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In Review

Watch the cream pour into your coffee, cloudily curling and swirling through the darkness.

It鈥檚 a more enigmatic process than you might think.

鈥淔luid mixing, and fluid dynamics generally, is a great example of how common, everyday things can really be subtle and intricate and complicated,鈥� says Douglas Kelley, an assistant professor of mechanical engineering.

He calls fluid mixing 鈥渄evilishly difficult to predict, control, and understand.鈥� But he鈥檚 working to make sense of it, through his research on the space and time dynamics of fluid flows and the materials that mix within them. In some cases鈥攏otably, in his work on ocean currents and phytoplankton鈥攈is interest is purely scientific. For other work, he puts on what he calls his 鈥渆ngineer鈥檚 hat鈥� and pursues applications, as in his work on fluid flow in liquid batteries, an emerging technology that could transform the electric grid.

While the foundations of fluid mixing research鈥攂asic thermodynamics and the conservation of energy, momentum, and mass鈥攈ave been understood for a century and a half, fundamental questions remain.

Kelley teaches his graduate students how to derive the equations that underpin the science. 鈥淏ut the math is beautiful in that you can鈥檛 solve it, and all of these surprises come out,鈥� he says. 鈥淎nd that鈥檚 why it鈥檚 鈥榙evilishly difficult.鈥� Nobody can solve this stuff straight up. Instead, we do experiments. And sometimes we get really surprised.鈥�

In their mixing laboratory in Hopeman Engineering Building, he and his team are investigating phytoplankton, microscopic marine plants that perform photosynthesis as they float in the ocean. They鈥檙e at the base of nearly every marine food chain.

And the tiny organisms play a pivotal role beyond the ocean鈥檚 buffet. They鈥檙e also one of the earth鈥檚 largest carbon sinks. When trees decay, the carbon dioxide they absorbed returns to the atmosphere. But when phytoplankton die, they sink to the bottom of the ocean鈥攁nd the carbon dioxide they captured is taken out of circulation for about 10,000 years, the time it takes for the ocean to turn over.

鈥淚f you want to make accurate climate models, it鈥檚 really important to keep track of where that carbon dioxide is going,鈥� Kelley says.

To help do that, he and his students carry out chemical reactions in the lab that model the replication of phytoplankton in the tumult of marine currents. They鈥檝e found that the fate of phytoplankton in the ocean is much like that of a flame. If you blow gently on a lit match, you can make the flame grow. But blow too hard and you extinguish it.

鈥淲e鈥檝e found a very similar phenomenon in our reactive mixing experiments,鈥� he says. When the team models a gentle fluid flow, that encourages phytoplankton growth. But a fast flow dilutes things so quickly that it kills off phytoplankton growth. 鈥淪o there鈥檚 a 鈥榖lowout鈥� threshold,鈥� he says. The research has just been published in Physics Review Letters, a top journal in the field.

Kelley鈥檚 winding educational and career path has cultivated his capacity to think as both a scientist and an engineer. An electrical engineering major as an undergraduate, he earned a doctorate in physics and then completed postdoctoral work, first in a mechanical engineering department and then in a materials science department. 鈥淎nd now I teach mechanical engineering, but I don鈥檛 have any mechanical engineering degree,鈥� he says. He calls Rochester鈥檚 mechanical engineering department science focused. 鈥淪o for somebody like me, who has straddled science and engineering, it鈥檚 a great place to be.鈥�

He brings an engineer鈥檚 mind to his work on liquid metal batteries, a technology that鈥檚 being designed for grid-scale energy storage by a start-up company called Ambri, based in Cambridge, Massachusetts. The present electrical grid has almost no capacity to store electricity. Energy that鈥檚 not being used during the relatively cool nights and mornings of a hot summer, for example, can鈥檛 be saved for blisteringly hot afternoons, when air conditioners are running at full capacity.

But liquid batteries鈥攋ust about four inches in size individually, but stacked together in groups the size of shipping containers to support the grid鈥攁re able to store a lot of energy and deliver it quickly. Using his fluid flow expertise, Kelley is investigating how battery performance and efficiency can be enhanced, and how the fluid mixing inside the batteries can be gauged to know, for instance, how a battery will perform at a certain temperature after it has been charged and discharged a certain number of times.

At a fundamental level, there are many commonalities between his oceanic and battery projects, he says鈥攏one more so, perhaps, that their unpredictability.

鈥淵ou try to anticipate as many of the surprises as you can鈥� if you鈥檙e working as an engineer and 鈥渨ant to do practical things and control stuff,鈥� he says. 鈥淏ut then you can put on your scientist鈥檚 hat, too鈥攁nd just enjoy the surprises.鈥�

鈥擪athleen McGarvey锘匡豢