|
ramé-hart instrument co. February 2015 Newsletter |
| Mainstreaming Superhydrophobicity |
|
The other day Walt, our master machinist,
came to work and commented on a news story he saw about some new laser
machining method that makes metal surfaces superhydrophobic. The same
day, without looking for it, I saw an article on CNN about the same
technology.1 Since then several other friends and family
members have told me they saw a story somewhere on this same topic. (They know that I'm into superhydrophobicity.) It turns out that researchers at the University of Rochester have developed a novel method of using short-burst laser blasts to modify a metal surface making it superhydrophobic. If you're still working on a design to reinvent the toilet, you might want to check out this new technology.2 Since this method modifies the material rather than coating it, it will likely last much longer than other methods for making surfaces non-wetting. In any case, it has been a news story that has reached the mainstream media. Pharrell, the singer, was at CES (Consumer Electronic Show) in Las Vegas earlier this month making a plug for a superhydrophobic coating used on smart phones.3 This is not the first time mainstream news sources have highlighted the nano-coating technologies from companies like HzO.4 NeverWet, a spray-on liquid repelling treatment, is another product that has been highlighted in the news recently.5 (My Puma Clyde sneakers are still keeping my feet dry in puddles but not with the same efficiency I experienced when they were freshly treated.6 Time for a retreatment.) And the product can be purchased at Home Depot. On YouTube there are over 70,000 videos with the word "superhydrophobic" in the title. Several have gone viral scoring millions of views. Gradually the general public is becoming more aware of and interested in the benefits of making things superhydrophobic. The day may not be far away when people talk about the contact angle of their cell phone screens.
1
http://www.cnn.com/2015/01/22/us/feat-metal-repels-water-rochester/
|
| Dynamic Contact Angle |
|
Many times when we refer to contact angle,
we are really referring to static contact angle which is the most common
type of contact angle measurement. In short, we place a small drop of
liquid (commonly water) onto a surface. We then measure the angle formed
at the three-phase line where the drop touches the surface. Our
DROPimage software does all of this rather automatically. A high contact
angle over 90° indicates poor wetting and is called hydrophobic. A low
contact angle under 90° indicates better wetting and is called
hydrophilic. Most of the time, if the drop is observed over time,
nothing additional happens although if you watch closely enough, the
contact angle may decrease slowly as the drop evaporates.
On some surfaces, however, a sessile drop will spread over time. We refer to this behavior as dynamic wetting and we can capture the change in contact angle over time by taking many contact angle measurements at a fixed time interval. In the video below, we measure the dynamic contact angle of a 5µl water drop on a treated synthetic flouropolymer surface by taking 100 measurements at a rate of 10 measurements per second. Thus we are watching the dynamic contact angle behavior over a 10 second period. The initial contact angle at the time the drop is deposited on the surface is 88.45°. Ten seconds later the contact angle measures 80.61°, a drop of nearly one degree per second. Further, it's observed that the left side of the drop remains pinned while the right side of the drop slips. This indicates that the surface may not be homogenous. The purpose of the video is not solely to demonstrate an example of dynamic contact angle, but to illustrate how the ramé-hart DROPimage Advanced program can be employed to easily capture and replay such behavior. In addition to creating an experiment to capture the change in contact angle over time, we are also saving each of the 100 frames to separate image files and then we show how to use Recalculate command to replay the video and recalculate the contact angle measurements in batch mode from the saved images. We also show how to generate a report that shows all 100 contact angle measurements. The video is a useful tutorial for anyone who wishes to measure dynamic contact angle or use Recalculate to replay and recalculate contact angle measurements. We should point out that in addition to the spreading drop as shown in the video there are several other types of dynamic contact angle measurements: the tilting base method for measuring advancing and receding contact angles as well as roll-off angle and the add/remove volume method which is also used for measuring advancing and receding contact angles.1 1 See our July 2009 newsletter for discussion of these types of dynamic contact angle. For a video example of the tilting base method, see this video: http://youtu.be/UWMwSppjv6s. For an example of the add/remove volume method using the ramé-hart Automated Dispensing System, see this video: http://youtu.be/1wh0VtnCIEs. |
|
Regards,
Carl Clegg |
