Dynamic interfacial tension near critical point of a solvent–antisolvent mixture and laminar jet stabilization

The hydrodynamic behavior of a solvent jet (ethanol) injected into a pressurized antisolvent (CO2) fluid was experimentally investigated for a pressure range from far below to far above the critical point of the solvent–antisolvent mixture (CPM). The study shows that, at pressures sufficiently below the CPM, four traditional flow regimes (dripping flow, symmetric wave jet flow, sinuous jet flow, and atomization) can be identified depending upon the jet velocity and nozzle size. At pressures near the CPM where the equilibrium interfacial tension is extremely small (for pressure slightly below the CPM) or absent (for pressures above the CPM), all of these flow regimes can still be identified, indicating the existence of a dynamic interfacial tension (DIT). At pressures far above the CPM, only gas-like jets without any jet interfaces or droplet formation can be seen. The DIT near the CPM can be estimated from the measured stability curves (dependence of jet length on jet velocity) and our proposed analytic modeling. The zero-time DIT (ZTDIT) is insufficient to explain the jet behavior near the CPM since its relaxation time (microseconds) is very short in comparison with the jet breakup time (milliseconds). Hence another mode (or other modes) of DIT should exist. In this study, a new mode of DIT is proposed, namely, the nonisothermic DIT, which is caused by the enthalpy of mixing of two miscible fluids (such as solvent–antisolvent mixture near the CPM). The nonisothermic DIT enhances the jet stability for exothermic mixing while it reduces the jet stability for heat absorption, i.e. endothermic mixing. Both modes of DIT complement each other for jet stabilization. The ZTDIT stabilizes the small initial section of the jet while the nonisothermic DIT would stabilize (in case of exothermic mixing) the rest of the jet until its breakup.