Timo Betz, University of Münster, Germany
The physics of active motion in biological systems
Living biological systems are continuously reorganizing their structure to perform their function. The mechanical activity plays here an important role, as the constant generation of forces drives fluctuations as well as controlled motion of intracellular particles, membranes and even whole cells. From a physical point of view, this active motion drives the system far away from thermodynamic equilibrium, which can be measured as a violation of equilibrium quantities such as the fluctuation dissipation theorem. Quantifying the out-of-equilibrium components provides the possibility to model the active molecular processes. We measure the energy and the forces actively applied and model these with an active Langevin approach. On the scale of multicellular systems and tissue we are interested in collective motion which is still poorly understood. Here we use 3D tracking and hydrodynamic models to quantify and understand morphological changes ranging from tumor invasion up to embryonal development.
Raphael Wittkowski, University of Münster, Germany
Modeling the dynamics of active soft matter
Active soft matter contains constituents that are able to move autonomously. Through the energy dissipation of these "active" constituents, active soft matter is intrinsically out of thermodynamic equilibrium. As a consequence, many of the standard laws and methods from thermodynamics can typically not be applied to active soft matter. Instead, new theoretical methods that are applicable far from equilibrium are required. This lecture addresses multiscale methods that allow to derive models for active-soft-matter systems. The focus of the lecture is on field-theoretical methods for modeling the collective dynamics of suspensions of active colloidal particles.
K. John, Laboratoire Interdisciplinaire de Physique CNRS / Université Grenoble-Alpes
Derivation of continuous equations for an isotropic active elastic network
The standard active gel theory postulates phenomenological continuous equations for the density, stress, and local orientation fields on the basis of symmetry and linear irreversible thermodynamics arguments [1]. In most cases, the gels considered behave as liquids on long time scales, but 'solid' active gels, which do not flow at long time under an externally applied shear, have also been studied [2]. In this work, we derive the large-scale continuous description of an isotropic elastic network in the presence of force dipoles and momenta (a crude modeling of an actin network with molecular motors) which generate an active stress. The main results of this explicit coarse-graining procedure are two-fold. First, the derivation yields non-linear terms able to saturate the instability reported in linear active gel theory. Second, activity (i.e., the strength of force dipoles and momenta) not only generates new 'active' terms with respect to the passive case, but also 'renormalizes' the passive elastic properties of the medium. This change of the elastic properties leads for example to a shear instability for extensile force dipoles, while standard active gel theory yields an instability for contractile dipoles [2].
References
[1] K. Kruse, J.F. Joanny, F. Jülicher, J. Prost, and K. Sekimoto, Eur. Phys. J. E 16, 5 (2005).
[2] S. Banerjee and M.C. Marchetti, Soft Matter 7, 463 (2011).
C. Wagner, Department of Experimental Physics, Saarland University, Germany
3D tomography of blood flow
Red blood cells (RBCs) are very soft objects that can pass capillaries smaller than the cell’s diameter. Due to their high deformability, they couple strongly with the flow and can adopt many different shapes. For their quantitative characterization we developed a new confocal 3D imaging technique for fluorescent stained RBCs. Our approach allows us to recover the full 3D representation of moving RBCs under conditions prevailing in the micro-vasculature. As a key feature, we employ a micro-fluidic channel which is tilted by a small angle with respect to the objective. Image slices are then assembled to recover the volumetric representation of individual cells. We found two equilibrium cell shapes under certain flow condition: the so called 'slipper' and the 'croissant' shape. Numerical simulations are in good agreement with experimental observations [1]. In addition, high throughput data of classical 2-D microscopy combined with an adaptive neural network allow us to obtain the full phase diagram of red blood cell shapes as a function of the flow rate [2]. In larger channels, we use the confocal technique to characterize the margination of single rigidified RBCs in a suspension of healthy RBCs. Margination of e.g. white blood cells or platelets at the vessel walls is a haemodynamic key mechanism of our immune system. Our confocal observation technique allows us to characterize the distribution of hard vs. soft cells in full time and space resolution for the first time. Again numerical simulations are in good agreement although some quantitative differences remain that need further investigations.
Figure adapted from [1]: top: 3-D measurement of a flowing red blood cell at two different flow velocities. bottom: numerical simulation.
References
[1] S. Quint. et al. (2017). Applied Physics Letters, 111(10), 103701.
[2] A. Kihm et al. (2018). PLOS Computational Biology
Prof. Markus Bär, Physikalisch-Technische Bundesanstalt, Berlin
Pattern formation in active matter: Bacterial swarming and microswimmer suspensions
Active matter contain a large number of active agents which consume energy in order to move. Examples include bacterial swarms and suspensions of microswimmers. The talk surveys two representative examples for spatiotemporal self-organisation in active fluids: (i) formation of polar clusters and nematic bands [1-5] in the swarming of bacteria gliding on a surface and (ii) the so called “mesoscale turbulence” in dense suspensions of swimming bacteria. We show that many aspects of the swarming dynamics of bacteria on a surface are reproduced in agent-based [1-4] and continuum models of self-propelled rods [5]. Mesoscale turbulence in bacterial suspensions [6] is qualitatively modelled by agent-based models with self-propelled particles with competing alignment interactions [7, 8]. In a more quantitative approach bacterial turbulence is found to be caused by the interplay of active motion, local alignment of swimmers and long-range hydrodynamic interactions with the help of a continuum model derived from the dynamics of individual swimmers [9, 10].
[1] F. Peruani, A. Deutsch and M. Bär. Nonequilibrium clustering of self-propelled rods. Phys. Rev. E 74, 030904, 2006.
[2] F. Ginelli, F. Peruani, M. Bär and H. Chaté. Large-scale collective properties of self-propelled rods.Phys. Rev. Lett. 104, 184502, 2010.
[3] F. Peruani, J. Starruß, V. Jakovljevic, L. Søgaard-Andersen, A. Deutsch and M. Bär. Collective Motion and Nonequilibrium Cluster Formation in Colonies of Gliding Bacteria. Phys. Rev. Lett. 108, 098102, 2012.
[4] F. Peruani and M. Bär. A kinetic model and scaling properties of non-equilibrium clustering of self-propelled particles. New J. Phys. 15, 065009, 2013.
[5] R. Großmann, F. Peruani and M. Bär. Mesoscale pattern formation of self-propelled rodswith velocity reversal. Phys. Rev. E 94, 050602, 2016.
[6] J. Dunkel, S. Heidenreich, K. Drescher, H. H. Wensink, M. Bär and R. E. Goldstein. Fluid Dynamics of Bacterial Turbulence. Phys. Rev. Lett., 110 228102, 2013.
[7] R. Großmann, P. Romanczuk, M. Bär and L. Schimansky-Geier. Vortex arrays and mesoscale turbulence of self-propelled particles. Phys. Rev. Lett. 113, 258104, 2014.
[8] R. Großmann, P. Romanczuk, M. Bär and L. Schimansky-Geier. Pattern formation in active particle systems due to competing alignment interactions. Eur. Phys. J - Special Topics 224, 1325-1347, 2015.
[9] S. Heidenreich, J. Dunkel, H. Klapp and M. Bär. Hydrodynamic length-scale selection in microswimmer suspensions. Phys. Rev. E 94, 020601, 2016.
[10] H. Reinken, S. H. L. Klapp, M. Bär and S. Heidenreich. Derivation of a hydrodynamic theory for mesoscale dynamics in microswimmer suspensions. Phys. Rev. E 97, 022613, 2018.
Stefan Karpitschka, MPI Göttingen
Elastocapillarity - Mechanics at small length scales
Mechanical interactions of solids at micrometer length scales are ubiquitous in nature and play a major role in self-organization of cell tissues, in embryonic development, or in wound healing. However at small scales, the behavior of solids may deviate significantly from well-known macroscopic bulk mechanics. Similar to liquids, surface tension effects become dominant, giving rise to the emerging field of elastocapillary mechanics. In this lecture, I will introduce the fundamental concepts of elastocapillarity and demonstrate its significance in recent experimental work.
Chaouqi Misbah, Laboratoire Interdisciplinaire de Physique), Université Grenoble Alpes and CNRS
Pattern formation: wavelength selection, coarsening and front propagation
General questions on pattern formation will be discussed. The talk will be focused first on the longstanding puzzle of wavelength selection. Indeed, unlike equilibrium systems for which an optimization principle exist (e.g. maximum entropy for an inoculated system), there is no such principle in non equilibrium systems. Attempts to resolve this issues will be presented. One particular situation in non equilibrium systems is the fact that some systems exhibit (i) coarsening (increase of wavelength with time), (ii) no coarsening, I which the wavelength is fixed in time and (iii) interrupted coarsening (increase of wavelength with time until a crayon size and then the system ceases to coarsen. We will see that for some classes of nonlinear equations it is possible to show which scenario prevail without solving time-dependent equations. Finally, a discussion will be devoted to front propagation. Indeed, patterns can take place locally in space, and then invade the whole system via front propagation. We will see one major dilemma of front propagation in none-quilibruim systems in which the front velocity can be selected (from an infinite set of solutions) from a subtle mechanism. Example will be borrowed from physics, chemistry, biology, including living fluids and active matter.
Salima Rafai, CNRS Grenoble, France
Flowing Active Suspensions: Plankton as a model active particle
Suspensions of motile living organisms represent a non-equilibrium system of condensed matter of great interest from a fundamental point of view as well as for industrial applications. These are suspensions composed of autonomous units - active particles - capable of converting stored energy into motion. The interactions between the active particles and the liquid in which they swim give rise to mechanical constraints and a large-scale collective movement that have recently attracted a great deal of interest in the physical and mechanical communities. From the industrial point of view, microalgae are used in many applications ranging from the food industry to the development of new generations of biofuels. The biggest challenges in all these applications are the processes of separation, filtration and concentration of microalgae. There is therefore a real need for a better understanding of the flow of active material in order to ensure optimal control of these systems.
Our recent work on microalgae suspensions will be presented. The micro alga Chlamydomonas Reinhardtii uses its two anterior flagella to propel itself into aqueous media. It then produces a random walk with persistence that can be characterised quantitatively by analysing the trajectories produced. Moreover, in the presence of a light stimulus, it biases its trajectory to direct it towards the light: this phenomenon is called phototaxis. By coupling experiments and modelling, we propose to extract from the hydrodynamic characteristics of this microalga the generic properties of microswimmer suspensions. How does the swimming of an active particle couple with a flow? How is the dispersion of active particles affected by hydrodynamic interactions?
References
Matthieu Martin, Alexandre Barzyk, Eric Bertin, Philippe Peyla, Salima Rafaï. Photofocusing: Light and flow of phototactic microswimmer suspension. Physical Review E , 2016, 93 (5), pp.051101(R).
Xabel Garcia, Salima Rafaï, Philippe Peyla. Light Control of the Flow of Phototactic Microswimmer Suspensions. Physical Review Letters, 2013, 110, pp.138106.
Salima Rafaï, Levan Jibuti, Philippe Peyla. Effective Viscosity of Microswimmer Suspensions. Physical Review Letters, 2010, 104, pp.98102.