Structural forces in liquid crystals

This article is a short review of our work on forces in a liquid crystalline medium that has been carried out in recent years. Using an atomic force microscope (AFM), the force between the glass substrate and ≈10 µm glass sphere has been measured. In all experiments, the glass surfaces have been treated for a homeotropic alignment to achieve controlled orientation of the liquid crystal, confined between the surfaces. Force measurements with different liquid crystals have been done in their isotropic phase, but close to the phase transition to an ordered liquid crystal phase. We have found two different mechanisms that influence the force between objects immersed in a liquid crystal. The first is due to a pretransitional partial ordering of the liquid crystal close to the phase transition. Forces of this type are relatively weak and their range is limited by the correlation length of the liquid crystalline order. The second mechanism is a phase transition in the gap between the ordering surfaces. In this case, the force appears suddenly at certain distance and is strongly temperature dependent. The force in this case is stronger, but limited to a narrow temperature range, just above the phase transition.