Active particles

Colloidal swimmers, or active particles, constitute a new class of nonequilibrium model systems because they consume energy to move. They can furthermore be used as models to understand the behavior of biological microswimmers, and also hold great promise for applications such as targeted drug delivery.
In the Kraft lab we develop new types of active particles and quantitatively investigate their motion, interactions, and collective behavior.

A robust and widely employed way to create colloidal swimmers is to half-coat particles with platinum and disperse them in hydrogen peroxide. The catalytic decomposition of hydrogen peroxide on the platinum patch then propels the particles, but precisely how this propulsion mechanism works is still under debate. We discovered that nearby substrates, to which the particles have a strong affinity, affect the swimming speeds of the particles. In particular, we found a quantitative relationship between the slip of the substrate and the velocity of the particles. The swimmers furthermore adopt a constant height above the substrate, which we revealed by analyzing their active motion.

We studied the interactions of these catalytically propelled particles in a 1D enviornment. To do so, we leveraged their surface affinity and confined them to spherical posts. We found a rich variey of behaviors, from moving at stable distances to the dynamic formation, breaking and rearrangement of trains and chains.

Publications

• S. Ketzetzi, R. Doherty, J. de Graaf, D.J. Kraft, Slip length dependent propulsion speed of catalytic colloidal swimmers near walls, Physical Review Letters, , (2020)
• S. Ketzetzi, J. de Graaf, D.J. Kraft, Diffusion-based height analysis reveals robust microswimmer-wall separation, Physical Review Letters, 125(23), 238001 (2020)
• S. Ketzetzi, M. Rinaldin, P. Dröge, J. de Graaf, D.J. Kraft, Activity-induced interactions and cooperation of artificial microswimmers in one-dimensional environments, Nature Communications, 13 (1), 1772 (2022)

Self-propelling particles can be used as a model system for understanding the behavior of biological microswimmers. However, most synthetic microswimmers are spherical, whereas many biological microswimmers are more complex in shape. We use 3D microprinting to create anisotropic active particles and study how a different particle symmetry affects their motion, interactions, and collective behavior. For example, our microscopic version of 3D Benchy boat swims in circles due to its approximately L-shape.

Publications

• R.P. Doherty, T. Varkevisser, M. Teunisse, J. Hoecht, S. Ketzetzi, S. Ouhajji, D.J. Kraft, Catalytically propelled 3D printed colloidal microswimmers, Soft Matter, 16, 10463-10469 (2020)
• S. Riedel, L. Hoffmann, L. Giomi, D.J. Kraft, Designing highly efficient lock-and-key interactions in anisotropic active particles, Nature Communications (accepted), , (2024)