Chen Xuemei, Hamlington Benjamin D, Shen Amy Q
Mechanical and Aerospace Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
Langmuir. 2008 Jan 15;24(2):541-6. doi: 10.1021/la701844s. Epub 2007 Dec 15.
In this paper, we focus on the isotropic-to-nematic phase transition in a liquid-crystal droplet. We present the results of an experiment to measure the growth of the nematic phase within an isotropic phase liquid-crystal droplet. Experimentally, we observe two primary phase transition regimes. At short time scales, our experimental results (R(t) approximately t0.51) show good agreement with a Stefan-type model of the evolution of the nematic phase within the isotropic phase of a liquid crystal. As time progresses, the growth of the nematic phase is restricted by increased confinement of the droplet boundary. During this stage of growth, the nematic phase grows at a slower rate of R(t) approximately t0.31. The slower growth at later stages might be due to a variety of factors such as confinement-induced latent heat reduction; a change of defect strength during its evolution; or interactions between the defect and the interface between the liquid crystal and oil or between adjacent defects. The presence of two growth regimes is also consistent with the molecular simulations of Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) who identify an early stage domain regime and a late stage confinement regime. For the domain and confinement regimes, Bradac et al. (Bradac, Z.; Kralj, S.; Zumer, S. Phys. Rev. E 2002, 65, 021705) obtained growth exponents of 0.49 +/- 0.05 and 0.25 +/- 0.05. These are remarkably close to the values 0.51 and 0.31 observed in our experiments.
在本文中，我们聚焦于液晶微滴中的各向同性向向列相转变。我们展示了一项实验的结果，该实验用于测量各向同性相液晶微滴中向列相的生长情况。通过实验，我们观察到两种主要的相变机制。在短时间尺度下，我们的实验结果（R(t) 约为t^0.51）与液晶各向同性相中向列相演化的斯特凡型模型吻合良好。随着时间推移，向列相的生长受到微滴边界限制增加的制约。在这个生长阶段，向列相以较慢的速率R(t) 约为t^0.31生长。后期生长较慢可能是由于多种因素导致的，例如限制诱导的潜热减少；其演化过程中缺陷强度的变化；或者缺陷与液晶和油之间的界面或相邻缺陷之间的相互作用。两种生长机制的存在也与布拉达克等人的分子模拟结果一致（布拉达克，Z.；克拉伊，S.；祖默，S.《物理评论E》2002年，65卷，021705期），他们确定了一个早期的畴区机制和一个后期的限制机制。对于畴区和限制机制，布拉达克等人（布拉达克，Z.；克拉伊，S.；祖默，S.《物理评论E》2002年，65卷，021705期）得到的生长指数分别为0.49±0.05和0.25±0.05。这些值与我们实验中观察到的0.51和0.31非常接近。
