Equilibrium surface force measurements in thin liquid crystal films

Surface force measurements provide deep insights into the structure and response of liquid crystals (LCs) near surfaces and in nanoscale confinement geometries. Thin LC films in the nematic, smectic-A, and cholesteric phases generate equilibrium repulsive forces that are a direct expression of the order, elasticity, and defect-mediated plasticity of the phase. On the other hand, surface interactions modify the molecular order over nanoscale distances and can produce new structure and response in nanoscale LC films. In this review, we discuss equilibrium surface force measurements obtained in thin LC films using the surface forces apparatus (SFA) and the atomic force microscope (AFM). The results illustrate general theoretical concepts related to surfaces and confinement, such as phase frustration, heterophases, capillary condensation, and defect nucleation, many of which are yet to be explored in ferroelectric, spontaneously modulated, and chiral LC phases. These concepts are relevant to technological applications as nanoscale LC films can effectively modulate colloidal interactions in response to external stimuli in composite materials and are increasingly used in nanoscale photonic devices as polarisation-controlling elements.