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A. Callegari and F. Schmidt won the Poster Prize at the 729. WE Heraeus Seminar on Fluctuation-induced Forces

Gold flake suspended over a functionalized gold-coated substrate. (Image by F. Schmidt.)
Agnese Callegari and Falko Schmidt share one of the three Poster Prizes of the 729. WE-Heraeus-Seminare on Fluctuation-induced Forces.

The two complementary posters focused on the experimental and theoretical/numerical aspects of a system constituted by a micron-sized gold flake suspended in a solution of water-lutidine at critical concentration above a gold-coated substrate. The dynamic of such a system is driven by the interplay of Casimir-Lifshitz forces and critical Casimir forces, which, under convenient circumstances, are the keystone to prevent stiction.

The other two Poster Prizes were awarded to Ariane Soret ( University of Luxembourg, with the poster: Forces Induced by Quantum Mesoscopic Coherent Effects) and Fred Hucht (University of Duisburg-Essen, with the poster: The Square-Lattice Ising Model on the Rectangle).

The Poster Prizes recipients’ names were announced during the closing session on 17 February. Each prize consisted in 100 EUR, which in the case of Agnese and Falko will be shared equally between the two. Andrea Gambassi, who made the announcement on the behalf of the organizers, amusingly mentioned the custom of equally sharing the Nobel Prize.

The Wilhelm and Else Heraeus Foundation is a private institution that supports scientific research and education with an emphasis on physics. It was established in 1963 by Dr. Wilhelm Heinrich Heraeus and his wife Else Heraeus. The Wilhelm and Else Heraeus Foundation is Germany’s most important private institution funding physics.

Flash Talk by A. Magazzù at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 15 February 2022

Schematic of the experimental setup used in the experiment. (Image by A. Magazzù.)
Controlling the dynamics of colloidal particles by critical Casimir forces
Alessandro Magazzù
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
15 February 2022, 14:50 CET

Critical Casimir forces can play an important role for applications in nano-science and nano-technology, owing to their piconewton strength, nanometric action range, fine tunability as a function of temperature, and exquisite dependence on the surface properties of the involved objects. Here, we investigate the effects of critical Casimir forces on the free dynamics of a pair of colloidal particles dispersed in the bulk of a near-critical binary liquid solvent, using blinking optical tweezers. In particular, we measure the time evolution of the distance between the two colloids to determine their relative diffusion and drift velocity.

Invited Talk by G. Volpe at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 14 February 2022

Sketch of the experimental setup for the measurement of nonadditivity of critical Casimir forces. (Image by S. Paladugu.)
Experimental Study of Critical Fluctuations and Critical Casimir Forces
Giovanni Volpe
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
14 February 2022, 16:35 CET

Critical Casimir forces (CCF) are a powerful tool to control the self-assembly and complex behavior of microscopic and nanoscopic colloids. While CCF were theoretically predicted in 1978 [1], their first direct experimental evidence was provided only in 2008, using total internal reflection microscopy (TIRM) [2]. Since then, these forces have been investigated under various conditions, for example, by varying the properties of the involved surfaces or with moving boundaries. In addition, a number of studies of the phase behavior of colloidal dispersions in a critical mixture indicate critical Casimir forces as candidates for tuning the self-assembly of nanostructures and quantum dots, while analogous fluctuation-induced effects have been investigated, for example, at the percolation transition of a chemical sol, in the presence of temperature gradients, and even in granular fluids and active matter. In this presentation, I’ll give an overview of this field with a focus on recent results on the measurement of many-body forces in critical Casimir forces [3], the realization of micro- and nanoscopic engines powered by critical fluctuations [4, 5], and the creation of light-controllable colloidal molecules [6] and active droploids [7].

References

[1] ME Fisher and PG de Gennes. Phenomena at the walls in a critical binary mixture. C. R. Acad. Sci. Paris B 287, 207 (1978).
[2] C Hertlein, L Helden, A Gambassi, S Dietrich and C Bechinger. Direct measurement of critical Casimir forces. Nature 451, 172 (2008).
[3] S Paladugu, A Callegari, Y Tuna, L Barth, S Dietrich, A Gambassi and G Volpe. Nonadditivity of critical Casimir forces. Nat. Commun. 7, 11403 (2016).
[4] F Schmidt, A Magazzù, A Callegari, L Biancofiore, F Cichos and G Volpe. Microscopic engine powered by critical demixing. Phys. Rev. Lett. 120, 068004 (2018).
[5] F Schmidt, H Šípová-Jungová, M Käll, A Würger and G Volpe. Non-equilibrium properties of an active nanoparticle in a harmonic potential. Nat. Commun. 12, 1902 (2021).
[6] F Schmidt, B Liebchen, H Löwen and G Volpe. Light-controlled assembly of active colloidal molecules. J. Chem. Phys. 150, 094905 (2019).
[7] J Grauer, F Schmidt, J Pineda, B Midtvedt, H Löwen, G Volpe and B Liebchen. Active droploids. Nat. Commun. 12, 6005 (2021).

Flash Talk by F. Schmidt at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 16 February 2022

Title slide of the presentation. (Image by F. Schmidt.)
Casimir-Lifshitz forces vs. Critical Casimir forces: Trapping and releasing of flat metallic particles
Falko Schmidt
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
16 February 2022, 14:50 CET

Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occuring even at zero temperature. Their generalization at finite temperature and in material media are referred to as Casimir-Lifshitz forces. These forces are typically attractive, leading to the widespread problem of stiction between the metallic parts of micro- and nanodevices. Recently, repulsive Casimir forces have been experimentally realized but their use of specialized materials stills means that the system can not be controlled dynamically and thus limits further implementation to real-world applications. Here, we experimentally demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to prevent stiction due to Casimir-Lifshitz forces. We show that critical Casimir forces can be dynamically tuned via temperature, eventually overcoming Casimir-Lifshitz attraction. We study a microscopic gold flake above a flat gold-coated substrate immersed in a critical mixture. Far from the critical temperature, stiction occurs because of Casimir-Lifshitz forces. Upon approaching the critical temperature, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. By removing one of the key limitations to their deployment, this experimental demonstration can accelerate the development of micro- and nanodevices for a broad range of applications.

Flash Talk by A. Callegari at 729. WE Heraeus Seminar on Fluctuation Induced Forces, Online, 14 February 2022

Potential energy landscape for a flake suspended on a patterned substrate. (Image by A. Callegari.)
Theoretical and numerical study of the interplay of Casimir-Lifshitz and critical Casimir force for a metallic flake suspended on a metal-coated substrate
Agnese Callegari
729. WE-Heraeus Stiftung Seminar on Fluctuation-induced Forces
14 February 2022, 14:50 CET

Casimir-Lifshitz forces arise between uncharged metallic objects because of the confinement of the electromagnetic fluctuations. Typically, these forces are attractive, and they are the main cause of stiction between microscopic metallic parts of micro- and nanodevices. Critical Casimir forces emerge between objects suspended in a critical binary liquid mixture upon approaching the critical temperature, can be made either attractive or repulsive by choosing the appropriate boundary conditions, and dynamically tuned via the temperature.
Experiments show that repulsive critical Casimir forces can be used to prevent stiction due to Casimir-Lifshitz forces. In a recent work, a microscopic metallic flake was suspended in a liquid solution above a metal-coated substrate [1]. By suspending the flake in a binary critical mixture and tuning the temperature we can control the flake’s hovering height above the substrate and, in the case of repulsive critical Casimir forces, prevent stiction.
Here, we present the model for the system of the metallic flake suspended above a metal-coated substrate in a binary critical mixture and show that repulsive critical Casimir forces can effectively counteract Casimir-Lifshitz forces and can be used to control dynamically the height of the flake above the surface. This provides a validation of the experimental results and a base to explore and design the behavior of similar systems in view of micro- and nanotechnological applications.

References
[1] F. Schmidt, A. Callegari, A. Daddi-Moussa-Ider, B. Munkhbat, R. Verre, T. Shegai, M. Käll, H. Löwen, A. Gambassi and G. Volpe, to be submitted (2022)

Invited Talk by G. Volpe at UFS Day 10.02.22

DeepTrack 2.0 Logo. (Image from DeepTrack 2.0 Project)

Deep learning for experimental soft matter
Giovanni Volpe
Invited Talk at UFS Day 10.02.22
Online
10 February 2022
14:00 CET

After a brief overview of artificial intelligence, machine learning and deep learning, I will present a series of recent works in which we have employed deep learning for applications in experimental soft matter.

Presentation by H. Bachimanchi at Prof. Metzler’s group at the University of Potsdam, 4 February 2022

Tracking of the planktons. (Image by H. Bachimanchi.)
Characterising plankton behaviours using deep learning powered inline holography
Harshith Bachimanchi
Presentation at Prof. Ralf Metzler’s Theoretical Physics group at University of Potsdam (Online)
4 February 2022, 14:15 CET

Digital holographic microscopy is a powerful label-free imaging technique for studying biological specimens. The complex optical fields of microscopic objects can be stored in the form of interference patterns and can be reconstructed by using the principles of holography. Recently, we have developed a digital inline holographic microscope with a deep learning powered analysis to track planktons through generations, and continuously measure their three-dimensional position and dry mass. By bringing planktons of different trophic levels together, we were able to perform a quantitative assessment of trophic interactions between planktons such as feeding events, biomass transfer from cell to cell, etc. In this talk, I will be giving a short overview of our method and present some of our recent results.

Directed Brain Connectivity Identifies Widespread Functional Network Abnormalities in Parkinson’s Disease published in Cerebral Cortex

Visual display of the nodes that show significant differences between controls and participants with PD in network measures using the anti-symmetric correlation method. (Image by the Authors.)
Directed Brain Connectivity Identifies Widespread Functional Network Abnormalities in Parkinson’s Disease
Mite Mijalkov, Giovanni Volpe, Joana B Pereira
Cerebral Cortex 32(3), 593–607 (2022)
doi: 10.1093/cercor/bhab237

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by topological abnormalities in large-scale functional brain networks, which are commonly analyzed using undirected correlations in the activation signals between brain regions. This approach assumes simultaneous activation of brain regions, despite previous evidence showing that brain activation entails causality, with signals being typically generated in one region and then propagated to other ones. To address this limitation, here, we developed a new method to assess whole-brain directed functional connectivity in participants with PD and healthy controls using antisymmetric delayed correlations, which capture better this underlying causality. Our results show that whole-brain directed connectivity, computed on functional magnetic resonance imaging data, identifies widespread differences in the functional networks of PD participants compared with controls, in contrast to undirected methods. These differences are characterized by increased global efficiency, clustering, and transitivity combined with lower modularity. Moreover, directed connectivity patterns in the precuneus, thalamus, and cerebellum were associated with motor, executive, and memory deficits in PD participants. Altogether, these findings suggest that directional brain connectivity is more sensitive to functional network differences occurring in PD compared with standard methods, opening new opportunities for brain connectivity analysis and development of new markers to track PD progression.

Antonio Ciarlo joins the Soft Matter Lab

(Photo by A. Argun.)
Antonio Ciarlo joined the Soft Matter Lab on 31th January 2022.

Antonio is a PhD student at the Physics Department of the University of Naples, Italy.

He will be working on the modelling and the analysis of the data of his experiments with intracavity optical trapping.

He will stay in our lab for six months.

Keynote Talk by G. Volpe at IUPAP Conference on Condensed Matter Physics and Optics, 20 January 2022


Deep learning for microscopy, optical trapping, and active matter
Giovanni Volpe
Keynote Talk at IUPAP conference on Condensed Matter Physics and Optics
Online
20 January 2022
15:00 PST

After a brief overview of artificial intelligence, machine learning and deep learning, I will present a series of recent works in which we have employed deep learning for applications in photonics and active matter. In particular, I will explain how we employed deep learning to enhance digital video microscopy, to estimate the properties of anomalous diffusion, to characterize microscopic force fields, to improve the calculation of optical forces, and to characterize nanoparticles. Finally, I will provide an outlook for the application of deep learning in photonics and active matter.