Direct Measurement of Interaction Forces Between Islands on Freely Suspended Smectic C Films Using Multiple Optical Tweezers

Abstract

Smectic liquid crystals can be made to form freely suspended films, two-dimensional systems locally quantized in thickness by an integral number of smectic layers, on which islands, circular regions of greater thickness than the surrounding film area, can be generated. In smectic C films, each such island is accompanied by a topological defect pair, an s = +1 topological defect inside and an s = −1 defect nearby on the background film. The distortions of the in-plane orientational order of the smectic C director field result in elastic interactions between the islands, with a short-range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation. We demonstrated previously that such islands can be manipulated using optical tweezers. Using an acousto-optically scanned infrared laser system to generate dynamically controllable, multiple optical traps, we have now directly measured the repulsive and attractive elastic interaction forces between smectic C* islands and have compared the results quantitatively with theory. We find that the interactions between islands are much smaller in the racemic smectic C case than in the chiral smectic C*, an effect we attribute to long-range coulombic forces arising from polarization charges.