On screen chemistry with Jonathan Hare
In the film Shanghai noon, Chon Wang (Jackie Chan), a Chineseman in the US and his new-found friend, Roy O'Bannon (Owen Wilson) a small-time robber, are trying to escape from an old-style Wild West jail which has metal bars from ceiling to floor. Wang does a very odd thing. He takes off his shirt, tears it up into strips and urinates on them. Holding them up he proclaims: 'when the shirt gets wet it doesn't break'. Then knotting some of the cloth around a pair of prison bars and using a broken -off wooden chair leg as a lever, he tightens the knot which bends the bars so he and his mate can get through and escape! So is this possible? Is wet cloth really stronger than dry cloth?
A trip to the launderette
To start my investigations, I went down to the local launderette to ask the staff what they thought. They weren't sure if wet clothes were stronger but they seemed to think that wet clothes were less likely to tear or get damaged in their machines (apart from wool perhaps).
I then tested strips of cotton by hanging weights from them made from buckets which I could slowly fill with water (not urine) to make them heavier. On average, a heavier weight was required to break the wet cloth than the dry cloth. So the wet cloth did seem to be stronger. Also the leverage obtained by the cloth knot and wooden stick was enough to bend a steel bar 2.4 m long, similar to those used in the jail in Shanghai noon. (Note: I don't think the jail-break stunt would have worked if there had been a horizontal bar welded about halfway up, as is the case in modern jails.)
And the chemistry?
Cotton and paper are mainly composed of cellulose, a very large molecule (a polymer) made up of several hundred glucose molecules linked by an O atom (see structure). Now wet paper is definitely not stronger than dry paper. Paper absorbs a lot of water, making it heavier and separating the fibres - so it falls apart. So what could be happening in the wet cotton fibres to make them stronger?
If molecules attract each other, the resulting intermolecular forces can sometimes be considerable. In water, for example, hydrogen bonding is so strong that at room temperature and pressure it is a liquid rather than a gas. With this in mind, could the forces between the closely spaced cellulose molecules in the cloth fibres be enhanced by hydrogen bonding when wet? Could this explain the increased strength of Wang's cloth?
Ask the experts
Recently, I was giving an end of conference talk to a group of professional scientists. Just for fun I described the Shanghai noon clip and asked them what they thought about the hydrogen bonding idea. I was amazed at the debate the question started. Some scientists thought the hydrogen bonding would be significant while others were equally adamant it could not be.
The general conclusion was that the molecules making up the cotton fibres were probably too widely and randomly spaced on a molecular scale for the short range intermolecular forces to have an effect. So hydrogen bonding is probably unlikely to account for the increased strength of wet cloth.
However, in cotton the cellulose fibres are not just pressed or glued together as they are in paper but are twisted around each other like a fine rope, making it very strong. Perhaps the most plausible explanation is that the water causes the fibres to swell, increasing the friction between them and thus making the cloth harder to tear... which was, after all, the conclusion the staff in the local launderette came to.
Dr Jonathan Hare, The CSC Centre, Chemistry Department, University of Sussex, Brighton BN1 9ET
This article was originally published in The Mole