Simultaneous influence of contact angle, capillary number, and contraction ratio on droplet dynamics in hydrophobic microchannel
Le Hung Toan, Thanh Tung Nguyen, Van T. Hoang, Jyh‐Wei Lee
Abstract
Abstract In droplet-based microfluidic systems, droplet motion and behavior largely depend on fluid properties, flow conditions, and microchannel geometry. This study presents a model to predict droplet dynamics within a contraction microchannel, considering various factors, such as contact angle ( θ ), capillary number (Ca), and contraction ratio ( C ), within defined ranges, through the employment of a three-dimensional numerical simulation method. Droplets may undergo trap, squeeze, or breakup as they traverse the contraction microchannel, contingent upon the aforementioned factors. The transition from trap to squeeze is determined by the critical value of capillary number ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>C</mml:mtext> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mtext>a</mml:mtext> </mml:mrow> <mml:mrow> <mml:mtext>I</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ). Within the squeezed regime, the contact angle influences droplet deformation and velocity within the contraction microchannel. However, when capillary number ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mtext>C</mml:mtext> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mtext>a</mml:mtext> </mml:mrow> <mml:mrow> <mml:mrow> <mml:mtext>II</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) in contraction microchannel increases to a specific threshold corresponding to the contraction ratio, the droplet no longer in contact with the wall and becomes independent of the contact angle, a phenomenon referred to as detachment. Moreover, the contact angle’s impact on the transition of droplets from squeeze to breakup is contingent upon the contraction ratio. This study provides a comprehensive overview of droplet behavior within microfluidic systems, offering valuable insights for the optimization and design of microsystems.