Chocolate-Inspired Theory Predicts Thickness of Coatings

Chocolatiers have been perfecting the art of the bonbon since the 1600s, passing down techniques for crafting a smooth chocolaty shell. Now, a theory and a simple fabrication technique derived by MIT engineers may help chocolate artisans create uniformly smooth shells. The research should have uses far beyond the chocolate shop - by knowing just a few key variables, engineers could predict the mechanical response of many other types of shells, from small pharmaceutical capsules to large airplane and rocket bodies. The fabrication technique involved drizzling liquid polymer over dome-shaped molds and spheres such as ping pong balls. The researchers allowed the liquid to coat each mold and cure over 15 minutes. They then peeled the resulting shell off the mold and observed that it was smooth, with a nearly uniform thickness throughout. They developed a simple formula to estimate the final thickness of a shell, which essentially equals 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. The formula boils down to the following relationships: The larger a mold's radius, the longer it takes for fluid to flow to the bottom, resulting in a thicker shell; the longer the curing time, the faster the fluid will drain to the bottom, creating a thinner shell.

Transcript

00:00:06 Inspired by the making of chocolate confections like hollow chocolate eggs and bonbons, a group of MIT researchers have developed a rapid fabrication technique and a theory that accurately predicts the final thickness of a shell of a known material given the original rheological properties of the

00:00:22 material and the geometry of the mold used for the coating. Their technique consists of pouring a liquid polymer onto curved surfaces, otherwise known as the mold, until they are completely coated and solidified. The specific properties of each polymer used can be distinguished by their varying colors and they

00:00:42 can be poured over molds with varying radii to yield different results. Using a set of variables the researchers are able to predict precisely how thick the polymer shell will be once it is in its final state. For example, they know the final shell will be thicker for larger values of the

00:01:04 radius of the mold or the initial viscosity of the polymer. The shell will be thinner if the polymer takes longer to cure. With their technique, the researchers can readily and accurately predict this final shell thickness based on design parameters. Once the coating fully hardens,

00:01:25 it is removed, and measured, and examined under the microscope confirming their predictions. An interesting feature of this fabrication mechanism is that the resulting, elastic shells, are nearly free of any imperfections and their thickness is nearly uniform from the pole to their equator.

00:01:44 Overall their technique is simple and versatile. It does not involve high-end equipment and allows for a robust, rapid, and precise fabrication technique that has the potential for practical applications such as artificial vesicles, smart skins, as well as protecting and package films on curved components.