Whenever Timo van Overveld walks along the beach, he can't help but look at the ridges formed by the sand. The retreating sea leaves behind beautiful structures, especially at low tide. But these short, regular wave patterns are much more complex than they look, Van Overveld explains. He started his doctoral research four years ago at the Environmental Fluid Mechanics Lab. Here, he wanted to capture the hows and whys of these sand ripples in a model, to fathom the transport of sediment along the Dutch coastline. Many simulations and experiments later, the model exists. It is much more fundamental than was envisaged, which makes it useful in other fields where particle interactions, the formation of patterns and flow play a role.

Ripples

“You must know them, those ripples in the sand along the shoreline.” Van Overveld's explanations are accompanied by enthusiastic movements of his hand. He's certainly attuned to rhythm and pattern. As well as being a passionate researcher, for more than ten years he has been the guitarist in the successful cover band IMPAKT. He is energized by “taking a breather from the academic bubble”, by the band's up-tempo delivery of its repertoire of more than a hundred covers. And he puts all his fresh energy into his research. “Sand ripples are a fascinating sight. Even though it's very regular, the pattern is never the same in any two places, at any two moments. What you're seeing is the result of a very dynamic system. The undulating sea causes the grains of sand to form patterns and this action changes the seabed. For their part, these sand ripples change how the water flows, and a complicated interaction comes into play. Researchers have been trying for decades to capture in a model the movement of sand and other sediment along the coastline, but this level of complexity means it's not easy.”

And so Overveld decided to reduce the pattern formation to the very smallest scale. He set out to identify the fundamental interactions between one or two particles and the surrounding flow of water. In simulations he was able to describe the behavior of a single particle, but when there were two particles, he saw an added phenomenon. “We see something that is more than the sum of the separate parts. It's due to the back and forth movement, and it happens in the sea too. Just above the seabed the waves cause the direction of flow to oscillate, so on average the particles stay in the same place. But the simulations allow us to see that a vortex forms in the wake of the particles. On a longer time scale this causes an added flow, the residual flow.”

Quaking box of marbles

As is often the case in physics, where ‘one, two, many,’ is a familiar notion, Van Overveld's logical next step after two particles was many: a handful of sand. This, it was thought, would provide some answers. How did the detail as he had described it translate into a big pattern? And what exactly were the characteristics of that pattern? Simulations don't lend themselves to studies involving more than a hundred particles, so Van Overveld threw hundreds of marbles into a transparent box of water and started it quaking. Using an overhead camera, he made videos of what happened to the marbles. His marbles video ultimately became the winning movie in the Burgers Gallery of the J.M. Burgerscentrum, the national research school for fluid mechanics. “It's like magic, a pattern forms within seconds. We did so many experiments. Everything that could be varied, we varied it. The size of the marbles, the numbers, the frequency, and the amplitude of the quaking.”

They saw - depending on the parameters - all kinds of patterns, but two variations predominated: long, thin chains and broader bands. In the literature, Van Overveld came across two research groups who, independently of each other, had described these patterns. Experiments by the English group had resulted in chains; the other research group, in Italy, had produced broad bands. Van Overveld was convinced they were both looking at the same phenomenon. He continued to develop his theory, based on his own experiments and simulations, and attempted to form an overarching model. And he did. “We saw that little vortices were playing a key role. Under some conditions these vortices are big enough to enable two chains to merge into a single thicker band; under other conditions this doesn't happen. It's all to do with hydrodynamics. Just look at the beach during a storm, the sand ripples look very different then.”

Shaking up nanoparticles

Thanks to Van Overveld's research it has become clear why certain patterns emerge, so now scientists can investigate how to influence this pattern forming. This will expand the model's range of possible uses, he explains. “We now know how to make any given pattern. Using my research, the Italian researchers can go ahead and develop a model capable of predicting sediment transport; this prospect has already generated a good collaborative relationship. But the model can also be used for more industrial applications, such as improving the mixing of chemical substances. We now know more about what kind of shaking results in optimum mixing. And we're working with the TU/e group Colloidal Soft Matter to translate this theory to nanoparticles. They recognized their own structures in our collection of patterns. It'll be very interesting to investigate whether we can tune the interactions between nanoparticles in a similar way, in order to develop new structures, materials and material properties. This development in particle dynamics is really shaking up multiple fields.”