Marian Smoluchowski Institute of Physics, Department of Statistical Physics, Jagiellonian University, 30-348 Kraków, Poland;
Faculty of Physics, Mathematics, and Computer Science, Tadeusz Kościuszko Cracow University of Technology, 30-084 Kraków, Poland.
Proc Natl Acad Sci U S A. 2018 Oct 30;115(44):E10303-E10312. doi: 10.1073/pnas.1721786115. Epub 2018 Oct 11.
The response of the nematic twist-bend ([Formula: see text]) phase to an applied field can provide important insight into the structure of this liquid and may bring us closer to understanding mechanisms generating mirror symmetry breaking in a fluid of achiral molecules. Here we investigate theoretically how an external uniform field can affect structural properties and the stability of [Formula: see text] Assuming that the driving force responsible for the formation of this phase is packing entropy, we show, within Landau-de Gennes theory, that [Formula: see text] can undergo a rich sequence of structural changes with the field. For the systems with positive anisotropy of permittivity, we first observe a decrease of the tilt angle of [Formula: see text] until it transforms through a field-induced phase transition to the ordinary prolate uniaxial nematic phase (N). Then, at very high fields, this nematic phase develops polarization perpendicular to the field ([Formula: see text]). For systems with negative anisotropy of permittivity, the results reveal new modulated structures. Even an infinitesimally small field transforms [Formula: see text] to its elliptical counterpart ([Formula: see text]), where the circular base of the cone of the main director becomes elliptic. With stronger fields, the ellipse degenerates to a line, giving rise to a nonchiral periodic structure, the nematic splay-bend ([Formula: see text]), where the two nematic directors are restricted to a plane. The three structures-[Formula: see text], [Formula: see text], and [Formula: see text]-with a modulated polar order are globally nonpolar. But further increase of the field induces phase transitions into globally polar structures with nonvanishing polarization along the field's direction. We found two such structures, one of which is a polar and chiral modification of [Formula: see text], where splay and bend deformations are accompanied by weak twist deformations ([Formula: see text]). Further increase of the field unwinds this structure into a polar nematic ([Formula: see text]) of polarization parallel to the field.
向列-扭曲双稳态 ([Formula: see text]) 相对外场的响应可以为这种液体的结构提供重要的见解，并可能使我们更接近于理解非手性分子液体中手性破缺的机制。在这里，我们从理论上研究了外场如何影响 [Formula: see text] 的结构特性和稳定性。假设形成这种相的驱动力是堆积熵，我们在 Landau-de Gennes 理论中表明，[Formula: see text] 可以在外场作用下经历丰富的结构变化序列。对于介电各向异性为正的系统，我们首先观察到 [Formula: see text] 的倾斜角减小，直到它通过场诱导的相变转变为普通的扁长单轴向列相 (N)。然后，在非常高的场强下，这种向列相产生垂直于场的极化 ([Formula: see text])。对于介电各向异性为负的系统，结果揭示了新的调制结构。即使是一个无穷小的场也会将 [Formula: see text] 转变为它的椭圆对应物 ([Formula: see text])，其中主向矢圆锥的圆形基底变为椭圆。随着场强的增强，椭圆退化为一条线，产生非手性周期性结构，即向列展曲 ([Formula: see text])，其中两个向列指向被限制在一个平面内。这三种结构-[Formula: see text]、[Formula: see text] 和 [Formula: see text]-具有调制的极序，全局是非极性的。但是，外场的进一步增强会诱导出具有沿场方向非零极化的全局极性结构的相变。我们发现了两种这样的结构，其中一种是 [Formula: see text] 的极性和手性变体，其中展曲和弯曲变形伴随着弱的扭曲变形 ([Formula: see text])。进一步增加外场会将这种结构展开为具有平行于场的极化的极性向列 ([Formula: see text])。
