|M.Sc Student||Sarit Feldman|
|Subject||Pattern Formation of Pentane on a Water Substrate near|
|Department||Department of Physics||Supervisors||Professor Emeritus Lipson Stephen|
|Dr. Raz Eli|
|Full Thesis text|
When a material is inserted into a closed cell containing a substrate on which it can float, and the material vapor pressure is close to saturation, there are different ways in which the material will accumulate on the substrate. In the case of alkanes on water, it depends on the chain length of the alkane. In general, short alkanes create a wetting layer, while long alkanes only form drops. The behavior of pentane, which is of intermediate length, is more complex. It form drops at low temperatures, a uniform thick layer at high temperatures, and in between has an intermediate state of coexistence between a layer and flat drops.
We conducted research on pentane on a water surface at different temperatures, all in the range corresponding to the intermediate state, in the presence of air and in vacuum too, a condition in which such an experiment hadn’t yet been performed. Our goal was to find the contact angles that the flat pentane drops formed with the surface. These drops have been observed in a previous experiment, and independently their contact-angle range had been numerically estimated, but contact angles of pentane on water had not been measured before.
An optical setup was built for imaging the water surface wetted by pentane. Light incident on the surface, is reflected and redirected. The beam of light is divided into two beams differing in their optical path length. Both beams are recorded simultaneously. The differences between the image of one beam compared to the other enable us to find the phase on either recorded plane. The phase on the water surface is then found by simulating light propagation to its image plane and subtracting parasite quadratic phase whose origin is not in the water surface.
The contact angle of the drop is calculated from the phase of the reflected light by comparison to the expected phase of light reflected by a flat drop. The results obtained don’t show significant dependence of the contact angles on temperature, but the range of the contact angles is in agreement with the range that was numerically estimated in previous work. The phase profiles of the drops are qualitatively correct in that they have the expected shape for a drop on a flat surface. In addition, calibration of the optical setup shows accuracy of 3.5 to 10 percent for phase retrieval from an object with abrupt phase shifts.