Droplet Formation at the T-Junction
The microfluidic T-junction is one of the simplest and most robust geometries for generating monodisperse droplets. The dispersed phase fluid enters through a side channel perpendicular to the main channel carrying the continuous phase. As the dispersed phase emerges into the main channel, it forms a growing finger that partially or fully blocks the flow, building pressure upstream until the neck pinches off and a droplet detaches.
Flow Regimes and the Capillary Number
The capillary number Ca = μv/γ governs the transition between formation regimes. At low Ca, surface tension dominates and the emerging finger blocks the channel, leading to the squeezing regime where droplet size depends mainly on Qd/Qc. At intermediate Ca, the dripping regime produces droplets that pinch off before blocking the channel. At high Ca, viscous forces elongate the dispersed phase into a thin jet that breaks up via the Rayleigh-Plateau instability far downstream.
Scaling Laws and Size Control
In the squeezing regime, droplet volume scales as V ~ w³(1 + αQd/Qc), giving remarkably uniform droplets with coefficients of variation below 2%. This predictability makes T-junctions ideal for applications requiring precise volume control — from single-cell encapsulation to digital PCR. The generation frequency scales inversely with droplet volume, enabling kilohertz-rate production of picoliter to nanoliter droplets.
Applications in Biotechnology
Droplet microfluidics has revolutionized high-throughput biology. Each droplet serves as an isolated microreactor for single-cell genomics, enzyme evolution, or drug screening. Companies like 10x Genomics use droplet generators to encapsulate individual cells with barcoded beads, enabling single-cell RNA sequencing of millions of cells. The combination of small volumes, rapid mixing, and massive parallelism makes droplet microfluidics a cornerstone of modern bioanalysis.