When solute molecules in a liquid evaporate at the surface,
concentration
gradients can lead to surface tension gradients and provoke fluid
convection at the interface, a phenomenon commonly known as the Marangoni
effect. Here, we demonstrate that minute quantities of ethanol in
concentrated sodium hydroxide solution can induce pronounced and long-lasting
Marangoni flow upon evaporation at room temperature. By employing
particle image velocimetry and gravimetric analysis, we show that
the mean interfacial speed of the evaporating solution sensitively
increases with the evaporation rate for ethanol concentrations lower
than 0.5 mol %. Placing impermeable objects near the liquid–gas
interface enforces steady concentration gradients, thereby promoting
the formation of stationary flows. This allows for contact-free control
of the flow pattern as well as its modification by altering the objects
shape. Analysis of bulk flows reveals that the energy of evaporation
in the case of stationary flows is converted to kinetic fluid energy
with high efficiency, but reducing the sodium hydroxide concentration
drastically suppresses the observed effect to the point where flows
become entirely absent. Investigating the properties of concentrated
sodium hydroxide solution suggests that ethanol dissolution in the
bulk is strongly limited. At the surface, however, the co-solvent
is efficiently stored, enabling rapid adsorption or desorption of
the alcohol depending on its concentration in the adjacent gas phase.
This facilitates the generation of large surface tension gradients
and, in combination with the perpetual replenishment of the surface
ethanol concentration by bulk convection, to the generation of long-lasting,
self-sustaining flows.