# Chocolate-inspired technique helps researchers develop better polymer shells

For centuries, chocolatiers have been trying to develop the perfect chocolate coating for bonbons, honing their skill to the point of artistic performance. But scientists believe they can take things even further. A group of MIT researchers believe they’ve come up with the perfect chocolate coating technique, a technique that could have many applications outside the food industry.

Tartufo, a desert covered in chocolate. Photo by Anna Fox

Bonbons can come in a large variety of shapes, sizes and tastes – but the most loved ones are without a doubt small candies coated in chocolate. The first reports about bonbons come from the 17th century, when they were made at the French royal court.

“Think of this formula as a recipe,” says Pedro Reis, the Gilbert W. Winslow Associate 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 a much better, quantitative understanding of what’s going on, and one can now be predictive.”

Reis and his team were inspired by videos of chocolatiers making bonbons and other chocolate shells. They pour the chocolate into molds, allowing excess chocolate to flow out, creating a shell of uniform thickness. But Reis was curious: was there a way to accurately predict the thickness of the resulting shell? He set out to explore this seemingly frivolous question, alongside lead author and graduate student Anna Lee, postdoc Joel Marthelot, and applied mathematics instructor Pierre-Thomas Brun, along with colleagues from the team of François Gallaire at the Swiss Federal Institute of Technology in Lausanne, Switzerland.

Initially, Lee and Marthelot used an analogous technique to experimentally create their own shells, using not chocolate but a polymer solution that they drizzled over dome-shaped molds and spheres.

They found that again and again, the coating had equal thickness on all sides (they cut the balls in half to test this). So they set out and determined the mathematical formula for the thickness of the shell, which is basically the square root of the fluid’s viscosity, times the mold’s radius, divided by the curing time of the polymer, times the polymer’s density and the acceleration of gravity as the polymer flows down the mold.

It sounds like a complicated formula, but it boils down to this: the bigger the mold, the thicker the shell, because it takes the fluid longer to flow to the bottom. The longer the curing time, the thinner the shell will be. Armed with that knowledge, they could go crazy with polymer models and see how to obtain shells of the desired thickness.

“You could go in the lab and lay down tons of ping pong balls and test various initial conditions, which is what Anna and Joel have been doing to some extent, but with numerics, you can get really creative,” Brun says.

Ultimately, they found that by tampering with the curing time, they can create much thicker coatings, which can be significant not only for the materials industry, but also for medical purposes

“By waiting between mixing and pouring the polymer, we can increase the thickness of a shell by a factor of 11,” says Lee.
“This flexibility of waiting gives us a simple parameter we can tune, depending on what we want for our final goal,” Reis says. “So I think ‘rapid fabrication’ is how we can describe this technique. Usually that term means 3-D printing and other expensive tools, but it could describe something as simple as pouring chocolate over a mold.”

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