Synopsis: Science of Slosh 
 
 An image analysis program reveals the biomechanics of walking with coffee. 
 
 H. C. Mayer and R. Krechetnikov, Phys. Rev. E (2012) 
 
 Each morning, blurry-eyed physicists try to solve a frustratingly complex mechanical problem: how to walk with a full cup of coffee, without letting it slosh over the sides. Writing in Physical Review E, Hans Mayer and Rouslan Krechetnikov at the University of California, Santa Barbara, report their study of the biomechanics of walking with coffee and the factors that lead to spills. 
 
 The sloshing of liquid in a cylindrical container, like a mug, is similar to the motion of a pendulum: the natural frequencies of oscillation depend on the liquid’s height and diameter (and, of course, gravity). In a typical mug, 7 cm in diameter and 10 cm tall, the lowest frequency oscillation of the coffee rocking back and forth in the cup is easily excited by walking at a normal pace. 
 
 This gives an intuitive explanation of why coffee spills, but Mayer and Krechetnikov have found that noise—potentially caused by uneven steps or small jerks of the cup — plays an important role in amplifying the natural oscillations of coffee into a full-blown spill. They set up an image analysis program to track coffee levels in cups carried by human subjects, who were asked to either focus on keeping the coffee from spilling, or to walk without paying attention. 
 
 Staying focused makes it less likely a walker will spill his coffee, but Mayer and Krechetnikiv are still testing if this is because the vigilant coffee carrier acts as feedback control that is activated at the first signs of coffee tipping toward the rim. – Jessica Thomas ||||| Tuesday, November 18, 2014 
 
 
 
 
 
 Credit: Julius Schorzman via Wikimedia Commons 
 
 
 
 
 
 Anyone who has ever carried a tray full of pint glasses without getting their feet wet knows that such a feat is hard work , but perhaps our sympathies should go out to the baristas in coffee shops instead. New research has concluded that carrying coffee without spilling is harder than beer since the foam on the surface of beer dampens sloshing.A team of physicists at Princeton and NYU Polytechnic School of Engineering set up an experiment which jolts three identical pint glasses carrying Guinness, Heineken, and black coffee, and measures the resulting oscillations.This video is the team's entry to the annual APS Gallery of Fluid Motion competition and explains their whole analysis.The pint of Guinness, which has the largest head of foam, sloshes the least, while the pint of coffee sloshes so much that it spills over the edge of the tall glass.To test this more rigorously, the team lead by Howard Stone of Princeton studied the sloshing of liquid in a thin, clear container, with various amounts of foam added on top. Each trial showed the characteristic motion of damped harmonic oscillation , but the amount of time it took for the sloshing to stop depended upon the height of the foam on top.It turns out that foam thicker than about 5 layers of bubbles is enough to damp out nearly all of the motion at the top of the foam. The team was also able to mathematically relate the amount of damping to the height of the foam, as shown in the video.These very practical results from a simple physics experiment could make things a lot easier for those in the industry of liquid transport. The researchers are presenting their work at next week's APS Division of Fluid Dynamics annual meeting.But the barista at your local coffee shop may still face challenges — after all, not everyone wants to order a cappuccino. In that case, the coffee sloshing research which won a 2012 Ig Nobel prize might be helpful.--By Tamela Maciel , also known as "pendulum"
– Transporting a round of beers from the bar to your table is an achievable task, but you probably wouldn't want to carry three cups of coffee without lids. Scientists at Princeton and NYU are explaining why: When it comes to beer, we have the bubbles to thank, the Smithsonian reports. Using high-speed cameras, the researchers recorded motion on the surface of several drinks: coffee, amber beer, and Guinness. Foam reduced sloshing, and the experts say it's because the foam absorbs the energy of the moving liquid. More foam meant less sloshing. That means that if you're looking for an easy-to-carry beer, Guinness may be your best bet. Why, you ask, are scientists spending their time on this? Well, it's not just to have a better night out. The study, an abstract says, "has promising applications in numerous industrial applications such as the transport of liquid in cargoes." And believe it or not, this isn't the first time the physics of coffee spillage has received study, Physics Central reports: A couple of years ago, "blurry-eyed physicists" tried to figure out how to walk with a full cup of coffee without spilling any, as the American Physical Society noted. (Some of us respond very inappropriately when our beer is spilled.)