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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.

Jesús Domínguez’s secondment at the University of Gothenburg

Jesus Manuel Antunez Dominguez during his secondment in the University of Gothenburg. Image by L. Natali.
Jesús Manuel Antúnez Domínguez visited the University of Gothenburg (Sweden) from the 27th of October to the 11th of November 2021 during a two weeks secondment.

The University of Gothenburg is the academic collaborator of his industrial PhD project at Elvesys (France). During the secondment, he could meet his academic supervisor Dr. Caroline Beck Adiels and the rest of her group, the Biological Physics Lab, but also, the Soft Matter Lab researchers who work closely with them. The secondment served to become familiar with the facilities available at the University and it was a great opportunity to participate in outreach activities and present the progress of his project to establish new collaborations for the research about Active Matter.

Audrey Nsamela presented a poster at MicroTAS 2021 hybrid conference in Palm Springs

The Miniaturized Systems for Chemistry and Life Sciences (or MicroTAS) conference took place this year on October 10-14th in a hybrid configuration, both online and in-person in Palm Springs (USA). This conference unite top researchs groups from all over the world and present the most recent advances in MEMS. Audrey attended the conference online from Paris and presented her poster on the development of a sperm sorting platform including chemotaxis guidance.

Audrey Nsamela and Jesús Domínguez both present a poster at MNF conference in Toulouse

The Micro-Nano-Fluidics meeting took place in Toulouse (France) in September 2021. This conference was organized by a french research group and covered mainly 6 topics: Nanofluidics, Chemical Engineering, Flow-waves interactions, Flow chemistry, Diagnostics and clinics, Organ-on-Chip. Audrey and Jesús attended the 2 days conference and presented their poster. Audrey’s poster was focused on the development of a microfluidic platform for sperm sorting, while Jesús’s poster described his work on microfluidic droplet generation for bacteria encapsulation and biofilm studies. This national conference was a great opportunity for both ESRs to meet other researchers in microfluidics and discuss about applications in Active Matter.

Group picture.

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.

Presentation by D. Bronte Ciriza at the 19th Electromagnetic and Light Scattering Conference

Comparison between the Geometrical Optics (GO) method and the Neural Network (NN) for the optical forces and torques calculation. The NN improvement in speed and accuracy could help to study the motion of active particles in optical landscapes. Image by D. Bronte Ciriza.
Machine learning to enhance the calculation of optical forces in the geometrical optics approximation
David Bronte Ciriza, Alessandro Magazzù, Agnese Callegari, Maria A. Iatì, Giovanni Volpe, Onofrio M. Maragò
Submitted to: ELS-XIX (2021)
Date: 14 July
Time: 12:50 CEST

Short Abstract:
We show how machine learning can improve the speed and accuracy of the optical force calculations in the geometrical optics approximation

Extended Abstract:
Since the pioneering work by Ashkin in the 1970’s, optical forces have played a fundamental role in fields like biology, nanotechnology, or atomic physics. In all these fields, numerical simulations are of great help for validating theories, for the planning of experiments, and in the interpretation of the results. However, the calculation of the forces is computationally expensive and prohibitively slow for numerical simulations when the forces need to be calculated many times in a sequential way.

Recently, machine learning has been demonstrated to be a promising approach to improve the speed of these calculations and therefore, to expand the applicability of numerical simulations for experimental design and analysis. In this work we show that machine learning can be used to improve not only the speed but also the accuracy of the force calculation in the geometrical optics regime, valid when the particles are significantly bigger than the wavelength of the incident light. This is first demonstrated for the case of a spherical particle with 3 degrees of freedom and later expanded to 9 degrees of freedom by including all the relevant parameters involved in the optical forces calculation. Machine learning is proved as a compact, accurate, and fast approach for optical forces calculation and presents a tool that can be used to study systems that, due to computation limitations, were out of the scope of the traditional ray optics approach.