Cesar Augusto López “Fluorescent visualization of Marangoni flow”

Cesar Augusto López , SOBIO-MMOI

The release of a drop of dish-washing on the surface of pure water covered with pepper produce the transport of the pepper grains to the edge of the container. The flow that carries the grains away is known as the Marangoni Flow and is the result of the interfacial tension difference between the point where dish-washing liquid was added and clean water far away. Fluid flows induced by gradients of interfacial tension plays an important role in many natural and industrial processes, but the visualization of the surfactant spreading has been a long-standing experimental challenge since is necessary the introduction of tracers that could affect the properties of the flow. This makes surfactants with fluorescent properties ideal for this kind of studies because allow to observe in a direct way the Marangoni flow through the fluorescent light emission of the surfactant molecules. Due to their long excited-state lifetime and their sensitivity to the local enviroment, pyrene and its derivates have been extensively used as chromophores in fluorescence studies.

For our study, we have synthesized a pyrene based amphiphilic molecule and in the first part of this talk, we are going to present the results obtained for its concentration-dependent behaviour in aqueous solution. Surface tension measurements along with fluorescence techniques allow us to identify three concentration-dependant regimes. For a low surfactant concentration, molecules are dissolved in monomeric form. At intermediate concentrations, labile aggregates are formed, which, with surfactant concentration increase give way to micelles formation. In the second part of the talk, we will address the effects of the surfactant flow rate and water layer thickness on the Marangoni flows. The results show that the Marangoni zone size increasing initially with both parameters and after that a plateau is observed.