WHAT FORCES BIND LIQUID-CRYSTALS

The surface freezing transitions in four liquid crystals (9O.4, 4O.7, 7O.7 and <(14)over bar>S5) have been studied to determine the temperat ure dependence and the finite-size behavior of their effective interfa ce potentials. The effective interface potentials are simply related t o the attractive pair potentials between the molecules, and were used to probe the forces that bind these liquid crystals. Our measurements provide strong evidence for the two attractive forces expected theoret ically in liquid crystals, long-range van der Waals and short-range ex ponential forces, and also admit the possibility of the recently predi cted ''thermal-Casimir'' forces.