Fig. 2: Regimes of droplet formation.

a Phase diagram of double hollow-core emulsification in the plane of dimensionless air pressure \(\hat{P}=P/{P}^{* }\) and capillary number of the oil, \({{Ca}}_{2}\). The Hele-Shaw channel has a width \(w=150\,\upmu \rm{m}\) and a height \(b=5\,\upmu \rm{m}\). The aqueous (continuous) phase flow rate \({q}_{3}\) = 100 \(\upmu \rm{L/h}\). Seven emulsification regimes are observed: SE shown in (b), and the subsequent oil droplets with uniform size distribution with diameter around \(2b\) following the air core shrinkage, observed downstream of the reservoir, shown in (c); BE shown in (d), and the oil droplets with size ranging from submicrons to tens of microns observed in the reservoir shown in (e), the T-junction emulsification (TE) as shown in (f), and the double T-junction emulsification (TE2) shown in (g). Black, dark gray and light gray dots in (a) correspond to biphasic emulsion formations, and the corresponding micrographs are shown in Supplementary Fig. S2. Blue, green and black solid lines in (a) showing the boundaries among different regimes are for eye guidance