|
ramé-hart instrument co.
November 2013 Newsletter |
| Electrowetting |
|
This past month we set out to discover the
exciting realm of electrowetting. Our quest began about a month ago when
a pair of our customers began using our instruments to measure the
effects of electrowetting on different materials they are developing. At
that point we decided we needed to better understand the phenomenon.
We began by attempting to create a simple electrowetting experiment in our own lab. To be honest, our original intention and hope was to be able to replicate something like the experiment shown in the video below - with vivid differences between the wetting and dewetting cycles. However, I must confess that our own experiment fell somewhat short of this.
We first obtained some Kapton tape made from polyimide which was a recommended material due to its hydrophobic and dielectric properties. We used this tape to cover our specimen stage in order to insulate it from any electrical charge. Next, we placed a piece of steel material on top of the tape and then used a single layer of Kapton tape to tape the steel down to the stage. We used banana clips to apply the ground to the plate and then the positive charge to the needle which we left in the drop to act as an electrode. We applied a current and watched. Nothing happened. We checked our voltage and also we checked all of our connections. Everything was set up correctly, but no wetting or dewetting was occurring.
In theory, what we expected to happen when voltage is applied to the droplet phase is the drop should wet out resulting in a lower contact angle as illustrated in the above diagram. The simplest way to understand this behavior is in terms of the forces that are introduced by the electrical field - the drop is pulled down to the surface by the energy stored in capacitor form between the electrode and the drop.
After doing some troubleshooting, we determined that our power source was set to a DC current of about 24V which is not enough to make a significant charge. We also determined that our Kapton tape was too thick (1 mil). So, we made some changes to our system, switched to AC and greatly increased the voltage. At this point we were now able to use electrowetting to reduce the contact angle by just over 20°. See the illustration above. However, in our case, when we removed the electrical charge, the drop did not dewet and return to its original state - rather, it stayed at 71°. We believe that this is due to a somewhat large contact angle hysteresis. More on that in last month's newsletter. To achieve the wetting and dewetting behavior shown in the video, a hydrophobic surface with a low hysteresis would work better. Amorphous fluoropolymers work best such as Teflon AF, Cytop, or FluoroPel - these materials have been designed and optimized for electrowetting applications. A higher voltage may also help. In any case, we were successful in demonstrating the effect of electrowetting. In case we did not mention it, the external phase is air. The contact angle in the above analysis was measured using a ramé-hart Model 400. However, any ramé-hart goniometer or goniometer / tensiometer could be used for this application. With any new technology, the full range of potential applications is not fully discovered. However, early work in the field has explored adjustable lenses, electronic displays (such as those used in tablet e-readers), oil spill cleanup technology, and for use in microfluidic applications. Researchers at the University of Akron are developing a method for picking up micro objects that uses electrowetting.1 They are also using a ramé-hart goniometer to measure the contact angle in their studies. |
| Eighth International Conference on Materials Technologies and Modeling |
|
We are pleased to announce MMT-2014 which
will be held the last week of July, 2014 at Ariel University in Israel.
Ariel University, which is hosting the conference and Ben Gurion
University, which is a contributor, are both ramé-hart customers. What's
most exciting about this conference is that they will have a
special section on wetting phenomena headed by Dr. Edward Bormashenko. This section will cover, but is
not limited to these topics: fundamentals of wettability, experimental
techniques intended for investigation of wetting, contact angle
hysteresis: experiment and theory, the phenomenon of
superhydrophobicity: experiment, theory, applications,
superoleophobicity, wetting of re-entrant structures, dynamic wetting,
anisotropic wetting, electrowetting, superhydrophilicity,
icephobicity, wetting transitions, surfaces with a tunable wettability,
and liquid marbles. For more details and registration information,
please proceed to:
http://tinyurl.com/lpx5pqu. |
|
Regards,
Carl Clegg |
