Thursday, 14 November 2013

The Surface Tension Principles in Splashback

There is a great episode of Curb Your Enthusiasm where Larry David has to pee at his employee's home.  He is taking a drug that makes him pee like a race horse and subsequently he gets splash back.  Of course he could have sit down as he does in other episodes but this would not be as funny and make a good story.  So standing he gets splashback and some of that goes onto a painting of Jesus on the wall next to the toilet.  Jesus looks like he is crying and Catholics once again to believe in miracles.  Someone should teach them about surface tension as well.  However, this phenomenon of splashback is only recently being understood by some American scientists.

The team used high speed cameras and filmed jets of artificial piss striking toilet walls and spraying back.   The team used different angles and changed the distance from the toilet and determined that closeness and angle of the attack reduced the splash back.

Firstly it depends largely on the distance.  When a guy starts to pee droplets are made right away coming out of the urethra due to a phenomenon called the Plateau-Rayleigh instability in which a falling stream breaks into smaller packets with the same volume but reduces the surface area due to the liquid wanting to minimise the surface area.  This phenomenon is evident in ink jet and electrospray technology in Mass Spectrometry instruments.  To control the distance obviously a man could sit down to pee and in the above example with Larry David occasionally he does do this as mentioned in other episodes.

Also changing the angle helps as well to deal with the surface tension affects caused by scattering.  For example if you aimed directly at the water (as it seems in the above video that Larry David did) you will have a nasty splashback.  However, if you aim at the bowl which have a very hydophilic surface it may help a little.  The best is likely a tried and tested technique of adding some toilet paper to soften the blow.  Ultimately, it will be better to make superhydrophobic coatings that will make droplets on the surface and roll off before splashing back.

I am not clear whether the surface tension of the urine (which varies from   55 - 62 mN/m) plays a significant factor in the splashback and how the difference between the surface tension of the lower droplets are affected by the surface tension of the bulk water in the toilet.    The surface tension is lowered by the bile fats, the urea and some small proteins that are excreted.    Potentially pee which has a higher surface tension would give more splashback.  

Six Techniques to Reduce Back Splash 

  1. Get close and make a smooth unbroken stream.  
  2. Angle downwards or sideways instead of straight (for urinals especially)  if the urinal has a fly, aim for it.
  3. Add some toilet paper
  4. Hydrophobic coatings for toilets 
  5. Sit down to pee.
  6. Push it all out at once and be weary of the end of the stream