SURFACE TENSION

Samaresh  Goswamy 

WHAT  ARE  NEEDED:

*   A Plastic  Bowl  of  medium  size 

*  Water 

*  A Plastic  shaped  Arrow shaped  float with a'V' cut

*  some detergent  powder 

WHAT  TO  DO  :

*  Float the  Plastic  shaped  Arrow on the  Water in  the  Bowl 

* ADD a pinch of  detergent  powder in  the  water near the  'V' notch of  the  Plastic  Arrow 

WHAT  TO  OBSERVE  :

*As a  pinch  of  detergent  powder  is added  near the  V notch of  the  Arrow  it starts moving  forward. 

 WHAT  IS  GOING  ON  :

Due to  surface  tension  the  free surface  of  water behaves like  a  thin stretched membrane where equal  forces of surface  tension  acts in all sides.As you  float the  Plastic  Arrow  in water,  it is  kept on  the  free water  surface. Equal  forces of surface  tension  acts on all sides on it causing  no resultant  force,and  you  see it remains  stationary  on the  surface. 

When you  add a  pinch  of  detergent  powder in the  water  near  the   V  notch at the  rear end of  the  Arrow,  the  detergent  powder  goes in the  water  and  makes  a  thin  soap film, which  spread  around  the  rear end.This reduces  the  surface  tension  of that  particular  portion  than anywhere. AT this  stage,  forces  due to  surface  tension acting from  all sides can  no more  remain  equal,  as forces  acting on rear end became  less  than  the  forward forces. So a resultant  forward  force starts  acting  on the  Arrow DUE to  this  resultant  forward  force of surface  tension, the  Arrow  moves  forward  like  that  of a boat.

Prof. Samaresh Goswamy