Scientists from the California Institute of Technology (Caltech) have revealed the physical mechanism responsible for growing structures of nanopillars on polymer layers with great precision and in diverse shapes.
This nanofluidic process, developed by Sandra Troian, professor of applied physics, aeronautics and mechanical engineering at Caltech, and published in the scientific journal Physical Review Letters, may in the future replace the lithography methods used today to prepare three-dimensional nano- and microstructures in applications of optical, photonic and biofluidic devices.
The production of large and precise nanoarrays relies on standard photolithographic replication methods, which include optical (ultraviolet radiation) or chemical treatment, using strong materials that etch and dissolve, alternately, pieces of silicon (silicon) and other materials. Photolithography is used today, for example, to produce integrated circuits and microelectromechanical devices.
However, the many cycles of melting and consumption of the material cause a lot of roughness in nanostructures, a result that limits their performance.
"This process is also fundamentally two-dimensional, and therefore the production of three-dimensional structures requires the construction of layer upon layer, over and over again," explains the researcher.
In order to reduce costs, production times and the resulting roughness, researchers examined alternative methods by which molten layers could go through structural design and solidification processes in their original location, in one step.
About a decade ago, research groups in Germany, China and the United States encountered a strange phenomenon while using methods involving temperature changes. When molten polymer nanolayers were inserted into a tiny gap between two pieces of steel that had different temperatures, arrays of nanopillars began to form independently.
These protrusions continued to grow until they reached the upper piece of mold; The finished columns were, on average, several hundred nanometers high and spaced a few microns apart.
Sometimes, these columns were joined together to form patterns that looked like bi-ring chains; In other cases the columns grew at regular intervals from each other while obtaining honeycomb-like arrays. Once the temperature of the system was returned to room temperature the structures solidified in place to form forms that aggregated independently.
In 2002, researchers in Germany, who came across this phenomenon, hypothesized that the pillars grow out of extremely minute - but completely real - pressure fluctuations that exist in the surface, which only seems smooth and calm to us. They proposed that the differences in the pressures prevailing on the surface are due to tiny changes in the way in which individual quanta of fluctuating energy, called phonons, are scattered from the surface complexes.
"In this research group's model, the scientists believe that the difference in the acoustic resistance between the air and the polymer is the one that gives rise to an imbalance in the phonon flux, which creates radiation pressure that causes instability of the layer and thus leads to the formation of the columns," the researcher notes. "Their mechanism is the acoustic equivalent of the Casimir effect (a physical effect predicted by the Dutch physicist Hendrik Casimir in 1948 that describes the attraction between two conductive plates that are not charged with an electric charge. This phenomenon is the result of quantum fluctuations of the void between the plates and also exists for geometric configurations difference from emitters, between two conductors) is quite familiar to physicists dealing with the nanometer level."
However, the researcher, who was aware of heat effects in the nanometer range - and who knew that the progress of these phonons is actually implausible in amorphous molten polymers, which lack any periodic order - immediately realized that a different mechanism might be hidden in this system.
To determine the practical reason for the formation of the nanopillars, her research team developed a fluid dynamics model of the same kind of thin molten nanolayer in a temperature gradient. Their model, the researcher explains, "demonstrated instability of self-aggregation that allowed for the recovery of the unusual structures," and showed that the nanopillars, in fact, arise not as a result of pressure fluctuations but as a result of a simple physical process known as thermocapillary flow.
In capillary flow (the law of capillarity, or capillary force) the force of attraction, or the adhesion, between particles existing in the same liquid (for example, water) produces surface tension, the same force responsible for the stability of a drop of water. Since surface tension tends to reduce the surface area of the liquid, it sometimes functions as a stabilizing mechanism for deformations created as a result of other forces. Temperature differences across the liquid interface create differences in interfacial tension. In most fluids, colder areas will have a higher surface tension than warmer areas – and this non-uniformity can cause the fluid to flow from the hotter to colder areas – a process known as thermocapillary flow. In her past, the researcher has already used such forces for microfluidic applications to move liquid droplets from one point to another.
"You can see this phenomenon very well if you move an ice cube in a track of the figure eight under a metal foil coated with a liquid such as glycerol," she explains. "The liquid flows towards the cube during its movement in the track. This way you can write your name, and voila – you have a new form of thermocapillary lithography!”
In the published article, the researchers showed how this phenomenon could, theoretically, overcome all other forces operating in nanometer dimensions, and also showed that the phenomenon is not unique to polymeric layers. In temperature-drop experiments, they explain, the tips of these tiny pillars in the polymer layer experience a slightly lower temperature than the liquid surrounding them, due to their proximity to the colder piece of mold.
"The interfacial tension of the tip of the developing pillar is slightly higher, and this tiny difference produces a very strong force acting parallel to the air/polymer interface, directing the liquid to the cooler surface. The closer the tip gets to the surface, the colder it becomes, thus increasing the self-instability," explains the researcher.
In the end, says the researcher, "the activity can end in receiving very tall pillar structures. The only limit to the height of these nanopillars is the space between the pieces of the mold."
In computer models, the researchers were able to use controlled changes in the temperatures of the colder material to precisely shape the resulting replicated shapes in the nanosurface.
Using one of these models, they created a three-dimensional nanorelief of the Caltech institute symbol.
The team of researchers has now begun laboratory experiments through which they hope to produce nano-optical and nano-photonic components in a variety of configurations. "We are now interested in making very smooth surfaces - the smoothest possible - and three-dimensional shapes that are difficult for contractors today using normal lithography methods," adds the researcher.
"The research carried out in our institute is a good example of how a basic understanding of the principles of physics and mechanics can lead to unexpected discoveries with far-reaching and practical applications," says the head of the Department of Engineering and Applied Sciences at Caltech.
