Drop-to-Drop Transfer of Magnetic Activity: Imparting Significant Heterogeneity and Reactivity to a Drop via Functionality Transfer from Another Drop

Abstract

Interactions between nanoparticles (NPs) and liquid drops have been widely utilized to manipulate both drops and NPs. However, achieving their separation without altering the intrinsic state of either component has remained a significant challenge in NP-laden drop systems. In our previous work (ACS Appl. Mater. Interfaces 2024, 16, 64253–64263), we addressed this challenge by extracting magnetic nanoparticles (MNPs) from the interior of an MNP-laden drop as a compact aggregate under the influence of an external magnetic field. In the present paper, we develop a technique that enables the seamless transfer of this extracted MNP aggregate from one liquid drop (the source drop) into the interior of a second liquid drop (the receiver drop). Remarkably, the aggregate, which can be tracked throughout the transfer process, is transferred nearly in its entirety, while neither drop undergoes any permanent deformation throughout the process. To the best of our knowledge, this is the first demonstration of a seamless drop-to-drop transfer of NPs, or, more broadly, the transfer of one drop’s functionality to another (here magnetic activity) without permanently altering the morphology of either drop. Beyond facilitating the NP transfer, the process transforms the receiver drop into a heterogeneous system that exhibits a variety of intriguing phenomena. For example, the transferred NPs can self-assemble into either skyscraper-like solid structures or magnetically responsive liquid layers. The nature of these assemblies depends on whether the MNP transfer occurs once or multiple times, and these assemblies display markedly different dynamics when subjected to magnetic fields of varying orientations, as well as oscillating and rotating magnetic fields. Furthermore, the transfer process converts the receiver drop into a highly stable microreactor for chemical reactions, enabling reaction products to form and persist while remaining minimally exposed to external disturbances that could otherwise compromise their stability.