Magnetoresponsive Surfaces for Manipulation of Nonmagnetic Liquids: Design and Applications

Magnetic actuation provides a remote, nondestructive, and real-time way for controllable liquid manipulation, which has promising technological applications for areas ranging from digital microfluidics, biochemical assays, and microreactors, to liquid collection. However, conventional magnetic liquid manipulation usually relies on incorporating magnetic particles into a liquid to empower its motion in response to an external magnetic field, resulting in considerable limitations of magnetic actuation in various applications. Recently, a range of magnetoresponsive surfaces (MRSs) with elaborately designed surface structures and/or compositions have enabled on-demand liquid manipulation using magnetic fields, even when the liquids do not contain any magnetic particles. Here, the state-of-the-art of MRSs capable of manipulation of nonmagnetic liquids is reviewed. Their preparation, different manipulation modes, including directed propulsion, spreading, and rebound, and the underlying working mechanisms are discussed. Based on the working principles, MRSs are classified into three categories, including surfaces with magnetic bendable microstructures, surfaces with switchable topographies, and surfaces infused with ferrofluids. Their applications in microfluidics, microreactors, liquid distributors and pumps, fog collection, and anti-icing are presented. Finally, key challenges and the future prospects of MRSs are provided.