Microscopic spheres that can self-propel through a liquid move faster over a hydrophobic surface than they do over a hydrophilic surface. ‘This was a surprise,’ says Stefania Ketzetzi, the researcher who discovered the effect. Understanding how these artificial spheres move is important for understanding the behaviour of other microswimmers, for example, algae, bacteria and future microrobots.
Tiny synthetic spheres that self-propel in a liquid usually move near surfaces, such as the walls of their containers. Experiments with these spheres showed that the walls in fact influence their speed. The same experiments also demonstrated that the distance between the spheres and the walls is very robust. ‘It turned out that it is important to not just look at the microswimmers, but also at the surrounding surfaces, to fully understand their motion,’ says Ketzetzi.
The microswimmers Ketzetzi uses measure barely three micrometres. A part of each sphere is covered with a thin layer of platinum. When put in a hydrogen peroxide solution, the platinum acts as a catalyst, accelerating the chemical reaction of hydrogen peroxide into water and oxygen. This reaction, which occurs on the platinum-coated side, creates a fluid flow that propels the particle forward. ‘Because they propel themselves, we call them active particles,’ says Ketzetzi.
Ketzetzi was working on a project with active particles on polymer-coated surfaces. To her surprise, the microswimmers moved more slowly than they did on the more commonly used glass surface. Ketzetzi: ‘We decided to investigate different surfaces to see what properties cause this change in movement.’
It turned out that the slipperiness of the surface impacts the microswimmer’s speed. The particles are propelled by the fluid flows around them. When they swim close to a surface, flows are also created on these surfaces. These flows move more easily over a slippery, hydrophobic surface. Hydrophilic surfaces are less slippery and therefore offer more resistance, slowing down flows and thus also the propulsion and the microswimmer’s speed.
‘If we can fully understand how active particles work, we can program them into doing tasks. This means that, besides using them as models for fundamental research, we can potentially also use them for real-life applications.’
Ketzetzi made another surprising discovery when looking at microswimmers moving along a surface. ‘We measured at what distance from the surface the particles move, and discovered that this “height” stayed constant; it was not influenced by such factors as the size or charge of the particles.’ They keep moving parallel to the wall, unlike passive particles. This constant distance results from the activity of the particle and its interaction with the wall.
Clean water and drug delivery
‘So far, experiments have focused on the swimmers,’ says Ketzetzi. ‘But it turns out that nearby walls are important as well. This will shift the way we think about the experiments, and could help us discover more about the propulsion mechanism.’
Knowing how active particles respond to their surroundings is important for future applications. Ketzetzi: ‘Synthetic microswimmers could, for example, be used to clean polluted waters or for targeted drug delivery in the human body. In most cases, they will have to be able to self-propel in complex environments with walls and other obstacles.’
Stefania Ketzetzi specialised in atmospheric physics in Greece. She then moved to Amsterdam, where she switched to soft matter physics. After graduating, she came to Leiden as a PhD candidate to work on active matter. ‘I think it’s cool to work with actively moving particles that perform directed motion. Their behaviour is very different from passive particles and they have the prospect of new behaviours and exciting applications.’ Once she finishes her PhD in 2021, she hopes to continue in active matter research