EVOLUTION CRITERION IN NONEQUILIBRIUM AND A VARIATIONAL PRINCIPLE FOREQUILIBRIUM STATES OF FREESTANDING LIQUID-CRYSTALLINE FILMS

Starting out with the thermodynamic balance equations of liquid crysta lline films and the second law of thermodynamics, an analog to the glo bal surface free energy is derived whose time derivative is negative d efinite out of equilibrium and zero in equilibrium. This evolution cri terion allows one to formulate a variational principle for the equilib rium states by minimizing the modified global surface free energy deri ved above. Thus the quantity which is extremal in equilibrium in the c ase of free-standing crystalline films is not presupposed ad hoc, but derived from an evolution criterion based on the second law.