Generation and Observation of Long-Lasting and Self-Sustaining Marangoni Flow

When solute molecules in a liquid evaporate at the surface,concentrationgradients can lead to surface tension gradients and provoke fluidconvection at the interface, a phenomenon commonly known as the Marangonieffect. Here, we demonstrate that minute quantities of ethanol inconcentrated sodium hydroxide solution can induce pronounced and long-lastingMarangoni flow upon evaporation at room temperature. By employingparticle image velocimetry and gravimetric analysis, we show thatthe mean interfacial speed of the evaporating solution sensitivelyincreases with the evaporation rate for ethanol concentrations lowerthan 0.5 mol %. Placing impermeable objects near the liquid-gasinterface enforces steady concentration gradients, thereby promotingthe formation of stationary flows. This allows for contact-free controlof the flow pattern as well as its modification by altering the objectsshape. Analysis of bulk flows reveals that the energy of evaporationin the case of stationary flows is converted to kinetic fluid energywith high efficiency, but reducing the sodium hydroxide concentrationdrastically suppresses the observed effect to the point where flowsbecome entirely absent. Investigating the properties of concentratedsodium hydroxide solution suggests that ethanol dissolution in thebulk is strongly limited. At the surface, however, the co-solventis efficiently stored, enabling rapid adsorption or desorption ofthe alcohol depending on its concentration in the adjacent gas phase.This facilitates the generation of large surface tension gradientsand, in combination with the perpetual replenishment of the surfaceethanol concentration by bulk convection, to the generation of long-lasting,self-sustaining flows.