Motorized droplets thanks to feedback effects

According to an age-old saying, the whole is often more than the sum of its parts. After all, a sandwich made of bread, lettuce and mayonnaise tastes better than its individual components. A team of physicists from HHU, TU Darmstadt and Sweden's University of Gothenburg has determined that this adage is also true in the realm of physics, and that combining individual parts can create something with entirely new properties.

The research project involved combining different atoms and larger particles and studying the effects they have on each other. It is ultimately a typical example of what the matter that surrounds us is composed of. The researchers extended this general principle of combination to include additional feedback processes, thus creating new kinds of dynamic structures referred to as 'positive feedback loops'.

Specifically, they combined two different types of colloid particles -- in a water-lutidine heat bath. They irradiated the bath with lasers, and the light from the lasers brought the liquid near the particles to the critical point. The fluctuations are particularly strong at this point, allowing droplet-like structures to form that in turn surround the particles.

Inside the droplets, the two types of colloid particles heat up to different temperatures. This results in effective forces that contradict Newton's fundamental law of motion (actio = reactio) to propel the droplets forwards. This means that the colloid particles induce the formation of droplets that encapsulate the colloids and are in turn propelled by the particles. This feedback loop results in novel superstructures with a self-organised colloidal motor. The researchers adopted the term 'droploids', a portmanteau of the words 'droplets' and 'colloids', to describe these superstructures.

The research team combined theoretical and experimental approaches, with the system modelling performed in Düsseldorf and Darmstadt, while the colleagues in Gothenburg verified the findings using real-life experiments, thus confirming the theoretical models.

Prof. Dr. Hartmut Löwen, Head of the Institute of Theoretical Physics II at HHU, had this to say: "It's important here that the process can be controlled entirely by laser illumination. This makes it possible to steer the system externally so that it is flexible for different applications."

Prof. Dr. Benno Liebchen, leader of the "Theory of Soft Matter" working group at TU Darmstadt, explained the actual use of the droploids as follows: "Besides justifying a novel concept for micromotors, the droploids and the non-reciprocal interactions involved could serve as important ingredients for generating future biomimetic materials."