Basu Abhirup, Clary Matthew R, Tracy Joseph B, Hall Carol K, Velev Orlin D
Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States.
Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States.
ACS Nano. 2024 Jul 15. doi: 10.1021/acsnano.4c06285.
Magnetic field-directed colloidal interactions offer facile tools for assembly of structures that range from linear chains to multidimensional hierarchical architectures. While the field-driven assembly of colloidal particles has commonly been investigated in unbounded media, a knowledge gap remains concerning such assembly in confined microenvironments. Here, we investigate how confinement of ferromagnetic nanoparticles in microspheres directs their magnetic assembly into hierarchical architectures. Microdroplets from polydimethylsiloxane (PDMS) liquid precursor containing dispersed iron oxide magnetic nanoparticles (MNPs) were placed in a static magnetic field leading to the formation of organized assemblies inside the host droplets. By changing the MNP concentrations, we revealed a sequence of microstructures inside the droplets, ranging from linear chains at a low MNP loading, transitioning to a combination of chains and networked bundles, to solely 3D bundles at high MNP loading. These experimental results were analyzed with the aid of COMSOL simulations where we calculated the potential energy to identify the preferred assembly conformations. The chains at high MNP loading minimized their energy by aggregating laterally to form bundles with their MNP dipoles being out-of-registry. We cured these PDMS droplets to immobilize the assemblies by forming soft microbeads. These microbeads constitute an "interaction toolbox" with different magnetic macroscale responses, which are governed by the structuring of the MNPs and their magnetic polarizability. We show that thanks to their ability to rotate by field-induced torque under a rotating field, these microbeads can be employed in applications such as optical modulators and microrollers.
磁场引导的胶体相互作用为从线性链到多维层次结构的结构组装提供了便捷工具。虽然胶体颗粒的场驱动组装通常在无界介质中进行研究，但在受限微环境中的此类组装仍存在知识空白。在此，我们研究了微球中铁磁纳米颗粒的受限情况如何将其磁性组装引导成分层结构。将含有分散的氧化铁磁性纳米颗粒（MNP）的聚二甲基硅氧烷（PDMS）液体前驱体的微滴置于静磁场中，导致主体微滴内部形成有组织的组装体。通过改变MNP浓度，我们揭示了微滴内部的一系列微观结构，从低MNP负载时的线性链，过渡到链与网络束的组合，再到高MNP负载时的仅三维束。借助COMSOL模拟对这些实验结果进行了分析，我们在模拟中计算了势能以确定优选的组装构象。高MNP负载时的链通过横向聚集形成束来最小化其能量，其MNP偶极失配。我们固化这些PDMS微滴以通过形成软微珠来固定组装体。这些微珠构成了一个具有不同宏观磁性响应的“相互作用工具箱”，其由MNP的结构及其磁极化率决定。我们表明，由于它们在旋转场下能够通过场致转矩旋转，这些微珠可用于诸如光调制器和微滚轮等应用中。
