A study on texture change on Langmuir monolayrs by optical tweezers

构建了布儒斯特角显微镜与光钳的组合系统。分子取向的整数S=1点错向织构在激光加热下分裂成两个S=1/2缺陷。我们用连接这两个缺陷的c指向矢的π壁的线张力与连续统取向弹性能的竞争来解释这一行为。当线张力引起吸引时，取向弹性能调节半整数倾斜度之间的排斥相互作用。激光对织构的局部加热在取向有序的固相中产生各向异性应力，迫使分子取向进入单层平面，并在那里通过降低π壁线张力来实现。用Brewster角显微镜观察到了固相中连接半整数位错对的静态和动态弦缺陷织构。静态弦缺陷结构与两种具有四个和两个黑刷的点倾斜并存。利用局部激光加热，可以观察到在S=1点位错分裂为分数位错的过程中，连接两个半整数位错的弦缺陷的产生和湮灭的动力学。通过研究分子的取向弹性与弦的线张力和倾斜力之间的竞争，对这些动力学进行了分析。上述结果表明，光钳是研究固体朗缪尔单分子膜微观操纵和微观流变学的有力工具。

The combined system of Brewster angle microscope with optical tweezers has been constructed. Integer s=1 point disclination textures of the molecular orientation of a pentacosadiynoic acid Langmuir monolayr split into two s=1/2 defects upon laser heating. We explain this behavior by a competition of the line tension of a π-wall of the c-director connecting both defects with the continuum orientation elastic energy. While the line tension causes an attraction, the orientation elastic energy mediates a repulsive interaction between the half-integer disclinations. Laser heating of the texture locally induces an anisotropic stress in the orientationally ordered solid phase, forcing the molecular orientation into the monolayr plane and there by reducing the π-wall line tension. The static and dynamic string defect textures connecting pairs of half-integer disclinations have been observed by Brewster angle microscopy in the solid phase of pentacosadiynoic acid Langmuir monolayers. The static string defect structures have appeared coexisting with two kinds of point disclinations that have four and two black brushes. The use of local laser heating has allowed one to observe kinetics of creation and annihilation of string defects connecting the two-half-integer disclinations in the splitting process of an s=1 point disclination into fractional disclinations. These kinetics have been analyzed by studying the competition between the orientational elasticity of the molecules and the line tension of the string and the drag force of the disclinations. The above results demonstrate that optical tweezers are a powerful tool for the study of micromanipulations and microrheology in the solid Langmuir monolayrs.