Hydrophobic Vs Superhydrophobic Coatings: An In-Depth Probe

Superhydrophobic coatings

In the past few years, the scientific community together with the world’s coatings industrial sector has seen the introduction of a range of both hydrophobic coatings and superhydrophobic coatings.  Each coating displays exceptional water repellent features.

 

The advantages of these hydrophobic and superhydrophobic coatings include:

  1. Enhanced self-cleaning functionality.
  2. Improved moisture and corrosion resistance.
  3. Reduction in dirt retention.
  4. Extended life expectancy for the coating and substrate.

 

Superhydrophobic Coating Defined

An accepted definition of a superhydrophobic coating is, “a coating that has a water contact angle of greater than 150 degrees and a sliding angle of fewer than 10 degrees.”

You can create superhydrophobic materials using the following methods:

  • Coating a surface with a superhydrophobic material.
  • Nanostructuring a surface.
  • Applying nanoparticles to a surface.
  • Adopting a combination of these techniques.

 

Hydrophobic Coating Defined

A definition of a hydrophobic is one that, “tends to repel or fail to mix with water with a contact angle of fewer than 150 degrees and a sliding angle of fewer than 10 degrees.”

 

Superhydrophobic Coatings In Nature

Nature has many examples of superhydrophobic and hydrophobic applications. One of the most studied surfaces is that of the Lotus leaf. The contact angle of water on the surface of a Lotus leaf is greater than 150°. The Lotus leaf’s self-cleaning properties are derived from the hydrophobic water-repellent double structure of its surface. This enables the contact area and the subsequent adhesion force between the surface and the water droplet to reduce significantly.  This results in a self-cleaning process which allows water to readily roll off the surface of the leaf, whilst collecting dust deposits on the way.

This micron size double structure on the surface of the plant is comprised of needle-like projections from the surface that are covered by wax. The wax-covered projections are 10 to 20 µm in height and 10 to 15µm in width. These waxes are hydrophobic and form the top layer of the double structure. Some plants display contact angles of up to 160° and are hence categorised as super-hydrophobic. So, only 2 to 3 percent of the surface of a water droplet is in contact with the leaf’s surface. Since the surface contact area is less than 0.6 percent, this produces the leaf’s self-cleaning properties.

 

It’s All About Contact Angles

Water has a higher surface tension than most common solvents currently used in the materials industry. Science attributes this phenomenon to the high attraction of water molecules display for each other as a result of their hydrogen bonding.

A superhydrophobic coating is a nanoscopic surface layer that repels water. An important factor determining the hydrophobicity of a coating is the microscopic geometry of the surface. Droplets hitting this kind of coating can fully rebound in the shape of a column or pancake.  Hence, the angle with which a drop of water makes contact with a surface makes all the difference to its behaviour when it hits a surface coating, be it either hydrophobic or superhydrophobic.

To maximize the surface hydrophobicity of a coating, its surface energy should be as low as possible. A low surface energy, together with a complementary surface structure maximizes hydrophobicity.  A high surface tension liquid such as water will have maximum hydrophobicity thus displaying poor wetting properties on a coated surface that has a low surface energy.

Several factors shape the contact angle of a water droplet on the surface of a coating. These include the macro, micro, and nano-surface profile, together with the surface tension of the coating on which the water droplet rests. This surface tension is the elastic tendency of liquids that makes them adopt the smallest possible surface area.

 

The Benefits Of The Technology

In materials science, moisture is the underlying cause of almost all processes that damage contemporary building materials. From cracking to the onset of rusting to the emergence of unsightly scale deposits on surfaces. The porous nature of most mineral substrates leads them to soak up water containing contaminants hitting their surface via capillary suction.

Preventing water from penetrating into a substrate structure can completely eliminate the risk of short and long-term damage to the substrate due to moisture intrusion.

Coatings that offer either a hydrophobic or a superhydrophobic surface can impart a host of benefits to the surface and substrate that the coating is applied to. These properties range from hydrophilic or water-loving coatings, to hydrophobic through to superhydrophobic coatings, which are highly water-repellent.  Applying either a hydrophobic or a superhydrophobic surface treatment can vastly improve the performance characteristics of virtually any component via its easy to apply nanoscale coating.

These nanoscale coatings bond with an extensive range of classic or modern industrial materials.  These include complex metal alloys, a new generation of plastic compounds, vitreous materials, and even highly sensitive microelectronic silicon components.

Thanks to their water repellent properties, hydrophobic coatings make treated surfaces extremely easy to clean.  Additionally, deposits and residues left by water, dirt, and even oil cannot stick to or even smudge these surfaces once they have been treated.

These superhydrophobic coatings have the additional benefit of being oleophobic. As such, one pass of a clean cloth or absorbent paper over the surface is all you need to clean a treated surface.

 

High-Performance Superhydrophobic Coatings

Superhydrophobic coatings are used in ultra-dry surface applications. These coatings cause an almost imperceptibly thin layer of air to form on top of the treated surface. However, as superhydrophobic coatings are optically neutral, they are invisible to the naked eye.

The coating can be sprayed onto objects to make them waterproof. In addition to being waterproof, the spray improves anti-corrosion and anti-icing properties.  Moreover, surfaces will resist the accretion of grit and grime enhancing its self-cleaning capabilities.  This makes it ideal for protecting circuits and grids.

Superhydrophobic coatings have important applications in aviation and maritime environments. They can yield significant reductions in drag from skin friction for aircraft bodies and ships’ hulls. This increases fuel efficiency while reducing maintenance costs. They can also reduce corrosion and prevent marine organisms from growing on a ship’s hull in the marine environments.

In the automotive industry, superhydrophobic coatings have potential applications to vehicle windshields to prevent rain droplets from clinging to the glass. The coatings also streamline the removal of salt deposits without the need to use fresh water.

Superhydrophobic coatings rely on a delicate nanostructure for their water-repellent properties. Abrasion and cleaning can easily damage this structure.  As such, superhydrophobic coatings are mainly applied on an industrial scale on items such as electronic components, which are not prone to wear.

 

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Last Word

In the past few years, researchers have successfully developed a range of hydrophobic and superhydrophobic coatings. The advantages of these coatings include enhanced self-cleaning functionality, improved moisture and corrosion resistance, a reduction in dirt retention, as well as extended life expectancy for the coating and substrate. Today, superhydrophobic coatings are proving to be exceptionally useful in protecting delicate electronic components.

 

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