Scientists facilitate self-assembly of nanoparticles

With the scaling down of many devices, nano and microparticles have gained prestige in the world of technology. However, when dealing with devices that use such particles, the arrangement of the particles is an important factor.

Realizing this importance, Carnegie Mellon’s Nadine Aubry, head of the mechanical engineering department and Pushpendra Singh, an engineering professor at the New Jersey Institute of Technology (NJIT), have developed a new technique for the self-assembly of nano- and micro- particles.

Microparticles have sizes that are measured in micrometers, where one micrometer is 10 to the power of negative six meters.

Nanoparticles are even smaller. They are generally less than 100 nanometers in at least one dimension (one nanometer is 10 to the power of negative nine meters).

The team’s research paper, titled “Micro- and nanoparticles self-assembly for virtually defect-free, adjustable monolayers,” was published last month in the Proceedings of the National Academy of Sciences USA journal. As mentioned in the paper, the team’s research deals with the self-assembly of particles in two-dimensions, or “monolayers,” placed at a liquid interface.

A monolayer is a single layer of particles, and a liquid interface is a surface where two liquids come in contact with one another. The free surface of a liquid, which is in contact with air, is also considered to be a liquid interface. Self-assembly of particles at liquid interfaces was previously based on capillary forces. Capillary forces cause particles at the surface of a liquid to attract one another.

Aubry explained these forces by giving the analogy of a breakfast cereal. “When you have your bowl of milk with cereal, you put the cereal on top of the milk. If you wait long enough, you are going to see that the [flakes] gather. That is because of the capillary force which makes the particles attract one another,” she said.

Although this technique has been in use for quite some time, it has a number of drawbacks. One of the major drawbacks is that it does not work well for small particles that have a radius less than 10 microns.

“If you have very small particles, they don’t cluster together because their weight is not large enough,” Singh said. Aubry mentioned two other drawbacks of this technique.

Firstly, using this technique makes it difficult to arrange particles into regular patterns. “When we want to make a material, we want it to be as regular as possible,” Aubry said. Therefore, the final materials may be defective.

Secondly, after self-assembly the particles touch each other. Many devices require the particles to have gaps between them.

Such an arrangement cannot be achieved by relying solely on capillary forces. Aubry and Singh’s new method provides a solution to all of these problems. In their model, electrodes are placed above and below the surface of the liquid so that an electric field can be generated perpendicular to the free surface of the liquid that the particles are on.

Aubry explained that the electric field generates electrostatic forces acting on the particles, which can be split up into vertical and lateral components.

The vertical component of the force moves the particles in its direction, causing capillary forces to arise in the lateral direction. Unlike capillary forces generated without the use of an electric field, these capillary forces are capable of moving even small, light-weight particles.

Along with these capillary forces, lateral repulsive forces are also generated, which prevent the particles from coming close enough to touch each other. A balance between these repulsive forces and the attractive capillary forces results in a situation where the particles remain at a specific distance from each other.

“That is how we can make a monolayer in which the distance between the particles is non-zero,” Aubry said.

Not only can the precise distance between the particles be calculated, but the distance can also be changed by carefully manipulating the forces acting on the particles.

The applications of this new technique are numerous.

“The fact that we can control the gap between the particles means that we can also control the mass transfer of gas or liquid or other particles in the direction normal to the interface,” Aubry said.

This property makes such materials extremely useful in drug delivery, the process of delivering pharmaceuticals to humans. Patches through which the medicines are delivered can be manufactured using materials like these. By controlling the distances between the particles, the speed at which drugs are delivered can be controlled.

Another application is in the field of optics.

“[This technique] could be useful for making photonic materials. These are materials where the distance between the particles is comparable to the wavelength of light. So, you can manipulate light using these photonic materials,” Singh said.
Although the development of this technique is a remarkable feat, the researchers intend to delve deeper into their area of study.

Aubry mentioned that future projects include manipulating different kinds of particles like drops, biological cells, and particles with different shapes or sharp edges.

Aubry and Singh are also trying to use this technique to separate clustered biological cells, so that they can be viewed more easily and efficiently.