  An interface between two miscible fluids is transient, existing as a
non-equilibrium state before complete molecular mixing is reached. However,
during the existence of such an interface, which typically be at short
timescales, composition gradients at the boundary between the two liquids cause
stresses effectively mimicking an interfacial tension. Here, we combine
numerical modelling and experiments to study the influence of an effective
interfacial tension between a colloidal fibre dispersion and its own solvent on
the flow in a microfluidic system. In a flow-focusing channel, the dispersion
is injected as core flow that is hydrodynamically focused by its solvent as
sheath flows. This leads to the formation of a long fluid thread, which is
characterised in 3D using Optical Coherence Tomography and simulated using a
volume of fluid method. The simulated flow and thread geometries very closely
reproduce the experimental results in terms of thread topology and velocity
flow fields. By varying the effective interfacial tension numerically, we show
that it clearly influences threading dynamics and that it can be described by
an effective capillary number. Furthermore, we demonstrate that the applied
methodology provide means to measure the ultra-low but dynamically highly
significant effective interfacial tension.
