Study on Marangoni instabilities of condensed domains of Langmuir monolayers

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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.