Bruno Andreotti, Oliver Bäumchen, François Boulogne, Karen E. Daniels, Eric R. Dufresne, Hugo Perrin, Thomas Salez, Jacco H. Snoeijerhi, and Robert W. Style:
"Solid capillarity: when and how does surface tension deform soft solids?"
Soft Matter 12 (2016) 2993
[Journal URL], [BibTeX], [Abstract]

@article {andreotti-sm-16,
  title = {Solid capillarity: when and how does surface tension deform soft solids?},
  author = {Andreotti, Bruno AND B\"{a}umchen, Oliver AND Boulogne, Fran\c{c}ois AND E. Daniels, Karen AND Dufresne, Eric R. AND Perrin, Hugo AND Salez, Thomas AND Snoeijerhi, Jacco H. AND Style, Robert W. },
  journal = {Soft Matter},
  volume = {12},
  year = {2016},
  pages = {2993--2996},
  doi = {10.1039/C5SM03140K},
  publisher = {Royal Society of Chemistry},
}

Soft solids differ from stiff solids in an important way: their surface stresses can drive large deformations. Based on a topical workshop held in the Lorentz Center in Leiden, this Opinion highlights some recent advances in the growing field of solid capillarity and poses key questions for its advancement.

Sergej Püschel-Schlotthauer, Tillmann Stieger, Michael Melle, Marco G. Mazza, and Martin Schoen:
"Coarse-grained treatment of the self-assembly of colloids suspended in a nematic host phase"
Soft Matter 2 (2016) 469
[Journal URL], [BibTeX], [Abstract]

@article {pueschel-sm-2016,
  title = {Coarse-grained treatment of the self-assembly of colloids suspended in a nematic host phase},
  author = {P{\"u}schel-Schlotthauer, Sergej AND Stieger, Tillmann AND Melle, Michael AND Mazza, Marco G. AND Schoen, Martin},
  journal = {Soft Matter},
  volume = {2},
  year = {2015},
  pages = {469--480},
  DOI = {10.1039/C5SM01860A},
  publisher = {Royal Society of Chemistry},
}

The complex interplay of molecular scale effects, nonlinearities in the orientational field and long-range elastic forces makes liquid-crystal physics very challenging. A consistent way to extract information from the microscopic, molecular scale up to the meso- and macroscopic scale is still missing. Here, we develop a hybrid procedure that bridges this gap by combining extensive Monte Carlo (MC) simulations, a local Landau–de Gennes theory, classical density functional theory, and finite-size scaling theory. As a test case to demonstrate the power and validity of our novel approach we study the effective interaction among colloids with Boojum defect topology immersed in a nematic liquid crystal. In particular, at sufficiently small separations colloids attract each other if the angle between their center-of-mass distance vector and the far-field nematic director is about 30°. Using the effective potential in coarse-grained two-dimensional MC simulations we show that self-assembled structures formed by the colloids are in excellent agreement with experimental data.

Gal Schkolnik, Matthias Schmidt, Marco G. Mazza, Falk Harnisch, and Niculina Musat:
"In Situ Analysis of a Silver Nanoparticle-Precipitating Shewanella Biofilm by Surface Enhanced Confocal Raman Microscopy"
PLoS ONE 10 (2015) e0145871
[Journal URL], [BibTeX], [Abstract]

@article{schkolnik-plosone-2015,
  title = {In Situ Analysis of a Silver Nanoparticle-Precipitating Shewanella Biofilm by Surface Enhanced Confocal Raman Microscopy},
  author = {Schkolnik, Gal AND Schmidt, Matthias AND Mazza, Marco G. AND Harnisch, Falk AND Musat, Niculina},
  journal = {PLoS ONE},
  volume = {10},
  issue = {12},
  year = {2015},
  pages = {e0145871-1--e0145871-23},
  doi = {10.1371/journal.pone.0145871},
  publisher = {Public Library of Science},
}

Shewanella oneidensis MR-1 is an electroactive bacterium, capable of reducing extracellular insoluble electron acceptors, making it important for both nutrient cycling in nature and microbial electrochemical technologies, such as microbial fuel cells and microbial electrosynthesis. When allowed to anaerobically colonize an Ag/AgCl solid interface, S. oneidensis has precipitated silver nanoparticles (AgNp), thus providing the means for a surface enhanced confocal Raman microscopy (SECRaM) investigation of its biofilm. The result is the in-situ chemical mapping of the biofilm as it developed over time, where the distribution of cytochromes, reduced and oxidized flavins, polysaccharides and phosphate in the undisturbed biofilm is monitored. Utilizing AgNp bio-produced by the bacteria colonizing the Ag/AgCl interface, we could perform SECRaM while avoiding the use of a patterned or roughened support or the introduction of noble metal salts and reducing agents. This new method will allow a spatially and temporally resolved chemical investigation not only of Shewanella biofilms at an insoluble electron acceptor, but also of other noble metal nanoparticle-precipitating bacteria in laboratory cultures or in complex microbial communities in their natural habitats.

Marco Rivetti, Thomas Salez, Michael Benzaquen, Elie Raphaël, and Oliver Bäumchen:
"Universal contact-line dynamics at the nanoscale"
Soft Matter 11 (2015) 9247
[Journal URL], [BibTeX], [Abstract]

@article { rivetti-sm-2015,
  title = {Universal contact-line dynamics at the nanoscale},
  author = {Rivetti, Marco AND Salez, Thomas AND Benzaquen, Michael AND Rapha{\"e}l, Elie AND B{\"a}umchen, Oliver},
  journal = {Soft Matter},
  volume = {11},
  issue = {48},
  year = {2015},
  pages = {9247--9253},
  doi = {10.1039/C5SM01907A},
  publisher = {The Royal Society of Chemistry},
}

The relaxation dynamics of the contact angle between a viscous liquid and a smooth substrate is studied at the nanoscale. Through atomic force microscopy measurements of polystyrene nanostripes we simultaneously monitor both the temporal evolution of the liquid–air interface and the position of the contact line. The initial configuration exhibits high curvature gradients and a non-equilibrium contact angle that drive liquid flow. Both these conditions are relaxed to achieve the final state, leading to three successive regimes in time: (i) stationary contact line levelling; (ii) receding contact line dewetting; (iii) collapse of the two fronts. For the first regime, we reveal the existence of a self-similar evolution of the liquid interface, which is in excellent agreement with numerical calculations from a lubrication model. For different liquid viscosities and film thicknesses we provide evidence for a transition to dewetting featuring a universal critical contact angle and dimensionless time.

Andrés Córdoba, Tillmann Stieger, Marco G. Mazza, Martin Schoen and Juan J. de Pablo:
"Anisotropy and probe-medium interactions in the microrheology of nematic fluids"
J. Rheology 60 (2016) 75
[Journal URL], [BibTeX], [Abstract]

@article {stieger-molphys-2015,
  title = {Anisotropy and probe-medium interactions in the microrheology of nematic fluids},
  author = {Córdoba, Andrés AND Stieger, Tillmann AND Mazza, Marco G. AND Schoen, Martin AND de Pablo, Juan J.},
  journal = {Journal of Rheology},
  volume = {60},
  issue = {1},
  year = {2016},
  pages = {75--95},
  doi = {10.1122/1.4935849},
  publisher = {American Institute of Physics},
}

A theoretical formalism is presented to analyze and interpret microrheology experiments in anisotropic fluids with nematic order. The predictions of that approach are examined in the context of a simple coarse-grained molecular model which is simulated using nonequilibrium molecular dynamics calculations. The proposed formalism is used to study the effect of confinement, the type of anchoring at the probe-particle surface, and the strength of the nematic field on the rheological response functions obtained from probe-particle active microrheology. As expected, a stronger nematic field leads to increased anisotropy in the rheological response of the material. It is also found that the defect structures that arise around the probe particle, which are determined by the type of anchoring and the particle size, have a significant effect on the rheological response observed in microrheology simulations. Independent estimates of the bulk dynamic modulus of the model nematic fluid considered here are obtained from small-amplitude oscillatory shear simulations with Lees–Edwards boundary conditions. The results of simulations indicate that the dynamic modulus extracted from particle-probe microrheology is different from that obtained in the absence of the particle, but that the differences decrease as the size of the defect also decreases. Importantly, the results of the nematic microrheology theory proposed here are in much closer agreement with simulations than those from earlier formalisms conceived for isotropic fluids. As such, it is anticipated that the theoretical framework advanced in this study could provide a useful tool for interpretation of microrheology experiments in systems such as liquid crystals and confined macromolecular solutions or gels.

Laura Stricker and Jürgen Vollmer:
"Impact of microphysics on the growth of one-dimensional breath figures"
Physical Review E 92 (2015) 042406
[Journal URL], [BibTeX], [Abstract]

@article {stricker-pre-2015,
  title = {Impact of microphysics on the growth of one-dimensional breath figures},
  author = {Stricker, Laura AND Vollmer, J\{"u}rgen},
  journal = {Physical Review E},
  volume = {92},
  issue = {4},
  year = {2015},
  pages = {042406},
  doi = {10.1103/PhysRevE.92.042406},
  publisher = {American Physical Society},
}

Droplet patterns condensing on solid substrates (breath figures) tend to evolve into a self-similar regime, characterized by a bimodal droplet size distribution. The distributions comprise a bell-shaped peak of monodisperse large droplets and a broad range of smaller droplets. The size distribution of the latter follows a scaling law characterized by a nontrivial polydispersity exponent. We present here a numerical model for three-dimensional droplets on a one-dimensional substrate (fiber) that accounts for droplet nucleation, growth, and merging. The polydispersity exponent retrieved using this model is not universal. Rather it depends on the microscopic details of droplet nucleation and merging. In addition, its values consistently differ from the theoretical prediction by Blackman and Brochard [Phys. Rev. Lett. 84, 4409 (2000)]. Possible causes of this discrepancy are pointed out.

Tillmann Stieger, Sergej Püschel-Schlotthauer, Martin Schoen, and Marco G. Mazza:
"Flow-induced deformation of closed disclination lines near a spherical colloid immersed in a nematic host phase"
Mol. Phys. 0 (2015) 1
[Journal URL], [BibTeX], [Abstract]

@article {stieger-molphys-2015,
  title = {Flow-induced deformation of closed disclination lines near a spherical colloid immersed in a nematic host phase},
  author = {Stieger, Tillmann AND P{\"u}schel-Schlotthauer, Sergej AND Schoen, Martin AND Mazza, Marco G.},
  journal = {Molecular Physics},
  volume = {0},
  issue = {0},
  year = {2015},
  pages = {1--17},
  doi = {10.1080/00268976.2015.1096973},
  publisher = {Taylor & Francis},
}

We present nonequilibrium molecular dynamics simulations of a spherical colloidal particle with a chemically homogeneous surface immersed in a nematic liquid-crystal host phase. This setup is then placed between planar and atomically structured substrate surfaces that serve to fix the nematic far-field director . The substrates are separated by a sufficiently large distance such that they do not interfere directly with the environment of the colloid. Because of a mismatch between and the local homeotropic anchoring of molecules of the liquid crystal (i.e., mesogens) at the surface of the colloid circular defect (Saturn) rings ℓ arise if the host is in thermodynamic equilibrium (i.e., in the absence of flow). The size of these rings depends on the range of the mesogen-colloid interactions which we model via an attractive Yukawa potential. As Poiseuille flow is initiated, ℓ is deformed. The degree of deformation is analysed quantitatively in terms of characteristic geometric parameters fitted to suitable projections of ℓ. Our results suggest that smaller ℓ are shifted downstream while approximately maintaining their circular shape, whereas larger ones exhibit an elastic deformation in addition. We provide a simple geometric argument to predict the downstream shift of smaller, circular ℓs in excellent agreement with the simulation data over the range of steady-state flows considered.

Julie Murison, Robabeh Moosavi, Michael Schulz, Burkhard Schillinger, and Matthias Schröter:
"Neutron Tomography as a Tool To Study Immiscible Fluids in Porous Media without Chemical Dopants"
Energy Fuels 29 (2015) 6271
[Journal URL], [BibTeX], [Abstract]

@article {murison-ef-2015,
  title = {Wetting, spreading, and adsorption on randomly rough surfaces},
  author = {Murison, Julie AND Moosavi, Robabeh AND Schulz, Michael AND Schillinger, Burkhard AND Schr{\"o}ter, Matthias},
  journal = {Energy and Fuels},
  volume = {29},
  issue = {10},
  year = {2015},
  pages = {6271--6276},
  doi = {10.1140/epje/i2012-12043-8},
  publisher = {American Chemical Society},
}

We present the first study of fluid distribution inside porous media imaged by neutron tomography. We demonstrate that this technique has matured sufficiently to deliver pore level results. The major advantage of neutron tomography is the contrast mechanism of using deuterated phases. This allows high contrast imaging without the need to add large amounts of inorganic salts as dopants, required to achieve adequate contrast for X-ray tomography studies. Measurements were performed at the Antares beamline (MLZ, Garching) with a voxel size of 11.8 μm. We propose this technique as a useful tool for studying mutliphase phenomena in porous media where the results are known to depend on the salinty and species of ions present, such as low salinity water, surfactant, and polymer flooding.

Bernhard Altaner, Artur Wachtel, and Jürgen Vollmer:
"Fluctuating currents in stochastic thermodynamics. II. Energy conversion and nonequilibrium response in kinesin models"
Physical Review E 92 (2015) 042133
[Journal URL], [BibTeX], [Abstract], [ArXiv]

@article {altaner-pre-2015,
  title = {Fluctuating currents in stochastic thermodynamics. II. Energy conversion and nonequilibrium response in kinesin models},
  author = {Altaner, Bernhard AND Wachtel, Artur AND Vollmer, J\{"u}rgen},
  journal = {Physical Review E},
  volume = {92},
  issue = {4},
  year = {2015},
  pages = {042133},
  doi = {10.1103/PhysRevE.92.042133},
  publisher = {American Physical Society},
}

Unlike macroscopic engines, the molecular machinery of living cells is strongly affected by fluctuations. Stochastic thermodynamics uses Markovian jump processes to model the random transitions between the chemical and configurational states of these biological macromolecules. A recently developed theoretical framework A. Wachtel, J. Vollmer, and B. Altaner, Phys. Rev. E 92, 042132 (2015) provides a simple algorithm for the determination of macroscopic currents and correlation integrals of arbitrary fluctuating currents. Here we use it to discuss energy conversion and nonequilibrium response in different models for the molecular motor kinesin. Methodologically, our results demonstrate the effectiveness of the algorithm in dealing with parameter-dependent stochastic models. For the concrete biophysical problem our results reveal two interesting features in experimentally accessible parameter regions: the validity of a nonequilibrium Green-Kubo relation at mechanical stalling as well as a negative differential mobility for superstalling forces.

Artur Wachtel, Jürgen Vollmer, and Bernhard Altaner:
"Fluctuating currents in stochastic thermodynamics. I. Gauge invariance of asymptotic statistics"
Physical Review E 92 (2015) 042132
[Journal URL], [BibTeX], [Abstract]

@article {wachtel-pre-2015,
  title = {Fluctuating currents in stochastic thermodynamics. I. Gauge invariance of asymptotic statistics},
  author = {Wachtel, Artur AND Vollmer, J\{"u}rgen AND Altaner, Bernhard},
  journal = {Physical Review E},
  volume = {92},
  issue = {4},
  year = {2015},
  pages = {042132-1--042132-11},
  doi = {10.1103/PhysRevE.92.042132},
  publisher = {American Physical Society},
}

Stochastic thermodynamics uses Markovian jump processes to model random transitions between observable mesoscopic states. Physical currents are obtained from antisymmetric jump observables defined on the edges of the graph representing the network of states. The asymptotic statistics of such currents are characterized by scaled cumulants. In the present work, we use the algebraic and topological structure of Markovian models to prove a gauge invariance of the scaled cumulant-generating function. Exploiting this invariance yields an efficient algorithm for practical calculations of asymptotic averages and correlation integrals.We discuss how our approach generalizes the Schnakenberg decomposition of the average entropy-production rate, and how it unifies previous work. The application of our results to concrete models is presented in an accompanying publication.