Non-equilibrium properties of an active nanoparticle in a harmonic potential on ArXiv

Non-spherical nanoparticle held by optical tweezers. The particle is trapped against the cover slide.

Non-equilibrium Properties of an Active Nanoparticle in a Harmonic Potential
Falko Schmidt, Hana Šípovà-Jungová, Mikael Käll, Alois Würger & Giovanni Volpe
arXiv: 2009.08393

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.

Dynamics of an active nanoparticle in an optical trap

Dynamics of an active nanoparticle in an optical trap
Falko Schmidt, Hana Šípová-Jungová, Mikael Käll, Alois Würger, Giovanni Volpe

Click here to see the slides.
Twitter Link: here.

Active matter systems in non-equilibrium conditions have recently gained great interest from many disciplines such as micro and nanomachines and in living organisms. Probing the dynamics of active Brownian particles (ABPs) under confinement such as found in biological systems gives insight into their non-equilibrium processes. Although previous studies [1-4] have shown the effect of confinement on ABPs on the microscale and macroscale investigating dynamics on the nanoscale remains challenging where thermal fluctuations typically prevail. Here, we are investigating experimentally and theoretically a nanoscopic particle in the harmonic potential of an optical trap and driven away from equilibrium by self-induced concentration gradients. We find that a nanoparticle performs fast orbital rotation at finite distance from the trap center and its probability density shifts from a Gaussian to a skewed distribution. Furthermore, we show that by transfer of spin angular momentum from the trapping beam the direction of the particle’s rotation can be controlled. We develop a theoretical model of this system which reveals that the driving mechanism of such fast rotation is the particle’s non-sphericity providing insight for the development of future nanoscopic engines.


[1] S. C. Takatori et al., Nat. Comm., 7, 10694 (2016)
[2] O. Dauchot & V. Démery, Phys. Rev. Lett., 122, 068002 (2019)
[3] A. Pototsky & H. Stark, EPL, 98, 5004 (2012)
[4] F. Schmidt et al., Phys. Rev. Lett., 120, 068004 (2018)

Poster Session
Time: June 22nd 2020
Place: Twitter

POM Conference
Time: June 25th 2020
Place: Online

Poster Slides

Falko Schmidt – POM Poster – Page 1
Falko Schmidt – POM Poster – Page 2
Falko Schmidt – POM Poster – Page 3
Falko Schmidt – POM Poster – Page 4

Soft Matter Lab presentations at the Photonics Online Meet-up, 22 June 2020

Six members of the Soft Matter Lab (Aykut Argun, Falko Schmidt, Laura Pérez-Garcia, Saga Helgadottir, Alessandro Magazzù, Daniel Midtvedt) were selected for poster presentations at the Photonics Online Meet-up (POM).

POM is an entirely free virtual conference. It aims to bring together a community of early career and established researchers from universities, industry, and government working in optics and photonics.

The meeting, at its second edition, will be held on June 25th 2020, 9-14.30 Central European Time. The virtual poster session will take place on June 22nd, on Twitter and virtual reality.

The poster contributions being presented are:

Aykut Argun
Enhanced force-field calibration via machine learning
Twitter Link: here.

Falko Schmidt
Dynamics of an active nanoparticle in an optical trap
Twitter Link: here.

Laura Pérez-García
Optical force field reconstruction using Brownian trajectories
Twitter Link: here.

Saga Helgadottir
DeepTrack: A comprehensive deep learning framework for digital microscopy
Twitter Link: here.

Alessandro Magazzù
Controlling the dynamics of colloidal particles by critical Casimir forces
Twitter Link: here.

Daniel Midtvedt
Holographic characterisation of subwavelength particles enhanced by deep learning
Twitter Link: here.

Link: Photonics Online Meet-up (POM)

Start-up “Lucero” Semi-finalist in SPIE Startup Challenge

Our idea Lucero, has reached the semi-final for the SPIE Start-up challenge, where will pitch in front of a jury at Photonics West in San Francisco, CA, USA on the 4th of February 2020.

Lucero will compete, among other 41 semifinalists, for cash prizes and business support.

In addition, Lucero was awarded one of the three Early Stage Entrepreneurship Travel Grants to attend the semi-final.

The start-up is aiming to make cutting-edge laser technology easy to use and available to anyone by combining it with commercial microscope. The product and software combo utilizes optical tweezers in a brand-new way – and bridges the gap between physics and other scientific fields that would greatly benefit from easier access to this tool.

In December, Lucero was ranked among the best 5 business ideas in West Sweden.

Team components: Christopher Jacklin, Rich Zapata Rosas, Felix Mossberg, Falko Schmidt, Alejandro Diaz Tormo and Martin Mojica-Benavides.

Links: Lucero Homepage

Start-up “Lucero Bio” among the best 5 business ideas in West Sweden

Falko Schmidt and other researchers at the University of Gothenburg, in collaboration with Business students at the Chalmers School of Entrepreneurship, have received early acclaimfor their Start-up idea “Lucero Bio”.

Lucero Bio was ranked among one of the top 5 business ideas in West Sweden by Venture Cup Sweden. Out of the 376 ideas that were submitted to the competition, nearly half came from the western region of Sweden.

The start-up is aiming to make cutting-edge laser technology easy to use and available to anyone by combining it with commercial microscope. The product and software combo utilizes optical tweezers in a brand-new way – and bridges the gap between physics and other scientific fields that would greatly benefit from easier access to this tool.

Team components: Christopher Jacklin, Rich Zapata Rosas, Felix Mossberg, Falko Schmidt, Alejandro Diaz Tormo and Martin Mojica-Benavides.

More information:
Press release, in Swedish.
Top 20 list of the 2019 winners, in Swedish.

Poster presentation by F. Schmidt at the Light at the Nanoscale conference, 5 December 2019

Light-induced phase separation power novel micro machines
Falko Schmidt, and Giovanni Volpe
Light at the Nanoscale Conference, Chalmers University, Gothenburg, Sweden
5 December 2019, 16:30-18:30

Focused laser light is used to trap a micronsized absorbing particle around its beam center where it performs constant orbital rotation. The light-induced local phase separations create a concentration gradient on which the particle moves along. Large patches of absorbing material on the particle’s surface give rise to a torque required for steady rotation1

Phase separation is a phenomena that commonly exists in nature, from the freezing of ice to the intrinsic mechanism of the cell to order matter. We are exploiting phase separations to produce new types of miniaturised machines, in particular micron and nano sized engines1as well as to form self-assembled colloidal molecules2. We control their behaviour using only light and varying its ambient temperature making this a simple tool to study complex matter3. This will enhance the development of future medicine where nano robots deliver drugs specifically to the local infection side.

References:1. F. Schmidt et al. Microscopic engine powered by critical demixingPhys Rev Lett 120, 068004, 2018

2. F. Schmidt et al. Light-controlled assembly of active colloidal molecules, J Chem Phys150, 094905, 2019

3. S. Bo et al. Measurement of anomalous diffusion using recurrent neural networksPhys Rev E 100, 010102(R), 2019

Anomalous Diffusion Measurement with Neural Networks published in Phys Rev E

Measurement of Anomalous Diffusion Using Recurrent Neural Networks

Measurement of Anomalous Diffusion Using Recurrent Neural Networks
Stefano Bo, Falko Schmidt, Ralf Eichborn & Giovanni Volpe
Physical Review E 100(1), 010102(R) (2019)
doi: 10.1103/PhysRevE.100.010102
arXiv: 1905.02038

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 (RNN) 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 RNN 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 sub-diffusive colloids trapped in speckle light fields and super-diffusive microswimmers.

Falko Schmidt attends the 69th Lindau Nobel laureate meeting

Picture from the open discussion with Steven Chu (Nobel Prize Physics 1997) on the left. 69th Lindau Nobel Laureate Meeting 02.07.2019 Photo/Credit: Patrick Kunkel/ Lindau Nobel Laureate Meetings Open Exhange
Picture of the boat ride to Mainau Island with Donna Strickland (Nobel Prize Physics 2018) on the left. 69th Lindau Nobel Laureate Meeting, 04.07.2019, Lindau, Germany
Picture/Credit: Julia Nimke/Lindau Nobel Laureate Meetings
Picture of the open discussion with David Gross (Nobel Prize Physics 2004) on the left. 69th Lindau Nobel Laureate Meeting 03.07.2019 Photo/Credit: Patrick Kunkel/ Lindau Nobel Laureate Meetings Open Exchange David J. Gross

Falko Schmidt, and Jalpa Soni have been selected to attain the 69th Lindau Nobel Laureate meeting in Lindau, Germany from the 30th June till 5th July 2019.

The Lindau meeting is a platform where 600 young scientists around the world meet former Nobel laureates (as well as Turing-award winners). There they can exchange scientific ideas and experiences, inspire each other and connect for a more interdisciplinary scientific community. These are the three incentives that make this meeting a unique experience.

Falko Schmidt had the privilege to attend it and shares the following insight:

“For me, the Lindau meeting was a unique experience where I was able to meet peers across many disciplines, share ideas and experiences beyond my field of active matter and received much feedback on career choices and daily life as a PhD. Especially fruitful were the many possibilities to engage with senior scientists such as the Nobel laureates which with their humour, insight and advice deepened my passion about science. Personally, I would consider my best encounters with Steven Chu and William Phillips (Nobel Prize in Physics in 1997 on laser cooling),  Donna Strickland (Nobel Prize in Physics in 2018 on ultra-fast lasers), and Stefan Hell (Nobel Prize in Chemistry in 2014 on super-resolution microscopy). I am very grateful for the possibility of attending this meeting and would like to thank the Lindau Nobel committee and Söderbergs Foundation who  were selecting and sponsoring me.
From now on, in times of struggle, I will always look back to this meeting and remember why we all love doing science.”