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The Coming Era Of Self-Assembly Using Microfluidic Devices

Researchers are assessing the potential of an entirely new way to make exotic materials based on microfluidic self-assembly.

When it comes to building microscopic devices, one of the most promising ideas is to exploit the process of self-assembly. In this way, complex structures can be created by combining building blocks under natural circumstances.

This kind of self-assembly mechanism dominates at the molecular scale, where it is responsible for the construction of most biomolecules. At the heart of this mechanism is Brownian motion which effectively mixes and jiggles molecules so that they rapidly find their place in incipient structures. This is a powerful process that can form hugely complex machines such as the ribosome, a molecular device for synthesizing proteins.

One recent idea is to use colloids rather than molecules as the building blocks for even more advanced structures. Colloids are insoluble, nanometre-scale particles mixed in water. These particles can be chemically engineered to bind together to form specific structures.

But as the building blocks become bigger, it takes longer for Brownian motion to jiggle the blocks into the appropriate locations. So self-assembly take significantly longer. In fact, it can take thousands of seconds to synthesise a single colloidal molecule. At that rate, a millimetre cube containing 1 billion colloidal molecules would take 30 years to form.

Clearly, that is far too slow for any kind of industrial process, so chemists have been looking for ways to speed up colloidal self-assembly. Today, Bingqing Shen at ESPCI (the Paris Institute of Technology) in France and a few buddies say they’ve found an entirely new way to assemble structures on this scale that should reduce manufacturing time by orders of magnitude.

The new approach is to place the particles in a fluid flow and see whether this helps them assemble. The particles they want to assemble are tiny droplets of fluorinated oil placed in water. These droplets are 50 micrometres in diameter, about the width of a human hair.

The team injects a number of these droplets in a row into a flow of water that passes through a microfluidic chamber. It is easy to imagine that when the flow of liquid is steady and smooth, the droplets simply follow each other in line.

But Shen and co have observed something entirely different. As the droplets move through the chamber, they begin to assemble into specific shapes that depend on the number and types of particles involved but not on the detail of the initial conditions. Consequently, these shapes are entirely reproducible.

By injecting droplets with different compositions in various sequences, the team has produced all kinds of shapes including pyramids, tetrahedrons and even a three-dimensional spiral structure using magnetic drops in a magnetic field.

This kind of behaviour raises an interesting question. Given that the fluid flow is smooth, what causes the droplets to assemble in this way? “The self-assembly process is not driven by Brownian fluctuations because it would take days, while only a few seconds suffice in the experiments,” they say.

Instead, Shen and co think an entirely new hydrodynamic effect is at work. Their key discovery is that the self-assembling process works when the walls of the chamber are relatively close together but stops when the walls are moved further away. “We propose here that the physical origin of the phenomenon is linked to the presence of the top and bottom walls of the self-assembly channel,” they say.

In other words, the walls must be close to enough to interact with the droplets, slowing them down. This creates local changes in pressure in the fluid around the droplets that causes them to coalesce and tumble. It is this movement that allows them to self-assemble.

Crucially, this process is orders of magnitude faster than with Brownian motion. And because it is possible to produce the structures in parallel using many self-assembling chambers at the same time, this could dramatically increase the rate at which these kinds of materials are manufactured. “In our case, producing 1 billion structures in a one thousandfold parallelized device would take 30 days instead of 30 years,” say Shen and co.

Of course, this is still a long time but the team says there is considerable room for improvement. For example, they hope to significantly increase the flow speeds through the self-assembly chambers and they should be able to increase the parallelisation using tinier droplets and smaller mixing chambers. In this way, Shen and co say production rates could be increased by orders of magnitude. “Down the road, production rates of millions of clusters per second can be envisioned,” they say.

Having made these clusters, the final step is to assemble them into a solid material, which is done by drying them, possibly in the presence of an electric or magnetic field to align the clusters appropriately.

Shen and co say that it should be possible to change the droplet chemistry in such a way to manipulate the physical properties of the bulk material in ways that are currently difficult or impossible to do. This includes changing the refractive index, the electrical conductivity, the magnetic susceptibility and so on.

What these guys are proposing is an entirely new form of manufacturing based on clever chemistry and the powerful technique of self-assembly. Just where this will lead is not yet clear but it will be interesting to watch nonetheless.

Ref: arxiv.org/abs/1409.4009 : Self-Assembly Driven By Hydrodynamic Interactions In Microfluidic Devices

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