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Scientists invented a new “WRAP” droplet manipulation method

Magnetic actuation of additive-free droplets on slippery surfaces injected with ferrofluid. The working force is provided by a ferromagnetic wet ridge that spontaneously forms around the deposited droplets. Credit: Jianqiang Zhang et al.

Precise manipulation and transport of micro-sized droplets is a difficult task, but it is very important for biomedical and industrial applications. A research team led by scholars at City University of Hong Kong has succeeded in developing a new droplet manipulation method called “WRAP” that can transport droplets of various sizes and compositions by means of electromagnets or programmable electromagnetic fields. The researchers believe that this innovative method has great potential for the development of next-generation microfluidics and the detection of surface-attached respiratory droplets containing COVID-19 and other pathogens. increase.


The research team was led by Dr. Yao Xi, an associate professor at the Jockey Club College of Veterinary Medicine and Life Sciences in City U.Their findings were published in the journal Nature Communications..

Transporting micro-sized droplets on a solid surface with great accuracy, flexibility and speed has long been a major challenge for scientists. Compared to other methods of operation based on light, heat, or surface charge, magnetic operation has many advantages, including quick response, high driving force, and programmable control. However, one of the key problems with traditional magnetic working methods is the contamination caused by the addition of magnetic particles to the droplets, which greatly limits their use.

“Successful droplet manipulation methods should enable the operation of powerful, reversible, freely programmable droplets with as few restrictions as possible on the composition and size of the droplets,” Yao said. The doctor pointed out. Aiming to overcome these challenges, a research team led by Dr. Yao and his collaborators has succeeded in developing a “magnetic-free” Wet Ridge Assistance Program (WRAP) magnetic actuation method.

An experiment demonstrating the action of micro-sized droplets sprayed onto a ferrofluid-injected surface when the electromagnet (below the center of the surface) is on. Credit: Jianqiang Zhang et al

The research team has achieved breakthrough results by using ferrofluids. Ferrofluid is a liquid containing minute iron particles (ferromagnetic particles) suspended in a solvent. It is highly magnetized in the presence of a magnetic field.

First, the team injected ferrofluid into the porous surface. When the droplet lands on a porous surface, the ferrofluid automatically “wraps” the droplet due to surface tension, thus forming a magnetically responsive ferrofluid ridge that surrounds the droplet by capillary force.

When an external electromagnetic field is applied, the ferrofluid ridge is magnetically attracted and travels with the droplet based on the direction of the magnetic field. Therefore, the transport path can be programmed programmatically through the design of an external magnetic field.

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

Dr. Yao explained that the WRAP actuation method is applicable as long as the wet ridge is formed. This means that the transport of droplets is no longer restricted by droplet size, surface tension, or composition.

In the experiment, the research team used electromagnets to accurately and programmatically control the formation, movement, merging, and mixing of droplets. “The typical applications of these WRAP-based droplet manipulations are promising for the development of new digital microfluidics,” said Dr. Yao.

City U scientists invent new droplet manipulation method

Demonstration of the potential of the WRAP method in increasing the sensitivity of biomedical analysis. In the absence of magnetic action, some microdroplets randomly coalesced, randomly distributed, and showed a fluorescent signal slightly above the noise level (left image). Magnetically actuated to coalesce all the microdroplets into a grid pattern, showing a strong, regularly distributed fluorescent signal (right image). Credit: Jianqiang Zhang et al.

The team also used the WRAP method to collect a large number of micro-sized droplets and merge them into large droplets before discovering excellent signal enhancement in bioanalysis. The WRAP method can control the movement of large numbers of micro-sized respiratory droplets from patients and integrate them into larger test samples, potentially facilitating the detection of pathogens such as coronavirus from the respiratory tract. I have. splash Landing on Water surface By combining with advanced biochemical tests such as immunoassays and nucleic acid assays. Dr. Yao believed that this could improve the accuracy (or sensitivity) of the test.


Movement of small water droplets controlled by a magnet


For more information:
Jianqiang Zhang et al, Wet Ridge Assistance Programmed Magnetic Actuation of Droplets on Ferrofluid Infused Surfaces, Nature Communications (2021). DOI: 10.1038 / s41467-021-27503-1

Quote: Scientists have obtained a new “WRAP” droplet manipulation method from https: //phys.org/news/2022-02-scientist-droplet-method.html on February 11, 2022 (February 2022). 11th) was invented

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Scientists invented a new “WRAP” droplet manipulation method

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