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Colloids Interfaces and Surfaces 1. what is the principle behind the Lotus effect? Mention possible applications. 2. If you connect two soap bubbles of different sizes, through a glass tube, what will you observe? Explain what are the theoretical considerations behind it 3. Explain why drops are formed when a thin stream of water is flowing from an open faucet- What would be different if instead of water you have a water/cthanol stream? 4. why does water rise in glass capillaries and not in TEFLON capillaries with the same dimenSions? 5. Explain in detail two methods for measuring surface tenSion. Can you incasure also contact angle with the same method? 6. The diagrams below represent capillaries of varying conStruction and arrangement The diameter of the capillary portion is the same in each case and all the capillaries are conStructed of glass, unless otherwise indicated. The equilibrium risc for water is shown at the left. Draw meniscuses in each figure to correSpond to (a) the level reached by water rising up the clean, dry tube and (b) the level to which the water would recede after having been sucked upto the end of the capillary. The meniscus in the capillary may be assumed to the spherical in shape 7. Give two practical examples for which capillary rise is a fundamental issue and justify. 8. Compare two open systems at the same temperature (a) bowl containing water without stirring (b) same system under Stirring forming air bubbles dispersed in the liquid phaSe If you had the chance to incasure the humidity level justabove the water surface in case (a) and inside the bubble in case (b), where would you find larger values? Give a clear theoretical justification. 9. Carbon molecular sieve (CMS) membranes are expected to separate vapors (or gases) by size. However when working with a vapor feed mixture some of the very successful CMS membranes are more permeable for butane than methanc. How the equation of Young-Laplace can be used to explain it and how increasing the pore size would affect the performance. 10. You should develop a water-proof textile and can use fibers of the following polymers' PTFE, PET or polycthylene (contactanglo 150, 79 and 103°, respectively). Which material you would choose? Justify. How can texture affect the performance? Explain 11. Explain how detergency works. 12. Why do surfactant form micelles in solution? 13. What is the main reason for foam instability? How can you retard the break of soapbubbles? 14. What is Ostwald ripening? 16. A surfactant is known to lower the surface tension of water and also is known to adsorb at the water-oil interface but to adsorb appreciably at the water-fabric interface- Explain briefly whether this detergent should be useful in (a) waterproofing of fabrics or (b) in detergency and the washing of fabrics.

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