Guest Post by Sarah Jensen from the Ask an Engineer series, published by MIT’s School of Engineering

Photo: Reggie35

Photo: Reggie35

Since the time of Aristotle, researchers and amateur scientists alike have batted about the counter-intuitive theory that hot water freezes faster than cold. The notion even has a name: the Mpemba effect, named for a Tanzanian schoolboy who in 1963 noticed that the ice cream he and his classmates made from warm milk froze quicker than that made from cool milk.

“No matter what the initial temperature of water is, it must be brought to the freezing point before it will change state and become ice,” says Prakash Govindan, most recently a postdoctoral associate in MIT’s Mechanical Engineering Department. It will actually take more time and energy to freeze hot water because it must be brought down further in temperature until it reaches the freezing point, about 0°C.

Govindan suggests conducting a simple experiment to demonstrate that hot and cold water will behave as logic predicts. “Fill two identical containers with hot and cold tap water from the kitchen sink and see which freezes first,” he says. Interestingly, he points out, the rates of change in this experiment will not be the same. “When you set them in the freezer, the freezer will work harder to bring the temperature of the hot water down, so initially the rate of heat transfer will be faster in the hot water.” However, the other container will be cooling at the same time (if not at quite the same rate).

When the temperature of the water in each container reaches just about 0°C it will undergo the same changes as it moves from a liquid to a solid, and it will take the same amount of time to begin forming tiny ice crystals. At that point, each mixture of liquid and ice will be at a uniform temperature, and as more heat is taken from the mixtures, the thermodynamic principle of latent heat kicks in: The water continues to convert to a solid state, but no longer changes in temperature. “As long as you have a mixture of liquid water and solid ice, the temperature will remain at 0 until all the water is frozen,” says Govindan.

It’s never been convincingly proven than hot water and cold water behave differently from each other at any step of the freezing process, despite the ongoing fascination with the Mpemba effect. In early 2013, Europe’s Royal Society of Chemistry even held a competition for the best explanation of the theory. The winner speculated that hot water indeed freezes more quickly if the cold water is first supercooled. But logic triumphs when it comes the plain ordinary water that comes from the household faucet. Most likely to impact the freezing point of water is the presence of impurities such as salt, dissolved solids, and gases—and the ingredients of homemade ice cream. 

Thanks to Khubaib Mukhtar of Pakistan for this question. Visit the MIT School of Engineering’s Ask an Engineer site for answers to more of your questions.


Photo: Fleece Traveler

Guest Post by Elizabeth Dougherty from the Ask an Engineer series, published by MIT’s School of Engineering

Yes, but you can’t generate enough to power the roller coaster with itself…

“Imagine a straight-line, one-hill roller coaster. It’s boring to ride, but it’s a useful example,” says Aaron Johnson, a PhD student in aeronautics and astronautics. If you start your ride at ground level, the first challenge is getting the coaster to the top of the hill. This takes energy, so you expend what you need against gravity and friction to get the coaster to the top. When the coaster crests the hill, all of the energy you have just expended against gravity is now stored as potential energy—though exactly how much depends on the height of the hill and the weight of the cars and passengers.

In an ideal world, Johnson says, a perfectly efficient energy collector on a frictionless coaster in a vacuum should be able to harness that potential energy and convert it to enough kinetic energy on the way down to drive you up a hill of exactly the same height. But in the real world, energy collection is more complicated. Much of the potential energy you have just gathered is going to scatter. Friction generates heat energy in the track and wheels, and drag buffets the cars and passengers, heating them and the air around them and dissipating more of your energy. This dispersion means realizing all of your potential is nearly impossible.

Interestingly, not all of this energy needs to be completely lost—whether it’s from a roller coaster or any other moving vehicle. One way to collect some of the energy that dissipates from moving vehicles is through something called regenerative braking, as used in hybrid cars such as the Toyota Prius. Regenerative brakes use energy normally lost to heat and friction during braking to charge batteries that power the car. “It works, but you’re never going to get enough energy to bring you back up the hill again,” says Johnson. “You’ve lost some of it to heat, plus the regenerative braking process isn’t 100 percent efficient.” (In Pittsburgh, engineers recently created just such a demonstration coaster and used the collected energy to power a display of amusement park lights.)

Another approach is to add a turbine to the coaster to collect wind energy. Airplanes do this. In an emergency, such as a power failure, a plane drops a so-called ram air turbine from a hatch. The turbine collects wind energy to power hydraulics and critical instruments. “It’s very inefficient, so it’s usually only for emergency use,” says Johnson.

While no solution is perfect, regenerative brakes and turbines are constantly being reworked and redesigned to become more efficient. For instance, in turbines, the shape and twist of the blade matters. “There’s a lot of people trying to design the most efficient blade possible,” says Johnson.

Visit the MIT School of Engineering’s Ask an Engineer site for answers to more of your questions.


Despite the cold outside, inside MIT’s CityFarm bell peppers, eggplants, and tomatoes are ripening for an early February harvest. Unlike conventional farming methods, many of CityFarm’s plants are being grown with air.

Photo: Aleszu Bajak

Founded by Caleb Harper MArch ’14, CityFarm is an MIT Media Lab initiative designed to explore the large-scale adoption of both aeroponics and hydroponics to “invent the future of agriculture,” according to their website. Unlike traditional farming, which irrigates and uses soil as structural support, an aeroponic plant’s roots are fed a mineral-enriched mist and protected in boxlike chambers. Plants are exposed to spectrally-optimized LED lights and are constantly evaluated to ensure optimum growing conditions.

The results? A head of romaine lettuce can grow in 19 days. By comparison, it takes 80 days to grow the same lettuce through traditional farming and 22 days using hydroponics, which submerges the roots in water.

Aeroponics uses 98 percent less water than conventional farming, and plants can grow 365 days a year inside and in much smaller areas. That’s a whole lot more veggies even during Boston’s chilliest winters.

Harper and his team of farmers celebrate their first harvest this past November.

Harper and his team of farmers celebrate their first harvest this past November.
Photo: Aleszu Bajak

With recent estimates that 60 percent of the world’s population will reside in cities by 2030, aeroponics might be an increasingly common growing method in cities. Harper predicts city dwellers will be able to pick up their berries, lettuce, and vegetables at local growing sites right in their neighborhoods.

Back at CityFarm, the lab’s 1,500 plants provide Harper’s team with detailed data on embodied energy—exactly how much energy a plant needs to grow. Small radio transmitters on the lights, misters, and other equipment submit information on each kilowatt of energy used. Eventually equipment will tweet this data.

Such detailed tracking of energy inputs and produce outputs is new to farming. Often the energy required to power the tractor or transport tomatoes to the grocery store is rarely factored into the true energy requirements to grow produce. Harper hopes to change that.

Harper envisions launching an Open Agriculture Initiative in the next couple months with CityFarm hosting an open source platform of farming data to improve the evaluation of aeroponics and other farming methods. “We’re providing an economic and data-driven back bone for fundamental agricultural change,” he said.

Caleb Harper checks on lettuce plants in the CityFarm. Photo: Kent Larson

He sees MIT leading the way in the new technology-agriculture space and encouraging cross-discipline scholarship between tech and agriculture universities. “I’m not competing with agriculture, I’m really providing networked intelligence and technological optimization that wasn’t there before.”

For now, Harper continues to taste test his lettuce plants in preparation for the upcoming harvest. “I have become a lettuce connoisseur,” he jokes.

For farm updates and news on the latest harvest event follow CityFarm on Twitter at @MITCityFARM.


MIT’s Department of Earth, Atmospheric and Planetary Sciences takes the pulse of the weather and planetary changes from the ocean depths to deep space. Learn what phenomena influence today’s weather as well as the future of the planet. Here are a few research highlights:

oceans-EArthBigger Storms Ahead

For the past 40 years—as far back as satellite records show—the frequency and intensity of tropical cyclones has remained relatively stable: About 90 of these storms spin through the world each year. But according to a report by MIT’s Kerry Emanuel, the coming century may whip up stronger and more frequent storms.

Not Too Hot, Not Too Cold?

Some 40 billion Earth-like planets have been discovered in the so-called Goldilocks Zone. NPR’s On Point with Tom Ashbrook reports on research by Sara Seager, astrophysicist and planetary scientist, and colleauges in “Earth 2.0? Billions of Reasons Why It’s Possible.”

Weather in a Tank

Fluid dynamics plays a central role in determining Earth’s climate. Ocean currents and eddies stir up contents from the deep, while atmospheric winds and weather systems steer temperature and moisture around the globe. A demonstration called Weather in a Tank—a clear circular basin of water on a rotating platform that simulates Earth’s spin—illustrates weather phenomena such as atmospheric cyclones, fronts, jets, and ocean currents and eddies.

And for a little history…

Modern meteorology arrives at MIT.

Modern meteorology arrives at MIT.

Wind, War and Weathermen

Learn how a Swedish bon vivant let MIT introduce modern meteorology to America—just in time to help the Allies win World War II.

When the Butterfly Effect Took Flight

While simulating weather patterns 50 years ago, Edward Lorenz, SM ′43, ScD ′48, overthrew the idea of the clockwork universe with his ground-breaking research on chaos.


Clean technology, or cleantech, has had some growing pains in the past several years thanks to a lackluster economy, political turmoil, and some dire existential questions posed about its future.

Questions about cleantech’s future, including that of wind, solar, hydro-power, and other alternative energy sources, were put to Steven Koonin PhD ’75, President Obama’s former Under Secretary for Science and current director of NYU’s Center for Urban Science and Progress on 60 Minutes this week.

Steven Koonin PhD '75.

Steven Koonin PhD ’75.

In her story, CBS correspondent Leslie Stahl focused on Silicon Valley investor Vinod Khosla, who has spent billions of dollars of his own money to support alternative fuel startups, many of which have relied on stimulus funds from the U.S. government.

Koonin, who supported many such ventures during his term, gave candid responses to Stahl about cleantech’s successes and failures four years after the stimulus funding was approved.

“I think there are significant developments that have come out of that spending that impact our energy system now, new technologies demonstrated,” Koonin said. “I think it was good value for the money.”

Asked about the Solyndra scandal and the failure of other clean-energy startups like Range Fuels, Abound Energy, A123 and Fisker to get off the ground, Koonin had mixed feelings.

“There are parts of [cleantech] that are on life support right now… I put some of the major blame on the government, both the executive branch and Congress, for an inability to set a thoughtful and consistent energy policy.”

Khosla responded to critiques of the cleantech industry with vehemence. “I am trying. And they’re sitting there doing nothing. They’re being the nay-sayers, the pundits who say why it can’t be done. But they won’t try.

Koonin has also grown weary of the startup mentality of investors who do not have the patience for a payoff 20 or 30 years down the road. Wangxiang, a company that acquired many stimulus-funded American ventures after they failed to earn profits, was cited as an example of the contrasting long-term view that many Chinese firms can take.

Tesla motors, a firm focused on creating top of the line electric and hybrid vehicles, was also praised for its success. The company, for which many MIT alums work, was #1 on the Nasdaq index of the 100 largest nonfinancial companies in 2013.


In her teens, Yoky Matsuoka SM ’95, PhD ’98 wanted to be a pro tennis star. When she first learned of the field of robotics, she dreamed of building a robot tennis partner you could adjust to hit harder or easier on you depending on your mood.

At MIT, Matsuoka met Professor Rodney Brooks at the Computer Science and Artificial Intelligence Laboratory and found a greater cause: optimizing robotics to help disabled people. There, Matsuoka developed a theory on machines and robotics: “You need to let them learn what you’re bad at,” she says. 12-20-13 nest_thermostat_iphone_app

Now, Matsuoka has applied that theory to energy. As VP of technology for Nest, she has developed the Nest Learning Thermostat, an easy to use, self-learning device that gets to know you and your home and how to heat or cool it most efficiently.

Since coming to market in 2011, Nest has saved billions of dollars in energy bills for thousands upon thousands of households around the world. This fall, Nest announced that the thermostat, which retails for $249, was in close to 1% of U.S. homes (about 1.1 million) and was endorsed by 20 U.S. utility companies.

Last week, Ellen DeGeneres even gave viewers an on-air tutorial on how Nest works.

Employing Matsuoka’s theory, Nest learns to do what people are bad at: turning off or reducing their energy systems when they’re away from home. The thermostat’s algorithms learn the patterns of each user and adjust schedules based on how the user turns up or down the heat (or AC) in the first few weeks of use.

When away from home, users can employ Nest’s smartphone app to change settings or monitor the temperature. The app also produces reports on energy usage and makes recommendations.

This year, Nest added a smoke and carbon monoxide detector to its catalog of smart devices for the home, one which is selling tens of thousands of units each month. Nest’s CEO, Tony Fadell, knows a thing or two about smart devices, having worked on eighteen generations of the iPod and three of the iPhone at Apple.

“We’re now sitting at the intersection of machine learning and human learning,” says Matsuoka. “I’m extremely interested in enhancing human ability in allowing them to achieve what they want, utilizing machine learning to enhance lives.”

For those who fear the Internet of things–objects being smarter than people or machines running amok and ruling the world, Matsuoka offers this simple rule: “People who want to share more to get more should be able to opt in to [devices] like this. Those who want to withhold data should be able to withhold it. Give that freedom and power to the person to decide. We can do this right.”


Update 1/13/14: Google acquired Nest in January 2014 for $3.2 billion in cash.



Guest Post from the Ask an Engineer series, published by MIT’s School of Engineering

Photo: Theen Moy

Photo: Theen Moy

“If you want to capture energy and use it to power something, it’s true that you can get energy out of a speed bump,” says Amos Winter, an assistant professor in MIT’s Department of Mechanical Engineering and director of its Global Engineering and Research (GEAR) Lab. “You need to think about how you transfer that energy, and how much energy you can actually capture.”

A speed bump pushes the car’s wheels upwards, and when it bounces back to the ground, kinetic energy is converted to potential energy absorbed in its shock absorbers. Capturing some of that energy sounds plausible at first blush. Using piezoelectric sensors, you can generate power by converting some of that force into an electrical charge. “You can produce high voltage, but pretty low current,” says Winter. “The energy of the spark of a piezoelectric barbecue lighter, for example, is less than the energy you put into pulling the trigger.”

The core principle involves work: the application of force over a distance. A car’s force multiplied by the distance it depresses the piezos equals the amount of useful work that can be done – and a car tire rolling over a piezo will depress it only a few microns. Winter suggests a more efficient way of capturing that energy. “Instead of making a speed bump, make a speed divot,” he says. “A flat surface that can be depressed easily will sink down when you drive over it and you can use the weight of the car to spin some sort of generator.” Another scenario is a compressible speed bump made of flexible plastic or rubber filled with water. When a car drives over it, water is squeezed out and can be pumped into a water tower high in the air. “You could run that water through a turbine and produce electricity with it,” says Winter.

Such models, of course, require full-size cars, full-size roadways, and full-size water towers. Scaling down the project to exhibit in the school auditorium is another story. The size of the display may be smaller, but the challenge is much larger. “You could construct a small speed bump and push down on it with a section of a car tire to light up an LED,” suggests Winter. “But a model using Hot Wheels cars? Forget it. They’d never deflect the speed bump enough to produce any energy.”

In the end, the most important thing to keep in mind is that creating energy isn’t just about applying force; it all goes back to the principle of work. “You need force multiplied by a distance,” says Winter. “That’s what shock absorbers do, and a more interesting design could be built around that.” He suggests checking out Levant Power, a company founded by Shakeel Avadhany ’09 and Zack Anderson ’09 to develop and commercialize shock absorber energy recovery.

Authored by Sarah Jensen. Thanks to Jonathan from Atlanta, Georgia, for this question. Visit the MIT School of Engineering’s Ask an Engineer site for answers to more of your questions, and ask your own. 




Benjamin Glass ’07, SM ’10 and Adam Rein MBA ’10 combined their research savvy in aeronautical engineering and business  to launch Altaeros Energies, Inc. in 2010, a company whose first prototype fit in the back of a Volkswagen they drove to a field in rural Massachusetts for testing.

At first glance, it looks like a small blimp.

Photo: Altaeros Energies, Inc.

Altaeros mechanical engineer Eph Lanford ’11, SM ’11 stands at the foot of the airborne turbine during a test run. Photo: Altaeros Energies, Inc.

But floating 2,000 feet in the sky, Glass and Rein’s product is actually an aerostat, or tethered balloon–the same kind that are used for lifting telecommunications or surveillance equipment high into the air. Within their giant, donut-shaped dirigible sits a wind turbine.

Last year, their dream to think outside the wind-farm box grew even bigger. Supported by MassChallenge and the MIT Venture Mentoring Service, Glass and Rein built a 60-foot diameter airborne wind turbine and launched it in the sky above Limestone, Maine.

The turbine captures many times more wind power than a traditional turbine, its creators say. It is also far cheaper and can be operated simply in remote locations that have no other power source, channeling wind energy back to the ground through its heavy-duty tether. One balloon-turbine unit can generate 30-100 megawatts of electricity, enough to power a small village.

Footage of the Altaeros test last year attracted hundreds of thousands of views, and now they are courting government, military, and private interests to scale up their design and make portable, airborne wind farms a reality. According to Glass, Altaeros is now at work on its first commercial-scale demonstration in Alaska.

“What we’re trying to do,” Rein said in a Reuters interview this May, “is instead of looking for where the sun or the winds are strong, bringing cheap renewable energy anywhere in the world where folks are having trouble getting renewable energy.”

The airborne turbine also solves the problem of building expensive offshore platforms, such as the ones required for the Cape Wind project currently in development.

“We’ve taken the approach of trying to design a system that will basically direct itself,” says Glass, who dreamed up the tethered turbine in his Energy Ventures Class in 2009. “Without any input or any control the aerodynamics of the system, just by the forces on the shroud and on the fins will direct it into the wind even as the wind direction changes.”

Many had doubts the turbine would work at first, but Glass and Rein, who partnered with Harvard Law student Alain Goubau, believed in their project. After seeing natural disasters like the earthquake in Haiti and the Fukashima Power Plant meltdown in Japan, their work has raised new possibilities. The turbine could be used in remote locations, or even above cities, when major power outages occur.


Update 3/21/14: Altaeros was awarded a $1.3 million grant by the Alaska Energy Authority for an 18-month test of its turbines. Read New York Times coverage of the project.

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Mix one part water, one part web 2.0, one part Madison Avenue, and one part celebrity dazzle. The result is charity:water, a New-York nonprofit that’s found success mixing these components to get water to the world’s most impoverished populations.

In the past six years, charity:water has succeeded in bringing fresh water to more than three million people, funding nearly 9,000 local water projects in 20 countries.

Vice-president of growth Yukari Matsuzawa MBA ’05 joined charity:water after four years helping Twitter and Google expand their reach in Japan. Matsuzawa’s current role brings her to communities in need of water across four continents, fulfilling the organization’s ambitious role to be the funding clearinghouse for all water projects worldwide.

“I joined this organization because I saw the potential in how the 100% model, proof and brand enables everyone to make a difference in this water crisis,” Matsuzawa says.

Yukari Matsuzawa MBA '05.

Yukari Matsuzawa MBA ’05.

Touting a no-overhead model with dedicated donors covering admin fees, charity:water crowdsources both its donor base and its water projects, letting potential donors create accounts, manage campaigns, and creatively fundraise while doing much the same for potential water projects. When a project that donors have helped fund from my.charitywater accounts reaches completion, they are sent coordinates and aerial views of the new well or water basin.

The organization focuses on water projects one at a time on its website. This month’s feature project, Gram Vikas, is in India.

Since 2006, charity:water has partnered with nearly 9,000 such locally-run water projects in the world, funding well-digs that have brought more than 3 million people fresh water. By 2015, Matsuzawa hopes to double its reach. She believes that this model is the best, most scalable way to prevent 90 percent of the world’s water-related deaths.

Charity:water appeals to millennials, reports the New York Times, quite differently than organizations like the Peace Corps appealed to idealists a half-century ago. Using microfinancing and short pop-videos to entertain and educate its target audience has won it acclaim from leading philanthropists and celebrity endorsements from Jessica Biel to Tony Hawke.

Christine Lee ’09 launched a charity:water project last year. “I had learned about how critical clean water is from a D-Lab, my favorite class at MIT.  As noted in my campaign page, a trip to Ghana with D-Lab showed me first-hand how much I undervalued the easy access to clean water we enjoy.”

“Their mission and vision is simple and clear: give access to clean water to people in need,” says Omar Fernandez ’10, who asked that his MIT graduation gifts be given to charity:water. “I’m a big fan of looking for the things that can be changed that would trigger a positive chain reaction of events and giving a community access to clean water is one of those.”


TR35MIT Technology Review recently announced its annual TR35—the Top 35 Innovators Under 35. And similar to past years, the MIT community has a strong presence on the always-anticipated list.

Christine Fleming '04 (photo via TR)

Christine Fleming ’04 (photo via TR)

At least seven MITers—including five alumni, one graduate student, and two Institute assistant professors—were named to the thirteenth annual roll call of the world’s best young innovators who are driving the next generation of technology breakthroughs.

The MIT names listed below represent varied backgrounds including developing-world technology, nuclear power, and body-machine interfaces.

  • Leslie Dewan ’07, PhD ’13, chief science officer and co-founder, Transatomic Power, has designed a nuclear “waste-annihilating molten-salt reactor” that may be able to consumer nuclear waste and never melt down.
  • Christine Fleming ’04, assistant professor, Columbia University, has created a device that shows real-time, high-resolution images of a beating heart during cardiac procedures.
  • Roozbeh Ghaffari ’01, MEng ’03, PhD ’08, co-founder and director, MC10, is an expert in the science and technology of body-machine interfaces and body-integrated devices.
  • Rebeca Hwang ’02, MEng ’03, CEO, YouNoodle, connects entrepreneurs through YouNoodle-organized technology competitions.
  • Steve Ramirez, doctoral student in Brain and Cognitive Sciences, is a “brain inceptor” focused on finding where memories are located throughout the brain.
  • Amos Winter SM ’05, PhD ’11, assistant professor, Department of Mechanical Engineering, focuses on technology in the developing world and has developed a wheelchair sturdy enough for uneven outdoors terrain.
  • Feng Zhang, assistant professor, Department of Brain and Cognitive Sciences, is using genomic research to dispel misconceptions and inaccuracies about mental illness.
Amos Winter SM ’05, PhD ’11

Amos Winter SM ’05, PhD ’11

According to the magazine, the TR35 is a nearly year-long process where hundreds of candidates (who must be younger than 35 on Oct. 1, 2014) are first narrowed down to 100 finalists. A panel of judges rate each finalist’s originality and scope of impace, and the editors take the judges score into account when selecting the list of 35.

The panel of 15 judges who rated the finalists includes six MIT alumni: David Berry ’00, PhD ’05, a partner at Flagship Ventures; MIT Associate Professor Edward Boyden ’99, MEng ’99; Professor Yet-Ming Chiang ’80, ScD ’85; Jennifer Elisseeff PhD ’99, a professor at Johns Hopkins; John Rogers SM ’92, SM ’92, PhD ’95, a professor at the University of Illinois; and MIT Senior Lecturer Ken Zolot.

Did we leave any MIT community members off our list? Are there any other MIT alumni or faculty that should have made the TR35? Let us know in the comments below or on Facebook or Twitter.