Inspired by a centuries-old technique used by chocolatiers to create chocolate shells for bonbons and other sweets, engineers have developed a new technique for making polymer films that are both uniform and predictable. According to the researchers, the new theory and method can not only allow confectioners to precisely control the thickness of bonbon casings, but can be more generally applied to create polymer shells for everything from drug capsules to rocket bodies.
One way of making bonbons is to cast hollow spheres, eggs, or other shapes by pouring molten chocolate over molds. This sees the chocolate form a film, which hardens to form a smooth, uniform shell for various fillings.
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After viewing chocolate making videos, an MIT team, along with colleagues from the Swiss Federal Institute of Technology, noticed how uniform these chocolate shells were and wanted to know if a similar technique could be used to create thin polymer shells. To test their idea, they used liquid polymers and poured them over dome and sphere-shaped molds of various diameters to form thin, rubbery shells after about 15 minutes of curing. Like their chocolate counterparts, the shells were smooth, uniform, and free of defects.
The team cut the films off the molds, then measured them and studied them under a microscope. They found the films were free of defects and of uniform thickness. By studying the dynamics of how the shells were formed, the researchers isolated the factors that affect the final product, such as the mold size, the speed that the liquid polymer flows, and the curing time of the polymer.
What the engineers found was that the amount of polymer poured or the height from which it was poured did not alter the thickness of the shell, but they were able to come up with a mathematical formula that could predict how changing certain factors would affect the thickness of the film.
According to MIT, the formula says that the thickness equals the square root of the fluid's viscosity, multiplied by the mold's radius, divided by the curing time of the polymer, multiplied by the polymer's density and the acceleration of gravity as the polymer flows down the mold. For example, the larger the mold, the longer it takes the liquid polymer to flow over it, which makes the shell thicker. On the other hand, a smaller mold or thinner polymer makes for a thinner shell.
"Think of this formula as a recipe," says Pedro Reis, a professor of mechanical engineering and civil and environmental engineering at MIT. "I'm sure chocolatiers have come up with techniques that give empirically a set of instructions that they know will work. But our theory provides a much better, quantitative understanding of what's going on, and one can now be predictive."
The team followed this part of the research by creating mathematical models to test the formula. This allowed them to study the effects of the many possible variables without having to try out every permutation in the lab. What they found was that the length of time it took the polymer to set was a key factor. By allowing the polymer to cure for a few minutes before pouring, they could make the film thicker and with less run off. Conversely, by pouring the polymer earlier, the shell became thinner.
According to MIT, the new technique will help chocolatiers to produce chocolate shells that are smooth and with precisely tailored thickness, as well as sparking new interest in shell mechanics in engineering. In addition, it could allow for rapid prototyping of items without relying on expensive 3D printing techniques.
"This flexibility of waiting gives us a simple parameter we can tune, depending on what we want for our final goal," says Reis. "So I think 'rapid fabrication' is how we can describe this technique. Usually that term means 3D printing and other expensive tools, but it could describe something as simple as pouring chocolate over a mold."
The team's results were published in Nature Communications.
The video below gives an overview of the MIT team's research.