops on tilted hydrophobic textures is demonstrated, obtained by ion track etching followed by a hydrophobic treatment. Preliminary trends concerning the advancing and receding contact angles are established, each with and against stroke direction. In rough agreement with Cassie-Baxter theory, the cosines of these four contact angles depend linearly on the wetted area fraction. The etched tracks are randomly distributed on the surface of polycarbonate disks and inclined by 30 degrees with respect to the surface, whereby the aspect ratio of individual etched cones is larger than 10. The morphology of the resulting surface is characterized by randomly shaped flat tops overhanging on one side and gradually falling off on the other side. The area fraction of the supporting tops can be calculated from the number of impinging ions per unit area and the cross section of the etched ion tracks. The top layer of the texture consists of flat, horizontal, irregularly shaped tops supporting water drops in the Cassie-Baxter state. With increasing etching time, the texture becomes increasingly clefted. To fabricate the textures, we irradiated polycarbonate with 5 x 10(7) (80)Br(7+) ions/cm(2) of 30 MeV total energy (having a range of about 20 microm in polycarbonate) at a tilt angle of 30 degrees with respect to the sample surface and etched the latent ion tracks selectively. The textured surface is made hydrophobic by carbondifluoride radicals (CF(2)) resulting from the decay of octafluorocyclobutane, C(4)F(8), in a plasma reactor. The goal of the report is to show that the tilt orientation of a superhydrophobic surface leads to advancing and receding contact angles depending on the orientation with and against the stroke direction. In addition, a rotating movement is demonstrated qualitatively by floating a rotationally asymmetric disk on an ultrasonic bath, similarly treated after an irradiation with (1.2 +/- 0.4) x 10(7) (129)Xe(27+) ions/cm(2) of 8.3 MeV/nucleon at an angle of 45 degrees, whereby the superhydrophobic side of the disk points downward to the water of the ultrasonic bath.

Atomic Layer Deposition as Pore Diameter Adjustment Tool for Nanoporous Aluminum Oxide Injection Molding Masks

The wetting properties of polypropylene (PP) surfaces were modified by adjusting the dimensions of the surface nanostructure. The nanostructures were generated by injection molding with nanoporous anodized aluminum oxide (AAO) as the mold insert. Atomic layer deposition (ALD) of molybdenum nitride film was used to control the pore diameters of the AAO inserts. The original 50-nm pore diameter of AAO was adjusted by depositing films of thickness 5, 10, and 15 nm on AAO. Bis(tert-butylimido)-bis(dimethylamido)molybdenum and ammonia were used as precursors in deposition. The resulting pore diameters in the nitride-coated AAO inserts were 40, 30, and 20 nm, respectively. Injection molding of PP was conducted with the coated inserts, as well as with the non-coated insert. Besides the pore diameter, the injection mold temperature was varied with temperatures of 50, 70, and 90 degrees C tested. Water contact angles of PP casts were measured and compared with theoretical contact angles calculated from Wenzel and Cassie-Baxter theories. The highest contact angle, 140 degrees , was observed for PP molded with the AAO mold insert with 30-nm pore diameter. The Cassie-Baxter theory showed better fit than the Wenzel theory to the experimental values. With the optimal AAO mask, the nanofeatures in the molded PP pieces were 100 nm high. In explanation of this finding, it is suggested that some sticking and stretching of the nanofeatures occurs during the molding. Increase in the mold temperature increased the contact angle.

Effects of geometrical characteristics of surface roughness on droplet wetting

Surface roughness is known to alter the wettability on a solid substrate. In general, either Wenzel or Cassie-Baxter theory is adopted to describe the apparent contact angle. Following the minimum free energy pathway associated with the imbibition process, we have derived a generalized expression for the apparent contact angle on a textured surface and the liquid-gas contact area within the groove that plays a key role. Depending on the geometrical characteristics of the grooves, the surface wetting falls into three regimes: (i) single stable state which is either Wenzel (completely wetted roughness) or Cassie-Baxter (completely nonwetted roughness) state, (ii) two stable states (Wenzel and Cassie-Baxter) separated by an energy barrier, and (iii) single stable state with partially wetted roughness. The sufficient condition for each regime is derived and several groove geometries are given to show the free energy path. Alteration in the geometric parameters may lead to the wetting crossover. We also show that the Cassie-Baxter can occur at a hydrophilic surface for particular pore shapes.

Effect of Three-Phase Contact Line Topology on Dynamic Contact Angles on Heterogeneous Surfaces

Cassie-Baxter theory has traditionally been used to study liquid drops in contact with microstructured surfaces. The Cassie-Baxter theory arises from a minimization of the global Gibbs free energy of the system but does not account for the topology of the three-phase contact line. We experimentally compare two situations differing only in the microstructure of the roughness, which causes differences in contact line topology. We report that the contact angle is independent of area void fraction for surfaces with microcavities, which correspond to situations when the advancing contact line is continuous. This result is in contrast with Cassie-Baxter theory, which uses area void fraction as the determining parameter, regardless of the type of roughness.

Tunable Two-Dimensional Non-Close-Packed Microwell Arrays Using Colloidal Crystals as Templates

In this paper we demonstrate a facile and efficient way to fabricate poly(dimethylsiloxane) (PDMS) molds with hexagonal non-close-packed (ncp) arrangements of microwells by casting PDMS prepolymer onto two-dimensional (2D) ncp colloidal crystals. The templates of the 2D ncp colloidal crystals were fabricated via coupling lift-up soft lithography and solvent-swelling. We found that the depths of the microwells together with the lattice spacing can be adjusted by the sphere interstices and chemical composition of the 2D ncp colloidal crystals. The relationship of the surface character of the templates with the depths of the microwells can be explained by the wetting behavior of PDMS prepolymer on the rough surface. Contact angle measurements are consistent with the experimental results of the microwells in depth and agree well with the Cassie-Baxter theory. There are at least three advantages of the approach. First, the depth and distance of the microwells can be controlled. Second, PDMS molds can be easily peeled from the surfaces of the templates, which results in reusing the original templates to make new molds. Third, this method can be applied to other materials, such as photopolymerizable resin or thermosetting resin. The potential application of the microwells is as microlenses to make a pattern or as microvials in bioanalytical techniques.

Superhydrophobicity and Superhydrophilicity of Regular Nanopatterns

The hydrophilicity, hydrophobicity, and sliding behavior of water droplets on nanoasperities of controlled dimensions were investigated experimentally. We show that the "hemi-wicking" theory for hydrophilic SiO(2) samples successfully predicts the experimental advancing angles and that the same patterns, after silanization, become superhydrophobic in agreement with the Cassie-Baxter and Wenzel theories. Our model topographies have the same dimensional scale of some naturally occurring structures that exhibit similar wetting properties. Our results confirm that a forest of hydrophilic/hydrophobic slender pillars is the most effective superwettable/water-repellent configuration. It is shown that the shape and curvature of the edges of the asperities play an important role in determining the advancing angles.