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 100°F 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.