IYNT 2017 NANJING Team BULGARIA Problem 16 Paper wrinkles Lachezar Dimitrov BULGARIA

The problem When a piece of paper dries after being wet, it can get wrinkled. Investigate and explain this phenomenon. Our aims are to explain this effect and to compare it with wetting by another liquid (cooking oil and ethanol).

the composition of paper Paper is composed of cellulose biopolymer that forms long chains. Cellulose is synthesized in plants from bonded glucose residues. The chains are held together by weak hydrogen bonds. paper under microscope cellulose chains with hydrogen bonds glucose The main chemical in paper is cellulose. It is a biopolymer, produced in plant cells. It is accumulated in big amounts in wood mass, but the most pure natural cellulose is cotton - over 90%. The chemical is composed of bonded glucose residues- up to 30 000 arranged in long strain chains. It's important to say that glucose has many hydroxyl groups attached. Paper may contain additives that improve its water resistance or some colorants and optical brighteners. Paper contains long cellulose chains arranged in determinated way. The chains bond together because of weak hydrogen bonds that arise between oxygen and hydrogen atoms from hydroxyl groups. These bonds are much weaker than the covalent ones between the other atoms. https ://c1.staticflickr.com https://upload.wikimedia.org https://i.stack.imgur.com/JUzVl.png

interaction with water

Hydrogen bonds Hydrogen bonds are interactions between covalently bonded hydrogen to highly electronegative atom and another electronegative atom. They arise between hydroxyl groups in cellulose chains in paper and hold them together. These hydrogen bonds hold polymer chains together Hydrogen bonds between water molecules

Paper – water interaction Water permeates into the paper and interact with hydroxyl groups in cellulose chains. It breaks the hydrogen bonds and enfolds the chains. This takes place when water is absorbed by paper. Hydrogen bond breakage facilitated by water molecules Hydrogen bond bridge Wall fiber

How do wrinkles appear When hydrogen bonds are broken nothing keeps the cellulose chains together and they scatter. When water escapes the polymer chains are left in a different position and the wrinkles are formed.

Experimental part In our experimental part we will show wrinkle forming on paper by water. We also want to know if other liquids can do the same. Our liquids are cooking oil and ethyl alcohol. Cooking oil is a liquid fat. It contains glycerin and fatty acid residues. The long carbon chains make it hydrophobic so it doesn’t interact with water and hydroxyl groups. Ethanol has polar and non polar side.

Set up In our experiments we used: -different kinds of paper -water -oil

Water wetting Water permeates thought different papers in different rates but almost all of them get wrinkled. wet samples almost dry samples

Ethanol wetting Ethanol makes wrinkles on paper as water does because it has similar structure. wrinkles from water wrinkles from ethanol

Oil wetting Samples wetted with oil don’t get wrinkled. But oil permeates into the paper easily because there is a space between cellulose chains.

Conclusion We found that non polar liquids don’t make wrinkles on paper but permeate into it because of the paper structure. Also some papers get wet faster than others - the rate depends on paper processing.

Thank you for your attention! 謝謝