| Abstract
| - We report definitive evidence for an effective interfacial tension between two types of miscible fluids using spinning-drop tensiometry (SDT). Isobutyric acid (IBA) and water have an upper critical solution temperature (UCST) of 26.3°C. We created a drop of the IBA-rich phase in the water-rich phase below the UCST and then increased the temperatureabove it. Long after the fluids have reached thermal equilibrium, the drop persists. By plotting the inverse of the dropradius cubed (r-3) vs the rotation rate squared (ω2), we confirmed that an interfacial tension exists and estimated itsvalue. The transition between the miscible fluids remained sharp instead of becoming more diffuse, and the dropvolume decreased with time. We observed droplet breakup via the Rayleigh−Tomotika instability above the UCSTwhen the rotation rate was decreased by 80%, again demonstrating the existence of an effective interfacial tension.When pure IBA was injected into water above the UCST, drops formed inside the main drop even as the main dropdecreased in volume with time. We also studied 1-butanol in water below the solubility limit. Effective interfacialtension values measured over time were practically constant, while the interface between the two phases remains sharpas the volume of the drop declines. The effective interfacial tension was found to be insensitive to changes in temperatureand always larger than the equilibrium interfacial tension. Although these results may not apply to all miscible fluids,they clearly show that an effective interfacial tension can exist and be measured by SDT for some systems.
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