A study of the effective interfacial tension between miscible fluids by spinning drop tensiometer and microfluidics

A miscible system is a system in which two fluids can completely dissolve in one another. A sharp concentration gradient can be observed in miscible systems. We studied the concentration gradient or miscible interface between IBA (isobutyric acid) and water, a miscible system near a consulate point (close to the system's upper critical solution temperature [UCST]). The original hypothesis was that the sharp concentration gradient of IBA/water was due to barodiffusion, a diffusion effect driven by pressure. We tested this hypothesis by studying IBA/water at five different rotation rates and three different temperatures. At 20°C, increasing rotation acceleration from 6000 to 15000 rpm resulted in increasing dissolution rate, thus demonstrating that barodiffusion did not cause the sharp concentration gradient. However, the rotation acceleration did not affect the dissolution rate at higher temperatures. Increasing the temperature from 20°C to 27°C caused EIT (effective interfacial tension) to decrease. Since surfactants generally lower the interfacial tension between immiscible fluids, we tested an anionic and cationic surfactant and evaluated how its concentration within cmc (critical micelle concentration) affected the EIT of a miscible system. With increasing surfactant concentration, the EITs generally decreased. At 20°C, the ITs of IBA/water systems using surfactants were slightly higher than IBA/water systems without surfactant, which is unusual. At 30°C, increasing and decreasing the rotation rate resulted in the averaged EIT and radii getting higher. We had some unusual behavior in the microfluidic device that we did not observe in the SDT (spinning drop tensiometer) because of mixing and the microsystem was done on a smaller scale so that larger effects from surface tension occurred, but some behaviors were the same, thus indicating that the behavior of the IBA/water system was not solely due to the instrument used.