The effects of hydrophilic surfactant concentration and flow ratio on dynamic wetting in a T-junction microfluidic device

摘要

An initial study was conducted on the influence of the hydrophilic surfactant concentration and two-phase flow-rate ratio on the aqueous-phase wetting ability (dynamic contact angle) of a water-in-oil (W/O) emulsion in a T-junction poly(methyl methacrylate) microfluidic device. Cetyltrimethyl ammonium bromide (CTAB) was used as the hydrophilic surfactant for the aqueous phase while the organic phase was pure corn oil. Static contact angles and interfacial tensions were measured for emulsions with surfactant concentration ranging from 0 to 0.5% (w/w). Contacts angles for an oil drop on PMMA in water transition from <90°to >90°at CTAB concentrations near the transition concentration of 1 x10-2 % (w/w), suggesting a change in the surface energy and a transition from a slightly hydrophobic to hydrophilic PMMA surface. Microchannel flow experiments were conducted to identify an analogous dynamic transition in wetting character, and special attention was paid to the CTAB concentration range between 3.0 x10-3 3 x 10-2 to focus on the wetting transition observed in the static case. At concentrations below 1 x 10-2 %, the microchannel walls are hydrophobic and stable W:O emulsions are formed. As CTAB increases, “sticky” type flow ensues and both phases wet the channel walls. At the transition concentration, a spontaneous phase-inversion was observed, producing stable O:W plugs. Above the transition concentration, the flow-type returns to W:O plugs, and advancing and receding dynamic contact angles for the aqueous plugs are observed. Dynamic contact angles and wetting responded more to changes in the aqueous phase flow rate than to changes in the organic (continuous) phase flow rate and receding contact angles of the plugs transition from <90°to >90°at a W:O flow ratio near unity. Finally, an equation of state approach was used to estimate the interfacial energies of the emulsions and plug shapes at higher CTAB concentrations. The results indicate that emulsions in which both phases exhibit partial wetting are lowest in energy and changes to either or both of the liquid-liquid and liquid-solid interfacial energies can significantly affect the emulsion type and wetting character.