Institut für Theoretische Physik
Fakultät für Physik und Geowissenschaften
Page: Soft Condensed Matter Theory Group - Research


Recent Highlights

What is swim pressure? -- Microscopic derivation of the hydrodynamics of active-Brownian-particle suspensions
in Phys. Rev. E 95, 052142 (2017)

by S. Steffenoni, G. Falasco & K. Kroy

PRE We derive the hydrodynamic equations of motion for a fluid of active particles described by underdamped Langevin equations that reduce to the active-Brownian-particle model, in the overdamped limit. The contraction into the hydrodynamic description is performed by locally averaging the particle dynamics with the nonequilibrium many-particle probability density, whose formal expression is found in the physically relevant limit of high friction through a multiple-time-scale analysis. This approach permits us to identify the conditions under which self-propulsion can be subsumed into the fluid stress tensor and thus to define systematically and unambiguously the local pressure of the active fluid.

Unlocking the Mysteries of Sandy "Megaripples"
Science blog about our recent paper in Phys. Rev. E 95 (2017) 022902

Polymer crumpling through molecular crowding
in Phys. Rev. Lett. 113 (2014) 238302

by S. Schoebl, S. Sturm, W. Janke & K. Kroy

PRL Molecular crowding in animal cells, and how it affects the fundamental processes of life, has been a much debated topic in the biophysical literature over the last decade but proved difficult to quantify. Now writing in Physical Review Letters, a collaboration of researchers in the Saxon Research Group FOR 877 reveals a convenient new way to overcome this difficulty. By a combination of computer simulations and theory, the study shows that snapshots of semiflexible polymers or fibers embedded into a crowded background are, on average, virtually indistinguishable from those of more flexible ones in free solution. This insight allows both to theoretically predict polymer conformations in crowded environments and to employ them as novel quantitative probes of molecular crowding.

Theory of rapid force spectroscopy
in Nature Communications 5 (2014) 4463

by J. T. Bullerjahn, S. Sturm & K. Kroy

Nature Communications In dynamic force spectroscopy, single (bio-)molecular bonds are actively broken to assess their range and strength. At low loading rates, the experimentally measured statistical distributions of rupture forces can be analysed using Kramers' theory of spontaneous unbinding. The essentially deterministic unbinding events induced by the extreme forces employed to speed up full-scale molecular simulations have been interpreted in mechanical terms, instead. Here we start from a rigorous probabilistic model of bond dynamics to develop a unified systematic theory that provides exact closed-form expressions for the rupture force distributions and mean unbinding forces, for slow and fast loading protocols. Comparing them with Brownian dynamics simulations, we find them to work well also at intermediate pulling forces. This renders them an ideal companion to Bayesian methods of data analysis, yielding an accurate tool for analysing and comparing force spectroscopy data from a wide range of experiments and simulations. (popular summary in German)

Rapid internal contraction boosts DNA friction
in Nature Communications 4 (2013) 1780

by O. Otto, S. Sturm, N. Laohakunakorn, U. F. Keyser & K. Kroy

Nature Communications Macroscopic objects are usually manipulated by force and observed with light. On the nanoscale, however, this is often done oppositely: individual macromolecules are manipulated by light and monitored with force. This procedure, which is the basis of single-molecule force spectroscopy, has led to much of our quantitative understanding of how DNA works, and is now routinely applied to explore molecular structure and interactions, DNA-protein reactions and protein folding. Here we develop the technique further by introducing a dynamic force spectroscopy set-up for a non-invasive inspection of the tension dynamics in a taut strand of DNA. The internal contraction after a sudden release of the molecule is shown to give rise to a drastically enhanced viscous friction, as revealed by the slow relaxation of an attached colloidal tracer. Our systematic theory explains the data quantitatively and provides a powerful tool for the rational design of new dynamic force spectroscopy assays.

Not-So-Recent Highlights

100th NJP video abstract

Resolving the Stiffening-Softening Paradox in Cell Mechanics
in PLoS ONE 7 (2012) e40063

by L. Wolff, P. Fernandez and K. Kroy

PLoS ONE Despite their notorious diversity, biological cells are mechanically well characterized by only a few robust and universal laws. Intriguingly, the law characterizing the nonlinear response to stretch appears self-contradictory. Various cell types have been reported to both stiffen and soften, or ``fluidize'' upon stretch. Within the classical paradigm of cells as viscoelastic bodies, this constitutes a paradox. Our measurements reveal that minimalistic reconstituted cytoskeletal networks (F-actin/HMM) exhibit a similarly peculiar response. A mathematical model of transiently crosslinked polymer networks, the so-called inelastic glassy wormlike chain (iGWLC) model, can simulate the data and resolve the apparent contradiction. It explains the observations in terms of two antagonistic physical mechanisms, the nonlinear viscoelastic resistance of biopolymers to stretch, and the breaking of weak transient bonds between them. Our results imply that the classical paradigm of cells as viscoelastic bodies has to be replaced by such an inelastic mechanical model.
See also the reduced schematic model: Phys. Rev. E 86 (2012) 040901(R)
Editor's suggestionHot Brownian Motion in PRL 105 (2010) 090604

by D. Rings, R. Schachoff, M. Selmke, F. Cichos, K. Kroy

teasing nanoparticles with lasers We derive the Markovian description for the non-equilibrium Brownian motion of a heated particle in a simple solvent with a temperature-dependent viscosity. Our analytical results for the generalized fluctuation-dissipation and Stokes-Einstein relations compare favorably with measurements of laser-heated gold nano-particles and provide a practical rational basis for emerging photothermal tracer and nano-particle trapping and tracking techniques.
Here are some related news items in English and German and at PHYSORG, and our video abstract of a related recent paper (credit for professional help with the production goes to K. Krauel, F. Meier and W. Scheible from the ZMK and to S. Auschra).
Tube Width Fluctuations in F-Actin Solutions in PRL 105 (2010) 037801
by J. Glaser, D. Chakraborty, K. Kroy, I. Lauter, M. Degawa, N. Kirchgeßner, B. Hoffmann, R. Merkel, M. Giesen

tube radius distribution We determine the statistics of the local tube width in F-actin solutions, beyond the usually reported mean value. Our experimental observations are explained by a segment fluid theory based on the binary collision approximation. In this systematic generalization of the standard mean-field approach, effective polymer segments interact via a potential representing the topological constraints. The analytically predicted universal tube width distribution with a stretched tail is in good agreement with the data.
Here is a related news item in English and German

Glass Transition and Rheological Redundancy in F-Actin Solutions
by C. Semmrich, T. Storz, J. Glaser, R. Merkel, A. R. Bausch, and K. Kroy

title page The unique mechanical performance of animal cells and tissues is attributed mostly to their internal biopolymer meshworks. Its perplexing universality and robustness against structural modifications by drugs and mutations is an enigma in cell biology and provides formidable challenges to materials science. Recent investigations could pinpoint highly universal patterns in the soft glassy rheology and nonlinear elasticity of cells and reconstituted networks. Here we report observations of a glass transition in semidilute F-actin solutions, which could hold the key to a unified explanation of these phenomena. Combining suitable rheological protocols with high-precision dynamic light scattering, we can establish a remarkable rheological redundancy and trace it back to a highly universal exponential stretching of the single-polymer relaxation spectrum of a ``glassy wormlike chain''. By exploiting the ensuing generalized time-temperature superposition principle, the time domain accessible to microrheometry can be extended by several orders of magnitude, thus opening promising new metrological opportunities.
Learn more about Wormlike and Glassy Wormlike Chains in this review paper

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