Thomas Speck

Dynamical clustering and phase separation in suspensions of self-propelled colloidal particles.

We study experimentally and numerically a (quasi-)two-dimensional colloidal suspension of self-propelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.

Thermodynamics of a colloidal particle in a time-dependent nonharmonic potential.

We study the motion of an overdamped colloidal particle in a time-dependent nonharmonic potential. We demonstrate the first lawlike balance between applied work, exchanged heat, and internal energy on the level of a single trajectory. The observed distribution of applied work is distinctly non-Gaussian in good agreement with numerical calculations. Both the Jarzynski relation and a detailed fluctuation theorem are verified with good accuracy.

Restoring a fluctuation-dissipation theorem in a nonequilibrium steady state

In a nonequilibrium steady state, the violation of the fluctuation-dissipation theorem (FDT) is connected to breaking detailed balance. For the velocity correlations of a driven colloidal particle we calculate an explicit expression of the FDT violation. The equilibrium form of the FDT can be restored by measuring the velocity with respect to the local mean velocity.

Fluctuation-dissipation theorem in nonequilibrium steady states

In equilibrium, the fluctuation-dissipation theorem (FDT) expresses the response of an observable to a small perturbation by a correlation function of this variable with another one that is conjugate to the perturbation with respect to energy. For a nonequilibrium steady state (NESS), the corresponding FDT is shown to involve in the correlation function a variable that is conjugate with respect to entropy. By splitting up entropy production into one of the system and one of the medium, it is shown that for systems with a genuine equilibrium state the FDT of the NESS differs from its equilibrium form by an additive term involving total entropy production. A related variant of the FDT not requiring explicit knowledge of the stationary state is particularly useful for coupled Langevin systems. The a priori surprising freedom apparently involved in different forms of the FDT in a NESS is clarified.

Ecology and Biomechanics : A Mechanical Approach to the Ecology of Animals and Plants

Tree Biomechanics and Growth Strategies in the Context of Forest Functional Ecology, M. Fournier, A. Stokes, C. Coutand, T. Fourcaud, and B. Moulia Diversity of Mechanical Architectures in Climbing Plants: An Ecological Perspective, N. Rowe, S. Isnard, F. Gallenmuller, and T. Speck The Role of Blade Buoyancy and Reconfiguration in the Mechanical Adaptation of the Southern Bullkelp Durvillaea, D.L. Harder, C.L. Stevens, T. Speck, and C.L. Hurd Murrays Law and the Vascular Architecture of Plants, K.A. McCulloh and J.S. Sperry Plant-Animal Mechanics and Bite Procurement in Grazing Ruminants, W.M. Griffiths Biomechanics of Salvia Flowers: The Role of Lever and Flower Tube in Specialization on Pollinators, M. Reith, R. Classen-Bockhoff, and T. Speck Do Plant Waxes Make Insect Attachment Structures Dirty? ExperimentalEvidences for the Contamination Hypothesis, E. Gorb and S. Gorb Ecology and Biomechanics of Slippery Wax Barriers and Wax Running to Macaranga - Ant Mutualisms, W. Federle and T. Bruening Nectar Feeding in Long-Proboscid Insects, B.J. Borrell and H.W. Krenn Biomechanics and Behavioral Mimicry in Insects, Y. Golding and R. Ennos Interindividual Variation in the Muscle Physiology of Vertebrate Ectotherms: Consequences for Behavioral and Ecological Performance, C.A. Navas, R.S. James, and R.S. Wilson Power Generation during Locomotion in Anolis Lizards: An Ecomorphological Approach, B. Vanhooydonck, P. Aerts, D.J. Irschick, and A. Herrel Implications of Microbial Motility on the Water Column Ecosystems, K. Christensen-Dalsgaard The Biomechanics of Ecological Speciation, J. Podos and A. Hendry

Microscopic theory for the phase separation of self-propelled repulsive disks

Motivated by recent experiments on colloidal suspensions, we study analytically and numerically a microscopic model for self-propelled particles lacking alignment interactions. In this model, even for purely repulsive interactions, a dynamical instability leading to phase separation has been reported. Starting from the many-body Smoluchowski equation, we develop a mean-field description based on a novel closure scheme and derive the effective hydrodynamic equations. We demonstrate that the microscopic origin of the instability is a force imbalance due to an anisotropic pair distribution leading to self-trapping. The phase diagram can be understood in terms of two quantities: a minimal drive and the force imbalance. At sufficiently high propulsion speeds there is a reentrance into the disordered fluid.

Effective Cahn-Hilliard Equation for the Phase Separation of Active Brownian Particles

The kinetic separation of repulsive active Brownian particles into a dense and a dilute phase is analyzed using a systematic coarse-graining strategy. We derive an effective Cahn-Hilliard equation on large length and time scales, which implies that the separation process can be mapped onto that of passive particles. A lower density threshold for clustering is found, and using our approach we demonstrate that clustering first proceeds via a hysteretic nucleation scenario and above a higher threshold changes into a spinodal-like instability. Our results are in agreement with particle-resolved computer simulations and can be verified in experiments of artificial or biological microswimmers.

Einstein Relation Generalized to Nonequilibrium

The Einstein relation connecting the diffusion constant and the mobility is violated beyond the linear response regime. For a colloidal particle driven along a periodic potential imposed by laser traps, we test the recent theoretical generalization of the Einstein relation to the nonequilibrium regime which involves an integral over measurable velocity correlation functions.

Measurement of Stochastic Entropy Production

Using fluorescence spectroscopy we directly measure entropy production of a single two-level system realized experimentally as an optically driven defect center in diamond. We exploit a recent suggestion to define entropy on the level of a single stochastic trajectory [Seifert, Phys. Rev. Lett. 95, 040602 (2005)10.1103/PhysRevLett.95.040602]. Entropy production can then be split into one of the system itself and one of the surrounding medium. We demonstrate that the total entropy production obeys various exact relations for finite time trajectories.

Integral fluctuation theorem for the housekeeping heat

The housekeeping heat Qhk is the dissipated heat necessary to maintain the violation of detailed balance in nonequilibrium steady states. By analysing the evolution of its probability distribution, we prove an integral fluctuation theorem exp[−βQhk] = 1 valid for arbitrary-driven transitions between steady states. We discuss Gaussian limiting cases and the difference between the new theorem and both the Hatano–Sasa and the Jarzynski relation.