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Abstract

The sessile drop method was used to measure the wetting angles of carbonate-silica melt droplets as a proxy for surface tension. Melts were synthesized by sintering pure anhydrous Na₂CO₃ powder with SiO₂ powder. The final weight percentages of Na₂O, SiO₂, and CO₂ of each melt were calculated. Samples of approximately 0.03g of each melt composition were melted on a substrate of alumina at 1000°C until the droplets had fully relaxed. The droplets were quenched at room temperature and wetting angle measurements were taken. Droplets of melt compositions of 62 wt% SiO2 and less produced wetting angles of 3 to 9°. Droplets of melt composition of 71 wt% produced drastically higher wetting angles of 23°. These results indicate that SiO₂ content does not significantly affect the wetting angle of the Na₂SiO₃- SiO₂ system until SiO₂ composes 71 wt% of the of the melt composition. The phase diagram of the SiO₂-Na₂CO₃ system is used to estimate the varying amounts of liquid and solid Na₂CO₃, solid Na₂SiO₃, liquid Na₂Si₂O⁰₅, and solid SiO₂ in each melt. Due to its molecular structure and high number of non-bridging oxygens/tetrahedral (NBO/T) Na₂CO₃ likely is the controlling component of the surface tension of the melts below 71 wt% SiO₂. Once the melt composition exceeds 62 wt% SiO₂,, Na₂Si₂O₅ controls the surface tension of the melt via the introduction of a bridging oxygen that affects the interactions between the liquid molecules and increases the wetting angle.