Evidence for the Existence of an Effective Interfacial Tension between Miscible Fluids:  Isobutyric Acid−Water and 1-Butanol−Water in a Spinning-Drop Tensiometer

Abstract

We report definitive evidence for an effective interfacial tension between two types of miscible fluids using spinningdrop 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 temperature above it. Long after the fluids have reached thermal equilibrium, the drop persists. By plotting the inverse of the drop radius cubed (r -3 ) vs the rotation rate squared (ω 2 ), we confirmed that an interfacial tension exists and estimated its value. The transition between the miscible fluids remained sharp instead of becoming more diffuse, and the drop volume decreased with time. We observed droplet breakup via the Rayleigh-Tomotika instability above the UCST when 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 drop decreased in volume with time. We also studied 1-butanol in water below the solubility limit. Effective interfacial tension values measured over time were practically constant, while the interface between the two phases remains sharp as the volume of the drop declines. The effective interfacial tension was found to be insensitive to changes in temperature and 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.

Instabilities of Diffuse Interfaces

“…From a plot of r −3 vs. ω 2 ( Fig. 2), Pojman et al determined the effective interfacial tension [34]. We confirmed this relationship for a single drop that was initially equilibrated at 24 0 C. The temperature was raised to 28 0 C. The EIT, assuming the density difference had not changed significantly because of diffusion, was 0.10×10 −3 N/m.…”

“…Nevertheless, the experimental confirmation of the existence of such phenomena remained arguable as well as adequate mathematical models to describe them. An important step in the development of these studies was related to the application of the spinning drop tensiometry to IBA-water systems [34]. It allows the investigation of miscible fluids with a sharp interface between them.…”

On the phase-field modelling of a miscible liquid/liquid boundary

“…[18], where the existence of the dynamic surface tension at a miscible interface was confirmed. The spreading of a miscible interface in a vertical cuvette was experi-60 mentally studied in Ref.…”

“…The term with Grashof number stands for the effect of barodiffusion (y is the vertical coordinate). The last term in (18) means that the surface energy effects define not only the interface 235 morphology but also the diffusion through the interfacial boundary.…”

All-aqueous multiphase microfluidics

“…In other words, the presence of local water flow may change the dynamic distribution of the incompatible solutes and the interfacial properties as well. 60,61 Therefore, the thickness of the w/w interface needs to be redefined and investigated, especially when nonequilibrium phases of ATPS are investigated.…”