
Recent Highlights
What is swim pressure?  Microscopic derivation of the hydrodynamics of activeBrownianparticle suspensions
in Phys. Rev. E 95, 052142 (2017)
by S. Steffenoni, G. Falasco & K. Kroy
We derive the hydrodynamic equations of motion for a fluid of active particles described by underdamped Langevin equations that reduce to the activeBrownianparticle model, in the overdamped limit. The contraction into the hydrodynamic description is performed by locally averaging the particle dynamics with the nonequilibrium manyparticle probability density, whose formal expression is found in the physically relevant limit of high friction through a multipletimescale analysis. This approach permits us to identify the conditions under which selfpropulsion 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
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
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 fullscale 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 closedform 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
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 singlemolecule 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, DNAprotein
reactions and protein folding. Here we develop the technique further by
introducing a dynamic force spectroscopy setup for a noninvasive 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.
NotSoRecent Highlights
100th NJP video abstract
Resolving the StiffeningSoftening Paradox in Cell
Mechanics in PLoS ONE 7 (2012) e40063
by L. Wolff, P. Fernandez and K. Kroy
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 selfcontradictory. 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 (Factin/HMM) exhibit a similarly peculiar response. A mathematical model of transiently crosslinked polymer networks, the socalled 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)
Hot Brownian Motion in PRL 105 (2010) 090604
by D. Rings, R. Schachoff,
M. Selmke, F. Cichos, K. Kroy
We derive the Markovian description for the nonequilibrium Brownian
motion of a heated particle in a simple solvent with a
temperaturedependent viscosity. Our analytical results for the
generalized fluctuationdissipation and StokesEinstein relations
compare favorably with measurements of laserheated gold nanoparticles
and provide a practical rational basis for emerging photothermal tracer
and nanoparticle trapping and tracking techniques.
Here are some related news items in
English and German
and at
PHYSORG,
and our 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 FActin 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
We determine the statistics of the local tube width in Factin
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 meanfield 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 FActin Solutions
by C. Semmrich, T. Storz, J. Glaser, R. Merkel, A. R. Bausch, and K. Kroy
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 Factin solutions, which could hold the key to a unified
explanation of these phenomena. Combining suitable rheological
protocols with highprecision dynamic light scattering, we can
establish a remarkable rheological redundancy and trace it back to a
highly universal exponential stretching of the singlepolymer
relaxation spectrum of a ``glassy wormlike chain''. By exploiting
the ensuing generalized timetemperature 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
For free downloads of our publications please see condmat/
