Dynamics and statistics of microswimmers in turbulence
University of Turin
The results of theoretical and numerical studies of swimming microorganisms in a turbulent flow is presented. Motivated by application to phytoplankton in the oceans, the phenomena of preferential sampling and clustering, and their dependence on the swimmers characteristics, is discussed. Finally the possibility to observe these effects in the laboratory is examined.
To be announced
Univ. ESPCI-Sorbonne University Paris-Diderot
To be announced
Recognizing the Role of the Boundary
Joost de Graaf
Microorganisms and man-made chemical swimmers have seen intense study over the past two decades. However there remain many open questions concerning the way the latter group achieves self-propulsion. Many theoretical studies have focused on bulk descriptions of the swimming speed and flow fields to understand the experimental results. Yet, chemical swimmers are almost always observed near a wall. In this talk, I will show that the presence of a boundary must be taken into account when interpreting experimental results. The properties of a nearby wall strongly influence the speed and even direction of chemical swimming and it is this understanding that can reconcile many seemingly conflicting results in the literature.
Using light to understand and control active matter
R. Di Leonardo
Sapienza University of Rome
Dense suspensions of swimming bacteria display striking motions that appear extremely vivid when compared to the thermal agitation of colloidal particles of comparable size. These suspensions belong to a broader class of non-equilibrium systems that are now collectively referred to as active matter. Fundamental research in the physics of active matter investigates the basic principles governing non equilibrium phenomena such as self-propulsion, collective behavior and rectification. From a more engineering point of view, however, active particles could potentially provide the active "atoms" of a new class of smart materials with unique response characteristics. Using advanced 3D optical imaging, micromanipulation and microfabrication tools, we study complex phenomena in active matter using direct and quantitative methods. I will review our recent work in this direction, from the fluid and statistical mechanics of bacterial movements in structured environments to the use of genetically modified bacteria as propellers for micromachines or as a "living" paint that can be controlled by light.
Irreversibility in active matter systems: Fluctuation theorem and mutual information
NORDITA Nordic Institute for Theoretical Physics
Active particle systems consist of individual entities (“particles") which have the ability to perform motion by consuming energy from the environment and converting it into a self-propulsion drive. Examples are suspensions of biological microorganisms or artificial microswimmers, such as bacteria and colloidal particles with catalytic surfaces. We consider such a Brownian particle which, in addition to being in contact with a thermal bath, is driven by active fluctuations. These active fluctuations do not fulfill a fluctuation-dissipation relation and therefore play the role of a non-equilibrium environment. Our main goal is to develop a trajectory-wise thermodynamic description as a natural generalization of stochastic energetics and thermodynamics for passive Brownian motion in a purely thermal equilibrium environment. After giving a short introduction to active matter, we discuss the main modeling concepts (Langevin equation) and recapitulate some basic results from the stochastic thermodynamics of passive Brownian motion, in particular that the log-ratio of path probabilities for observing a certain particle trajectory forward in time versus observing its time-reserved twin trajectory quantifies the entropy production in the thermal environment. We then calculate this path probability ratio for active Brownian motion and derive a generalized "entropy production", which fulfills an integral fluctuation theorem. We show that those parts of this "entropy production", which are different from the usual dissipation of heat in the thermal environment, can be associated with the mutual information between the particle trajectory and the history of the non-equilibrium environment.
Micron-scale active matter: from experiments to theory and back
Sapienza University of Rome
In this talk I will review how a number of experiments on active micro-particles have led to new theoretical concepts and, conversely, how theory has inspired new experimental works on active
systems. In particular I will show experiments on biological and synthetic active particles (i.e. swimming bacteria and catalytic particles respectively) exhibiting phenomena which cannot take place in equilibrium. For example I will show how two repulsive micro-beads (surrounded by swimming bacteria) display a marked tendency to be in contact, oppositely to what would happen for repulsive particles in equilibrium . I will also show how these experimental results have inspired new a new theoretical framework, going beyond the concept of “effective tempertaure”, leading to a “generalization” of the Boltzmann distribution for active system . In the following I will focus on active rectification effects showing how a simple model of active particles predicts that asymmetric energy barriers can accumulate active particles into specific spatial regions, with no favourable energy level, in striking contrast with equilibrium . I will discuss how experiments have verified this prediction showing that swimming bacteria and asymmetric micro fabricated structures can transport colloidal beads into target structures working as a “micro-conveyor
belt” . Finally I will show how Janus particles or swimming bacteria can be used to power micro-gears in various experiments [5,6] and how, although qualitatively understood, these systems are still challenging to be described theoretically.
 L. Angelani, C. Maggi, M. L. Bernardini, et al. Phys. Rev. Lett., 107, 138302, (2011)
 C. Maggi, U. M. B. Marconi, N. Gnan et al. Sci. Rep., 5, 10742, (2015)
 N. Koumakis, C. Maggi, R. Di Leonardo, Soft Matter, 10, 5695-5701, (2014)
 N. Koumakis, A. Lepore, C. Maggi, et al. Nature Communications, 4, 2588, (2013)
 C. Maggi, J. Simmchen, F. Saglimbeni, et al. Small, 12, 446-451, (2016)
 G. Vizsnyiczai, G. Frangipane, C. Maggi et al. Nature Communications, 8, 15974, (2017)
Clausius relation for active particles
Sapienza University of Rome & CNR
Many kinds of active particles, such as bacteria or active colloids, move in a thermostatted fluid by means of self-propulsion. Energy injected by such a non-equilibrium force is eventually dissipated as heat in the thermostat. Since thermal fluctuations are much faster and weaker than self-propulsion forces, they are often neglected, blurring the identification of dissipated heat in theoretical models. For the same reason, some freedom —or arbitrariness— appears when defining entropy production. Recently three different recipes to define heat and entropy production have been proposed for the same model where the role of self-propulsion is played by a Gaussian coloured noise and a mapping is performed from overdamped to underdamped dynamics. Here we compare and discuss the relation between such proposals and their physical meaning. One of these proposals takes into account the heat exchanged with a non-equilibrium active bath: such an “active heat” satisfies the original Clausius relation and can be experimentally verified. I finally discuss a possible solution to such a controversy, where entropy production is directly computed at the level of the original overdamped dynamics, where there are no ambiguities.
L. Caprini, U. Marini Bettolo Marconi, A. Puglisi, A. Vulpiani. Comment on "Entropy Production and Fluctuation Theorems for Active Matter". Phys. Rev. Lett. 121, 139801 (2018)
A. Puglisi and U. Marini Bettolo Marconi Clausius relation for active particles: what can we learn from fluctuations? Entropy 19, 356 (2017)
Umberto Marini Bettolo Marconi, Andrea Puglisi, Claudio Maggi. Heat, temperature and Clausius inequality in a model for active brownian particles. Scientific Reports 7, 46496 (2017)
Experiments of active suspensions in flow
Department of Biomedical Sciences, Humanitas University, Milan
The vast majority of microorganisms are exposed to fluid flow, whether in natural environments, the human body, or artificial systems. Flow plays an important role in a broad variety of microbial processes, including nutrient uptake and fertilization, as well as in many industrial applications, ranging from wastewater treatment to the production of biofuels. However, despite the pervasive occurrence and implications of a fluid dynamic environment, its influence on the transport of swimming microorganisms remains poorly investigated and understood. In this talk, I will present experimental tools and results of about dilute and concentrated suspensions of microorganisms in flow. These observations encompass a rich set of dynamics that emerges from the interaction of microbial motility, morphology and fluid flow and that can affect the microscale distribution of microorganisms in the environment.
Nonlinear response and fluctuations of a driven active particle in simple model fluids
Univeristà della Campania L. Vanvitelli
We present some recent results on the dynamics of a driven tracer particle beyond the linear regime, in two different model fluids. The driving mechanism can be either an applied external force or an internal energy consumption. In the latter case the driven particle represents a prototypical example of an active particle with infinite persistence time. We first focus on a lattice gas model -- where the tracer interacts via hard-core repulsion with a crowding particle bath -- which allows for analytical computations. In this model, two surprising phenomena can occur: negative differential mobility, namely a nonmonotonic force-velocity relation, and enhanced diffusivity induced by the crowding interactions. Then, we consider the dynamics of a driven interial particle in a steady laminar flow. Here we can observe the phenomenon of absolute negative mobility, where the tracer velocity is opposite to the applied external force. In this framework, we also study the dynamics of an active particle with finite persistence time and discuss a generalized fluctuation dissipation relation, involving the correlation with a non-equilibrium extra-term.
Active Brownian Particles: Theory and Application
Institute of Physics JGU Mainz
Active Brownian particles (ABPs) are a simple model system that combines directed motion with repulsive inter-particle forces. After introducing the model, I will present two experiments on colloidal Janus particles to which the model is applied. Both show aggregation into clusters, one under uniform illumination and the other controlling particle motility individually. In the second part of the talk, I will discuss thermodynamic considerations that lead to a modification of ABPs in which the propulsion speed is not constant but depends on the inter-particle forces. This modification allows an unambiguous identification of heat and thus entropy production.
Computational modelling of active emulsions
Center for Life Nano Science (CLNS)
Active droplets are a remarkable example of system that shows autonomous motion sustained by an internal energy supply provided by an active material. This is usually made of self-propelled units (such as microtubule-motor suspensions) whose emerging orientational order at macroscopic scale can be captured by order parameters used to describe liquid crystals. Such droplets may provide a useful model for moving cell fragments as well as for biomimetic active materials, of potential interest, for instance, in drug delivery. Their coarse-grained dynamics can be described by convection-relaxation equations, usually solved by means of suitable numerical methods. After an introduction on the computational model adopted to study the physics of an active droplet, recent numerical studies, performed via lattice Boltzmann simulations, will be reviewed. The dynamics and the rheology of active emulsions in the presence of a surfactant will be also discussed.