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April 2008

 
Lotusan Paint and Superhydrophobicity
Biomimicry involves the study of natural biological systems and how these systems can be used to design and engineer technological solutions. Velcro is an example of this transfer of technology from a natural system to a manmade system. George de Mestral, a Swiss engineer, was cleaning burrs from his dog's coat when he realized how the hooks of the burrs stuck to the loops of the fur.

Another example of how a natural system can be used to solve a technological problem is an amazing paint called Lotusan. A surface painted with Lotusan mimics the behavior of the superhydrophobic lotus leaf. Water will not adhere and in fact when it rolls off, it will pickup debris and makes the Lotusan-painted surface self-cleaning. In a commercial environment this not only reduces maintenance labor and improves the cleanliness of architectural surfaces, but also reduces the use of toxic and environmentally harsh cleaning solutions.


Amplified View of Water Drops on Lotus Leaf

Cassie's Law explains how the change to surface nanotopology (or roughness) can increase the contact angle. Water striders effortlessly float on water not because of any special chemical properties found on them, but because of the unique nanostructures found on their legs. Super hydrophobic films such as Lotusan paint also exploit this law. In addition to the self-cleaning property, Lotusan surfaces stay dry and are thus highly resistant to mold, mildew, and algae, and act as an excellent vapor barrier.

Lotusan is the brainchild of German inventor Wilhelm Barthlott who also claimed the use of "Lotus-Effect" which in the US is a registered trademark of Sto Corporation, the US arm of the German company Sto AG.

Lotusan is one of the first of many commercially available products that employs the lotus-effect to reduce wetting, provide self-cleaning, and promote other desirable characteristics. The Ferro Corporation has developed self-cleaning coatings for glass. Erlus-Lotus is the first commercial self-cleaning roof to use the lotus effect.

Nanopin film is an example of another new technology with superhydrophobic properties. Contact angles of 178° have been achieved on this amazing experimental material made up of many nanoscopic pins and cones based on a fractal structure.

There are myriad commercial applications for lotus-effect technology: toll-booth sensors, frying pans, raincoats and boots, shower walls, eye glasses, windshields, shingles, silverware and serving spoons, billboards and signs. As this exciting technology develops, look for many new products and applications that can benefit from a man-made Lotus-like surface.

Superhydrophobicity is easily measured with any ramé-hart contact angle goniometer and is typically defined by contact angles in excess of 150°. Additionally, the contact angle hysteresis is typically very low, under 5°, and roll-off angle (as measured with a ramé-hart tilting base) is also low, under 8°.
 

How to Measure Superhydrophobic Contact Angles with DROPimage
Measuring drops on superhydrophobic surfaces with a ramé-hart goniometer and DROPimage is very simple. Below are steps involved in measuring a drop on a superhydrophobic surface using our DROPimage Standard program and will work similarly with DROPimage Advanced.


Screen Shot of DROPimage Standard v2.2 Measuring Superhydrophobicity

1. Begin by verifying the calibration by performing a "check calibration" as detailed in the DROPimage User Guide. This will verify that the current calibration is valid and that both drop dimensions and contact angles are reported accurately.

2. Verify that the instrument is level using the spirit level at the base of the stage. This is especially important with superhydrophobic surfaces since drops tend to roll off very easily.

3. Use the Z-axis control on the instrument to set the stage so it's about 1/5 from the bottom edge of the image window as shown in the above screen shot. Since drops on superhydrophobic surfaces are nearly as high as they are wide, the baseline needs to be low enough to accommodate the entire drop.

4. Focus on the baseline.

5. Start the contact angle tool and click on Setup. Verify that the Tilt is set to 0.0. Use the Snap button to automatically set the baseline.

6. Now dispense the drop either with the standard microsyringe assembly or the optional Automated Dispensing System. Be careful to not disturb the drop after it's dispensed or it may roll off the sample.

7. If necessary, adjust your focus so the drop is in focus.

8. Click Start in the Contact Angle Tool dialog. Align the vertical line so that it passes through the center of the drop.

9. If you intend to use the CA data to measure surface energy, you will want to specify the Liquid and Solid phases. You can also optionally provide a Run Name which is for your reference only.

10. Click Measure and DROPimage will automatically generate a profile of the drop. The contact angle lines are measured and lines are placed in the image window for visual inspection. The left, right and mean contact angles are recorded and logged along with the drop height and width. Note that the width reported is at the interface of the solid and liquid, not the maximum width. You can use the Measure Tool if you wish to determine the maximum width.

11. If you plan to use the data later, for example to measure surface energy, click File > Save As... to create a CA file. Or, use File > Generate Log... to create a TXT file with the current set of data.

In our case we measured a CA of about 170°. There is no easier tool to measure superhydrophobic surfaces than the ramé-hart instrument goniometer.

Contact us today if you have any questions, require a quotation or product information, or would like to learn more about measuring any type of contact angle.

 

Regards,

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