Diffuse Interface

1
Diffuse Interface

• In this experiment the occurrence of a diffuse
  interface becomes apparent. When there are two
  regions of a liquid of vastly different
  concentrations in contact with each other then a
  diffuse interface between the two regions forms.
  The thickness of this region increases in time as
  diffusion reduces the concentration gradient.
  Such an interface does not have interfacial
  tension or a sharp boundary but it acts like an
  interface in many ways.

2
A thimble experiment on diffuse interfaces

• To see the diffuse interface, take a tall jar of
  water and put in rock salt (ice cream salt) heat
  this jar from below with a non-magnetic heater.
• After a few minutes you will observe a salty
  layer forming a diffuse interface between salty
  and fresh water. Take a light metallic thimble
  (we make ours from cookie sheets) and orient it
  so that its open end is down. Remove most but not
  all of the air in it and let the thimble sink.
  After some time it will rise up to the diffuse
  interface and then it will come back down in
  repeated oscillatory motion.
• If you have a thermometer you will observe a
  sharp change in the temperature at the diffuse
  interface

3
Thimble Explanation

• The rock salt used in this experiment will
  dissolve and create a salt saturated layer in the
  bottom of the beaker (since salt water is more
  dense than fresh water it will remain the bottom
  layer). This creates a diffuse interface between
  the salty water and the fresh water.
• As the beaker is heated the layer will move
  farther towards the air-liquid interface as more
  salt is dissolved and the temperature of the
  fresh water will be much less than the salt layer
  (as much as 100F difference in temperature)
• When the thimble is placed in the beaker, a small
  pocket of air is trapped in the thimble. Due to
  the weight of the thimble it will sink down into
  the salt layer which is much hotter than the
  fresh water layer. The heat is transferred from
  the salt water through the thimble and into the
  trapped air. This trapped air expands and makes
  the thimble more buoyant than the salt water and
  the thimble will rise. When the thimble reaches
  the cold fresh water, the air will then be cooled
  and then contract. The thimble sucks up salty
  water on its upward journey. So there is salty
  water in the thimble and fresh water outside the
  thimble and the thimble falls. The thimble will
  then sink into the hot salt water and the process
  is repeated is an oscillatory manner.