An experimental and analytical study on the influence of superhydrophobic micro-textured surfaces on liquid wetting phenomena (original) (raw)
Related papers
Microstructure Design for Artificial Superhydrophobic Surfaces
International Journal of Engineering and Advanced Technology, 2020
Superhydrophobic surfaces are the surfaces that do not allow the droplets of liquid to spread and wet it. Ideally, the droplets remain almost spherical in shape and with a very small angle of tilt, slide away from the surface. This occurs due to very high contact angle. A perfectly spherical droplet would make 1800 angle of contact, but practically this high contact angle is never possible for a stable droplet. The surfaces that make contact angle (CA)>90o are said to be hydrophobic surfaces. If CA is greater than 150o , the surface is known as superhydrphobic surface. This property of the surface is termed as superhydrophobicity. In this paper, the surface morphology to be engineered is studied, which is governed by certain principles. Theories of Thomas Young [1], Wenzel [2] and Cassie-Baxter [3] are reviewed and effect of micro and nano level of roughness, producing hierarchical structures is analyzed. Subsequently, the designing of such super hydrophobic surfaces is attempted.
Langmuir, 2016
We have considered in this work the Wilhelmy plate tensiometer to characterize the wetting properties of two model surface textures: (i) a series of three superhydrophobic micropillared surfaces, and (ii) a series of two highly water repellent surfaces microtextured with femtosecond laser. The wetting forces obtained on these surfaces with the Wilhelmy plate technique were compared to the contact angles of water droplets measured with the sessile drop technique, and to the bouncing behavior of water droplets recorded at a high frame rate. We showed that it is possible with this technique to directly measure triple line anchoring forces that are not accessible with the commonly used sessile drop technique. In addition, we have demonstrated on the basis of the bouncing drop experiments, wetting transitions induced by the specific test conditions associated to the Wilhelmy plate tensiometer for the two series of textured surfaces. Finally the tensiometer technique is proposed as an alternative test for characterizing the wetting properties of highly liquid repellent surface, especially in immersion conditions.
2010
The engineering of liquid behavior on surfaces is important for infrastructure, transportation, manufacturing, and sensing. Surfaces can be rendered superhydrophobic by microstructuring, and superhydrophobic devices could lead to practical corrosion inhibition, selfcleaning, fluid flow control, and surface drag reduction. To more fully understand how liquid interacts with microstructured surfaces, this dissertation introduces a direct method for determining droplet solid-liquid-vapor interfacial geometry on microstructured surfaces. The technique performs metrology on molten metal droplets deposited onto microstructured surfaces and then frozen. Unlike other techniques, this visualization technique can be used on large areas of curved and opaque microstructured surfaces to determine contact line. This dissertation also presents measurements and models for how curvature and flexing of microstructured polymers affects hydrophobicity. Increasing curvature of microstructured surfaces leads to decreased slide angle for liquid droplets suspended on the surface asperities. For a surface with regularly spaced asperities, as curvature becomes more positive, droplets suspended on the tops of asperities are suspended on fewer asperities. Curvature affects superhydrophobicity because microscopic curvature changes solid-liquid interaction, pitch is altered, and curvature changes the shape of the three phase contact line. This dissertation presents a model of droplet interactions with curved microstructured surfaces that can be used to design microstructure geometries that maintain the suspension of a droplet when curved surfaces are covered with microstructured polymers. Controlling droplet dynamics could improve microfluidic devices and the shedding of liquids from expensive equipment, preventing corrosion and detrimental performance. This dissertation demonstrates redirection of dynamic droplet spray with anisotropic microstructures.
Physics and applications of superhydrophobic and superhydrophilic surfaces and coatings
Surface Innovations, 2014
The terms superhydrophobicity and superhydrophilicity were introduced not very long ago, in 1996 and 2000, respectively. The former is used to describe exceptionally weak and the latter used to indicate strong interactions of materials and coatings with bulk water, controlled entirely by surface topography and material chemistry. An explosion of research on fabrication of superhydrophobic and superhydrophilic surfaces and coatings was noticed almost immediately after the concepts appeared in the technical literature, with hundreds of reports now published annually. The interest in this new class of surfaces/coatings is driven by an emerging market for water-repellant, snow-and ice-phobic products and formulations, water antifogging screens, windows and lenses, antifouling coatings, microfluidic devices, coatings for enhanced boiling heat transfer, foils for food packaging and many other products. The popularity of this emerging subdiscipline of surface chemistry can also be attributed to uncomplicated fabrication technologies that can produce superhydrophobic or superhydrophilic surfaces and coatings, in addition to the simplicity of the testing techniques used, such as contact angle measurements. In this article, the physics behind superhydrophobic and superhydrophilic effects are reviewed and several examples of applications of superhydrophobic and superhydrophilic surfaces and coatings are provided.
Superhydrophobic surface as a fluid enhancement material in engineering applications
2013
In this study, a superhydrophobic surface and its relation to the enhancement of the droplet fluid dynamics to the surface of the object materials was investigated. As the comparison, hydrophilic and uncoated surface of an object also investigated. The investigations used height of impact at 89 mm. The high quality speed camera is employed to investigate the droplet dynamic on a copper foil and a calcium fluoride surfaces. Both of the materials are coated with superhydrophobic and hydrophilic surfaces separately. The droplet diameter was analyzed using the program PHANTOM. The droplet contact angle was analyzed by the Goniometry method. The water was dropped on the calcium fluoride and the copper foil using a syringe (sharp tip) with initial droplet diameter of 1.9 mm. To record the droplet fluid shape, the photo micro sensor was placed inside the trigger box below the syringe. The results showed that the superhydrophobic surface both on copper foil and calcium fluoride enhanced the mobility of a droplet compared to the hydrophilic and the uncoated surfaces. The results showed that the maximum droplet diameter on the copper foil coated by the superhydrophobic, the hydrophilic and the uncoated surfaces are 4.7, 5.0, 5.2 mm, respectively; and for the calcium fluoride are 4.5, 5.1 and 5.5 mm, respectively. Meanwhile, the results for the droplet contact angle on the copper foil coated by the superhydrophobic, the hydrophilic and the uncoated surfaces are 20 o , 90 o , 160 o , respectively; and for the calcium fluoride are 25 o , 95 o , 165 o , respectively.
Wetting behavior on hybrid surfaces with hydrophobic and hydrophilic properties
Applied Surface Science, 2014
Hybrid surfaces consisting of a micropillar array of hydrophobic and hydrophilic sites were designed and fabricated to understand the effects of their unique surface morphology and chemistry on droplet condensation. Droplet impingement experiments have revealed that hybrid surfaces exhibit high contact angles, which is characteristic of purely hydrophobic surfaces. However, little is known about the wetting behavior of droplets that nucleate and grow on hybrid surfaces during condensation. In fact, condensed droplets display a distinct wetting behavior during the droplet growth phase which cannot be reproduced by simply impinging droplets on hybrid surfaces. In this study, hybrid surfaces with three different spacing ratios were subjected to condensation tests using an environmental scanning electron microscopy (ESEM) and a condensation cell under ambient conditions. For hybrid surfaces with spacing ratio below 2, droplets were observed to form on top and sides of the micropillars, where they grew, coalesced with adjacent droplets, and shed after reaching a given size. After shedding, the top surface remained partially dry, which allowed for immediate droplet growth. For hybrid surfaces with spacing ratio equal to 2, a different wetting behavior was observed, where droplets basically coalesced and formed a thin liquid film which was ultimately driven into the valleys of the microstructure. The liquid shedding process led to the renucleation of droplets primarily on top of the dry hydrophilic sites. To better understand the nature of droplet wetting on hybrid surfaces, a surface energy-based model was developed to predict the transition between the two observed wetting behaviors at different spacing ratios. The experimental and analytical results indicate that micropillar spacing ratio is the key factor for promoting different wetting behavior of condensed droplets on hybrid surfaces.
Adhesion and friction properties of micro/nano-engineered superhydrophobic/hydrophobic surfaces
Thin Solid Films, 2010
Hydrophobic micro/nano-engineered surfaces (MNESs) with good adhesion and frictional performances were fabricated by the combination of aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) and octadecyltrichlorosilane (OTS) coating. The AIC of a-Si technique was used to produce silicon micro/ nano-textured surfaces, while an OTS self-assembled monolayer was used to lower the surface energies of the textured surfaces. The wetting properties of the MNESs were studied using a video-based contact angle measurement system. The adhesion and friction properties of the MNESs were investigated using a TriboIndenter. This study shows that the adhesion and frictional performances of all MNESs are significantly improved compared to untreated silicon substrate surfaces, and the adhesion and frictional performances of the OTS-modified textured surfaces strongly correlate to their surface wetting property, i.e., the larger the water contact angle, the better the adhesion and frictional performances of the OTS-modified textured surfaces.
Wetting characteristics of liquid drops at heterogeneous surfaces
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1994
We!l-defined heterogeneous surfaces consisting of hydrophobic and hydrophilic regions were prepared on gold (a 2000 A gold film supported on an Si/SiO2/Ti substrate) by patterning self-assembled monolayers (SAMs), using an elastomer stamp. One surface was composed of alternating and parallel hydrophobic (2.5 ~) and hydrophilic (3 ~) strips, and the second surface consisted of alternating hydrophilic squares (3 ~ x 3 ~) separated by hydrophobic strips (2.5 ~). The wetting characteristics of these well-defined heterogeneous solid surfaces were examined by contact angle measurements. The contact angles for water drops, which varied in pH from 5.8 to 10.0, were measured with the strips both tangential to and normal to the three-phase contact line. The experimental contact angles are in good agreement with theory as calculated from the Cassie equation when the three-phase contact line is non-contorted (i.e. the three-phase contact line is situated along the hydrophobic strip). On the other hand, when the strips are normal to the drop edge, corrugation of the three-phase contact line affects the contact angle significantly. Contact angles, measured with the strips normal to the drop edge, were lower by 7-160 than those calculated from the Cassie equation. Analysis of these measurements, together with contact angle/drop size measurements for fully hydrophobic and hydrophilic surfaces, demonstrate the validity of a modified Cassie equation that includes a term describing the line tension contribution.