Benjamin Gorin did not like the graduation internship during his physics studies. “I did research into the interaction between light and matter and stood in a darkened laser lab for four months,” he says, laughing cautiously. “I learned a lot there, so I have no regrets. But sometimes I didn’t see daylight for whole days. I wouldn’t keep that up for years.”
Fortunately, a PhD research came his way that was a better fit: studying the impact of droplets on a hard surface. “It may sound crazy, but that interests me because I love surfing. I love water and water sports,” says Gorin, video calling from his apartment in Bordeaux. The research was a collaboration between the University of Amsterdam and the University of Bordeaux. He conducted most of his research in France. “I occasionally came to Amsterdam for specific experiments because they have better cameras and certain useful materials there.” He defended his dissertation on February 7 in Amsterdam.
During his PhD, Gorin again spent a lot of time in a lab, but this time without blackout curtains and with a high-speed camera with which he made hundreds of slow-motion recordings of different types and sizes of droplets that landed at different speeds on different surfaces. “I looked at how the drops hit the surface and how the spherical shape they have during the fall changes into a pancake when they hit the surface,” says Gorin enthusiastically. “That looks beautiful.”
Different conditions cause different drop impacts. For example, landing on a water-repellent surface, water droplets do not spread far. They remain as thick drops. While they spread on a water-loving surface into a thin layer.
Icing on aircraft
“We don’t just film for our own pleasure,” says Gorin. “We want to understand how droplets spread when they land and how their composition, size, speed, the surface and temperature influence this.”
This research is fundamental; so it has no direct applications. But it could be relevant for spraying paint or spraying crops, he says.
He also investigated “how droplets spread and freeze on cold surfaces,” says Gorin, down to tens of degrees below zero. “On a cold surface, a drop will spread less widely than on a warm one. We discovered that we can predict when that spreading will stop. The droplet freezes from below and we discovered that spreading stops when the frozen layer has reached a certain thickness.”
The freezing of droplets plays a role in ice formation, for example on aircraft. “If an aircraft flies through a cloud of cold droplets, they can freeze on the aircraft, which can lead to accidents. These clouds are now avoided. But it may be possible to design a surface without ice formation. To do this, we need to understand how exactly droplets freeze.”
Gradually, the drops that Gorin examined became less ‘droppy’. For example, he also filmed the impact of a hydrogel ball. “These water balls, so-called ‘water beads’, are often used to place flowers or plants. They are small balls that fill up when you put them in water. They then swell to an inch or two.” When he dropped the hydrogel balls from different heights, Gorin saw something surprising. “If they fall a little bit, and therefore have little speed, they bounce back, like a bouncy ball. But if they fall for longer, and therefore have a higher speed, they deform strongly on impact, just like a liquid drop. The hydrogel balls therefore behave like a bouncy ball or like a drop, depending on their speed.”
A kind of super sponges
To better understand this surprising behavior, Gorin looked for something with the structure of a hydrogel ball, but larger, to better see what was happening. Hydrogel balls consist of a network of polymers – chains of molecules – between which a lot of water can be retained, he explains. They are kind of like super sponges. “We then came up with the idea of using soft foam tennis balls, which you can buy in toy stores. They can also gorge themselves with a liquid. We then dropped the filled foam balls. It looked like what children sometimes do when they play with such foam balls: soak them in water and then throw them as hard as possible on the sidewalk so that the water splashes out. But more controlled.”
The foam balls indeed turned out to be comparable to the hydrogel balls. Gorin discovered that the liquid has an influence. “If we soaked it in water, it bounced less high than with oil,” he says. “This is because oil is more viscous and therefore flows less easily through the porous structure of the foam ball. The faster flowing water causes more energy loss, which means the ball doesn’t reach as high.”
Gorin substantiated and explained this playful research with solid, fundamental physics. Yet he does not want to continue in science. “Finding a job as a researcher is difficult. It’s too uncertain. That’s why I want to work in the industry. I don’t know exactly where yet, but with a physics PhD under my belt, I’m sure I’ll find something.”