One exemplar of the HEXBUGS used in the experiment. (Image by the Authors of the manuscript.)Active Matter Experiments with Toy Robots
Angelo Barona Balda, Aykut Argun, Agnese Callegari, Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 18 – 22 August 2024 Date: 22 August 2024 Time: 3:00 PM – 3:15 PM Place: Conv. Ctr. Room 6D
Active matter is based on concepts of nonequilibrium thermodynamics applied to the most diverse disciplines. Active Brownian particles, unlike their passive counterparts, self-propel and give rise to complex behaviors distinctive of active matter. As the field is relatively recent, active matter still lacks curricular inclusion. Here, we propose macroscopic experiments using Hexbugs, a commercial toy robot, demonstrating effects peculiar of active systems, such as the setting into motion of passive objects via active particles, the sorting of active particles based on their mobility and chirality. Additionally, we provide a demonstration of Casimir-like attraction between planar objects mediated by active particles.
Trajectory of a hexagonal cluster formed by a transparent particle (blu circle) and six light-absorbing particles (red circles) in a traveling sinusoidal optical pattern, in the absence of thermal noise. The direction of the motion of the optical pattern is given by the arrow. The trajectory’s duration is 30 s. (Image by A. Bergsten.)Chiral active molecules formation via non-reciprocal interactions
Agnese Callegari, Niphredil Klint, John Klint, Alfred Bergsten, Alex Lech, and Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 18 – 22 August 2024 Date: 19 August 2024 Time: 5:30 PM – 7:00 PM Place: Conv. Ctr. Exhibit Hall A
In 2019, Schmidt et al. demonstrated light-induced assembly of active colloidal molecules. They used two types of colloidal particles in a water-lutidine mixture: one transparent and one slightly absorbing light. In their experiment, this determined a non-reciprocal interaction between light-absorbing and transparent particles and promoted active molecule formation controlled by light. Beyond experimental details, we here explore the effects of this non-reciprocal interaction solely, showing its role in active molecule formation and self-propulsion. Simulation allows for the study of complex light profiles, enabling precise control over assembly and propulsion properties, relevant for targeted microscopic delivery.
Simplified sketch of the neural network used for the simulations of intracavity optical trapping. (Image by A. Callegari.)Neural networks for intracavity optical trapping
Agnese Callegari, Mathias Samuelsson, Antonio Ciarlo, Giuseppe Pesce, David Bronte Ciriza, Alessandro Magazzù, Onofrio M. Maragò, Antonio Sasso, and Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 18 – 22 August 2024 Date: 19 August 2024 Time: 5:30 PM – 7:00 PM Place: Conv. Ctr. Exhibit Hall A
Intracavity optical tweezers have been proven successful for trapping microscopic particles at very low average power intensity – much lower than the one in standard optical tweezers. This feature makes them particularly promising for the study of biological samples. The modelling of such systems, though, requires time-consuming numerical simulations that affect its usability and predictive power. With the help of machine learning, we can overcome the numerical bottleneck – the calculation of optical forces, torques, and losses – and reproduce, in simulation, the results in the literature and generalize to the case of counterpropagating-beams intracavity optical trapping.
Schematic of the scattering of a light ray on a Janus particle. (Image by A. Callegari.)Janus particles in a travelling optical landscape
Agnese Callegari, Giovanni Volpe
SPIE-OTOM, San Diego, CA, USA, 18 – 22 August 2024 Date: 19 August 2024 Time: 5:30 PM – 7:00 PM Place: Conv. Ctr. Exhibit Hall A
Janus particles possess dual properties that makes them very versatile for soft and active matter applications. Modeling their interaction with light, including optical force and torque, presents challenges. We present here a model of spherical, metal-coated Janus particles in the geometric optics approximation. Via an extension of the Optical Tweezers Geometrical Optics (OTGO) toolbox, we calculate optical forces, torques, and absorption. Through numerical simulation, we demonstrate control over Janus particle dynamics in traveling-wave optical landscapes by adjusting speed and periodicity.
Focused rays scattered by an ellipsoidal particles (left). Optical torque along y calculated in the x-y plane using ray scattering with a grid of 1600 rays (up, right) and using a trained neural network (down, right). (Image by the Authors of the manuscript.)Optical forces and torques in the geometrical optics approximation calculated with neural networks
David Bronte Ciriza, Alessandro Magazzù, Agnese Callegari, Gunther Barbosa, Antonio A. R. Neves, Maria Antonia Iatì, Giovanni Volpe, and Onofrio M. Maragò
SPIE-ETAI, San Diego, CA, USA, 18 – 22 August 2024 Date: 19 August 2024 Time: 1:55 PM – 2:10 PM Place: Conv. Ctr. Room 6D
Optical tweezers manipulate microscopic objects with light by exchanging momentum and angular momentum between particle and light, generating optical forces and torques. Understanding and predicting them is essential for designing and interpreting experiments. Here, we focus on geometrical optics and optical forces and torques in this regime, and we employ neural networks to calculate them. Using an optically trapped spherical particle as a benchmark, we show that neural networks are faster and more accurate than the calculation with geometrical optics. We demonstrate the effectiveness of our approach in studying the dynamics of systems that are computationally “hard” for traditional computation.
The Soft Matter Lab participates to the SPIE Optics+Photonics conference in San Diego, CA, USA, 18-22 August 2024, with the presentations listed below.
Agnese Callegari (for Alessandro Magazzù): Optically driven Janus microengine with full orbital motion control
20 August 2024 • 11:15 AM – 11:30 AM PDT | Conv. Ctr. Room 6D
Artist rendition of a disk-shaped microparticle trapped above a circular uncoated pattern within a thin gold layer coated on a glass surface. (Image by the Authors of the manuscript.)Our article, entitled Nanoalignment by Critical Casimir Torques, has been selected as a featured article by the editor at Nature Communications. This recognition highlights the significance of our research within the field of applied physics and mathematics.
The editors have included our work in their Editors’ Highlights webpage, which showcases the 50 best papers recently published in this area. You can view the feature on the Editors’ Highlights page (https://www.nature.com/ncomms/editorshighlights) as well as on the journal homepage (https://www.nature.com/ncomms/).
Screenshot from the Editors’ Highlights page of Nature Communications, dated 2 July 2024.
Artist rendition of a disk-shaped microparticle trapped above a circular uncoated pattern within a thin gold layer coated on a glass surface. (Image by the Authors of the manuscript.)Nanoalignment by Critical Casimir Torques
Gan Wang, Piotr Nowakowski, Nima Farahmand Bafi, Benjamin Midtvedt, Falko Schmidt, Agnese Callegari, Ruggero Verre, Mikael Käll, S. Dietrich, Svyatoslav Kondrat, Giovanni Volpe
Nature Communications, 15, 5086 (2024)
DOI: 10.1038/s41467-024-49220-1
arXiv: 2401.06260
The manipulation of microscopic objects requires precise and controllable forces and torques. Recent advances have led to the use of critical Casimir forces as a powerful tool, which can be finely tuned through the temperature of the environment and the chemical properties of the involved objects. For example, these forces have been used to self-organize ensembles of particles and to counteract stiction caused by Casimir-Liftshitz forces. However, until now, the potential of critical Casimir torques has been largely unexplored. Here, we demonstrate that critical Casimir torques can efficiently control the alignment of microscopic objects on nanopatterned substrates. We show experimentally and corroborate with theoretical calculations and Monte Carlo simulations that circular patterns on a substrate can stabilize the position and orientation of microscopic disks. By making the patterns elliptical, such microdisks can be subject to a torque which flips them upright while simultaneously allowing for more accurate control of the microdisk position. More complex patterns can selectively trap 2D-chiral particles and generate particle motion similar to non-equilibrium Brownian ratchets. These findings provide new opportunities for nanotechnological applications requiring precise positioning and orientation of microscopic objects.
An illustration of microscopic gold flakes on surface. (Image by F. Schmidt.)Tunable critical Casimir forces counteract Casimir–Lifshitz attraction
Falko Schmidt, Agnese Callegari, Abdallah Daddi-Moussa-Ider, Battulga Munkhbat, Ruggero Verre, Timur Shegai, Mikael Käll, Hartmut Löwen, Andrea Gambassi and Giovanni Volpe
SPIE-MNM, San Diego, CA, USA, 20 – 24 August 2023 Date: 23 August 2023
Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occurring 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 reliance on specialized materials prevents their dynamic control and thus limits their further applicability. 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 actively control microscopic and nanoscopic objects with nanometer precision. We demonstrate this by using critical Casimir forces to prevent the stiction caused by the Casimir–Lifshitz forces. 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 dominant Casimir–Lifshitz forces. Upon approaching the critical temperature, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. Our method provides a novel way of controlling the distances of micro- and nanostructures using tunable critical Casimir forces to counteract forces such as the Casimir–Lifshitz force, thereby preventing stiction and device failure. Due to the simplicity of our design the concept can be adapted to already existing MEMS and NEMS by, for example, controlling the temperature via light illumination.
Reference
Falko Schmidt, Agnese Callegari, Abdallah Daddi-Moussa-Ider, Battulga Munkhbat, Ruggero Verre, Timur Shegai, Mikael Käll, Hartmut Löwen, Andrea Gambassi and Giovanni Volpe, Tunable critical Casimir forces counteract Casimir-Lifshitz attraction, Nature Physics 19, 271-278 (2023)
Focused rays scattered by an ellipsoidal particles (left). Optical torque along y calculated in the x-y plane using ray scattering with a grid of 1600 rays (up, right) and using a trained neural network (down, right). (Image by the Authors of the manuscript.)Faster and More Accurate Geometrical-Optics Optical Force Calculation Using Neural Networks
David Bronte Ciriza, Alessandro Magazzù, Agnese Callegari, Gunther Barbosa, Antonio A. R. Neves, Maria Antonia Iatì, Giovanni Volpe, and Onofrio M. Maragò
SPIE-ETAI, San Diego, CA, USA, 20 – 24 August 2023 Date: 22 August 2023
Optical tweezers are an established and versatile tool for the optical trapping and manipulation of microscopic object using light. Modelling the interaction between particles and light, i.e., being able to calculate the optical force and torque the light exerts on the particle, is important to both understand the outcome of experiments and help designing the experimental setup to the obtain a certain outcome. Different modelling approximation and relative calculation techniques are employed depending on the size of the particle and the features of the trapping light. In this work, we will focus on the geometrical optics regime, which hold for particles whose size is significantly larger than the wavelength of the light. In this approximation, optical forces and torques are calculated by discretizing the trapping light beam into a set of rays. Each ray, impinging on the particle, is reflected and refracted multiple times and, in this scattering process, transfers momentum and angular momentum to the particle. However, the choice of the discretization, i.e., which and how many rays we use to represent a beam, sets a trade-off between calculation speed and accuracy. Here, we show that using neural networks allows overcoming this limitation, obtaining not only faster but also more accurate simulations. We demonstrate this using an optically trapped spherical particle for which we obtain an analytical solution to use as ground truth. Then, we exploit our neural networks method to study the dynamics of ellipsoidal particles in a double trap, a system that would be computationally impossible otherwise.
