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

Abstract

In this study, the effect of surface wettability on the two-phase immiscible fluids flow and dynamics of droplet pinch-off in a T-junction microchannel has been numerically investigated using the finite element method. A conservative level set method (CLSM) has been adopted to capture the interface topology in the squeezing regime (Cac<10-2) for wide flow rate ratio (1/10⩽Qr⩽10) and contact angle (120°⩽θ⩽180°). This study has revealed that the wettability is a dominant factor in determining the hydrodynamic features of the droplet. Based on the instantaneous phase flow profiles, the droplet formation stages are classified as: initial, filling, squeezing, pinch-off and stable droplet. Wettability effects are insignificant in the filling stage. However, the hydrophobic effects are more vital in the squeezing and pinch-off stages. In general, it is shown that engineering parameters have complex dependence on the dimensionless parameters (Cac,Qr,θ). Capturing the instantaneous interface evolution has revealed that the droplet shape is sensitive to the contact angle. Interface shape profiles transform from convex into concave immediately for hydrophobic conditions (120°⩽θ⩽150°) whereas slowly for the super hydrophobic conditions (150°<θ⩽180°). In contrast to the literature, the pressure in the dispersed phase is not constant, but it is an anti-phase with the pressure in the continuous phase. Maximum pressure in the continuous phase, and neck width of the interface are complex function of the governing conditions (Cac,Qr,θ). Comparison of 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 higher value. The interface neck width (2r) shows an increasing trend up to a threshold value and then decreases linearly with the contact angle.