{"id":8303,"date":"2016-04-05T06:07:45","date_gmt":"2016-04-05T06:07:45","guid":{"rendered":"http:\/\/revoscience.com\/en\/?p=8303"},"modified":"2016-04-05T06:07:45","modified_gmt":"2016-04-05T06:07:45","slug":"how-crispy-is-your-bonbon","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/how-crispy-is-your-bonbon\/","title":{"rendered":"How crispy is your bonbon?"},"content":{"rendered":"

New theory, inspired by chocolate coatings, predicts thickness of thin shells.<\/em><\/strong><\/p>\n

\n

\"The<\/a>
The team was initially inspired by videos of chocolatiers making bonbons and other chocolate shells. Pedro Reis and his team wondered whether there was a way to precisely predict the final thickness of chocolate and other shells that start out as a liquid film.
Image: Courtesy of the researchers<\/figcaption><\/figure>\n

CAMBRIDGE, MA<\/strong> — Since the 1600s, chocolatiers have been perfecting the art of the bonbon, passing down techniques for crafting a perfectly smooth, even chocolaty shell.<\/span>
\u00a0<\/span>
Now, a theory and a simple fabrication technique derived by MIT engineers may help chocolate artisans create uniformly smooth shells and precisely tailor their thickness. The research should also 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 team\u2019s results are reported today in the journal\u00a0<\/span>Nature Communications<\/em>.<\/span><\/span>
\u00a0<\/span>
The researchers developed a fabrication technique to quickly create thin, rubbery shells, which involved drizzling liquid polymer over dome-shaped molds and spheres such as ping pong balls. They allowed the liquid to coat each mold and cure, or solidify, over 15 minutes. They then peeled the resulting shell off the mold and observed that it was smooth \u2014 virtually free of noticeable defects \u2014 with a nearly uniform thickness throughout.<\/span>
\u00a0<\/span>
Combining this simple technique with the theory they derived, the team created shells of various thicknesses by changing certain variables, such as the size of the mold and the polymer\u2019s density. Surprisingly, they found that the shell\u2019s final thickness does not depend on the volume of liquid or the height from which it is poured onto the mold.<\/span>
\u00a0<\/span>
\u201cThink of this formula as a recipe,\u201d says Pedro Reis, the Gilbert W. Winslow Associate Professor of mechanical engineering and civil and environmental engineering at MIT. \u201cI\u2019m 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\u2019s going on, and one can now be predictive.\u201d<\/span>
\u00a0<\/span>
Reis hopes that the group\u2019s theory will reinvigorate studies in shell mechanics, a field that saw significant development in the 1950s and 60s.<\/span>
\u00a0<\/span>
\u201cThis is a really simple, robust, rapid prototyping technique, and we\u2019ve established design principles together with a predictive framework that characterizes the fabrication of thin shells,\u201d Reis says. \u201cI think that will be powerful. We\u2019re revisiting an old topic with new eyes.\u201d<\/span>
\u00a0<\/span>
Reis\u2019 co-authors include lead author and graduate student Anna Lee, postdoc Joel Marthelot, and applied mathematics instructor Pierre-Thomas Brun, along with colleagues from the team of Fran\u00e7ois Gallaire at the Swiss Federal Institute of Technology in Lausanne, Switzerland.<\/span>
\u00a0<\/span>
The thick and thin of it<\/strong><\/span>
\u00a0<\/span>
The team was initially inspired by videos of chocolatiers making bonbons and other chocolate shells. By simply pouring chocolate into molds, then inverting the molds to let excess chocolate drain out, chocolate makers create shells of relatively uniform thickness. Reis\u2019 team wondered whether there was a way to precisely predict the final thickness of chocolate and other shells that start out as a liquid film.<\/span>
\u00a0<\/span>
Lee and Marthelot used an analogous technique to experimentally create their own shells, using a liquid polymer solution that they drizzled over dome-shaped molds and spheres. After peeling the resulting shell off each mold, they cut the shell in half and found it was nearly the same thickness from top to bottom. But why?<\/span>
\u00a0<\/span>
To answer this question, Reis\u2019 team systematically characterized the coating dynamics in each of their experiments, including the physical properties of the polymer, the size of the mold, how fast the fluid flows down a mold, and the time it takes for the polymer to cure.<\/span>
\u00a0<\/span>
Based on their data, the researchers developed a simple formula to estimate the final thickness of a shell, which essentially equals the square root of the fluid\u2019s viscosity, times the mold\u2019s radius, divided by the curing time of the polymer, times the polymer\u2019s density and the acceleration of gravity as the polymer flows down the mold.<\/span>
\u00a0<\/span>
The formula boils down to the following relationships: The larger a mold\u2019s 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.<\/span>
\u00a0<\/span>
Like waiting for chocolate<\/strong><\/span>
\u00a0<\/span>
The researchers, led by Gallaire, then developed a numerical and analytical model to further test their simple mathematical formula, exploring more complex experimental configurations that are not easily feasible in the lab.<\/span>
\u00a0<\/span>
\u201cYou 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,\u201d Brun says.<\/span>
\u00a0<\/span>
For instance, in their models, the researchers explored complex coating patterns and the effects of changing a polymer\u2019s curing time.<\/span>
\u00a0<\/span>
Ultimately, through modeling and experiments, Reis and his colleagues found they were able to control the thickness of a shell by shortening the polymer\u2019s curing time. After mixing the polymer, they simply waited for it to thicken up before pouring it onto a mold. Because the polymer was already slightly solidified, it didn\u2019t take that much longer to fully cure. The result: More polymer solidified onto the mold rather than draining off.<\/span>
\u00a0<\/span>
\u201cBy waiting between mixing and pouring the polymer, we can increase the thickness of a shell by a factor of 11,\u201d says Lee.<\/span>
\u00a0<\/span>
\u201cThis flexibility of waiting gives us a simple parameter we can tune, depending on what we want for our final goal,\u201d Reis says. \u201cSo I think \u2018rapid fabrication\u2019 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.\u201d<\/span>
\u00a0<\/span>
This research is supported in part by the National Science Foundation.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

Now, a theory and a simple fabrication technique derived by MIT engineers may help chocolate artisans create uniformly smooth shells and precisely tailor their thickness.<\/p>\n","protected":false},"author":6,"featured_media":8304,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[23,17],"tags":[],"class_list":["post-8303","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-food","category-research"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0-300x200.jpg",300,200,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",600,400,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",600,400,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",540,360,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",95,63,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",639,426,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",96,64,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2016\/04\/MIT-chocolate-egg_0.jpg",150,100,false]},"author_info":{"info":["Amrita Tuladhar"]},"category_info":"Food<\/a> Research<\/a>","tag_info":"Research","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/8303","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=8303"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/8303\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/8304"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=8303"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=8303"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=8303"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}