Magnetic Droplet Manipulation on Open Surfaces

ABSTRACT Manipulation of liquids on a smaller scale enables applications in various fields, particularly diagnostics and medicine. Microfluidic platforms perform this task using existing comprehensive approaches and are exploring new methods for manufacturing efficient, cost‐effective, and versatile devices. Liquids are often analyzed on an open surface in droplet form (digital microfluidics), like a blood drop placed in a biosensor for measurement or diagnostics, or in continuous form, like liquids flowing in closed channels. Droplet manipulation on an open surface is controlled by interaction with the surface or by external forces. Droplets can be excited with various external energy sources for reconfiguration or translocation. The magnetic field is a non‐invasive manner to exert forces on smaller scales, widely used in microfluidics. The literature reports numerous works on magnetically responsive surfaces, magnetic liquid spheres, magnetically charged droplets, and ferrofluid‐based liquid‐liquid interactions. Here, we provide a brief but comprehensive overview of magnetic digital microfluidics, focusing on fluid handling physics at the micrometre scale. We also highlight limitations and complexity of existing approaches to provide a comprehensive picture enabling researchers from different fields to familiarize themselves with current methods.