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ramé-hart instrument co. July 2015 Newsletter
There are circumstances when the standard
method of measuring the contact angle of a liquid sessile drop on a
surface is not ideal. For example, in the case of porous materials and
solids with high surface energy, such as hydrogel contact lenses, an
alternate method has been developed to determine contact angle. The solid is submerged in a liquid (e.g., water) and then a
bubble of gas (e.g., air) is released onto the underside of the surface
forming an inverted sessile drop. The contact angle of this bubble is
then measured. This method is referred to as captive bubble and offers
some compelling advantages for certain applications.
The above video shows how to measure a captive bubble contact angle using any current-generation ramé-hart instrument with DROPimage Advanced. The instrument was fitted with our Environmental Fixture (p/n 100-14) (see picture below) which provides an environment for a liquid external phase. We also used an Inverted Needle (p/n 100-10-13-22). The air can be dispensed using either the manual Microsyringe assembly or our Automated Dispensing System. In both cases, you'll want to prime the tubing and syringe with water. Then retract into the needle a small volume of air - slightly more than the desired bubble volume. If you attempt to run the Microsyringe or the Automated Dispensing System with no water in the lines and syringe, it will be nearly impossible to dispense the air with any precision since air is highly compressible.
DROPimage Standard can also be used to measure captive bubble. In the case of both DROPimage Advanced and DROPimage Standard, you'll need to turn on the "Captive Bubble" option in the Options menu. You will also want to turn on the Right Light option. The video walks through the steps for taking a captive bubble measurement.
Some advantages of the captive bubble method:
- It works well on hydrogels and other surfaces with a high surface energy.
- The drop does not evaporate.
- The surface is in a saturated condition which is ideal for porous materials.
- The external phase liquid can be modified (e.g., by adding surfactants) to observe dynamically the effects of different liquid concentrations on the captive bubble contact angle.
- Less contamination of the gas phase by air particles.
- The temperature of the liquid phase can be adjusted and monitored in order to determine the effects of temperature on wetting properties.1
- Special fixtures (Environmental Fixture and Inverted Needle) are required.
- Larger volumes of liquid are required than for the conventional liquid sessile drop method.
- The liquid must be optically translucent. Captive bubble does not work with crude oil, for example.
- Setup is a bit more sensitive and time-consuming.
- If the solid is permeable and swells with the external liquid, this can present special challenges. Think particle board.
1 Note that Environmental
Fixture is designed to work at ambient temperature conditions. If
temperature control is required for your captive bubble study, you'll
want to use an
Environmental Chamber (p/n 100-07) with a
Temperature Controller (p/n 100-50) fitted with a
Quartz Cell (p/n 100-07-50) and
Chamber Cover with Stage (p/n 100-09) along with the Inverted
|Call for Papers|
|The Tenth International Symposium on Contact Angle, Wettability and Adhesion is scheduled for July 13-15, 2016 here in New Jersey at the Stevens Institute of Technology. If you are interested in being a presenter, please contact the organizers. More details can be found here: http://www.ramehart.com/newsletters/call_for_papers_2016.pdf.|