Related Experiment Video
Updated: Dec 21, 2025

Aqueous Droplets Used as Enzymatic Microreactors and Their Electromagnetic Actuation
Published on: August 28, 2017
Trapping and Coalescence of Diamagnetic Aqueous Droplets Using Negative Magnetophoresis
S K Jain1, U Banerjee1, A K Sen1
1Micro Nano Bio-Fluidics Unit, Fluid Systems Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India.
Abstract:
The manipulation of aqueous droplets has a profound significance in biochemical assays. Magnetic field-driven droplet manipulation, offering unique advantages, is consequently gaining attention. However, the phenomenon relating to diamagnetic droplets is not well understood. Here, we report the understanding of trapping and coalescence of flowing diamagnetic aqueous droplets in a paramagnetic (oil-based ferrofluid) medium using negative magnetophoresis. Our study revealed that the trapping phenomenon is underpinned by the interplay of magnetic energy (Em) and frictional (viscous) energy (Ef), in terms of magnetophoretic stability number, Sm = (Em/Ef). The trapping and nontrapping regimes are characterized based on the peak value of magnetophoretic stability number, Smp, and droplet size, D*. The study of coalescence of a trapped droplet with a follower droplet (and a train of droplets) revealed that the film-drainage Reynolds number (Refd) representing the coalescence time depends on the magnetic Bond number, Bom. The coalesced droplet continues to remain trapped or gets self-released obeying the Smp and D* criterion. Our study offers an understanding of the magnetic manipulation of diamagnetic aqueous droplets that can potentially be used for biochemical assays in microfluidics.
Related Concept Videos
Diamagnetism
Diamagnetism was discovered by Anton Brugmans in 1778 when he observed that bismuth gets repelled by magnetic fields, thus theorizing that diamagnets get repelled by magnets....
Magnetic Damping
If, however, the bob is a slotted metal plate, the magnet produces a much smaller effect. When a slotted metal plate enters the field, an emf is induced by the change in flux; however, it is less effective because the slots limit the...
Colloidal precipitates
Coagulation
Precipitation and Co-precipitation
Ferromagnetism

