Foam can be seen in various places, from cream on top of Cappuccino to large‐scale structure of the Cosmos.
When you redraw U.S. county map, scaling the area of each county in proportion to its population while keeping adjacency, You can observe mixture of a few large megalopolis and many small counties in-between.
What dynamics work behind these "Foam"s ?
Geographical map painted in proportion to the data, Choropleth map, is often misleading when the data is non-geographical, such as population growth or economic activities.
Continuous Area Cartogram is a "redrawn map", to avoid misleading visual image, by deforming each area little by little while keeping neighbouring relationship, so as to represent data by its size.
Here are points, as far as I noticed, cartogram can show clearly.
Hispanics : In geo map, you can see highly dense Hispanic area along the Mexican border. But in cartogram you find two large hispanic cores on the both end of the border; Hidalgo county (McAllen) and Los Angels. Miami-Dade gets much bigger than in geo map.
Asians : Concentration of Asian in California is clear in geo map. However in cartogram, another large core of asians emerges around New York and Washington DC. Their concentration on both coasts is extraordinary. In Queens of NY and Santa Clala of California, they are going to outnumber Whites.
Population : In geo map, in the western half of the Central time zone and the eastern half of Mountain time zone (blue), you can see population there decreased from 2000 to 2010. However these areas are sparsely populated. The big decrease of population is remarkable in cartogram; Wayne county (Detroit) is the Hometown of U.S. Automobile industry and Michigan is the only state which shrinked this decade. Orleans Parish (Louisiana) was devastated by Hurricane Katrina in 2005.