Microfluidic emulsification: Process and formulation variables effects in flow behavior pattern on a flow-focusing device

Formulation and process variables in surfactant/oil/water (SOW) systems can strongly modify the water/oil interfacial tension leading to different behaviors. Pre-equilibration time of the SDS/toluene/1-butanol/NaCl((aq) )system was studied as a process variable and the flow regimes were described at different continuous and dispersed flow rates through a flow focusing geometry. Salinity and oil nature for the SDS/n-alkane/alcohol/ NaCl(aq) equilibrated system were studied as formulation variables at fixed dispersed flow rates, changing the continuous flow rate and the formulation variable in order to get O/W or W/O emulsions in hydrophilic and hydrophobic chips, respectively. Pre-equilibration time modifies the flow regimes depending on the differences observed in the interfacial tension at t = 0 and t = 24 h of equilibration. Under very hydrophilic conditions, when gamma(t= 24) > gamma(t=0), the entire droplet area is slightly higher for equilibrated systems. At WIII condition, when gamma(t=0 )> > gamma(t=24), droplets are formed only in jetting regime for not equilibrated systems. For salinity scan, monodispersed oil droplets were obtained at low salinities, S = 0-0.25 % and in the oil scan, water droplets were obtained when using cyclohexane and cyclohexane-hexane mixtures. In both cases, flow regimes are reported into the formulation maps, using the Hydrophilic Lipophilic Deviation as formulation variable (HLD vs flow rate ratio). Droplet size was fitted into the same equation (d(i) approximate to alpha Ca-c(-0.264)), and it is noteworthy that the interfacial tension in Ca c is lower than in most of reported works using flow-focusing geometries and both emulsions (W/O and O/W) were obtained with the same surfactant.