Abstract In the realm of microfluidics, the dynamics of droplets have become a focal point of attention and research for potential applications in Lab-on-a-Chip devices or microanalysis systems. The behavior of droplets is contingent upon the characteristics of the flow, liquid properties, and geometric parameters. Droplet behaviors during passage through a T-junction involve breakup and non-breakup(NB) regime. In this study, the simultaneous influence of capillary number ( Ca ), viscosity ratio ( λ ), and length ratio ( l/w ) on the droplet behavior within the T-junction microchannel is investigated through three-dimensional numerical simulations. A novel model is proposed that incorporates the viscosity ratio in predicting the critical Ca ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>C</mml:mi> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>cr</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> </mml:math> ) for the transition from NB to breakup regime, which was not considered in previous studies. The critical Ca is a function of the viscosity ratio and length ratio, determined and governed by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>C</mml:mi> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>cr</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo>></mml:mo> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mn>1</mml:mn> </mml:msub> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mfrac> <mml:mi>w</mml:mi> <mml:mi>l</mml:mi> </mml:mfrac> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>a</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:mrow> </mml:msup> </mml:mrow> <mml:mi>f</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> . The reliability of the proposed model is validated through numerical simulations and earlier theoretical predictive models. Additionally, the study investigates the influence of the viscosity ratio on droplet behavior. A newly observed regime, called the Trap regime, is identified in microchannels with a rectangular cross-section, typically occurring at low Ca s. Furthermore, a comparative analysis is conducted to highlight the differences in droplet breakup behavior between square and rectangular microchannel cross-sections.