Abstract
Using video microscopy, we investigated melting of a two-dimensional colloidal system, formed by glycerol droplets at the free surface of a nematic liquid crystalline layer. Analyzing different structure correlation functions, we conclude that melting occurs through an intermediate hexatic phase, as predicted by the KosterlitzThouless-Halperin-Nelson-Young(KTHNY) theory. However, the temperature range of the intermediate phase is rather narrow, . 1 C, and the characteristic critical power law decays of the correlation functions are not fully developed. We conclude that the melting of our 2D systems qualitatively occurs according to KTHNY, although quantitative details of the transition scenario may partly depend on the details of interparticle interaction.
Highlights
Two-dimensional (2D) ordering and the formation of periodic 2D crystal-like structure, less familiar than 3D crystallization, is still widespread enough and can even be observed in the cup of morning coffee as Benard convection cells [1]
Laboratory 2D systems are usually formed by colloidal particles that are large enough to be individually observable by optical means, so that the structure and even individual particle trajectories can be monitored by optical microscopy, making the complete structural and dynamical data readily experimentally accessible [2]
Charged dust particles in “dusty plasma” readily form the so-called Wiegner or Coulomb crystal, whose melting transition can be experimentally investigated by optical means [3]
Summary
Two-dimensional (2D) ordering and the formation of periodic 2D crystal-like structure, less familiar than 3D crystallization, is still widespread enough and can even be observed in the cup of morning coffee as Benard convection cells [1]. Similar conclusion followed from a recent work on charged millimeter-sized steel balls [20] Another experimental work on uncharged polymer microspheres [16], while confirming the existence of hexatic phase, concluded that the liquid-to-hexatic and hexatic-to-solid transitions are strongly first order transitions. There has been discovered a new class of self-ordering 2D colloidal systems, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (NLC) [21,22,23] These colloids, which interact via liquid crystal mediated elastic forces, readily form hexagonal crystals that exhibit melting transition at temperatures in the range Tmelting = 26÷28◦C, which are relatively far from the temperature of the nematic-isotropic liquid transition, TNI =36◦C. We perform an analysis of the melting transition in these systems in an attempt to test the applicability of KTHNY and its predictions to this novel class of 2D lattices
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