Publish at July 08 2019 Updated January 28 2022

Model Experimental System of a Polar Active Fluid [Thesis]

A physics thesis to help us understand human collective dynamics

School of fish

If you've made it past the somewhat technical effect of the title of this thesis, you are a true explorer and your time will not be wasted. Indeed, Julien Deseigne carries out an exciting thesis in physical sciences to reveal us the behaviors of the living.

He reminds us of some phenomena that are difficult to explain, such as the swarms of starlings, the huge schools of fish that assemble to apparently deceive predators by preventing them from targeting a prey on an indistinct and constantly moving mass. He further notes the behaviors of grasshoppers that assemble in swarms and those of humans meeting at traffic circles.

Affinities with matter

He teaches us that what appear to be distinct phenomena specific to the living may well follow general laws specific to the properties of fluid physics, vortex phenomena, and polarities. For example a law of interaction with a constant number of neighboring elements 6 to 7 for birds and this, whatever the distance with its neighbors.

For grasshoppers he finds that when the density of grasshoppers increases, the transition from a disordered movement of grasshoppers to a movement in the same direction of grasshoppers is observable. When this collective motion appears, the direction of rotation of the grasshoppers changes over time.

For pedestrians he relates the existence of forces that are considered repulsive and forces that tend to align pedestrian speeds. These observations are useful in designing public spaces that reduce the potentially devastating effects of a panic, or in understanding and organizing the Shibuya intersection in Tokyo.

For the author collective movements would emerge in systems where the elements have a tendency to move in one direction and sense of self. Density would control this setting in motion: "when it increases, larger and larger packets of particles advance in the same direction". The hypothesis that is posed is that of the appearance of these movements following the local interaction between the various elements. This observation would be generalized on a range of length scale going from the meter to the micrometer, observable on different scales of nature. It would thus be less the character of the living which would explain these formations of movements than properties related to the proximity. For the author these systems have two main characteristics:

  • First, the collective movements emerge from interactions between a large quantity of individuals. Matter is ultimately just a collection of elements that interact together.
  • For another, the individuals that lead to the collective motions are self-propelled because they are systems that produce work by dissipating the energy that is injected into them. This illustrates the fundamentally non-equilibrium aspect of this matter. It therefore has nothing to do with a phenomenon that would be peculiar to the ≪vivant≫

The "active matter" or active particle can be identified from three characteristics:

  • An active particle generates its motion by producing work from the energy available homogeneously over the whole system.
  • A motion at a direction proper to the particle corresponding to internal degrees of freedom of the particle.
  • The particle is not subject to any resultant force.

These elements of definitions being given, the author notes that other particles alien to the world of the living obey the same laws of motion. What he sets out to demonstrate in his thesis to make the world of the living and the world of inanimate elements meet.

In fine, he models a system with a polar active fluid based on interactions. Beyond the existence of the described order/disorder transition it is for the author particularly interesting to characterize the dynamics and structure of active fluids (in ordered and disordered regimes) and to compare with what is known about equilibrium fluids and granular fluids

And in education?

Such a research in fundamental physics is very indirectly related to the issues of training and education, however, it stimulates the reflection on the organization of social networks or learning platforms. Where are the polarities located? What are the edges that constrain movements? What fluidities can be observed? What swirling effects are observable in groups, networks or online forums? How are the neighborhoods between elements of educational systems organized? Where are the sources of energy located? How are the first movements initiated?

A detour to a thesis that is very foreign to the educational sciences may well raise new questions for the design of more fluid training devices or platforms of a new kind.


Model experimental system of a polar active fluid - Julien Deseigne

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