# Effective interfacial tension in flow-focusing of colloidal dispersions: 3D numerical simulations and experiments

## TL;DR

This study combines 3D numerical simulations and experiments to investigate how an effective interfacial tension influences flow dynamics in microfluidic flow-focusing of colloidal dispersions, revealing its impact on thread formation and providing measurement methods.

## Contribution

It introduces a combined numerical and experimental approach to quantify and analyze the effective interfacial tension in colloidal dispersion flows, a novel application in microfluidics.

## Key findings

- Simulations closely match experimental flow and thread geometries.
- Effective interfacial tension significantly affects threading dynamics.
- Methodology enables measurement of ultra-low interfacial tensions.

## Abstract

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.