Dual Stimuli-Responsive Pickering Emulsions from Novel Magnetic Hydroxyapatite Nanoparticles and Their Characterization Using a Microfluidic Platform

Stimuli-responsive emulsifiers have
emerged as a class of smart
agents that can permit regulated stabilization and destabilization
of emulsions, which is essential for food, cosmetic, pharmaceutical,
and petroleum industries. Here, we report the synthesis of novel “smart”
hydroxyapatite (HaP) magnetic nanoparticles and their corresponding
stimuli-responsive Pickering emulsions and explore their movement
under confined spaces using a microfluidic platform. Pickering emulsions
prepared with our magnetic stearic acid-functionalized Fe2O3@HaP nanoparticles exhibited pronounced pH-responsive
behavior. We observed that the diameter of emulsion droplets decreases
with an increase in pH. Swift demulsification was achieved by lowering
the pH, whereas the reformation of emulsions was achieved by increasing
the pH; this emulsification-demulsification cycling was successful
for at least ten cycles. We used a microfluidic platform to test the
stability of the emulsions under flowing conditions and their response
to a magnetic field. We observed that the emulsion stability was diminished
and droplet coalescence was enhanced by the application of the magnetic
field. The smart nanoparticles we developed and their HaP-based emulsions
present promising materials for pharmaceutical and petroleum industries,
where responsive emulsions with controlled stabilities are required.