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ramé-hart Newsletter

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August 2019
 

Self-Cleaning Surfaces
I recently purchased a new umbrella. To my surprise, it was treated with a hydrophobic coating. When the raindrops hit the umbrella's surface, the drops would immediately bead up and roll off - even in areas were positioned flat. After walking through a rain storm, the umbrella was still fully dry and could be wrapped up and stored without the need to leave it out to dry. In addition to being fully water-repellant, there is a self-cleaning component. As the drops roll off, dust and contaminants latch onto the drops and are removed. I was amazed and curious to know if the developer of the treatment used a ramé-hart instrument to test the wetting properties of the treated surfaces. I would not be surprised.


Self-cleaning umbrella cloth stays dry

From airplanes, cars, kitchens, toilets, and windows to umbrellas, the gamut of applications that could benefit from self-cleaning surfaces is wide. In addition to keeping surfaces free of liquids and contaminants, materials can last longer and require less or no ongoing maintenance and cleaning. Surfaces that are made to be self-cleaning fall into three categories: (A) superhydrophobic, (B) superhydrophilic, and (C) photocatalytic. 

Titanium dioxide was first used over 20 years ago to treat glass to make it self-cleaning. This method relies on a photocatalytic step and then changes the surface to become superhydrophilic. Since then titanium dioxide is also being used to create self-cleaning nanoparticles that can be embedded in other materials which then inherit the self-cleaning properties.

There is no surface that is superhydrophobic1 without some form of nano-topology or roughness. This is true of natural surfaces - like the the Lotus leaf - as well as manmade surfaces. Plasma treating, nanolithography, ion etching, and other methods are used to create roughness at the nanoscale. Without a roughness, contact angle is limited to under 120° and that's on a fluoropolymer like PTFE. Furthermore, the surface needs to promote the Cassie-Baxter state - that is, the drop sits on the top of the asperities with small air pockets in the nooks and crannies. In this state, the roll-off angle is very low which permits water drops to roll off easily and take with it any contaminants it picks up. While water can clean small particulates off superhydrophobic self-cleaning surfaces, it's not so effective at removing oils.

Surprisingly, surfaces can be made self-cleaning not only by making them superhydrophobic but also by making them the extreme opposite - superhydrophilic - that is, with a water contact angle at or approaching 0°. Although this uses a totally different approach, the results are often comparable. Water, instead of beading up and rolling off and taking debris with it will wet out infinitely and lift the contaminants up and off the surface allowing them to be washed away.


Lotus Leaf under magnification

In nature, the Lotus leaf is the gold standard for superhydrophobic self-cleaning surfaces. The term "Lotus effect" refers to the amazing self-cleaning properties of this aquatic plant. The hierarchical structure of the leaves - that is, small hairs on larger asperities - is incredibly difficult to replicate using current nanotechnology methods but yet produces the absolute highest contact angles anywhere - in nature or manmade.


The amazing gecko with self-cleaning sticky feet

Gecko feet, in addition to being super "sticky" are also self-cleaning. The little lizards can run around on dusty ceilings all day long and always come home with clean feet that still stick. I wish my kids could learn that trick. Gecko feet also benefit from a complex hierarchical structure. They have millions of small hairs called setae. Yet each of those small setae has a hair with a small flap on it which uses van der Waals forces to provide the stick. The self-cleaning property does not require water to roll off. Researchers are looking at ways to mimic this type of surface which is proving to be a challenging task.

Novel methods such as nanocasting, capillary nanolithography, and photolithography are being developed to more cost-effectively product self-cleaning surfaces. Contact angle plays a pivotal role in the development and testing of self-cleaning products. With our Automated Tilting Base product, we can measure roll-off angle as well as advancing and receding contact angle and contact angle hysteresis. When the contact angle hysteresis is low, the roll-off angle will be low and the self-cleaning properties will be high.

Bullets

  • Self-cleaning surfaces reduce maintenance costs and improve the aesthetics of surfaces.

  • Titanium dioxide is used to create self-cleaning surfaces using a photocatalytic process.

  • Other types of self-cleaning surfaces involve superhydrophobic and superhydrophilic treatments.

  • Superhydrophobicity can only be achieved by adding a roughness - this is typically a nanostructured surface that promotes the Cassie-Baxter state.

  • The Lotus leaf is the gold standard for superhydrophobicity.

  • A self-cleaning surface can also be made from a superhydrophilic treatment that causes water to spread out and lift the contaminants off the surface.

  • There are many amazing examples of self cleaning surfaces in nature. In addition to the Lotus leaf, Gecko feet are interesting to study for their self-cleaning nature.

  • Methods are being developed to create self-cleaning surfaces that are economical to produce and durable.

  • A ramé-hart contact angle goniometer can be used to measure the wetting properties of self-cleaning surfaces as they are studied and developed.

Notes
1 A surface is considered superhydrophobic if the water contact angle is greater than 150°.
 

Spares Kits
We currently offer (3) unique spares kits. One kit is for the Automated Dispensing and includes spare tubing sets, a spare syringe, and two boxes of spare disposable tips. The other two kits provide a spare microsyringe assembly, needles, and o-rings. For more information or to order a spares kit, visit our spare parts ordering site here today. If you have any questions regarding our spares kits or any of our products, please contact us today. Thank you for your business. 
 
 
Regards,

Carl Clegg
Director of Sales
Phone 973-448-0305
www.ramehart.com
Contact us

 

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