Assembling Matter with a Magnet
You have probably seen magnetic objects such as iron filings being moved around with a magnet. But can we manipulate non-magnetic objects with a magnet? The answer is "yes!", if we disperse them in a paramagnetic fluid.
Bartosz Grzybowski, director of the Non-equilibrium Energy Research Center (NERC) explains a paramagnetic fluid with a simple example. "Imagine holding a magnet close to a ping pong ball. Nothing will happen. Similarly, if you hold the magnet near a glass full of water with some salt added, such as magnesium chloride, nothing will happen. But, if you place the ping pong ball inside the glass of water and salt, and then hold the magnet close to it, the ball will start moving in the water."
Grzybowski and his team of scientists used this phenomenon to precisely assemble up to one hundred million magnetic and non-magnetic particles into unique structures. Their research, published in Nature, can be used on a wide range of materials and arrange small molecules, particles, and even living bacteria.
"Positioning of biological matter using a magnetic field is quite unprecedented, resulting in numerous biological applications," said Grzybowski. Ahmet Demirörs, the lead author of this work, explains that the assembly depends on the difference in the magnetic susceptibilities of the particles that are dispersed in solution and the dispersing medium or solution. Magnetic susceptibility indicates the degree of the magnetization of a material in response to an applied magnetic field. By adjusting the magnetic susceptibility contrast between the particles and the solution, the effect of the magnetic field on the particles can be tuned. Combining that with patterning of the magnetic field in desired shapes and length scales based on the size of the particles, the authors successfully constructed intricate two- and three-dimensional (3D) structures.
For example, using a nickel grid with controlled thickness and width to tune the magnetic field, they assembled silica particles (about five times smaller than red blood cells) into 3D shapes inspired by Istanbul's Blue Mosque. The strength of the field felt by the particles depends on the arrangement of the nickel grid. The field itself holds the particles in position, acting as a virtual magnetic mold. By changing the shape and positioning of the grid, the team was able to control the height and layout of the arcs. To prevent the assemblies from falling apart after the magnet was removed, chemical compounds were added to bond and fix the structure to the substrate and each other.
Demirörs and coworkers have also shown that their process can be used to place live bacteria in defined locations on the substrate. Furthermore, because dead bacteria tend to take up ions from the solution resulting in a change of their magnetic properties, they can be separated from live bacteria via this technique!
This idea of using magnetic molds to assemble particles has opened a whole new field of research in Grzybowski's laboratory at NERC. His scientists are modifying the process to work with much smaller particles (nano-sized). They would also like to further develop this technique for positioning and manipulating living cells for biomedical applications in the near future. Aside from the potential biological applications, one can imagine using this technique to separate salts with different magnetic properties. This can lead to extraction and enriching elements such as uranium for potential energy applications. Furthermore, the flexibility and simplicity of the technique for making multi-component structures renders it suitable for potentially fabricating photonic and electronic devices that can be used for harvesting renewable energy.
Demirörs AF, PP Pillai, B Kowalczyk, and BA Grzybowski. 2013. "Colloidal Assembly Directed by Virtual Magnetic Moulds." Nature 503:99-103. DOI: 10.1038/nature12591