Interview: The movements of objects in a fluid

What does your research project consist of?

The aim of the project is to understand how objects move in a fluid when there are a large number of them. We are interested in two classes of systems: so-called passive systems which need an external force in order to move, and so-called active systems which can move autonomously. When these objects move, they generally cause movements of fluids on a very large scale, regardless of their size. We try to understand how the collective movements of these objects are dependent on the resulting hydro-dynamic interactions.

This research can be illustrated by some simple examples. Let’s take the example of a group of cyclists.  They are cycling in a fluid, air, and the speed of each cyclist depends to a large extent on the distance and speed of his neighbours. However, our research is focused on much smaller systems. They are governed by different laws to the one we experience daily, which gives rise to often counter-intuitive behaviour.  

For example one bacterium, a self-propelled agent and so capable of moving on its own, will move in a random way when isolated in a fluid, whereas a group of several hundred bacteria may demonstrate highly coherent directed movement.

How is your research project innovative?

A number of international studies have already been carried out on the subject of the movement of active objects in a fluid. The innovative aspect is the fact that we have created model artificial systems which contain only the minimum ingredients necessary for the emergence of collective movements in a fluid. This type of system had not previously existed.

We created these systems by combining experiments and theory. On the experimental level, we relied heavily on our knowledge of new tools, namely micro-fluidic devices. These small circuits are fluidic devices consisting of channels whose diameter is comparable to that of a hair.  They allow us to monitor the geometry of flows and also to obtain a highly quantitative vision of the collective dynamics of very large numbers of particles immersed in fluids.

Why do you need the Emergence(s) funding?

Emergence(s) funding has been essential in developing this project. It has allowed us to purchase specific materials and above all to create a research team. This team is made up of several post-doctoral students. Two of them have already defended their theses and the five other theses are currently in progress.

Three patents have also been filed for the creation of micro-fluidic devices which can now be used mainly for bio-engineering type applications.

Has your research project progressed much in three years? Were you expecting these results?

Things have progressed a great deal in three years and the results are very different to those we were expecting.

First we tried to understand the movement of passive objects in a fluidic network (i.e. a network of obstacles which filter the particles). Thanks to these results, we were then able to look at active objects. We did not anticipate either the driving force or the geometry of our model microswimmers when the project was launched. Our current systems have easily exceeded the performances we tried to anticipate.

I would like to add that our study on the movement of passive objects transported in fluidic networks has been of considerable interest to industrialists, who are currently funding this research. A number of questions remain unanswered and there are new discoveries to be made. As regards our model microswimmers, we have only been able to produce and study them en masse since September. Another three to five years of research are still needed to understand the full extent of their collective behaviour.

How have you presented your project to the scientific community?

Several articles have been published on the subject in different physics journals. I also have a personal website (http://denis114.wordpress.com/) where I display the results of our research. We have also organised and communicated in international symposiums in order to present our work.

Company profile:

Team:
Denis Bartolo, team leader and permanent member

Date of award:
Emergences 2009

Base institute:
Ecole Supérieure de Physique et de Chimie Industrielle (Paris Industrial Physics and Chemistry Higher Educational Institution - ESPCI-ParisTech), Centre national de la recherche scientifique (National Centre for Scientific Research - CNRS), Université Paris Diderot and Université Pierre et Marie Curie.
Laboratory: PMMH, Physique et Mécanique des Matériaux Hétérogènes (Physics and Mechanics of Heterogenous Materials)

Title of the research:  
Soft Active Matter

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