Study on Marangoni instabilities of condensed domains of Langmuir monolayers

我们开发了布鲁斯特角显微镜和激光镊子相结合的系统。这一系统使人们能够在监视器的视场内观察到局部激光加热时，由Marangoni驱动的非平衡的朗缪尔单分子膜凝聚区的定向生长。利用该系统发现，凝聚域在水面上的定向生长主要由两种形态来分类。一种是分形域，另一种是枝晶结构。在后者中，选择性地生长主枝一侧的侧枝，即实现了不对称的树枝生长。侧枝生长的对称性破缺与临近侧枝的生长方向和生长速度有关。此外，枝晶的生长是由相邻分支的存在引发的，并且表现出生长区域的各种界面不稳定性。作为定向生长不稳定性的结果，观察到了磁区弯曲尖端的瞬时生长和生长磁区之间的波状边界。这种由Marangoni驱动的非平衡定向域的生长不稳定性可以由以下原因引起：1)在高密度定向排列的树枝状结构域同时生长的同时，两亲分子在亚相和/或相邻域之间的过饱和LE相界面处被竞争捕获；2)相邻凝聚域之间的静电排斥。相邻域之间分子的竞争俘获和静电斥力对Marangoni驱动的凝聚域的定向生长具有协同作用，从而有效地影响了单个域的生长速度，导致了有趣的调制域图。

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.