| 1. |
- Bo, Stefano, et al.
(författare)
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Measurement of anomalous diffusion using recurrent neural networks
- 2019
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Ingår i: Physical Review E. - : American Physical Society. - 2470-0045 .- 2470-0053. ; 100:1
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Tidskriftsartikel (refereegranskat)abstract
- Anomalous diffusion occurs in many physical and biological phenomena, when the growth of the mean squared displacement (MSD) with time has an exponent different from one. We show that recurrent neural networks (RNNs) can efficiently characterize anomalous diffusion by determining the exponent from a single short trajectory, outperforming the standard estimation based on the MSD when the available data points are limited, as is often the case in experiments. Furthermore, the RNNs can handle more complex tasks where there are no standard approaches, such as determining the anomalous diffusion exponent from a trajectory sampled at irregular times, and estimating the switching time and anomalous diffusion exponents of an intermittent system that switches between different kinds of anomalous diffusion. We validate our method on experimental data obtained from subdiffusive colloids trapped in speckle light fields and superdiffusive microswimmers.
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| 2. |
- Bronte Ciriza, David, et al.
(författare)
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Optically Driven Janus Microengine with Full Orbital Motion Control
- 2023
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Ingår i: ACS PHOTONICS. - 2330-4022. ; 10:9, s. 3223-3232
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Tidskriftsartikel (refereegranskat)abstract
- Microengines have shown promise for a variety of applications in nanotechnology, microfluidics, and nanomedicine, including targeted drug delivery, microscale pumping, and environmental remediation. However, achieving precise control over their dynamics remains a significant challenge. In this study, we introduce a microengine that exploits both optical and thermal effects to achieve a high degree of controllability. We find that in the presence of a strongly focused light beam, a gold-silica Janus particle becomes confined at the stationary point where the optical and thermal forces balance. By using circularly polarized light, we can transfer angular momentum to the particle, breaking the symmetry between the two forces and resulting in a tangential force that drives directed orbital motion. We can simultaneously control the velocity and direction of rotation of the particle changing the ellipticity of the incoming light beam while tuning the radius of the orbit with laser power. Our experimental results are validated using a geometrical optics phenomenological model that considers the optical force, the absorption of optical power, and the resulting heating of the particle. The demonstrated enhanced flexibility in the control of microengines opens up new possibilities for their utilization in a wide range of applications, including microscale transport, sensing, and actuation.
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| 3. |
- Grauer, J., et al.
(författare)
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Active droploids
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- 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. Active matter can spontaneously form complex patterns and assemblies via a one-way energy flow from the environment into the system. Here, the authors demonstrate that a two-way coupling, where active particles act back on the environment can give rise to novel superstructures, named as active droploids.
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| 4. |
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| 5. |
- Schmidt, Falko, 1992
(författare)
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Active Matter in a Critical State: From passive building blocks to active molecules, engines, and active droplets
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The motion of microscopic objects is strongly affected by their surrounding environment. In quiescent liquids, motion is reduced to random fluctuations known as Brownian motion. Nevertheless, microorganisms have been able to develop mechanisms to generate active motion. This has inspired researchers to understand and artificially replicate active motion. Now, the field of active matter has developed into a multi-disciplinary field, with researchers developing artificial microswimmers, producing miniaturized versions of heat engines and showing that individual colloids self-assemble into larger microstructures. This thesis taps into the development of artificial microscopic and nanoscopic systems and demonstrates that passive building blocks such as colloids are transformed into active molecules, engines and active droplets that display a rich set of motions. This is achieved by combining optical manipulation with a phase-separating environment consisting of a critical binary mixture. I first show how simple absorbing particles are transformed into fast rotating microengines using optical tweezers, and how this principle can be scaled down to nanoscopic particles. Transitioning then from single particles to self-assembled modular swimmers, such colloidal molecules exhibit diverse behaviour such as propulsion, orbital rotation and spinning, and whose formation process I can control with periodic illumination. To characterize the molecules dynamics better, I introduce a machine-learning algorithm to determine the anomalous exponent of trajectories and to identify changes in a trajectory’s behaviour. Towards understanding the behaviour of larger microstructures, I then investigate the interaction of colloidal molecules with their phase-separating environment and observe a two-fold coupling between the induced liquid droplets and their immersed colloids. With the help of simulations I gain a better physical picture and can further analyse the molecules’ and droplets’ emergence and growth dynamics. At last, I show that fluctuation-induced forces can solve current limitations in microfabrication due to stiction, enabling a further development of the field towards smaller and more stable nanostructures required for nowadays adaptive functional materials. The insights gained from this research mark the path towards a new generation of design principles, e.g., for the construction of flexible micromotors, tunable micromembranes and drug delivery in health care applications.
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| 6. |
- Schmidt, Falko, et al.
(författare)
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Dynamics of an active Nanoparticle in an optical trap
- 2021
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Ingår i: Optics InfoBase Conference Papers. - 2162-2701.
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Konferensbidrag (refereegranskat)abstract
- 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.
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| 7. |
- Schmidt, Falko, 1992, et al.
(författare)
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Light-controlled assembly of active colloidal molecules
- 2019
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 150:9
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Tidskriftsartikel (refereegranskat)abstract
- Thanks to a constant energy input, active matter can self-assemble into phases with complex architectures and functionalities such as living clusters that dynamically form, reshape, and break-up, which are forbidden in equilibrium materials by the entropy maximization (or free energy minimization) principle. The challenge to control this active self-assembly has evoked widespread efforts typically hinging on engineering of the properties of individual motile constituents. Here, we provide a different route, where activity occurs as an emergent phenomenon only when individual building blocks bind together in a way that we control by laser light. Using experiments and simulations of two species of immotile microspheres, we exemplify this route by creating active molecules featuring a complex array of behaviors, becoming migrators, spinners, and rotators. The possibility to control the dynamics of active self-assembly via light-controllable nonreciprocal interactions will inspire new approaches to understand living matter and to design active materials. Published under license by AIP Publishing.
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| 8. |
- Schmidt, Falko, 1992, et al.
(författare)
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Microscopic Engine Powered by Critical Demixing
- 2018
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Ingår i: Physical Review Letters. - 0031-9007. ; 120:6
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally demonstrate a microscopic engine powered by the local reversible demixing of a critical mixture. We show that, when an absorbing microsphere is optically trapped by a focused laser beam in a subcritical mixture, it is set into rotation around the optical axis of the beam because of the emergence of diffusiophoretic propulsion. This behavior can be controlled by adjusting the optical power, the temperature, and the criticality of the mixture.
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| 9. |
- Schmidt, Falko, 1992, et al.
(författare)
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Non-equilibrium properties of an active nanoparticle in a harmonic potential
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11926
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Tidskriftsartikel (refereegranskat)abstract
- 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 solution.
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| 10. |
- Schmidt, Falko, 1992, et al.
(författare)
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Tunable critical Casimir forces counteract Casimir-Lifshitz attraction
- 2022
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 19, s. 271-8
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Tidskriftsartikel (refereegranskat)abstract
- Casimir forces are normally attractive and cause stiction, that is, static friction preventing surfaces in contact from starting to move. Now, a system exhibiting tunable repulsive critical Casimir forces, relevant for the development of micro- and nanodevices, is demonstrated. In developing micro- and nanodevices, stiction between their parts, that is, static friction preventing surfaces in contact from moving, is a well-known problem. It is caused by the finite-temperature analogue of the quantum electrodynamical Casimir-Lifshitz forces, which are normally attractive. Repulsive Casimir-Lifshitz forces have been realized experimentally, but their reliance on specialized materials severely limits their applicability and prevents their dynamic control. Here we demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to counteract stiction due to Casimir-Lifshitz forces and actively control microscopic and nanoscopic objects with nanometre precision. Our experiment is conducted on a microscopic gold flake suspended above a flat gold-coated substrate immersed in a critical binary liquid mixture. This may stimulate the development of micro- and nanodevices by preventing stiction as well as by providing active control and precise tunability of the forces acting between their constituent parts.
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