We have been able to observe dislocations in liquid crystals by taking advantage of the characteristics of the second order smectic A-to-smectic C transition. The two materials N-p-heptyloxybenzylidene-p-heptylaniline and n-undecyl-p-azoxy-a-methyl-cinnamate (AMC 11) have been studied so far. In a series of micrographs at small temperature intervals we show how the dislocations become gradually visible when the phase transition point is approached from above or from below. We discuss the phenomenon by making a Landau development of the free energy density, allowing an order of magnitude estimation of the temperature range for observations. The Burgers’ vector of the dislocations is determined by an interference measurement and found equal to one unit of smectic layer thickness.
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Lagerwall et al. (1978) studied this question.