Symposium
Modern Simulation Approaches to Soft Matter
Friday, October 7, 2016
MPI for Polymer Research, Mainz, Germany
Hermann Staudinger Lecture Hall
08.45 – 09.00
Welcome by Managing Director, Paul Blom
09.00 – 09.30
Kurt Binder: “Nematic Order in Lyotropic Solutions of Semiflexible Polymers:
Simulation vs. Theory”
Kurt Binder, Sergei A. Egorov, Andrey Milchev, and Peter Virnau Mainz University
ABSTRACT: The nematic ordering of semiflexible polymers in solutions under good
solvent conditions is studied for a wide range of contour length, persistence length,
and monomer density, applying both Molecular Dynamics methods using large scale
GPU computing (up to 700 000 monomeric units) and density functional theory. It is
demonstrated that the scaled density of the isotropic to nematic transition depends
not only on the ratio of contour to persistence length, but there is also a nontrivial
dependence on the ratio of persistence length and chain diameter. The nematic
ordering can be described in terms of confinement of a chain by an effective cylinder,
oriented along the director. Each chain undergoes long range undulation fluctuations
(on the scale of the deflection length) in the confining cylinder. Both the cylinder
radius and the deflection length are related to the nematic order parameter and the
persistence length, but not to the contour length. Each cylinder can be shared by
many chains. Due to these long wavelength fluctuations, the nematic order
parameter increases with density considerably slower than density functional theory
would predict. An outlook on experiments also is given.
09.30 – 10.00
Philip Pincus: “Polyelectrolyte Brushes in the Presence of Multi-Valent Salts”
P. Pincus, UC Santa Barbara, Blair Brettman, The University of Chicago, Lei Liu and
Changbong Hyeo, Korea Institute for Advanced Study, Seoul
ABSTRACT: Both experiment and theory suggest that polyelectrolyte (PE) brushes are
strongly stretched by the osmotic pressure associated with the counterions that are
electrostatically bound within the brush volume. Experimentally this breaks down
when the neutralizing counterions are multi-valent. Indeed, as the mole fraction of
multi-valent counterions increases, the brush shrinks. Furthermore surface force
apparatus (SFA) observations of opposing brushes demonstrate a transition from
repulsive to attractive forces with increasing concentrations of multi-valent ions.
An attempt to rationalize this behavior in terms of salt bridges in addition to standard
PE brush behavior seems not to be viable. A more likely scenario suggested by
fluorescence labelled DNA brushes in the presence of spermidine and verified by
simulations is the development of in-plane heterogeneities. These surface micelles
are reminiscent of a neutral polymer brush in a poor solvent. That raises the question
of the mechanism: do the multi-valent ions effectively neutralize the brush leaving
hydrophobic monomers or rather is it driven by salt bridges.
10.00 – 10.30 Markus Deserno: “What Can We Learn about Lipid
Membranes by Stressing Them Out”
Markus Deserno, Carnegie Mellon University, Pittsburgh
ABSTRACT: Lipid membranes resist shape changes because they possess an intrinsic
curvature rigidity. From the earliest days of membrane science it has been known
that the associated bending modulus can be (and then has been) measured indirectly
by monitoring a membrane’s fluctuation spectrum. Hence, it is maybe quite surprising
that the more obvious method of actually deforming a membrane and measuring
how it resists is a much more recent invention. The most widely applicable
deformation that has been proposed is membrane buckling, and in this talk I will first
revisit how this permits a clean measurement of a membrane’s bending modulus.
However, it turns our that from buckling membranes we can learn a lot more than
just their rigidity. The possibility of accurately controlling a shape that has not just
curvature but curvature gradients enables access to (at least) two more very
interesting observables: first, the position of the pivotal plane; and second, the tilt
modulus of lipids. I will explain what these two other observables are, why they
matter, and how to extract them from a careful investigation of the lipid distribution
in buckled membranes.
10.30 – 11.00
Coffee Break
11.00 – 11.30
Ralf Everaers: “Simulating simple polymer models: From rubber elasticity to
chromosome structure”
Ralf Everaers, École Normale Supérieure de Lyon, CNRS, Lyon,
ABSTRACT: The Kremer-Grest bead-spring model is a near standard in Molecular
Dynamic simulations of universal properties, which emerge from the generic
ingredients of polymeric systems: connectivity, local liquid like monomer packing, and
the constraint that the backbones of chains cannot cross. To set the stage, I will
briefly review the role that the Kremer-Grest model played (and continues to play) in
validating fundamental concepts and approximations in the theoretical description of
the dynamics of entangled polymer liquids and networks. Then I will argue that the
predictive power of the approach will continue to increase with the available
computer (and to overtake theory) as models matching the coarse-grain microscopic
structure faithfully represent universal aspects of polymer behavior. In this spirit, we
have recently developed a one-parameter (Faller-Müller Plathe) Kremer-Grest “forcefield” for common polymer species.The second part of my talk will focus on the
complementary situation of “crumpled” polymers, whose conformations have to
respect global topological constraints enforcing the absence of knots and links. While
such systems are typically modeled as melts of non-concatenated ring polymers, the
most prominent representatives are probably (interphase) chromosomes, whose
reptation times of the order of centuries preclude the equilibration of the topological
state. I’ll sketch the present state of the fruitful interchange between theoretical
ideas and numerical approaches in the field and give a glimpse of our present
attempts to introduce more biological details into our simulations of drosophila
nuclei.
11.30 – 12.00
Davide Donadio: “Thermal energy transport and dissipation in complex
systems”
Davide Donadio, University of California Davis
ABSTRACT: We propose a new non-equilibrium molecular dynamics approach to
study vibrational energy relaxation in pump-probe spectroscopy, using the
generalized Langevin equation formalism.
A colored noise thermostat is used to selectively excite a set of vibrational modes,
leaving the other modes unperturbed, so to mimic the effect of a monochromatic
laser pump. Energy relaxation is probed by analyzing a subsequent dynamics in the
microcanonical ensemble, thus providing direct information of the typical timescales
and energy redistribution paths at the molecular scale.
The method is applied to hydrogen bonded molecular liquids, specifically partially
deuterated methanol and water.
12.00 – 12.30
Matthias Scheffler: “Strong Electron-Vibrational Coupling and Ab Initio
Theory of Heat Transport”
Matthias Scheffler, Fritz Haber Institute, Berlin
ABSTRACT: Thermal conductivity is a key characteristic of many materials,
particularly those used in the energy and environment sectors (thermoelectrics,
thermal-barrier coatings, catalysis, etc.). Despite its importance, the thermal
conductivity has been experimentally measured for only a tiny minority of substances.
Calculations of the thermal conductivity have only been possible, so far, by employing
severe approximations (e.g. perturbation theory based on the harmonic
approximation, or empirical (classical) force-field potentials). Recently, we developed
a first-principles formulation of the Green-Kubo method that allows the accurate
assessment of the non-radiative thermal conductivity of solid semiconductors and
insulators. It is based on the fluctuation-dissipation theorem and ab initio molecular
dynamics. It is valid for any temperature gradient achievable on earth. Accurate sizeand time-convergence are achieved with moderate computational effort. We
demonstrate the capabilities of the technique by investigating the thermal
conductivity of very-high and very-low heat-conductivity materials, namely diamond
Si and tetragonal ZrO2.
12.30 – 14:00
Lunch
14.00 – 14.30
Ludwik Leibler, ESPCI Paris: ”Vitrimers: five years after”
14.30 – 15.00
Christine Peter: “From the atomistic to the coarse grained level and back”
Christine Peter, Konstanz University
ABSTRACT: Bottom-up coarse graining techniques produce reduced-resolution
models that are tied to a higher-resolution description, usually with the idea of scalehopping in mind. Such multiscale approaches seem to be particularly appealing for
the investigation of problems that cannot easily be solved with either high- or lowresolution models alone - because they require both covering a wide range of lengthand time scales as well as retaining a high degree of chemical specificity.
One example are processes in neurodegenerative diseases where proteins undergo
transitions from intrinsically disordered to ordered conformational states upon
aggregation. Another example is biomineralization, where recent experimental
evidence suggests that processes, pathways, and intermediates are much more
complex than those described by simple theoretical models such as classical
nucleation theory.
In this talk, I will use biomolecular and biomaterials examples to illustrate some of of
the prevalent multiscaling challenges such as the representation of environmentinduced conformational transitions, of transitions between different ordered or
disordered phases, or the characterization of phase space sampling by models at
different levels of resolution.
15.00 – 15.30
Alexander Grosberg: „Casimir forces and tight knots“
Alexander Y Grosberg, New York University
ABSTRACT: Recent experiments and recent simulations seem to confirm a (relatively)
old idea that knots in a polymer chain self-tighten for entropic reasons, revealing
some interesting and subtle physics.
15.30 – 16.00
Coffee Break
16.00 – 16.30
Jean Francois Joanny: “Meiotic cytoplasmic streaming in C. Elegans
embryos”
Jean Francois Joanny, Institut Curie, Paris
ABSTRACT: Cytoplasmic streaming is a collective movement of the entire
cytoplasm observed in various types of cells. The mechanism of meiotic cytoplasmic
streaming (MeiCS) in the Caenorhabiditis elegans zygote is surprising as the flow
direction is not predetermined by cell polarity, and the direction occasionally reverses.
We show that the network structure of the endoplasmic reticulum (ER) is required
for the collective flow.
We propose a positive feedback mechanism, in which a local flow generated along a
microtubule is transmitted to neighboring regions through the ER and aligns
microtubules in a broader area to self-organize the collective flow.
Our
theoretical
model
reproduces
well
the
experimental
results
and predicts not only the emergence but also the reversal of the flow with a set of
experimentally realistic parameters.
We also discuss the contribution of meiotic streaming to cortical granule exocytosis.
The proposed model can account for different types of streaming, and thus provides
a general model for cytoplasmic streaming.
16.30 – 17.00
Friederike Schmid: ”Designing (smart) surfaces with polymer brushes”
Friederike Schmid, Mainz University
ABSTRACT: This talk is directed at people who have retained a secret love for
classical polymer physics based on the Edwards model. We have used self-consistent
field theory and Monte Carlo simulations of Edwards-type models to investigate a few
ideas how polymer brushes can be used to make smart surfaces, which change their
properties in response to external stimuli. In this context, we have also investigated
the influence of polydispersity. The simulations indicate that ''monodispersity''
corresponds in some sense to a multicritical state characterized by anomalously high
fluctuations, which can be reduced dramatically already by introducing small levels of
polydispersity.
17.00 – 17.30
Gary S. Grest: “Going up in time and length scales in modeling polymers”
Gary S. Grest, Sandia National Laboratories, Albuquerque
ABSTRACT: Polymer properties depend on a wide range of coupled length and time
scales, with unique macroscopic viscoelastic behavior stemming from interactions at
the atomistic level. The need to probe polymers across time and length scales and
particularly computational modeling is inherently challenging. Here new paths to
probing long time and length scales including introducing interactions into the
traditional bead-spring model that Kurt Kremer and I have worked on for the past
thirty years and coarse graining of atomistic simulations will be compared. Using
linear polyethylene as a model system, the degree of coarse graining with two to six
methylene groups per coarse-grained bead derived from a fully atomistic melt
simulation were probed. Using these models we were successful in probing highly
entangled melts and were able reach the long-time diffusive regime which is
computationally inaccessible using atomistic simulations. We simulated the
relaxation modulus and shear viscosity of well-entangled polyethylene melts for
scaled times of ~500 μs. Results for plateau modulus are in good agreement with
experiment. The long time and length scale is coupled to the macroscopic
viscoelasticity where the degree of coarse graining sets the minimum length scale
instrumental in defining polymer properties and dynamics. Results will be compared
to those obtained from the bead-spring model to demonstrate the additional insight
that can be gained from atomistically inspired coarse grained models.
Sandia National Laboratories is a multi-program laboratory managed and operated
by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation,
for the U.S. Department of Energy’s National Nuclear Security Administration under
contract DE-AC04-94AL85000.
17.30
Closing Remarks: Klaus Müllen
18.00
Informal Reception