Represented by a capital A, change in it can be defined as the work, in a reversible process, under constant volume and constant temperature.  
The Cushman group at Purdue has shown that changes in this value determine drive flow in swelling systems not pressure gradients and external fields.  Represented by a capital A, change in it can be defined as the work, in a reversible process, under constant volume and constant temperature.  FTP, name this quantity equal to internal energy minus the product of temperature and entropy, a state function named for a German physicist.
The Cushman group at Purdue has shown that changes in this value determine drive flow in swelling systems not pressure gradients and external fields.  
Helmholz free energy