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
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, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. This experimental demonstration can accelerate the development of micro- and nanodevices by preventing stiction as well as providing active control and precise tunability of the forces acting between their constituent parts.
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.
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.
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 . 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.
 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)
The article Active Droploids has been featured in a press release of the University of Gothenburg.
The study, published in Nature Communications, examines a special system of colloidal particles and demonstrates a new kind of active matter, which interacts with and modifies its environment. In the long run, the result of the study can be used for drug delivery inside the human body or to perform sensing of environmental pollutants and their clean-up.
Jens Grauer, Falko Schmidt, Jesús Pineda, Benjamin Midtvedt, Hartmut Löwen, Giovanni Volpe & Benno Liebchen
Nat. Commun. 12, 6005 (2021)
Active matter comprises self-driven units, such as bacteria and synthetic microswimmers, that can spontaneously form complex patterns and assemble into functional microdevices. These processes are possible thanks to the out-of-equilibrium nature of active-matter systems, fueled by a one-way free-energy flow from the environment into the system. Here, we take the next step in the evolution of active matter by realizing a two-way coupling between active particles and their environment, where active particles act back on the environment giving rise to the formation of superstructures. In experiments and simulations we observe that, under light-illumination, colloidal particles and their near-critical environment create mutually-coupled co-evolving structures. These structures unify in the form of active superstructures featuring a droplet shape and a colloidal engine inducing self-propulsion. We call them active droploids—a portmanteau of droplet and colloids. Our results provide a pathway to create active superstructures through environmental feedback.
The Soft Matter Lab is involved in six presentations at the OSA Biophotonic Congress: Optics in the Life Sciences 2021, topical meeting of Optical Manipulation and its Applications.
Moreover, three of the presentations were selected as finalists for the best student paper in the topical meeting of Optical Manipulation and its Applications.
Dynamics of an Active Nanoparticle in an Optical Trap Falko Schmidt, Hana Sipova-Jungova, Mikael Käll, Alois Würger, Giovanni Volpe
Submitted as OSA-OMA-2021, AF1D.2 Contribution Date: 16 April Time: 12:30 CEST
We investigate a nanoparticle inside an optical trap and driven away from equilibrium by self-induced concentration gradients. We find that a nanoparticle performs fast orbital rotations and its probability density shifting away from equilibrium.
Non-equilibrium properties of an active nanoparticle in a harmonic potential
Falko Schmidt, Hana Šípová-Jungová, Mikael Käll, Alois Würger & Giovanni Volpe
Nature Communications 12, 1902 (2021)
Active particles break out of thermodynamic equilibrium thanks to their directed motion, which leads to complex and interesting behaviors in the presence of confining potentials. When dealing with active nanoparticles, however, the overwhelming presence of rotational diffusion hinders directed motion, leading to an increase of their effective temperature, but otherwise masking the effects of self-propulsion. Here, we demonstrate an experimental system where an active nanoparticle immersed in a critical solution and held in an optical harmonic potential features far-from-equilibrium behavior beyond an increase of its effective temperature. When increasing the laser power, we observe a cross-over from a Boltzmann distribution to a non-equilibrium state, where the particle performs fast orbital rotations about the beam axis. These findings are rationalized by solving the Fokker-Planck equation for the particle’s position and orientation in terms of a moment expansion. The proposed self-propulsion mechanism results from the particle’s non-sphericity and the lower critical point of the solute.