산책할 때는 아메리카 말고 라떼를...Why a coffee is more likely to spill than a latte

Just five layers of bubbles made the liquid's waves much smaller. (Alban Sauret/Princeton University) 

액체가 든 용기를 흔들 때 액체의 움직임을 촬영한 모습. 거품층의 두께가 두꺼워질 수록 액체의 움직임은 줄어

든다. - 프린스턴대 제공

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아메리카노를 들고 거리를 걸을 때는 걸음걸이에 신경 써야 한다.  자칫 잘못하다간 뜨거운 커피가 컵 밖으로 넘치기 때문. 반면 우유거품이 잔뜩 든 라떼를 들고 있다면 이런 걱정을 할 필요가 없다.   미국 프린스턴대 연구팀은 액체 위에 얹은 거품이 액체의 움직임을 줄인다는 연구 결과를 ‘유체물리학회지(journal Physics of Fluids)’ 24일자에 발표했다.   에밀리 드레사이유 박사는 “라떼가 잘 넘치지 않는 현상을 보고 이번 연구의 아이디어를 얻었다”고 밝혔다. 같은 연구팀의 알방 소레 박사는 “이런 현상은 다른 음료에서도 흔하다”며 “거품이 풍부한 맥주인 기네스도 컵 밖으로 넘치는 일이 드물다”고 설명했다.   카페와 술집에서 나타난 현상을 과학적으로 설명하기 위해 연구팀은 실험을 설계했다. 유리 용기에 물을 넣고 여기에 밀도를 높이는 글리세롤을 첨가했다. 그 다음 주방세제를 이용해 지름 3mm짜리 거품으로 이뤄진 거품층을 만들어 액체 위에 얹은 뒤 용기를 흔들어 액체가 흔들리는 모습을 초고속카메라로 촬영했다.     영상을 분석한 결과, 거품층이 단 한 층만 있어도 액체의 움직임이 줄어드는 것으로 나타났다. 거품층의 두께가 두꺼울수록 액체의 움직임은 점점 감소했고, 5층 정도 쌓이면 액체의 움직임은 확연히 떨어졌다. 연구팀은 거품층이 액체의 운동에너지를 소실시켜 이런 현상이 생긴다는 사실도 수학적으로 설명했다.    드레사이유 박사는 “거품층으로 액체의 움직임을 잡는 방법은 액화가스나 로켓에 쓰는 추진연료를 안전하게 운송하는 데도 적용할 수 있을 것”이라고 밝혔다. 동아사이언스 신선미 기자 vamie@donga.com

By Rachel Feltman It doesn't take a team of physicists to figure out that foamy liquids are less splishy -splashy than those without bubbles, but one team of physicists did try to figure out why. Their work was reported Tuesday in the journal Physics of Fluids. The study authors, who worked together as part of a the complex fluid group at Princeton University, had noticed the phenomenon in the real world: Emilie Dressaire, who's now an assistant professor at NYU Polytechnic School of Engineering, started thinking about the integrity of foamy liquids when a Starbucks employee told her she wouldn't need a lid stopper to keep her latte from spilling. Her colleague Alban Sauret, now a researcher at the French National Center for Scientific Research, had recognized the same difference in foamy beers. When carrying pints of Guinness, he'd learned as a student in Europe, sloshing was minimal. In both cases, it seemed the foamy layer on top of the liquid was to thank for the lack of spillage. The experiment was what you'd expect: The team filled a glass container with water, dish soap, and glycerol, which is used to make liquids more viscous in the lab. Then they created uniform layers of tiny bubbles into the liquid. By rocking the containers back and forth using different motions, they were able to record the behavior of the liquids as they became foamier and foamier. They recorded the tiny waves produced by the shaking with a high-speed camera. Sure enough, their foam made the liquid more stable. Five layers of bubbles lowered the wave height by 10 times compared with plain liquid. But don't go asking for a latte with extra, extra foam. More than five layers didn't have much effect. That's probably because the foam is diffusing the energy of the slosh by way of friction against the glass container. Layers of foam on top don't really move or come into contact with the container, so they're not sharing the energy burden. "The potential applications are much bigger than just beer," Sauret said in a statement. The authors have hopes for industrial applications. "This study demonstrates that a relatively thin layer of foam effectively damps sloshing," they write in the study. "Our findings suggest that foam could be used in various industrial processes in which sloshing needs to be minimized" particularly in the transportation of dangerous liquids, such as hazardous waste or oil. http://www.washingtonpost.com/news/speaking-of-science/wp/2015/02/24/why-a-coffee-is-more-likely-to-spill-than-a-latte/

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