Colloquium on artificial microswimmers by Frank Cichos, PJ Lecture Hall, 8 Nov 18

Information Controlled Structure Formation in Artificial Microswimmer Systems
General Physics Colloquium by Frank Cichos, University of Leipzig, Germany

Abstract: Self-organization is the generation of order out of local interactions in non-equilibrium. It is deeply connected to all fields of science from physics, chemistry to biology where functional living structures self-assemble and constantly evolve all based on physical interactions. The emergence of collective animal behavior, of society or language are the result of self-organization processes as well though they involve abstract interactions arising from sensory inputs, information processing, storage and feedback resulting in collective behaviors as found, for example, in crowds of people, flocks of birds, schools of fish or swarms of bacteria.
We introduce such information based interactions to the behavior of self-thermophoretic microswimmers. A real-time feedback of swimmer positions is used as the information to control the swimming direction and speed of other swimmers. The emerging structures reveal frustrated geometries due to confinement to two dimensions. They diffuse like passive clusters of colloids, but posses internal dynamical degrees of freedom that are determined by the feedback delay and the noise in the system. As the information processing in the feedback loops can be designed almost arbitrarily complex systems with mixed feedback delays and noise will give rise to new emergent dynamics of the self-organized structures. The presented control schemes further allow the integration of machine learning algorithms to introduce an adaptive behavior of swimmers.

Place: PJ Lecture Hall

Seminar on ripples in thin films by Mazi Jalaal from Twente, PJ Lecture Hall, 9 oct 18

Ripples in Thin Films
Seminar by Mazi Jalaal
from the Physics of Fluids laboratory
at the University of Twente, the Netherlands, EU

We present experimental observations of capillary ripples at the contact line of a droplet, spreading on a pre-wetted surface.
We use Digital Holographic Microscopy to measure the micro-scale undulation of the thin film. By raising the capillary number, the amplitude of the undulations increases at first and subsequently decreases.
At critical values of the capillary number, the ripples disappear. Using linear stability analysis, we further provide theoretical counterparts for the experimental observations, explaining the non-monotonic dependency on the capillary number

Place: PJ Lecture Hall
Time: 9 October, 2018, 11:00

Seminar on cell differentiation by Mariana Benitez Keinrad from UNAM, Soliden 3rd floor, 8 oct 18

Cell differentiation and pattern formation in the transition to multicellularity: lessons from the microbial world
Seminar by Mariana Benitez Keinrad
from the Laboratorio Nacional de Ciencias de la Sostenibilidad,
Universidad Nacional Autónoma de México (UNAM), Mexico.

Multicellular development occurs in plants, animals and other lineages, and involves the complex interaction among biochemical, physical and ecological factors. Our group has focused on the study of microbial multicellular organisms, which have been considered useful models to study the evolutionary transition to multicelullarity. I present some of our theoretical and experimental work, and discuss the physical and chemical processes that, in coordination with molecular regulatory networks, appear to be relevant for cell differentiation, patterning and morphogenesis in microbial aggregates.

Place: Soliden 3rd floor
Time: 8 October, 2018, 12:15

Colloquium on active matter by Hartmut Löwen, PJ Lecture Hall, 13 sep 18

Physics of active soft matter
General Physics Colloquium by Hartmut Löwen, Heinrich-Heine Universität Düsseldorf​, Germany

​Abstract: Ordinary materials are “passive” in the sense that their constituents are typically made by inert particles which are subjected to thermal fluctuations, internal interactions and external fields but do not move on their own. Living systems, like schools of fish, swarms of birds, pedestrians and swimming microbes are called “active matter” since they are composed of self-propelled constituents. Active matter is intrinsically in nonequilibrium and exhibits a plethora of novel phenomena as revealed by a recent combined effort
of statistical theory, computer simulation and real-space experiments. The colloquium talk provides an introduction into the physics of active matter focussing on biological and artificial microswimmers as key examples of active soft matter [1]. A number of single-particle and collective phenomena in active matter will be adressed ranging from the circle swimming to inertial delay effects.​​

​[1] For a review, see: C. Bechinger, R. di Leonardo, H. Löwen, C. Reichhardt, G. Volpe, G. Volpe, Active particles in complex and crowded environments, Reviews of Modern Physics 88, 045006 (2016).

Place: PJ Lecture Hall

Alejandro V. Arzola visits the Soft Matter Lab. Welcome!

Alejandro V. Arzola is a Visiting Professor from the Universidad Nacional Autónoma de México in Mexico City. His visiting position is financed through the Linnaeus Palme International Exchange Programme.

Alejandro was born in Oaxaca in the south of Mexico. He studied for a PhD at the Universidad Nacional Autónoma de México (UNAM) in Mexico City, worked as a posdoctoral researcher at the Institutte of Scientific Instruments in Brno, Czech Republic, and at UNAM. Since 2014 he joined the group of Optical Micromanipulation at the Institute of Physics in UNAM.

He is interested in optical micromanipulation and related research fields. His latest research deals with the transport of Brownian particles in optical landscapes under breaking space-time symmetries, a system which is known in the literature as ratchets. He is also interested in the behavior of microscopic particles in structured light fields with spin and orbital angular momentum.

Seminar on non-conservative optical forces in speckle fields by Laura Pérez García from UNAM, Faraday, 26 jun 18

Non conservative optical forces of speckle fields generated with a SLM
Seminar by Laura Pérez García from the Universidad National Autónoma de México (UNAM).

Speckle patterns arise when a highly coherent light source impinges on a rough surface or when it propagates through an inhomogeneous media. This phenomenon appeared after the invention of the laser in the 70’s and, initially was considered as a feature to avoid in optical setups since it limits the imaging resolution. However, speckle patterns can give information about the process that generates it and also can be incorporated by researchers in astronomy, surface characterization, biology, medicine and chemical processes [1, 2, 3]. In particular, speckle has been used in the last years in the area of optical micromanipulation to study the interaction of colloidal particles in random potentials[4, 5]. It is important the use of speckle patterns since it has a wide range of characteristic lengths, optical vortexes and intrinsic robustness to misalignment.

We’ve studied speckle patterns generated by a spatial light modulator (SLM), emphasizing in the intensity distribution, its spatial properties and the dynamical properties of particles subjected to these fields. Specifically, I studied the dynamical behavior of 1.54μm and 1μm spherical polystyrene particles embedded in deionized water in the presence of a speckle light field. We generated the speckle pattern using a 532 nm-wavelength laser which impinged on an SLM, which projected random values for each pixel, and then redirected to an optical micromanipulation system. It is important to mention that, by varying the optical resolution of the system with a diaphragm, we allowed the interference between all the wavefronts.

We analyzed the particle’s trajectories in the overdamped regime as an approximation for the particle dynamics. We didn’t assume the existence of a scalar potential, so we can study the nonconservative nature of the optical forces[6]. Additionally, the mean squared displacement was calculated and com- pared with free diffusion, we observed different regimes, owing to the spatial features in the speckle patterns used.

  1.  J.C. Dainty. Laser speckle and related phenomena. Topics in Applied Physics. Springer-Verlag, 1984.
  2.  J.W. Goodman. Speckle Phenomena in Optics: Theory and Applications. Roberts & Company, 2007.
  3. H.J. Rabal and R.A. Braga. Dynamic Laser Speckle and Applications. Optical Science and Engineering. CRC Press, 2008.
  4. Florian Evers, Christoph Zunke, Richard D L Hanes, J ̈org Bewerunge, Imad Ladadwa, Andreas Heuer, Stefan U. Egelhaaf, Giorgio Giovanni Volpe, Giorgio Giovanni Volpe, and Sylvain Gigan. Particle dynamics in two-dimensional random-energy landscapes: Experiments and simulations. Physical Review E – Statistical, Nonlinear, and Soft Matter Physics, 88(2):3936, 2014.
  5. Giorgio Volpe, Giovanni Volpe, and Sylvain Gigan. Brownian motion in a speckle light field: tunable anomalous diffusion and selective optical manipulation. Scientific Reports, 4:3936, 2014.
  6. Pinyu Wu, Rongxin Huang, Christian Tischer, Alexandr Jonas, and Ernst Ludwig Florin. Direct measurement of the nonconservative force field generated by optical tweezers. Physical Review Letters, 103(10):4–7, 2009.

Place: Faraday room, Fysik Origo, Fysik
Time: 26 June, 2018, 15:00

Seminar on Langevin equation in the small mass limit by Jan Wehr from the University of Arizona, Nexus, 21 Jun 18

Langevin equation in the small mass limit: higher order approximations
Seminar by Jan Wehr from the University of Arizona, Tucson (AZ), USA.

Abstract: We study the Langevin equation describing the motion of a particle of mass m in a potential and/or magnetic field, with state-dependent drift and diffusion.  We develop a hierarchy of approximate equations for the position degrees of freedom that achieve accuracy of order m^{k/2} over finite time intervals for any positive integer k.  This extends the previous work in which effective equations for the position variables were derived in the limit when the mass goes to zero.  The work was done jointly with Jeremiah Birrell.

Place: Nexus, meeting room, Fysik Origo, Fysik
Time: 21 June, 2018, 11:00

Viridiana Carmosa Sosa visits the Soft Matter Lab. Welcome!

Viridiana Carmosa Sosa studied her bachelor and master degree in Physics in the National Autonomous University of Mexico. In those years, she was working with optical tweezers, structured laser beams, and cavitation bubbles. Nowadays, she is a PhD student at Sapienza University of Rome under the supervision of Roberto Di Leonardo, where she uses two-photon polymerization to fabricate microstructures that allow her to study the dynamics of active and non-active matter at the micron scale.

She will spend a week at the Soft Matter Lab to work together with Alessandro Magazzù on a joint project.

Francesco Patti visits the Soft Matter Lab. Welcome!

Francesco Patti is a PhD student in Physics at the University of Messina (started in October 2017). His master’s degree thesis was about “Theoretical study of the interaction between E.M. radiation and chiral nanomaterials” (July 2017) and now he is a visiting student at the Soft Matter Lab where he will work on modeling of optical forces in liquids and vacuum as well as modelling of passive and active stochastic systems“ (June-July 2018).