Rip currents: A spontaneous heat transfer enhancement mechanism in a wickless heat pipe

•An unusual analog of a terrestrial rip current in a wickless heat pipe operating in the ISS.•Driven by a pair of counterrotating vortices formed from Marangoni and capillary flows.•Experimental data are supported by a numerical model.•The current arises spontaneously as the heat input increased.•It serves as a natural way of enabling higher evaporation rates.