Modelling of a system boundary

A type of system boundary, e.g. a membrane in biology, a castle in sociology, a typhoon eye in meteorology and a skin and fur on an animal body, is defined as a shell (jieke in Chinese), if the system boundary plays roles of both defending the system and exchanging between the system and its environment. Investigates the system boundary. Derives the entropy balance equation of the shell and deduces the criterion of the exchange ratio entropy which carries material out through the shell of the system from the second law of thermodynamics of the system. Also derives a dynamic equation of the shell from control theory and gives an application of the equation to immigration control by the customs.