Fig. 4: Mechanisms and influencing factors of water molecule transport driven by Marangoni natural convection.

a Sketch of two OH-group models of water molecules at the oil-water interface. Upper layer: oil phase. Lower layer: water phase. The red dashed line represents the oil-water interface. Orange and blue arrows indicate the opposite flow direction of oil and water near the interface. Gray dashed lines represent the sharp interfacial region. b Sum frequency generation spectra of the oil-water and air-water interface in the range of 3000–3800 cm⁻¹ under 20 °C. Gray dashed boxes: free OH peaks. c Schematic of convection cells in oil/water phases. Arrows: flow direction. The balls in the oil phase indicate nanodroplets. Interfacial water molecules are identical to (a). d Comparison of infrared spectra (1300–1800 cm⁻¹) of oil phase at various T₀. Gray band represents peak width. e Local magnification (1530–1760 cm⁻¹) of peaks from (d) after spectra fitting. f Evolution of relative absorbance at 1648 cm⁻¹ of the oil phase during cooling. ΔAbsorbance is the absorption intensity after correction for the initial value. Temperatures corresponding to the maximum absorbance are labeled via white points and brown lines. g Three-dimensional infrared spectra of oil phase (two-phase cooling). The yellow dashed line indicates maximum absorbance. The black curve shows the evolution of absorbance at 1648 cm⁻¹. Coupled evolution of Ma_ow(h)/Ma_oa(i), Ra, and ΔAbsorbance (at 1648 cm⁻¹). The color-temperature correspondence is identical to that of (e, f). The white points correspond to those in (f). The red and blue arrows refer to stage I of rapid and stage II of slow water molecule transport, respectively.