Chun-Jen Chen attended the “Mobility, self-organization and swimming strategies” school, 18-29 October 2021

The school brought about a new collaboration on a simulation project with Vicsek-like model in complex environment (an example in upper with swimmers in black arrows showing their orientations), e.g. a turbulent flow (heatmap in upper panel), considered microswimmers that try to align to neighbors in a spatially-correlated noise (cartoon in lower). (Image by C-J Chen.)
From the 18th to the 29th of October 2021, Chun-Jen and two of his colleagues working on programmable active Janus colloids participated in the The 1st UCA FOX, UCA Fall program on Complex Systems 2021 “Mobility, self-organization and swimming strategies” hosted by Université Côte d’Azur in Nice and Fréjus.

The school was focused on three main topics: Swimming into complex environment – micro-swimming, Collective motion, and Machine learning applied to active particles, and covered a wide range of models regarding chemical and biological microswimmers.

The school was organised in lecture sections and project sessions for participants to collaborate in groups, in which PhD students and post-doctoral researchers could develop a research project in one of the three main topics of the school. Chun-Jen was involved in a simulation project on a Vicsek-like model in complex environment which is still ongoing.

Round Table on the Universality of Active Matter: from Biology to Man-made Models

A screenshot taken during the round table discussion of 20 September 2021.

On the 20th of September, the last round table of the Initial Training on Theoretical Methods took place. The discussion was let by the ESRs David, Sandrine, Liam, Carolina, Danne, and Laura. We were excited by the presence of an inspiring panel composed of Felix Ritort, Roberto Cerbino, Kirsty Wan, Fabio Giavazzi, Bernhard Mehlig, and François Nédélec.

The quote “If a system is in equilibrium, it’s probably death” ignited a lively and dynamic discussion around the topic of this final round table: “The universality of active matter: From biology to man-made models.”
Several topics were discussed ranging from active matter length scales and entropy production, to the equipartition theorem and universality. The session left us pondering about the definition of active matter: From single cells to the galaxy, where does the definition of active matter end? Our panelists conclude that it all depend on the question we ask ourselves. The round table was closed with a highlight of the most interesting avenues and opportunities in active matter, including the merge information and activity, realization of in vivo systems, as well as the manipulation of soft matter systems. Some inspiring words from one of the panelists let us realize: “We are the future of active matter.”

Round Table Discussion on Theoretical Models for Active Matter

A screenshot taken during the round table discussion of 16 September 2021.

The fourth round table of the theoretical training took place with the participation of our panelists: Hartmut Löwen, Joakim Stenhammar, Holger Stark and Ramin Golestanian. The organizing ESRs were Ayten Gülce Bayram, Chun-Jen Chen, Jérémie Bertrand, Jesus Manuel Antunez Dominguez, Ojus Satish Bagal, Alireza Khoshzaban and Umar Rauf. The discussion mainly addressed to “Theoretical Models for Active matter”.

The discussion started with how the activity is included in theoretical models and how activity terms change depending on the particles system. It is followed by the theoretical aspects of going from one particle to the many-particle system and the relevant interaction terms in the theoretical models. Next, we mentioned the challenges behind the solvent-particle interaction, especially where we have complex solvents like a viscoelastic solvent. In this regard, our guests pointed out the importance of hydrodynamics. The meeting was concluded with the final remarks of our guests on the discussion that we should keep in mind in our future studies on active matter physics.

Round Table Discussion on Fluids and Active Matter

A screenshot taken during the round table discussion of 13 September 2021.

In our third round table we had the pleasure of Gareth Alexander, Ignacio Pagonabarraga and Julia Yeomans as our guest panellists. This time the overall theme was “Fluids and Active Matter” and hosted by Chun-Jen Chen, Davide Breoni, Danne van Roon, Audrey Nsamela, Dana Hassan and Sandrine Heijnen. 

It started out with an interesting discussion regarding the motivation to get in and what amazes them the most in the field of active matter. Here it became clear that active systems can have their passive counterparts, and works for easy transitions from active to passive systems, but at the same time, such active systems still have the potential to answer many fundamental questions. From this topic, one of the key takeaways was that the project that you are currently working on should be the subject that amazes you the most. 

The next topic that stood as the centre of the discussion was turbulence. Turbulence is an interesting phenomenon where a lot of things are still unknown. The intriguing concept here was that real, or fluid-dynamical, turbulence is different from active turbulence. As a clarification, Julia Yeomans introduced the following comparison. Real turbulence is observed in a waterfall where the energy follows the Kolmogorov cascade. In active turbulence, the energy originates from the individual particles moving and does not follow the same energy trend  as real turbulence. 

As one of the final topics, we were wondering what are the main takeaways regarding active nematics, especially if it’s not your field. We got it set for you in four points. One, it is fundamentally unstable and therefore creates flows. Point number two, motile topological effects. Number three, the potential connection it has to biological systems and the ability to explain similar processes. Finally, number four, the fact that we are looking at non-equilibrium systems.

Round Table Discussion on Theoretical Aspects of Collective Behaviour

A screenshot taken during the round table discussion of 9 September 2021.

Today the second round table of the Initial Training on Theoretical Methods took place, entitled “Theoretical aspects of collective behavior”. The round table was hosted by ESRs David, Jesus, Ojus, Carolina, Alireza, Dana, and Umar. The inspiring group of speakers included Margarida Telo da Gama, Fernando Peruani, Nicoletta Gnan, and Claudio Maggi.

Many matters were discussed, ranging from the limits of collective behavior and the role of communication in emergence, to the compatibility between experiments and theory of collective behavior. Examples can be found in both natural and artificial environments, even combinations with varying degrees of active motion. This adds to the challenge of defining valuable, even if not accurate, models. At the core, collective behavior highlights how the system can be much more than just the sum of individual entities.

Round Table Discussion on Introduction to Theoretical Active Matter

A screenshot taken during the round table discussion of 7 September 20201.

The first round table in the theoretical training gave a chance to start an interesting discussion which will continue in the following meetings.

The organizing ESRs were Ayten Gülce Bayram, Laura Natali, Liam Ruske, Jérémie Bertrand, Davide Breoni and Audrey Nsamela. They welcomed and introduced the three guests of the session: Nuno Araújo from the University of Lisbon, Jan Wehr from the University of Arizona and Denis Bartolo from École normale supérieure de Lyon.

The round table started with a personal question to the speakers about their interests and motivations for working in theoretical active matter. Having different backgrounds, the answers were very different, Nuno was attracted by non-intuitive behaviors observed in active matter experiments, while Jan started from a purely mathematical point of view and then moved towards physics of active systems. Denis provided another motivation, being head of a lab that deals with both theory and experiments.

The following discussion focused on the interaction and hierarchy between theory, simulations, and experiments. They all agree that establishing a constructive collaboration with experimental groups is not easy, but at the same time, it can have many benefits for both sides. However, none of the three elements is necessary for the others: a good paper can be presenting a theory not connected with experiments, even if its possible applications are not foreseeable yet. Denis firmly pointed out the difference between the observations and the tools (theoretical, numerical, and experimental) employed to explain it.

We also had a few more specific questions for the speakers, such as the distinctions in thinking between mathematicians and theoretical physicists, the possible applications to financial markets, and the differences in modeling artificial flocks and human crowds, which are often controlled by non-hydrodynamic variables.

We concluded the meeting by asking every one of our guests their tips for communicating the theory of active matter to a larger public. Here the answers were more relaxed and can be summed up as: trying to avoid technical and mathematical details while explaining the importance of the research problems, also using more familiar examples such as simulations employed in animation movies.

Rond Table Discussion on: Advanced Control of Active Matter

The last round table of this workshop regarded the topic advanced control of active matter. As organizers of round table, Audrey Nsamela, Chun-Jen Chen, Sandrine Heijnen, Harshith Bachimanchi and Alireza Khoshzaban, we welcomed and introduced our esteemed guests, namely Jérémie Palacci from University of San Diego, Clemens Bechinger from Konstanz University, Frank Cichos from Leipzig University, and Lucio Isa from ETH Zurich.

The round table started out with a clarification on advanced control of active matter. Active matter can be controlled by numerous external stimuli but implementing control on individual particles or artificial entities is what qualifies as advanced control. Currently, the control of active matter is still far from the behavior and control micro-organisms have on that scale; hence a big challenge lies there for us. Jérémie Palacci introduced an interesting research topic where they found a way to regulate the swimming process of E. Coli by light illumination. Here genetic modification was used to control the proton pump involved in the energy transportation process.

Advanced control of active matter can be applied to model systems where the control is lacking, for example biological systems. In a biological system the control over an organism is limited to the external stimuli that are applied and won’t always result in the same reaction. Therefore, using active particles showing predictable and reproducible behaviors when exposed to a stimulus works perfectly to model and to probe different parameters and thus provide a deeper understanding of the system. The fact advanced control of active matter doesn’t have an application outside of modelling systems is something we shouldn’t be ashamed of.

We concluded the meeting by asking every one of our guests what the promising research directions in the advanced control of active matter are. All of them had a different perspective. Starting with  Clemens Bechinger, who was most invested in the further exploration of the applications for model systems. Lucio Isa is mainly looking forward to explore the different materials that we can use to create active material that can subsequently be controlled. Frank Cichos mentioned the importance of looking into new ways to create active particles. So far nature was able to achieve production of active entities with limited waste whereas human production is rather inefficient. Jérémie Palacci pointed out that the current man-made active matter systems are reacting to a strong signal in a well-controlled environment, where nature faces many more factors and still works. It would be interesting to design a system that is resistant to noise.

Round Table Discussion on: Collective Behavior

The fourth roundtable was an opportunity for all students to discuss the topic “Collective Behavior” on Zoom with a panel of guests: Clemens Bechinger from the University of Konstanz, Ivo Buttinoni from Heinrich Heine University in Dusseldorf and Caroline Beck Adiels from Gothenburg University. The event was organized by Daniela Pérez, Danne van Roon, Davide Breoni, Jérémie Bertrand, Laura Natali and Liam Ruske on March 24th.

Although the guests had different background they seemed to agree on the fact that complex behavior can emerge from an ensemble of entities that obey a small number of simple rules. Indeed, minimalistic models such as the Vicsek model account for phase transition from a disordered motion to large scale motion and more; phenomena that appear to be universal.

A question on the role of intelligence and communication in collective behavior started the discussion. Although some animals or colony of bacteria may seem intelligent (e.g. escaping from a predator in a clever way or making long-lasting symbiotic microfilms), we must bear in mind that collective behavior is… collective, and rarely arises from decisions made individually. It may be said that in the animal kingdom, the need for survival requires a need to adapt and therefore to be intelligent, but this need for intelligence can be outsourced and solved at the level of the group rather than hardwired in the physical brain of each animal (or human).

It is also conceivable that one of the entities acts as a leader and ignites a collective behavior. Giovanni Volpe made an interesting remark, stating that a leader is the one who defines the objective function to be optimized by the group. The idea of leadership in collective behavior of microscopic systems remain largely unexplored by physicists.

After one hour of fruitful discussion and back and forth between the students and the guests, the session was finished and we resumed our activities with a better understanding of collective behavior. We thank the panelists for their inputs and attendance!

Round Table Discussion on: Optics, Spectroscopy, Micro and Nanofabrication, and Nanotribology

On Tuesday 23 March the fourth round table of the initial training on experimental methods for active matter took place. The topic of the round table was “Optics, Spectroscopy, Micro and Nanofabrication, and Nanotribology”, and the discussion was led by Ayten Gülce Bayram , David Bronte Ciriza, Dana Hassan, Carolina van Baalen and Jesús Manuel Antúnez Domínguez.
The panelists included Maria Grazia Donato, Pietro Gucciardi, Antonino Foti, Shivaprakash Ramakrishna, and Felix Holzner.

The importance of the topic of the round table to the field of active matter was motivated by the panelists from different perspectives. The discussion ranged from the main differences and challenges that come along with working on the micro- and nanoscale, to how changing the dimensions of your system allows one to change the properties of a system’s response, as well as the challenges involved in bringing a product idea to the market. The main conclusion was that the nanoscale is exciting, but the smaller you get, the greater the challenge.

Round Table Discussion on: Machine Learning

The third round table session of the experimental training was about machine learning and its role in science, in particular physics and active matter. The panelists invited to the discussion were Carlo Manzo from Vic University, Benjamin Midtvedt and Saga Helgadottir from Gothenburg University, Onofrio Maragò and Alessandro Magazzù from CNR ICPF-Messina. The discussion was organized and lead by Jesus M. A. Dominguez, Davide Breoni, Liam Ruske, Chun-Jen Chen and Alireza Khoshzaban, who are students attending the training.

The discussion touched topics like the applications of machine learning in fields like optics, biophysics, medical research, the potentialities and the reliability of the method. Questions on when a machine learning approach is advisable and how cautious one must be when applying machine learning were also addressed. Current important logical and practical aspects of the method were also discussed, together with the need of testing machine learning applications against more classical ones. The panelists also stated the importance of reliably checking the results obtained to avoid biases that can lead to false conclusions.

After one hour of fruitful discussion we gained a broader perspective and a deeper understanding of machine learning.