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A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 7 9 STRESS RELIEF THANKS A LATTE Have you ever been mesmerized by the rich shades of color in a café latte? You’re not alone. Researchers at Princeton University, N.J., have revealed how this tiered structure develops when espresso is poured into hot milk. “The struc- ture formation in a latte is surprising because it evolves from the chaotic, initial pouring and mixing of fluids into a very organized, distinct arrangement of layers,” says graduate student Nan Xue. Analysis of his team’s experiments shows that the primary mechanism behind the layering is a phenomenon known as double-diffu- sive convection. It occurs when stacked-up fluids of different densities exchange heat through the movement of their constituent materials. Within a given mixture, denser, cooler liquids sink, while lighter, hotter liquids rise. This sinking and rising stops when the local density in a region within the latte approaches an equilibri- um. As a result, the fluid there must flow horizontally instead of vertically, creating distinct layers. engineering.princeton.edu . MINIATURE MONA LISA MADE OF DNA In 2006, Paul Rothemund of the California Institute of Technology (Caltech), Pasadena, developed a method to fold a long strand of DNA into a prescribed shape. The technique, called DNA origami, enables scientists to create self-assembling DNA structures that can carry any pattern, such as a 100-nm-wide smiley face. DNA origami revolutionized the field of nanotechnology. However, some applications require much larger DNA origami struc- tures. Now, scientists in the laboratory of Lulu Qian, assistant professor of bioengineering at Caltech, have developed an inexpensive method by which DNA origami self-assembles into large arrays with entirely customizable patterns, creating a sort of canvas that can display any image. To demonstrate, the team created the world’s smallest version of Leonardo da Vinci’s Mona Lisa. The team developed software that can take an image, divide it into small square sections, and determine the DNA sequences needed to make up those squares. Next, their challenge was to get those sections to self-assemble into a superstructure that recre- ates the Mona Lisa. The key was to assemble the tiles in stages using what they call “fractal assembly,” which is like assembling small regions of a puzzle and then assembling those to make larger regions, and finally completing the image. caltech.edu . 3D-PRINTED BICYCLE FITS LIKE A GLOVE As an avid cyclist, Patrik Paul had always dreamed of owning the perfect mountain bike—one uniquely suited to his center of gravity. Paul, now CEO of Slovakia-based Kinazo Design, recently used additive manu- facturing to print a bike specifically tailored to his body. Because the bike frame’s individual components are customizable, designers are able to match each frame to a rider’s particular needs. The company uses special body-fitting equipment to take into account factors such as arm and leg length, weight, and balance—and can print the frames in different sizes, wall thicknesses, inside structures, and other custom specifications. Be- cause Paul and his team needed as much printing space as possible, they chose the largest commercial 3D printer for metals on the market—the Concept Laser X Line 2000R. The machine has a build area of roughly 2.5 × 1.5 × 1.5 ft, which gives them enough space to print the largest part of the bike frame as a single part. kinazo.com . DNA rendering of the Mona Lisa viewed with atomic force microscopy. Courtesy of Qian Laboratory. Nan Xue pours coffee into milk to simulate a layering effect, a process with potential implications for industrial fluid flows, clima- tology, and ecology. Riding the Kinazo eBike is like stepping out in a custom- tailored suit and shoes, says company co-founder Patrik Paul.