A study on texture change on Langmuir monolayrs by optical tweezers

构造了布鲁斯特角显微镜与光镊的组合系统。五五二酸Langmuir单层分子取向的整数s=1点偏斜织构在激光加热下分裂成两个s=1/2缺陷。我们用连接两个缺陷的c-指向器π壁的线张力与连续取向弹性能的竞争来解释这种行为。当线张力引起吸引时，取向弹性能介导半整数位差之间的排斥相互作用。激光加热织构在取向有序的固相中局部产生各向异性应力，通过降低π-壁线张力迫使分子取向进入单层平面并在那里。用布鲁斯特角显微镜观察了五五二酸Langmuir单层膜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有四个和两个黑色刷的点偏斜共存。局部激光加热的使用使人们能够观察到在s=1点偏斜分裂成分数偏斜过程中连接两个半整数偏斜的弦缺陷的产生和湮灭动力学。这些动力学通过研究分子的取向弹性与弦的线张力和斜齿的阻力之间的竞争来分析。上述结果表明，光镊是研究固体朗缪尔单分子膜微操作和微流变学的有力工具。

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.