Magnetic Nanoparticle Aggregation and Complete De-encapsulation of Such Aggregates from a Liquid Drop Interior

Magnetic nanoparticles (MNPs) have been extensively used for drug delivery, on-demand material deposition, etc. In this study, we demonstrate the capability to extract such MNPs on-demand from a magnetic nanoparticle-laden drop (MNLD) (i.e., a drop of a stable aqueous dispersion of MNPs), suspended inside a highly viscous polymer (poly(dimethylsiloxane) or PDMS) medium in the presence of an externally applied magnetic field. The phenomena involve the aggregation of the MNPs inside the drop and the consequent extraction of the MNP aggregate out of the drop, with the drop retaining its original shape after the MNP aggregate extraction. We define this latter phenomenon as de-encapsulation. This is the first study that, to the best of our knowledge, demonstrates such precise, controlled, and on-demand removal of the NPs from the interior of a drop (where the NPs, which were originally inside the drop, breach the drop interface and get completely separated from the drop as an aggregate) without any permanent deformation of the drop. We quantify the effect of the changes in the MNP concentration and the drop volume in determining the de-encapsulation distance, which refers to the distance between the drop and the location of the magnet at the time instant when the MNP aggregate leaves the drop. We further identify the volume of the aggregates extracted from the drop and the mechanisms causing such de-encapsulation. We propose a theory to describe the process, and our theoretical predictions capture the experimental trends well. In addition, we also demonstrate multiple, back-to-back MNP aggregate extractions from a single MNLD at different sites, indicating the possibility that the MNLD can be used as an on-demand carrier and depositor of materials. Overall, our results, in addition to demonstrating the first-of-its-kind de-encapsulation of NPs (in the form of an aggregate) from the drop interior, demonstrate a method to control the dynamics, extraction, and targeted deposition of the MNPs.