• A theoretical model based on force balance accurately predicts the critical Capillary number for droplet breakup in T-junction microchannels.
• Increasing sidearm length ratio inhibits droplet breakup and induces asymmetric breakup regimes.
• Increasing outlet-to-inlet width ratio reduces the likelihood of droplet breakup.
• The model provides a predictive framework for controlling droplet dynamics in lab-on-a-chip applications.