We show that director fluctuations in nematics induce long-range interactions between walls, analogous to van der Waals forces, which are attractive with symmetric boundary conditions, but may become repulsive with mixed ones. They affect the swelling of lamellar phases in a nematic solvent. The same phenomenon takes place in other mesophases, particularly in smectics, where it gives rise to forces with longer range than van der Waals.

• Received 12 November 1990

DOI:https://doi.org/10.1103/PhysRevLett.66.1481