Thermodynamics of chemical Marangoni-driven engines

The goal of this paper is to perform a general thermodynamic study of Marangoni-driven engines in which chemical energy is directly transformed into mechanical motion. Given that this topic has not been discussed before, we will explore here the most basic and fundamental aspects of the phenomena at work, which leads to a number of interesting observations typical of controversies in classical thermodynamics. Starting with a discussion of a few key motivating examples of chemical Marangoni-driven phenomena - tears of wine, an oscillating pendant droplet, "beating" oil lens, and traveling waves in a circular container - and contrasting homogeneous versus inhomogeneous thermodynamic systems we naturally arrive at alternative ways of storing and generating energy with the help of inhomogeneities in the bulk and surface properties of the working media. Of particular interest here are systems with interfaces - hence, in this context we discuss the nature and efficiency of the corresponding thermodynamic cycles leading to work done as a result of a non-uniform distribution of surface tension, which is in turn induced by a non-uniform surface active substance (surfactant) distribution, for both soluble and insoluble surfactants. Based on the relevant physical parameters of the working medium we can also evaluate the isothermality, i.e. temperature variations, dissipative losses, energy output and efficiency, entropy generation, and the period of such cycles in real processes. The role of singularity formation at the interface for the existence of such thermodynamic cycles is unraveled as well. Finally the discussion is concluded with a few ideas for potential applications of Marangoni-driven engines.