What Forces Bind Liquid Crystals?

The surface freezing transitions in four liquid crystals (9O.4, 4O.7, 7O.7 and $\overline{14}$S5) have been studied to determine the temperature dependence and the finite-size behavior of their effective interface potentials. The effective interface potentials are simply related to 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 theoretically in liquid crystals, long-range van der Waals and short-range exponential forces, and also admit the possibility of the recently predicted “thermal-Casimir” forces.