Microfluidic Droplet Generator Parameter Estimator

The Garstecki Squeezing-Regime Model

In 2006, Garstecki et al. showed that droplet size in a confined microfluidic channel follows a beautifully simple scaling:

where d is the droplet diameter, w is the nozzle width, Qd is the dispersed-phase flow rate, Qc is the continuous-phase flow rate, and $\alpha$ is a geometry-dependent prefactor of order 1.

The key insight: in the squeezing regime, surface tension dominates over viscous stress, so the droplet length depends only on the flow-rate ratio, not on the absolute flow rates. This makes the squeezing regime extremely predictable and the default for biological applications.

Squeezing, Dripping, Jetting — The Three Regimes

Surfactant: The Unsung Hero of Droplet Stability

Surface-active molecules (PFPE-PEG for fluorinated oils, Span 80 for hydrocarbon oils) adsorb to the oil/water interface and lower $\sigma$, which:

1. Prevents coalescence — drops can touch without merging 2. Speeds up drop formation (lower $\sigma$ → higher Ca at same velocity) 3. Can reduce drop size (weaker surface tension → earlier pinch-off)

At least 0.5% surfactant is required for stable drops. Below that, any downstream contact — in a delay line, in a storage array, at a detector — causes coalescence and destroys the emulsion.