Originally Posted by
MasterRolf
Lather is a very complex mixture consisting of soap, water and air. You may think the soap has simply disolved in the water, but in fact the process is somewhat more complicted involving the release of a sodium ion from the salts that make up the soap. This produces a radical that has both a hydrophilic and a hydophobic side. The hydrophilic side wants to be wetted by the water while the hydrophobic side wants to get as far away from the water as possible. The net result is a cluster of water droplets surrounded by a fringe of soap radicals with the hydrophobic tails sticking out. These clusters are what trap the air to make a lather.
This alignment of the soap radicals takes a little time, thus allowing the mixture to set gives time for all of the molecular species to order themselves correctly. This produces the phenomenon you have noticed. Obviously the resulting optimum alignment is better able to trap air, which explains why additional whipping often improves the quality further.