A scientist invents a new method of manipulating droplets “WRAP”

Additive-free magnetic droplet actuation on a slippery ferrofluid-infused surface. The actuation force is provided by the ferromagnetic wetting ridge that forms spontaneously around the deposited drop. Credit: Jianqiang Zhang et al.

The precise manipulation and transport of micro-droplets is a difficult but crucial task for biomedical and industrial applications. A research team led by a researcher from the City University of Hong Kong (CityU) has successfully developed a new droplet manipulation method called “WRAP” that can transport droplets of different sizes and compositions by electromagnets or electromagnetic fields. programmable. The research team believes that this innovative method has great potential in the development of next-generation microfluidics and in the detection of respiratory droplets carrying COVID-19 and other pathogens landing on the surface.

The research team was led by Dr. Yao Xi, associate professor in the Department of Biomedical Sciences (BMS), Jockey Club College of Veterinary Medicine and Life Sciences, CityU. Their findings were published in the journal Nature Communication.

The flexible and rapid transport of micro-droplets on a solid surface with high precision has long been a great challenge for scientists. Compared to other actuation methods based on light, heat or surface charge, magnetic actuation has many advantages such as fast response, large driving force and programmable control. However, one of the key problems with the traditional magnetic actuation method is the contamination caused by the addition of magnetic particles in the droplets, which severely limits its applications.

“Successful droplet manipulation methods should enable powerful, reversible and freely programmable droplet actuation while imposing the fewest possible restrictions on droplet composition and size,” Dr. Yao emphasized. In an effort to overcome these challenges, the research team led by Dr. Yao and coworkers successfully developed a wetting ridge-assisted programmed magnetic actuation method “without magnetic additives”.







Experiment showing the actuation of micro-droplets sprayed onto the ferrofluid-infused surface when the electromagnet (below the center of the surface) was turned on. Credit: Jianqiang Zhang et al

The research team achieved this breakthrough using a ferrofluid. Ferrofluid is a liquid that contains suspended iron microparticles (ferromagnetic particles) in a solvent. It becomes strongly magnetized in the presence of a magnetic field.

First, the team infused ferrofluid into a porous surface. When the droplets land on the porous surface, the ferrofluid automatically “envelops” the droplets due to surface tension, and thus forms the magnetically responsive ferrofluid ridges surrounding the droplets due to capillary forces.

When an external electromagnetic field was applied, the ferrofluid ridges were magnetically attracted, moving with the droplets depending on the direction of the magnetic field. Thus, the transport path could be program controlled through the design of the external magnetic field.







Experiment showing the guided motion (an “M” path) of a single droplet under the programmed control of a series of electromagnets. Credit: Jianqiang Zhang et al.

Dr. Yao explained that the WRAP method of actuation was applicable as long as there was a wetting ridge formed. This means that droplet transport is no longer limited by droplet size, surface tension or composition.

In the experiments, the research team used electromagnets to precisely and programmatically control the generation, movement, merging and mixing of droplets. “These typical WRAP-based droplet manipulation applications hold promise in the development of new digital microfluidics,” Dr. Yao said.

CityU scientist invents new method for manipulating droplets

Demonstration of the potential of the WRAP method in improving the sensitivity of biomedical analysis. Without magnetic actuation, some microdroplets randomly coalesce and exhibit randomly distributed fluorescence signals slightly above the noise level (left image). With magnetic actuation, all the microdroplets coalesce to form a lattice pattern, showing strong and evenly distributed fluorescence signals (right image). Credit: Jianqiang Zhang et al.

The team also discovered a large signal enhancement in bioanalysis after collecting and merging many micro-droplets into larger ones using the WRAP method. As the WRAP method can monitor the movement of many patient respiratory micro-droplets and merge them into a larger test sample, it can potentially facilitate the detection of pathogens such as the coronavirus from respiratory droplets landed on a surface by combining with advanced technologies. biochemical tests such as immunoassays and nucleic acid assays. Dr. Yao thought that in this way the accuracy (or sensitivity) of the tests could be improved.


Movement of small water droplets controlled by means of a magnet


More information:
Jianqiang Zhang et al, Droplet wetting ridge-assisted programmed magnetic actuation on a ferrofluid-infused surface, Nature Communication (2021). DOI: 10.1038/s41467-021-27503-1

Provided by City University of Hong Kong

Quote: Scientist Invents New Droplet Manipulation Method “WRAP” (February 11, 2022) Retrieved February 11, 2022 from https://phys.org/news/2022-02-scientist-droplet-method.html

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