Evidence of a first-order smectic-hexatic transition and its proximity to a tricritical point in smectic films

Abstract

Experimental and theoretical studies of a smectic-hexatic transition in freely suspended films of 54COOBC compound are presented. X-ray investigations revealed a discontinuous first-order transition into the hexatic phase. Moreover, the temperature region of two phase coexistence near the phase transition point diminishes with film thickness. The coexistence width dependence on film thickness was derived on the basis of the Landau mean-field theory in the vicinity of the tricritical point (TCP). Close to TCP the surface hexatic ordering penetrates anomalously deep into the film interior.

Angular X-ray Cross-Correlation Analysis (AXCCA): Basic Concepts and Recent Applications to Soft Matter and Nanomaterials

“…While many of the known hexatic LC compounds exhibit a second-order or very weak first-order Sm-A–Hex-B phase transition [17,39,68,69], this transition in the freely suspended films of the n-pentyl-4′−n-pentanoyloxy-biphenyl-4-carboxylate (54COOBC) compound is clearly of the first order. In reference [70], an X-ray beam focused down to 2×2 μm 2 was used to scan the 100×100 μm 2 area of the 54COOBC film in transmission geometry (Figure 5). At each spatial position, AXCCA was utilized to evaluate the value of the fundamental BO order parameter C6=|Cfalse^6(q0)/Cfalse^0(q0)|.…”

“…[70,73] the observed effect was explained by an anomalously large penetration length L0 of the surface hexatic order into the interior of the film in the vicinity of the tricritical point. This influences the temperature width of the two phase coexistence even in thick smectic films consisting of thousands of molecular layers [70,73]. Thus, by varying the thickness of the LC films, one can tune the proximity of the system to the tricritical point and eventually switch between the first- and second-order Sm-A–Hex-B phase transition.…”

“…The white dashed line in ( d , e ) separates regions of Sm-A (blue) and Hex-B (green) phases. (adopted from Zaluzhnyy et al [70]). Reproduced with permission of American Physical Society.…”

“…Dependence of the temperature width of the Sm-A and Hex-B coexistence region on the LC film thickness. The solid line shows the result of fitting with the analytical equation sans-serifΔT∝(1−L0/L)2, where L0≈0.9 μm is the characteristic length scale (adopted from Zaluzhnyy et al [70]). Reproduced with permission of American Physical Society.…”

Fluctuation X-ray scattering from nanorods in solution reveals weak temperature-dependent orientational ordering

“…Practical implementation of these techniques became possible only recently, mostly due to advances in X-ray instrumentation (Kurta et al, 2016). X-ray cross-correlation analysis (XCCA) based on CCFs allowed details to be revealed of the structural arrangements in partially ordered soft-matter systems such as colloids (Wochner et al, 2009;Lehmkü hler et al, 2014;Schroer et al, 2015Schroer et al, , 2016, liquid crystals (Kurta et al, 2013c;Zaluzhnyy et al, 2015Zaluzhnyy et al, , 2017bZaluzhnyy et al, , 2018, polymer blends (Kurta et al, 2015) and mesostructures (Zaluzhnyy et al, 2017a;Mancini et al, 2016;Lhermitte et al, 2017). Correlations have also been explored in electron scattering (Treacy et al, 2005(Treacy et al, , 2007Treacy & Borisenko, 2012;Liu et al, 2013aLiu et al, , 2015 where the requirement for a small scattering volume can be conveniently achieved to observe the intensity fluctuations associated with atomic scale structures (Clark et al, 1983).…”