Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

A new look at wetting models: Continuum analysis

27.11.2012
The wetting model is a classical problem in surface science and biomimetic science. Professor LIU Jianlin and his collaborators from China University of Petroleum, Wuhan University and Fourth Military Medical University approached this old and classical problem from a new direction.

They stressed that it is the triple contact line and not the contact area of the droplet/solid interface that determines the macroscopic contact angle. The proposed continuum model, termed the mechanism-based model, can illustrate the contact line pinning effect at some wedges or phase interfaces between different materials. Their work, entitled "A new look on wetting models: continuum analysis", was published in SCIENCE CHINA Physics, Mech & Astro, 2012, Issue 11.

The concept of the contact angle dates back to the pioneering contribution of Young, which was a milestone in the characterization of the wetting property of a perfectly smooth and homogeneous solid surface. However, many experiments have shown that most wetting and dewetting behaviors of solid are not only related to the chemical components but also closely related the micro- and/or nano-structures of their surfaces. The combination of wetting properties and geometric topographies of the surface often renders a wealth of attractive phenomena in nature and daily life. For example, the "lotus effect", that is, the self-cleaning capability of lotus leaves, is due to the intrinsic hydrophobicity and micro/nanomorphologies of surfaces. Some aquatic creatures, such as the water strider, water spider and mosquito, resort to their superhydrophobic capacities to stand, walk and jump on still or even flowing water.

This skill originates from the special micro/nano setae and nanoridge structures on their legs, which can produce extremely large driving forces to support their body weight. The Stenocara beetle of the Namib Desert is a dew-collector, possessing special surface microstructures with variable hydrophilic and hydrophobic domains on its carapace. A new approach for achieving special wetting surfaces has been proposed and advanced by varying the chemical compositions and geometrical topographies.

A plethora of superhydrophobic surfaces with static contact angles larger than 150° were prepared to produce ultra-hydrophobicity with fractal or hierarchical surface structures, to mimic the biological materials. These designed rough and heterogeneous surfaces have already been put into use in various areas of industry, spanning from porous media, microfluidic devices, and self-cleaning paints to glass windows.

For these natural phenomena and superhydrophobic (or superhydrophilic) materials, a critical issue is how to predict the wetting behaviors properly, which is of great value to both fundamental science and engineering applications. As is well known, there are two classical models with which to examine the macroscopic contact angle on a rough or heterogeneous substrate; i.e., the Wenzel model and the Cassie–Baxter model. The Wenzel model indicates that part of the liquid completely pierces into the microstructures on the rough solid substrate and the geometric topographies amplify the hydrophobicity of hydrophobic surfaces and enhance the hydrophilicity of hydrophilic surfaces. The Cassie model deals with a substrate with several phases of different wetting properties, implying that the macroscopic contact angle is actually the weighted average function of different hydrophilicities of the materials.

Although Wenzel and Cassie models can successfully elucidate the superhydrophobic phenomenon of inhomogeneous substrates, some scientists doubted the accuracy of these results. They designed experiments to demonstrate that the contact line and not the contact area beneath the droplet is responsible for the macroscopic contact angle. These disputes suggest that the Wenzel and Cassie models are only valid in a certain range, but to what extent the two models apply is a key subject of investigation. Therefore, the present work is not to invalidate the Wenzel and Cassie models, but is directed toward a further understanding of the wetting mechanism based upon the triple contact line, from a new viewpoint of continuum mechanics. This theory of wetting is constructed on a strong theoretical basis, including consideration of the force equilibrium and energy, which deals with a more general substrate, say, a rough and chemically heterogeneous solid surface.

Using the energy formulation and considering the movable boundary condition, the authors derived the macroscopic contact angle on a rough and heterogeneous substrate, under the assumption that the Young's contact angle and interfacial energies are all field functions of the position of the contact line. The equation of the relationship shows that the macroscopic contact angle has no direct connection with the gravity of the droplet. Besides, the macroscopic contact angle strongly depends on the geometrical and chemical properties of the substrate, as it is a continuum field variable at any point. This means that different positions will have different contact angles. Finally, the result again emphasizes that the macroscopic contact angle is dependent only on the properties of the triple contact line and is independent of those of the area underneath the droplet. However, the classical Wenzel and Cassie models are both concerned with the geometrical and chemical properties of the contact zone, and not the contact point. In the following, the authors designed several substrates with special roughnesses and hydrophilicities, and compared the results predicted by the Wenzel model, Cassie model and the mechanism-based model. The results show that the mechanism-based model deviates from the classical models greatly for these types of substrates, and this conclusion suggests that the Wenzel and Cassie models only hold in a certain range.

The mechanism-based model can also be adopted to explore the pinning effect due to a wedge or different hydrophilicity phases. At a sharp wedge, the derivative of the substrate curve is discontinuous, and the macroscopic contact angle does not have a determinate value. In reality, there are no pure "sharp" or "jump" points, and these sharp tips have small curvature radii. In particular, a fine AFM tip may take a radius of several tens of nanometers. The curvature at the singular point can be enlarged, and in this narrow domain, the derivative of the substrate can change from zero to the value when the triple contact line is located on the inclined side of the wedge. In the real world, this transition zone near the sharp tip is too minute to be observed. Therefore, the triple contact line is often regarded as being "pinned" at this position, and only variation in the contact angle is macroscopically seen. Similarly, we can consider a substrate only including two phases with different contact angles. This pinning phenomenon is really a "black box" used to find of a solution employing the developed classical model. Mathematically, there is no wetting property jump at the interface of two different materials binding together. Indeed, there is also a transition zone owing to the atom diffusion, where the width is reported as several tens of micrometers. In this area, the gradient of the wetting property is very abrupt, spanning from a smaller value to a larger value, and the motion of the triple contact line cannot be observed by the naked eye. The only phenomenon is that the contact angle at this point (in fact in this area) continuously increases with an increasing droplet volume.

This study investigated the wetting model for a general substrate from a new viewpoint of continuum mechanics, and the analyses first showed how the Wenzel and Cassie models deviate from the experimental results, and then elucidated the motion of the triple contact line. The result shows that the contact angle is independent of the gravity of the droplet and the contact area underneath the droplet, and is only dependent on the property of the triple contact line . Several substrates with typical geometric topographies and chemical components were provided as examples to demonstrate the difference between the classical models and mechanism-based models. The derived model can also illustrate the pinning effects on a sharp wedge or at the interface between two different phases by employing continuum field theory. This continuum model also casts light on the pinning effect on a sharp wedge or at the interface between two different phases. The investigation revisits the fundamentals of wetting on rough and heterogeneous substrates, which will help in the design of super-hydrophobic materials and provide the predictions required to engineer novel microfluidic devices.

See the article: Liu J L, Xua R, Zhou X H. A new look on wetting models: continuum analysis. SCI CHINA Phys, Mech & Astro, 2012(11):2158-2166 http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s11433-012-4895-2

LIU Jianlin | EurekAlert!
Further information:
http://www.upc.edu.cn

More articles from Physics and Astronomy:

nachricht What happens when we heat the atomic lattice of a magnet all of a sudden?
18.07.2018 | Forschungsverbund Berlin

nachricht Subaru Telescope helps pinpoint origin of ultra-high energy neutrino
16.07.2018 | National Institutes of Natural Sciences

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

Machine-learning predicted a superhard and high-energy-density tungsten nitride

18.07.2018 | Materials Sciences

NYSCF researchers develop novel bioengineering technique for personalized bone grafts

18.07.2018 | Life Sciences

Why might reading make myopic?

18.07.2018 | Health and Medicine

VideoLinks
Science & Research
Overview of more VideoLinks >>>