Here is a kitchen sink (literally) experiment I did today. I filled it with water. Floated a polythene sheet in it. Put a few quarters, dimes, and pennies on the sheet. It wrinkles! The white sheet at the bottom of the sink is so that my photo of the wrinkles would be clearer. It looks obvious that wrinkles would form, and the experiment itself looks pointless, right?
Not so, if the film that you are floating in water is a polystyrene film of thickness between 31 and 233 nanometres. Huang et al (1) report results of such an experiment in the latest issue of Science.
In such ultra thin films of nanometre thickness, a simple experiment of this type, with the films being loaded by water droplets, and the measurements of the number of wrinkles and length of the wrinkle can lead to a determination of the elastic moduli of the films. Further, the time dependent relaxation of the wrinkle patterns can be used to obtain the viscoelastic properties of the films.
Of course, there are some important differences between my kitchen sink experiment and that of Huang et al which are worth emphasizing.
- As Huang et al note in their paper, there is a crucial difference between loading with a solid (where the weight of the solid is the key factor), and loading with a liquid (where the capillary force due to the interfacial surface tension is the key factor).
- Obtaining the elastic properties of the films means that when the water droplet is removed, the film gets back to its original shape; in my case, it did not happen–the plastic sheet got wrinkled permanently. It is interesting to note that Huang et al also observe some hysteresis.
- In my kitchen sink experiment, there was no possible relaxation mechanism; in fact, after while, water started flowing in to the ridges, sinking the entire sheet.
A commentary by Aline Miller(2) is also available for the paper; Miller explains how the experiment is used not only to obtain the elastic constants, but also the thickness of these films:
As the mass of the drop–and hence its radius–increased, both the number and the length of the wrinkles increased. In addition, as the film became thicker, the number of wrinkles decreased, whereas their length increased. The authors used these results to develop two scaling relationships (one for the length and one for the number of wrinkles), adapting arguments made by Cerda and Mahadevan(3). Combination of these two scaling relationships shows that if the length and number of wrinkles are known from experimental measurement, the elasticity and thickness of the film under study can be determined by a simple calculation.
A wonderful paper and a nice commentary. Have fun and happy reading!
References:
3. E. Cerda, K. Ravi-Chander, L. Mahadevan, Nature 419, 579 (2002); E. Cerda, L. Mahadevan, Phys. Rev. Lett. 90, 074302 (2003).
Entertaining Research 
August 2, 2007 at 9:47 pm |
Interesting. Any ideas on what this observation (and the underlying math) could be used for?
On a discontinuous note, check out S. Rajeev’s blog
http://sgrajeev.com/. He has some interesting posts on fluid mechanics and other things.
August 2, 2007 at 11:45 pm |
Dear Biswajit,
My initial idea was just to see what will happen if the plastic film thickness is of the order of a millimetre or less; in other words, I was just checking if the method would work only for nanometre thick films or if it can be used for much thicker ones.
I found that if the sheet is too thick, it just supported some coins before sinking, and did not form any wrinkles at all–so, there seems to be an upper limit in thickness above which no wrinkles form. But that thickness is of the order of a millimetre or so.
The plastic sheet that is used to wrap newspapers is the one I show in the photograph above. But, as you can see, it is not good for measurements.
If I can get still thinner (cellophane) paper, I might be able to get more regular (concentric) wrinkles; but probably, I have to have a large sheet and do the experiment on the bathtub. If I can do that, provided the deformation is not permanent, it might be a nice way of getting the thickness as well as elastic properties of the plastic sheet, which, I think, is sort of a cute experiment for learning about this method for students. Apart from that, I do not see much use for the experiments.
Of course, I may have to use equations meant for weight loading unless I manage to get wrinkles by placing water droplets. I tried loading with water droplets with my kitchen experiment. It did not work because the water droplets just ran off the surface!
And, by the way, thanks for the pointer; I will add Rajeev’s blog to my reading list and blogroll.
November 23, 2007 at 12:52 am |
[…] C Srinivasan on a method based on wrinkle formation for the determination of elasticity and thickness of ultrathin films (pdf) (which is based on the same paper which set me into doing some kitchen sink experiments). […]
August 25, 2011 at 8:16 pm |
[…] time readers of this blog know of a kitchen sink experiment I did nearly four years ago. Thanks to an email alert from my colleague and friend Shirish […]