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Posted: Feb 10, 2010

Magnetic liquid marbles as an alternative to microchannel-based fluidics

(Nanowerk Spotlight) Miniaturized chemical and analysis processes have many advantages, such as in reduced use of chemical reagents and solvents, precisely controlled reaction condition, much shortened reaction time and the ability to integrate into a digital device. The existing technique to manipulate small volumes of liquid, which is the essential technique to the miniaturized processes, is mainly based on channel-based microfluidics. Channel microfluidics indeed has many advantages, but it is hard to handle just a single liquid droplet.
In contrast with the microchannel-based fluidics, the manipulation of discrete droplets without using microfluidic channels is a new field. Here, a liquid droplet is not confined to a closed channel and there is no risk of being adsorbed on a channel wall.
A liquid marble, a liquid encapsulated by non-wetting powder, could be a new microfluidic device, which is especially useful for handling single liquid droplet. One of the challenges for using liquid marbles as microfluidic devices is the communication between the liquid droplet and the external devices/materials.
Researchers in Australia have been trying to develop 'field-responsive smart liquid marbles' which can be opened and closed reversibly on demand, such that the liquid in the marble can be easily taken and other liquid can also be added into the marble easily. The mechanically robust magnetic liquid marble, prepared by coating a water droplet with highly hydrophobic magnetite nanoparticles, can be actuated magnetically.
"We have found that when magnetic hydrophobic nanoparticles are used, the liquid marbles are responsive to magnetic fields," Tong Lin, associate professor at the Institute for Technology Research and Innovation at Deakin University, tells Nanowerk. "Depending on the magnitude of a magnetic field, the magnetic liquid marbles can either be actuated to move, or be opened and closed reversibly."
Reporting their findings in a recent issue of Advanced Materials ("Magnetic Liquid Marbles: Manipulation of Liquid Droplets Using Highly Hydrophobic Fe3O4 Nanoparticles"), Tong and his colleagues have demonstrated the easy preparation of magnetic liquid marbles and their novel properties that can be used for magnetic manipulations of water droplets in a controlled manner.
To fabricate the liquid marbles, the scientists synthesized hydrophobic magnetite nanoparticles with a size of about 10 nm. By rolling a small volume of water in the highly hydrophobic magnetite nanoparticles they then obtained the liquid marbles. Lin explains that the spontaneous attachment of the nanoparticles at the liquid/air interface can be understood by the minimization of the free energy of the surface.
He points out that sufficient mechanical strength is an essential requirement for liquid marbles in practical microfluidic devices.
"We demonstrated the mechanical robustness of the liquid marbles by impact deformation and rebound of the marbles from a glass surface" says Lin. "When landing on the glass surface, the initial spherical liquid marble was first deformed to a pancake-like shape and then retracted and bounced off the surface. The bouncing continued until the potential energy was consumed by the inertial flow and the surface friction. After the impact deformation, we noticed a trace of magnetite nanoparticles on the glass plate. Despite this, the liquid marble was still able to recover to its original shape within 16 milliseconds. It is important to note here that the loosely packed nanoparticle coating was flexible enough to deform itself to follow the contours of the droplet, making the liquid marble elastic and bouncy without leaking of the fluid inside."
Lin further notes that a recent study has shown that the mechanical robustness of liquid marbles created with nanoparticles is greater than those made from microparticle materials due to more uniform coverage of the liquid/air interface by nanoparticles.
Compared to other similar developments in this field, the main contribution of Lin's team is the introduction of a magnetic response to open and close the liquid marbles. This creates an opportunity to enable an external device to take a liquid sample from the marble or add a chemical reagent to the encapsulated liquid for various purposes. Because the process is driven by a magnetic field, the opening and closing of the marble can be handled precisely by an electronic device.
Although this area is in its infancy, and liquid marble technology is facing many technical challenges such as encapsulation ability, mechanical robustness, optical transparency, actuation technique, and long-term stability, magnetic liquid marbles can potentially be used to develop new 'lab-on-chip' devices for chemical or biological analyses, or 'smart' reactors for drug discovery.
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