The whole is often greater than the sum of the parts – this wisdom was attributed to Aristotle and has become common knowledge. A sandwich also tastes better than the individual parts of bread, salad and mayonnaise. A team of physicists from HHU, TU Darmstadt and the University of Gothenburg in Sweden has now discovered that this is also true in physics and that something qualitatively new emerges from the combination of individual parts.

The research project was about the combination of different atoms and larger particles and how they influence each other – ultimately a typical example of how the matter surrounding us is composed. The researchers extended the combination principle to include additional feedback processes and thus created novel dynamic structures. These are so-called “positive feedback loops”.

Specifically, they combined two different types of colloid particles – particles or droplets in a dispersion medium – in a liquid bath. They irradiated this bath with lasers whose light brought the liquid to the critical point near the particles. There, the fluctuations are particularly strong, allowing droplet-like structures to form, which in turn trap the particles.

Formation of droploids

Within the droplets, the two types of colloid particles heat up to different degrees. This ultimately results in effective forces that violate Newton's fundamental principle (actio = reactio) and thus push the droplets forward. So, on the one hand, the colloid particles cause the formation of droplets that trap the colloids and, on the other hand, are propelled by the particles. This feedback loop creates novel superstructures with a self-organised colloidal motor; the researchers named them “droploids”, a portmanteau of droplets and colloids.

Professor Dr. Benno Liebchen, head of the group Theory of Soft Matter at the Department of Physics at TU Darmstadt, and his cooperation partners developed a novel model to describe the droploids. This model explains the mutual influence of the dynamics of the temperature field, binary fluid and colloids and can predict all aspects of the experiments, from the formation of the droploids to their propulsion mechanism and speed, with astonishing accuracy.

The research team combined theoretical and experimental approaches: While the system was modelled in Düsseldorf and Darmstadt, the colleagues from Gothenburg were able to verify the results on the real experiment and thus confirm the theoretical models.

Prof. Dr. Hartmut Löwen, head of the Institute for Theoretical Physics II at HHU, emphasises: “It is important that the process is completely controlled by laser illumination. This way, the system can be controlled flexibly from the outside for various possible applications.”