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The Experiment

Why and How

1) What have been the reasons to set up an experiment at ZiF? What is the challenge and what are the technical problems raised by this?

Anne Cros, Patrick McGuire, Ricardo Lima, Patrice Le Gal and Elena Floriani (from left to right) Physics is made by starting from experimental facts, so that we found it interesting that people be gathered around an original experiment. This actually favours an interdisciplinary activity, as different people are led to apply their specific knowledge to interpret the experimental observations, without restricting discussion to their usual research subjects. This is indeed one of the challenges of the ZiF project.

Setting up the experiment was not an easy task anyway (which explains the fact that projects like this are seldom realised): we had to face the problem of design, manufacturing, transportation of material, installation of software, and of coordinating the work of the numerous persons involved in the project.

In fact, the experiment was conceived first as a collaboration between Pisa and Marseilles; it was then partially assembled in Marseilles and Bielefeld. At the end, all the different elements, that is, a mechanical part, an optical part as well as a hardware and software parts, were assembled at ZiF.

Moreover, as people used to experimental activity know, to overcome the technical difficulties related to the beginning of an experiment in one month only is a big challenge, too.

In a certain sense, and this was also the reason for choosing this particular experiment, we believe that it has a paradigmatic significance.


2) What do you mean by paradigmatic significance?

Leo Fronzoni and Patrice Le Gal When we look at the outcome of the experiment as it is visible in these pictures, we clearly identify domains with different patterns that move, grow or disappear as they interact or touch one another. These domains are made of regions of the liquid where the long molecules are oriented, roughly speaking, in the same direction. This collective behaviour which becomes relevant at a macroscopical scale (the size of a domain) is nevertheless the result of the microscopic interactions of the single molecules among them. Since one of the horizontal plates is rotating and the other one is at rest, the device forces each molecule to move in its own way. In spite of this almost individual behaviour a more global picture emerges from the individual background.

The fact that, just by adjusting one external parameter (the velocity of the moving disk), we can see so many different situations, ranging from a rather ordered to a completely disordered one, through successive and sudden dislocations of the local order of each region of the liquid, appears as a kind of universal scenario that may deserve attention for many other fields of research represented at ZiF. At least this is our belief.


3) In which ways did the interaction between theoreticians and experimentalists take place?

The room at ZiF The people who perform the experiment know the microscopic physics of liquid crystals much better than the theoreticians present at ZiF. They therefore tend to propose methods of analysis relying on the physical evolution equations of the system under study.

On the other hand, theoreticians have mostly proposed to characterize data, which are below the form of images, by global quantities such as entropy and complexity of the observed patterns.

Some discussions have been necessary to persuade the experimentalists that this could give a real insight into the phenomena observed, but now this seems to be one of the directions in which a first analysis will be oriented.

Moreover, it appears that not only the complexity of a "frozen" situation (the configuration considered at a fixed instant of time) is important, but, as time changes, the whole sequence of "photos" needs to be taken into account.


4) What are the research directions emerging from this experience?

The liquid crystal experiment: experimental device, camera, lamp and electronic controllers One of the problems will now be to define complexity in the specific physical situation of the experiment. This is a subject of active discussion since it appears that this definition should take into account the time and space scales typical of the phenomena observed and the limitations related to data acquisition.

Then, our goal will be to be able to modify the external parameters of the experiment in a way as to control the formation of particular patterns. In this sense, characterizing the data through complexity will help us to select the particular values of the parameters for which bifurcations take place: these are the regimes where the control mechanism should be applied.


You can view/print/download the whole report (PDF, approx. 4 MB) here.


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