Study on Marangoni instabilities of condensed domains of Langmuir monolayers

我们开发了一个联合系统的布鲁斯特角度显微镜与激光镊子。该系统使人们能够在局部激光加热的情况下，在监视器的视野内启动和观察Langmuir单层凝聚域的非平衡、marangoni驱动的定向生长。利用该系统发现，水面上凝聚域的定向生长主要分为两种形态。一种是分形结构，另一种是枝晶结构。在后一种方法中，选择性地生长主枝一侧的侧枝，即实现了不对称枝状生长。侧枝生长中对称性的破坏与邻近枝条靠近侧枝的生长方向和速度有关。此外，树突生长是由邻近分支的存在引发的，并表现出生长域的各种界面不稳定性。由于定向生长的不稳定性，观察到畴的弯曲尖端和生长畴之间的波浪边界的瞬态生长。这种由marangoni驱动的非平衡定向畴的生长不稳定性可以由以下原因引起：1)在树枝状畴高密度定向阵列同时生长时，在相邻畴之间的亚相和/或过饱和LE相的界面上，两亲分子的竞争性捕获；2)相邻凝聚畴之间的静电斥力。在marangoni驱动的凝聚态结构域的定向生长中，邻近结构域之间的竞争捕获和分子的静电斥力的协同效应有效地影响了单个结构域的生长速度，并导致了有趣的调制结构域图案。

We developed a combined system of Brewster angle microscopy with laser tweezers. This system enables one to initiate and to observe the nonequilibrium, Marangoni-driven directional growth of condensed domains of Langmuir monolayers in the field of view of the monitor upon local laser heating on them. By using the system directional growth of condensed domains on the water surface is found to be classified mainly by two types of morphologies. One is fractal domain, and the other is dendrite structure. In the latter one, side branches on one side to the main branch were selectively grown, I.e., asymmetric dendritic growth was realized. The breaking of symmetry in the growth of side branches is correlated with the growth direction and velocity of the neighboring branches approaching the side branches. Moreover, dendritic growth is triggered by the existence of neighboring branches and exhibits a variety of interfacial instabilities of growing domains. As the results of directional growth instabilities, the transient growth of curved tips of domains and wavy boundaries between grown domains were observed. Such Marangoni-driven growth instabilities of non-equilibrium directional domains can be caused by I) the competitive capture of amphiphilic molecules in the subphase and/or at the interface in the supersaturated LE phase between the neighboring domains in the simultaneous growth of high density of directional arrays of dendritic domains and by ii) electrostatic repulsion between the neighboring condensed domains. The cooperative effect of the competitive capture and the electrostatic repulsion of molecules between the neighboring domains on the Marangoni-driven, directional growth of condensed domains thus affects the growth rate of individual domains effectively and leads to the intriguing modulated domain patterns.