Thermocapillary and thermosolutal Marangoni convection of ethanol and ethanol–water mixtures in a microfluidic device

We have investigated Marangoni convection at an evaporating meniscus of pure ethanol and of ethanol–water mixtures. The experiments were performed in a microfluidic device consisting of 100μm\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$100\,{\upmu \text{m}}$$\end{document} wide channels of quadratic cross section and a cylindric reservoir of the same height. Tracer particles were added to determine the velocity field using particle image velocimetry. In the case of pure ethanol, thermocapillary convection rolls similar to those reported in the literature were observed. The velocity of the flow can be controlled by local laser heating, confirming the thermocapillary nature of the process. Ethanol–water mixtures show distinctly different patterns when compared to pure ethanol: the flow velocity of the symmetric vortices is significantly increased and almost insensitive to heating on the symmetry line. By heating near the contact line of the meniscus, the symmetric two-roll state can be reversibly switched into an asymmetric one-roll state with the flow at the interface pointing in the direction of the temperature gradient. A consistent interpretation is obtained based on the assumption that solutocapillary instead of thermocapillary forces is the main driving mechanism in the case of the mixtures.