CFD analysis of effects of surface wettability and flow rates on the interface evolution and droplet pinch-off mechanism in the cross-flow microfluidic systems

TL;DR

This paper numerically investigates how surface wettability and flow rates influence droplet formation and interface evolution in cross-flow microfluidic systems, revealing complex dependencies and new insights into pinch-off mechanisms.

Contribution

It introduces a detailed numerical analysis of wettability effects on droplet dynamics in microchannels, highlighting the significance of pressure variations and interface shape transformations.

Findings

Wettability significantly affects droplet shape and formation stages.

Pressure in dispersed phase varies anti-phase with continuous phase, contrary to previous literature.

Droplet pinch-off can be predicted using pressure sensors without flow visualization.

Abstract

This study has numerically investigated the effect of surface wettability on two-phase immiscible flow and dynamics of droplet pinch-off in a T-junction microchannel using finite element method. A conservative level set method (CLSM) has been adopted to capture the interface topology in squeezing regime ($Ca_c <10^{-2}$) for wide flow rate ratio ($1/10 \leq Q_r \leq 10$) and contact angle ($120^{\circ} \leq \theta \leq 180^{\circ}$). Based on the instantaneous phase profiles, droplet formation stages are classified as initial, filling, squeezing, pinch-off and stable droplet. Wettability effects are insignificant in filling stage. However, hydrophobic effects are more visible in squeezing and pinch-off stages. Engineering parameters have generally shown complex dependence on dimensionless parameters ($Ca_c$, $Q_r$, $\theta$). Capturing the instantaneous interface evolution has revealed droplet shape senstivity with the contact angle. Interface profiles transform from convex into concave immediately for hydrophobic ($120^{\circ} \leq \theta \leq 135^{\circ}$) whereas slowly for super hydrophobic ($150^{\circ} \leq \theta \leq 180^{\circ}$) conditions. In contrast to the literature, pressure in dispersed phase is not constant, but it is an anti-phase with pressure in continuous phase. Comparing the filling and pinch-off time based on the pressure and phase profiles has brought new insights that the droplet pinch-off mechanism can be elucidated by installing the pressure sensors even without the flow visualization and phase profiles. The interface curvature adopts a flattened to a more concave shape when the Laplace pressure varies from a smaller to a higher value. The interface neck width (2r) shows an increasing trend up to a threshold value and then decreases linearly with the contact angle.