Science Buddies: "Ask an Expert"

After yet another outstanding year of success in science fairs, I've decided to conduct my science fair project this year on a new experiment.

This year, I'm going to be testing Water on different surfaces. I'm going to be placing droplets of water on different surfaces and measure the angle at which the water meets the surface.

For instance, when there is a water drop on a leaf, the shape that is formed is more globule whereas when it is placed on a slide, it forms an oval shape.

I realize that the main reason for this is surface tension, however, I have a few questions. How does Surface tension affect water, a liquid? Does temperature affect this?

In school, we have a microscope that we will be using to measure and view the drops at a microscopic view in order to measure the angles, etc.

Aside from the questions about surface tension, would anybody have any suggestions?

THANKS.

In school, we have a microscope that we will be using to measure and view the drops at a microscopic view in order to measure the angles, etc.

I suspect that a microscope isn't going to be very useful. The optics are perpendicular to the plane of the viewing stage and the angle you wish to observe is an angle of inclination wrt to the stage. A "macro" lens on a digital camera is a much better instrument to view how the drop bends to meet the surface it is resting on.

How does Surface tension affect water, a liquid?

I would think that this would be something you would be researching and investigating in your project. There should be plenty of information online to get you started on your research.

Does temperature affect this?

This could also be an interesting thing to research and investigate. Temperature will affect the "vapor pressure" and rate at which liquid atoms can evaporate and condense. For liquids like water, which has a tripple point at various pressures where water can exist as a solid (ice), liquid, and gas (water vapor) there is definitely the potential for some small effects or interactions with surface tension to occur. I have no idea if these temperature effects could be measured. I suspect that mechanical vibrations from environmental factors would likely be more significant factors that might prove extremely challenging to eliminate to an extent that the temperature effects might become measurable.

Part of coming up with a good Science Project is figuring out if what you need to measure in order to prove or disprove your hypothesis can be done with the equipment you have, can borrow, or can afford and have the time to accomplish. My gut feeling is that the temperature effects are probably outside what can reasonably be done with what you have available to you and in the time you have to do the project.

Craig has some good advice about the problems using a microscope to measure your contact angles. Lighting is very sensitive. There is a common instrument used for measuring contact angle that uses microscopy and critical back-lighting. It's called a contact angle goniometer. You can learn more about it here: http://www.ramehart.com/goniometers/contactangle.htm

Here are some sources you might check out to figure out other ways to measure surface tension:

http://www.ksvinc.com/surface_tension1.htm

http://www.kibron.com/company/science-t ... echniques/

Good luck with your project.

Ed Neu
Buffalo, MN

Thank you for the thoughtful and detailed responses.

In regards to the microscope, I have developed an apparatus (with the assistance of my teacher) that will let me turn the microscope on it's side, allowing the camera to take pictures of the water droplet horizontally. I from there, can analyze the picture using a program known as ImageJ, which I've used in the passed to analyze images.

The links that were provided (above) were actually extremely helpful in my project. I'm currently trying to measure the surface tension of these solid surfaces, as the "aim" of my project (persay) is going to be if Surface Tension of surfaces is going to effect the contact angle. For instance, on a leaf, which has quite a "waxy" texture to it, when a water drop falls onto it, it typically has a very globule structure. I've also found a very interesting article that explains the dynamics of when a water droplet is "dropped" (literally) onto a surface, and how 3 different types of behavior can occur, as a result of that: Splashing, Spreading and Bouncing.

At the moment, I'm just coming up with ideas to support this project. However, it is not set in stone. Are there any suggestions that could be made to possible improve my project? If I were to stick with this, maybe a few different surfaces that would be useful? I was thinking a Slide Cover, as it's close to glass, a leaf, a plastic surface, a rock (possibly marble, similar to represent a counter top, etc.). Just trying to brainstorm some different ideas, in order to make this project as successful as my last two were.

Appreciate the responses, and looking forward to reading them in the future

Mike

Michael --

Very cool idea for a science project ... I hope to be able to see your results at some point. Since it seems like you're seeking help with idea generation, here are some more ideas for questions to ask and things to investigate. I'm just brainstorming randomly here; I haven't actually researched or thought through any of these carefully, and I don't have any specific expertise with this topic. Use this as food for thought.

* How does the *amount* of water affect the shape? In other words, what is the shape of the water if you place one droplet of water vs. two droplets vs. three droplets? How accurately are you able to make sure that "one droplet" really does contain the same amount of water in each experiment?

* Other surfaces to consider trying ... a copper (or other metal) plate, a slice of wax paper (preferably glued to a hard surface), smooth wood surface, surface of an inflated balloon (although that might be too challenging to measure), a magnet, ceramic.

* What about other liquids with different viscosities, like oil, vinegar, maple syrup? Do they change shape with different surfaces the same way water does?

* What happens to the shape of the droplet if you angle the surface? (i.e., Instead of horizontal, place the surface at different angles, like 10 deg., 20 deg., 30 deg., as long as it's not steep enough to make the droplet start to flow downhill.)

* Probably hard to come by, but if you can get your hands on a high-speed video camera with a macro lens, then you could study the dynamics (as you already mentioned) of the droplet striking the surface and bouncing, spreading, and splashing, and see if the dynamics vary depending on the surface.

* Probably a long shot, but can you measure the differences in shape of the droplet by trying to pass a laser beam through the droplet, and finding the angle at which the beam of light exits the droplet?

Hopefully this gets your creative juices flowing! Good luck!

--Rajeev

Very inventive! It sounds like you are essentially creating a goniometer with the microscope.

Surface tension (measured in force per unit length) can be related by the Young Equation which is explaiend in this link
http://en.wikipedia.org/wiki/Contact_angle which basically means:

Surface tension between solid (S) and vapor (V) - Surface tension between solid (S) and droplet (L) - Surface tension between droplet (L) and vapor (V) x cos theta (the contact angle) = 0

Ysv - Ysl - Ycostheta = 0

Wikipedia does a pretty good job of explaining how this phenomenon is really a result of the net force being equal to 0
"Surface tension is caused by the attraction between the liquid's molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which is balanced only by the liquid's resistance to compression, meaning there is no net inward force. However, there is a driving force to diminish the surface area. Thus the liquid squeezes itself together until it has the locally lowest surface area possible."