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A flock of birds is a group of active particles. The birds fly independently, but the sum of their interactions moves the flock more or less in the same direction. Microscopically small ”flocks”, such as bacteria populations, behave in a somewhat similar way.

In physics, autonomously moving—or self-propelled—particles from microbes to larger animals and synthetic particles are called agents or active particles. The new extensive research projects led by Professor Jaakko Timonen seek a breakthrough in controlling microscopic active particles with magnetic tweezers. The research focuses on particles that move in different ways: bacteria, microalgae, and synthetic active particles.

The aim is to develop a new type of magnetic tweezers to manipulate rapidly moving active particles in real-time and also to study the mechanisms underlying the interaction between the particles. Professor Timonen’s projects are funded by both the Academy of Finland and the European Research Council (ERC) Starting Grant.

In the first phase, Timonen’s team will examine the framework, or the room for manoeuvre, of a single active particle: how can it affect the collective action of a larger group of particles. The study involves mixing chemically synthesized magnetic nanoparticles with the active particles and seeing how their motion could be controlled with an external magnetic field. This micromanipulation technique based on magnetic forces is called magnetic tweezing.

‘We turn one actively moving agent to a different direction than the others and monitor in real-time how that changes the collective motion. If you compare this phenomenon to a flock of birds, we are looking for the smallest change in direction and amount of movement needed from a single bird to make the entire flock formation react and change course,’ says Jaakko Timonen.

Active particles move rapidly and unpredictably, however, making it difficult to manipulate them. These “intelligent” magnetic particles controlled with magnetic tweezers could be programmed to differentiate various types of microscopic objects, such as different cell types in microbiological research.

In the next phase of the project, active particles will be placed in magnetic fluid. The goal now is to control not individual particles, but the entire flock at the same time—indirectly through the magnetic fluid. The fluid acts as magnetic tweezers creating potential wells of energy to trap a large number of active particles.

‘When we have previously conducted similar studies on passive colloidal particles, they have ended up in simple formations at the bottom of the potential well. In our new study design, we replace passive particles with active ones. When the potential well becomes deeper, the particle density grows and there is less and less room to move. We want to study this interesting competitive situation where the particles try to move even though they are running out of space,’ says Timonen.

Further information:

Jaakko Timonen
Assistant Professor
Aalto University
jaakko.timonen@aalto.fi
tel. +358 44 230 5820