Microfluidic Synthesis of Janus Particles with Controlled Size and Hydrophilic/Hydrophobic Domains for Stable Pickering Emulsion Formation

Stable Pickering emulsions are important to systems where controlled confinement of an oil or water phase are critical to its applications. For example, micro-capsules encased by particles can serve as rigid vehicles for drugs or precious food ingredients. These coated emulsions can harbor oil in water (o/w) or even water in oil (w/o) phases. Nonetheless, the stability of the Pickering emulsions depend on the wetting properties of the particle. For this reason amphiphilic “Janus” particles (both hydrophilic and hydrophobic portions on same particle) have proven to be major assets in forming stable Pickering emulsions. The key to forming either o/w or w/o emulsions depends on the ratio of lengths of the hydrophobic to hydrophilic parts.

Polyethylene Glycol diacrylate (PEGDA), ethoxilated trimethylopropane triacrylate (ETPTA – monomer to be polymerized), 2-hydroxy-2-methyl-propiophenone (HMP – photo initiator), and water were mixed together to form the hydrophilic portion of the inner-phase. 99 wt% trimethylopropane triacrylate (TMPTA) and 1 wt% 2-hydroxy-2-methyl-propiophenone were added together to form the hydrophobic portion of the inner-phase. HMP would serve as the photo-initiator for polymerization under UV light. 95 wt% silicone oil (viscosity of 20 centi stokes) and 5 wt% Dow Corning Resin 749, as the surfactant served as the continuous phase. Janus particles were formed using flow of these solutions in a glass capillary microfluidic device. Flowrates were varied to control particle sizes (125- 400 microns) and hydrophilic to hydrophobic domain volume ratios (0.14 – 7). At a fixed surfactant concentration, variations of the ratio of inner water to oil flow rates lead to the formation of amphiphilic embedded particles of different sizes and hydrophobic portions. Using UV light, these droplets were cross-linked via photo-polymerization to form fairly monodispersed particles. It was demonstrated that Janus particles created with a hydrophilic to hydrophobic volume ratio of 7 favored an o/w emulsion while particles with a volume ratio of 0.14 favored w/o emulsion. The size of the emulsions were controlled by changing the size of the Janus particles. An effort was also made to discuss the emulsion final form in terms of the interfacial tension of the two particle domains with respect to the dispersed and or continuous phase.