  When two miscible fluids are brought into contact with each other, the
concentration gradients induce stresses. These are referred to as Korteweg
stresses and are analogous to interfacial tension between two immiscible
fluids, thereby acting as an effective interfacial tension (EIT) in
inhomogeneous miscible systems. EIT governs the formation of a viscous thread
in flow-focusing of two miscible fluids. To further investigate its
significance, we have studied thread formation of a colloidal dispersion
focused by its own solvent. Experiments are combined with three-dimensional
numerical models to systematically expand previous knowledge utilising
different flow-focusing channel setups. In the reference setup, the sheath
flows impinge the core flow orthogonally while in four other channel setups,
the sheath flows impinge the core flow at an oblique angle that is both
positive and negative with respect to the reference sheath direction. As an
initial estimate of the EIT, we fit the experimentally determined thread shape
in the reference setup to a master curve that depends on EIT through an
effective capillary number. By numerically reproducing these experimental
results, it is concluded that the estimated EIT is within 25% of the optimal
EIT value that can be deduced by iteratively fitting the numerical results to
the experimental measurements. Regardless of channel setups, further numerical
calculations performed using the optimal EIT evaluated from the reference setup
show good agreement with the experimental findings in terms of thread shapes,
wetted region morphologies, and velocity flow fields. The one-to-one comparison
of numerical and experimental findings unveil the crucial role of EIT on the
thread detachment from the top and bottom walls of the channel, bringing useful
insights to understand the physical phenomenons involved in miscible systems
with a high-viscosity contrast.
