Birefringence suppression near the smectic A–smectic C phase transition in de Vries-type liquid crystals

Abstract

Two smectic liquid crystal materials exhibiting a de Vries-type smectic A to smectic C transition with low layer shrinkage were studied using polarised microscopy. Birefringence and tilt angle was measured as a function of temperature using a novel camera and software system. The analysis showed unusual features in birefringence, which had a peak within the smectic A phase and decreased significantly with cooling for a ∼15 • C range above the smectic A-smectic C transition in both materials. The reduction in birefringence above the smectic A-smectic C transition was interpreted with a critical fluctuation model, which allowed calculation of the heat capacity critical exponent α. Exponents were not in a clear universality class, similar to previously reported results on other non-de Vries materials. However, a recently published mean field model for de Vries smectics provided an alternate explanation for birefringence suppression. Five analyses were performed on the birefringence and tilt angle data and were interpreted using new predictions of this mean field model. Our results for the observed reduced temperature range |T − T C | T C > 10 −3 , where T is temperature and T C is the smectic A-smectic C transition temperature, were consistent with all mean field theory predictions for a transition near or at a tricritical point.

Critical behavior of the optical birefringence at the nematic to twist-bend nematic phase transition

“…The decrease of the birefringence is due to the presence of strong fluctuations of the director in the nematic phase caused by the instantaneous formation of local heliconical states. Such fluctuations lead to local tilting of the molecules, and thus decrease the difference between the eigenvalues of the dielectric tensor at optical frequencies, which results in the smaller optical anisotropy of the phase [25][26][27]. The temperature range of strong tilt fluctuations for CB6OCB is consistent with those deduced from the NMR results [17].…”

N−SmA−SmCphase transitions probed by a pair of elastically bound colloids

“…Thus we conjecture that the interparticle separation D is coupled to the order parameter (θ ) of the SmA-SmC phase transition. It may be mentioned that several physical properties such as birefringence and elastic modulus of the SmA phase shows the effect of strong pretransitional fluctuations across SmA to the tilted phase (SmC or SmC*) transition [47][48][49][50]. In the present system we do not observe any significant pretransitional effect except a small and linear variation of D just above the SmC to SmA phase transition.…”

Observation of “de Vries-like” properties in bent-core molecules

“…The local maxima, so-called parabolic behaviour at the SmA phase followed by a dip in birefringence at the SmA–SmC transition is known to be a typical characteristic of compounds exhibiting the de Vries SmA–SmC transition. 53–56 Such a behaviour is theoretically explained to originate from the increasing fluctuations of the tilt angle as the SmA–SmC transition temperature is approached. 57,58 A quantitative estimation of the tilt fluctuations can be made from the temperature-dependent birefringence above the SmC to SmA phase transition.…”

“…In the absence of critical fluctuations and for small angles, Δ n in the SmA phase can be expressed as:where 〈 δθ 2 〉 is the mean square fluctuation of the tilt angle, Δ n ( T ) is the measured birefringence and Δ n o is the predicted birefringence in the absence of critical fluctuations. 53,56 The dashed line in Fig. 6b shows predicted birefringence Δ n o , which was obtained by extrapolating the data away from the pre-transitional suppression region of the SmA phase.…”

“…6b shows predicted birefringence Δ n o , which was obtained by extrapolating the data away from the pre-transitional suppression region of the SmA phase. 53,58 Fig. 7 shows the variation of the calculated mean square fluctuations 〈 δθ 2 〉 with the reduced temperature t = ( T − T AC )/ T AC .…”