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A special magazine about the challenging world of fluid dynamics

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Prof. Dr. Detlef Lohse (University of Twente):

“We’re living in the golden age of fluid dynamics”

From studying the behavior of drops to discovering that there is more than one state of turbulence, Professor Detlef Lohse is one of the world’s leading authorities on the way fluids behave and interact with their environment. What makes this subject so fascinating, and what are the big questions researchers are trying to answer?

Why does a coffee stain always have a dark rim around its edge? For a long time, nobody knew the answer. However, 20 years ago in Chicago scientists were on the brink of finding an answer. Detlef Lohse was working in Chicago at the time and vividly remembers the atmosphere. “The “coffee stain question” had become an iconic problem in physics. It attracted scientists from all kinds of disciplines.”

Together they discovered the answer: evaporation at the edges of the spilt coffee triggers a flow inside the liquid, which takes all the coffee particles to the edge. It was a landmark study that opened up many new research questions and had very practical implications for all kinds of industries, from paint manufacturers to molecular biologists.

Beautiful physics
For Lohse, the episode perfectly illustrates why fluid dynamics, the science of how fluids behave and interact, is such a fascinating field of study. “First of all it’s beautiful physics, with a lot of unsolved problems. At the same time, it’s a science with very practical implications for many industries. And finally, it’s a truly multidisciplinary field. There’s a very close interaction between theory, experiments and numerical models that you don’t see so pronounced in other branches of physics.”

Why does a coffee stain always have a dark rim around its edge? For a long time, nobody knew the answer. However, 20 years ago in Chicago scientists were on the brink of finding an answer. Detlef Lohse was working in Chicago at the time and vividly remembers the atmosphere. “The “coffee stain question” had become an iconic problem in physics. It attracted scientists from all kinds of disciplines.”

Turbulence: the final mystery?
Much of his own research focuses on an area that first piqued his interest as a PhD student: the question of turbulence, sometimes called the last major unsolved problem in classical physics. It’s hardly a new field of study. One of the first attempts to describe it is a set of drawings made by Leonardo da Vinci, standing on a bridge in Florence, captivated by the vortices and eddies in the river Arno below him.

What’s fascinating about turbulence, Lohse says, is that at first glance it isn’t a mystery at all. “The basic equations have been known for over 150 years. Yet actually solving these equations for any particular problem is extremely complex. The stronger the turbulence gets, the more difficult it becomes.” As another authority in the field once put it: “Scientists today are better able to explain the structure of stars than accurately predict turbulence in a liquid flowing through a pipe.”

The golden age

However, Lohse emphasizes that this is only one of many big questions in his field. “Fluid dynamics is very rich in relevant problems. It’s not like high-energy physics, where at one point everybody was hunting for the Higgs boson1. There’s no grand, central mystery in fluid dynamics. Instead, we have a continuous stream of specific questions and phenomena, each with their own specific challenges.”

And if fluid dynamics is rich in questions, it is currently also remarkably rich in answers. “It’s no exaggeration to say that this is the golden age of fluid dynamics”, says Lohse. “In the last two decades our field has made enormous progress, and we are on the brink of exploring whole new questions that as a PhD student I didn’t dream we’d ever be able to tackle.”

This “golden age” was made possible by two key technological innovations, the first being the rapid development of digital high-speed cameras. Lohse was among the first to spot the crucial potential of this technology. “My background is in statistical physics, so I reasoned: the more data I have, the better.”

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Whereas the first camera he worked with could capture around 2,000 digital images per second, the cameras Lohse now routinely works with generate a million images per second and beyond, and are even able to track thousands of particles as they move through a fluid. “We’re generating massive amounts of data that we simply couldn’t have collected 20 years ago. That has revolutionized our field. It allows us to study particular events in amazing detail, from the impact of a droplet on a thin film to the famous “crown” formation that occurs when a drop splashes onto a wetted surface.

Also known as the ‘God’ particle, the missing piece in the Standard Model of particle physics, which was first observed in 2012. It helps explain how all other particles acquire mass. Named after Peter Higgs, who was awarded the Nobel Prize for Physics in 2013.

The ouzo effect

Pour some water into a glass of ouzo or pastis and its appearance changes from transparent to murky. The reason? The higher proportion of water changes the solubility of the oil in ouzo. Tiny anise oil droplets form, which scatter the light and give the ouzo its characteristic milky look.

Detlef Lohse and his research group studied this process in more detail by filming an evaporating drop of ouzo on a hydrophobic surface. As the alcohol starts to evaporate, firstly at the edges, the proportion of water rises and the ouzo effect is triggered. Changes in surface tension then create strong convection in the drop, with the oil droplets reaching speeds of millimeters per second.

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