Particle encapsulation in aqueous ferrofluid drops and sorting of particle-encapsulating drops from empty drops using a magnetic field

Handling and manipulation of particle-encapsulating droplets (PED) have profound applications in biochemical assays. Herein we report encapsulation of microparticles in aqueous ferrofluid droplets in a primary continuous phase (CP) and sorting of PED from empty droplets (ED) at the interface of the CP in coflow with a second continuous phase using a magnetic field. We find that the encapsulation process results in a size contrast between the PED and ED that depends on the flow regime - squeezing, dripping, or jetting - which in turn is governed by the ratio of the discrete phase to the continuous phase capillary number, Ca r . The difference between the volume fractions of ferrofluid in the PED and ED, Δ α PED , is utilized for sorting, and is found to depend on the ratio of the capillary numbers, Ca r . The difference Δ α PED is found to be maximum in the jetting regime, suggesting that the jetting regime is most suitable for encapsulation and sorting. The sorting criterion is represented in terms of a parameter ξ , which is a function of the ratios of the magnetic force to the interfacial force experienced by the PED and ED. Our study revealed that sorting is possible for ξ < 0, which corresponds to Δ α PED > 0.25. The maximum sorting efficiency of our system is found to be ∼95% at a throughput of ∼100 drops per s. We report encapsulation of microparticles in aqueous ferrofluid droplets and sorting of particle-encapsulating droplets from empty droplets at the fluid-fluid interface using a magnetic field.