Die neuesten Veröffentlichungen

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.

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 (2015) Advance Article
[Journal URL], [BibTeX], [Abstract]

@article {pueschel-sm-2015,
  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 = {Advance Article},
  year = {2015},
  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.

Ian R. Jenkinson, Elisa Berdalet, Wie-Chun Chin, Stephan Herminghaus, Sophie Leterme, James G. Mitchell, Michael Orchard, Ri Qiu, Laurent Seuront, Peng Wang, Tim Wyatt and Li Zhuo:
"Micro- and nano-fluidics around HAB cells" in
A. Lincoln MacKenzie [Ed]: "Marine and Freshwater Harmful Algae".
Proceedings of the 16th International Conference on Harmful Algae, Wellington, New Zealand 27th-31st October 2014.
Cawthron Institute, Nelson, New Zealand and International Society for the Study of Harmful Algae
(ISSHA)
ISBN: 978-87-990827-5-9
[Book URL], [BibTeX], [Abstract]

@book {mackenzie-2015,
  booktitle = {Marine and Freshwater Harmful Algae},
  author = {MacKenzie, A. Lincoln},
  year = {2014},
  isbn = {978-87-990827-5-9},
  publisher = {Cawthron Institute, Nelson, New Zealand and International Society for the Study of Harmful Algae (ISSHA)},
}

Have you ever wondered how algae stay so clean? Most flowering-plant leaves also stay clean. Under air, films of water and "dirt" are repelled. Repulsion forces the water into droplets that easily roll off because these leaves are covered in hydrophobic nanometre (nm) to micrometre (µm) sized grooves and pillars, producing superhydrophobicity (SH) at the surface. Similarly, most algal cells bear a glycocalyx of organic fibrils that give surface structure, and are often hydrophobic. Glycocalyxes serve many functions, but whether they produce SH is poorly known. SH coatings are being developed to prevent fouling of ships and aquaculture structures without using toxins, so this technology could help understand how algae defeat fouling. Glycocalyxes are composed of exopolymeric secretions (EPS), and algae sometimes make the water more viscous using this tightly and more loosely bound EPS. EPS is also sometimes sticky. SH cuticles on copepods may change ambient fluid microdynamics by allowing slip at their surfaces, and facilitate filter feeding. By managing ambient viscosity and surface properties including slipping and sticking, algae may have the tools to engineer ambient fluidics and stay clean and unfouled.

Sabrina Haefner, Oliver Bäumchen, and Karin Jacobs:
"Capillary droplet propulsion on a fibre"
Soft Matter 11 (2015) 6921
[Journal URL], [BibTeX], [Abstract]

@article { hefner-sm-2015,
  title = {Capillary droplet propulsion on a fibre},
  author = {Haefner, Sabrina AND B{\"a}umchen, Oliver AND Jacobs, Karin},
  journal = {Soft Matter},
  volume = {11},
  issue = {35},
  year = {2015},
  pages = {6921--6926},
  doi = {10.1039/c5sm01228g},
  publisher = {The Royal Society of Chemistry},
}

A viscous liquid film coating a fibre becomes unstable and decays into droplets due to the Rayleigh–Plateau instability (RPI). Here, we report on the generation of uniform droplets on a hydrophobized fibre by taking advantage of this effect. In the late stages of liquid column breakup, a three-phase contact line can be formed at one side of the droplet by spontaneous rupture of the thinning film. The resulting capillary imbalance leads to droplet propulsion along the fibre. We study the dynamics and the dewetting speed of the droplet as a function of molecular weight as well as temperature and compare to a force balance model based on purely viscous dissipation.

Tilo Finger, Matthias Schröter, and Ralf Stannarius:
"The mechanism of long-term coarsening of granular mixtures in rotating drums"
New Journal of Physics 17 (2015) 093023
[Journal URL], [arXiv], [BibTeX], [Abstract]

@article {finger-njp-2015,
  title = {The mechanism of long-term coarsening of granular mixtures in rotating drums},
  author = {Finger, Tilo AND Schr{\"o}ter, Matthias AND Stannarius, Ralf},
  journal = {New Journal of Physics},
  volume = {17},
  issue = {9},
  year = {2015},
  pages = {093023-1--093023-10},
  doi = {10.1088/1367-2630/17/9/093023},
  publisher = {IOP Publishing Ltd and Deutsche Physikalische Gesellschaft},
}

Three fundamental segregation and pattern formation processes are known in granular mixtures in a rotating cylindrical drum: radial segregation, axial banding, and coarsening of the band pattern. While the mechanism for the first effect is well understood and for the second effect, several models have been proposed, the long-term coarsening mechanism remained unexplained so far. We demonstrate that the unidirectional flow between the bands in an axially segregated pattern is driven by small differences in size of the small beads at the band edges. Due to a process of microsegregation inside each band of small particles, which was so far unrecognized, this difference in diameter will be effective in all experiments with polydisperse beads. In consequence the stability of individual bands can be easily controlled by minor alterations of their composition. Our results make evident that a new mechanism as the driving force behind the axial particle flow has to be sought. We suggest possible hypotheses for such a mechanism.

Pree-Cha Kiatkirakajorn and Lucas Goehring:
"Formation of Shear Bands in Drying Colloidal Dispersions"
Phys. Rev. Lett. 115 (2015) 088302
[Journal URL], [BibTeX], [Abstract]

@article { kiatkirakajorn-prl-2015,
  title = {Formation of Shear Bands in Drying Colloidal Dispersions},
  author = {Kiatkirakajorn, Pree-Cha AND Goehring, Lucas},
  journal = {Physical Review Letters},
  volume = {115},
  issue = {8},
  year = {2015},
  pages = {088302-1--088302-5},
  doi = {10.1103/PhysRevLett.115.088302},
  publisher = {American Physical Society}
}

In directionally dried colloidal dispersions regular bands can appear behind the drying front, inclined at +/-45° to the drying line. Although these features have been noted to share visual similarities with shear bands in metal, no physical mechanism for their formation has ever been suggested, until very recently. Here, through microscopy of silica and polystyrene dispersions, dried in Hele-Shaw cells, we demonstrate that the bands are indeed associated with local shear strains.We further show how the bands form, that they scale with the thickness of the drying layer, and that they are eliminated by the addition of salt to the drying dispersions. Finally, we reveal the origins of these bands in the compressive forces associated with drying, and show how they affect the optical properties (birefringence) of colloidal films and coatings.

Maria Ekimova, Wilson Quevedo, Manfred Faubel, Philippe Wernet, and Erik T. J. Nibbering:
"A liquid flatjet system for solution phase soft-x-ray spectroscopy"
Structural Dynamics 2 (2015) 054301
[Journal URL], [BibTeX], [Abstract]

@article { ekimova-strdyn-2015,
  title = {A liquid flatjet system for solution phase soft-x-ray spectroscopy},
  author = {Ekimova, Maria AND Quevedo, Wilson AND Faubel, Manfred AND Wernet, Philippe AND Nibbering, Erik T. J.},
  journal = {Structural Dynamics},
  volume = {2},
  issue = {5},
  year = {2015},
  pages = {054301-1 -- 054301-13},
  doi = {10.1063/1.4928715},
  publisher = {AIP Publishing LLC},
}

We present a liquid flatjet system for solution phase soft-x-ray spectroscopy. The flatjet set-up utilises the phenomenon of formation of stable liquid sheets upon collision of two identical laminar jets. Colliding the two single water jets, coming out of the nozzles with 50 μm orifices, under an impact angle of 48° leads to double sheet formation, of which the first sheet is 4.6 mm long and 1.0 mm wide. The liquid flatjet operates fully functional under vacuum conditions (<10−3 mbar), allowing soft-x-ray spectroscopy of aqueous solutions in transmission mode. We analyse the liquid water flatjet thickness under atmospheric pressure using interferomeric or mid-infrared transmission measurements and under vacuum conditions by measuring the absorbance of the O K-edge of water in transmission, and comparing our results with previously published data obtained with standing cells with Si3N4 membrane windows. The thickness of the first liquid sheet is found to vary between 1.4–3 μm, depending on the transverse and longitudinal position in the liquid sheet. We observe that the derived thickness is of similar magnitude under 1 bar and under vacuum conditions. A catcher unit facilitates the recycling of the solutions, allowing measurements on small sample volumes (∼10 ml). We demonstrate the applicability of this approach by presenting measurements on the N K-edge of aqueous NH4+. Our results suggest the high potential of using liquid flatjets in steady-state and time-resolved studies in the soft-x-ray regime.

Ulf D. Schiller, Jean-Baptiste Fleury, Ralf Seemann and Gerhard Gompper:
"Collective waves in dense and confined microfluidic droplet arrays"
Soft Matter 11 (2015) 5850
[Journal URL], [BibTeX], [Abstract]

@article {schiller-sm-2015,
  title = {Collective waves in dense and confined microfluidic droplet arrays},
  author = {Schiller, Ulf D. AND Fleury, Jean-Baptiste AND Seemann, Ralf AND Gompper, Gerhard},
  journal = {Soft Matter},
  volume = {11},
  issue = {29},
  year = {2015},
  pages = {5850--5861},
  doi = {10.1039/c5sm01116g},
  publisher = {Royal Society of Chemistry},
}

Excitation mechanisms for collective waves in confined dense one-dimensional microfluidic droplet arrays are investigated by experiments and computer simulations. We demonstrate that distinct modes can be excited by creating specific ‘defect’ patterns in flowing droplet trains. Excited longitudinal modes exhibit a short-lived cascade of pairs of laterally displacing droplets. Transversely excited modes obey the dispersion relation of microfluidic phonons and induce a coupling between longitudinal and transverse modes, whose origin is the hydrodynamic interaction of the droplets with the confining walls. Moreover, we investigate the long-time behaviour of the oscillations and discuss possible mechanisms for the onset of instabilities. Our findings demonstrate that the collective dynamics of microfluidic droplet ensembles can be studied particularly well in dense and confined systems. Experimentally, the ability to control microfluidic droplets may allow the modulation of the refractive index of optofluidic crystals, which is a promising approach for the production of dynamically programmable metamaterials.

Fabian M. Schaller, Sebastian C. Kapfer, James E. Hilton, Paul W. Cleary, Klaus Mecke, Cristiano De Michele, Tanja Schilling, Mohammad Saadatfar, Matthias Schröter, Gary W. Delaney, Gerd E. Schröder-Turk:
"Non-universal Voronoi cell shapes in amorphous ellipsoid packs"
EPL 111 (2015) 24002
[Journal URL], [arXiv], [BibTeX], [Abstract]

@article {schaller-epl-2015,
  title = {Non-universal Voronoi cell shapes in amorphous ellipsoid packs},
  author = {Schaller, Fabian M. AND Kapfer, Sebastian C. AND Hilton, James E. AND Cleary, Paul W. AND Mecke, Klaus AND De Michele, Cristiano AND Schilling, Tanja AND Saadatfar, Mohammad AND Schr{\"o}ter, Matthias AND Delaney, Gary W. AND Schr{\"o}der-Turk, Gerd E.},
  journal = {Europhysics Letters},
  volume = {111},
  year = {2015},
  pages = {24002p1 -- 24002p6},
  publisher = {EPLA},
}

In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios α and global packing fractions φg. We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions φl, defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index β = β2,00 , points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape β of all cells with a given value of φl is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true.

Mark Ilton, Oliver Bäumchen, and Kari Dalnoki-Veress:
"Onset of Area-Dependent Dissipation in Droplet Spreading"
Phys. Rev. Lett. 115 (2015) 046103
[Journal URL], [BibTeX], [Abstract]

@article { ilton-prl-2015,
  title = {Onset of Area-Dependent Dissipation in Droplet Spreading},
  author = {Ilton, Mark AND B{\"a}umchen, Oliver AND Dalnoki-Veress, Kari},
  journal = {Physical Review Letters},
  volume = {115},
  issue = {4},
  year = {2015},
  pages = {046103-1--046103-5},
  doi = {10.1103/PhysRevLett.115.046103},
  publisher = {American Physical Society}
}

We probe the viscous relaxation of structured liquid droplets in the partial wetting regime using a diblock copolymer system. The relaxation time of the droplets is measured after a step change in temperature as a function of three tunable parameters: droplet size, equilibrium contact angle, and the viscosity of the fluid. Contrary to what is typically observed, the late-stage relaxation time does not scale with the radius of the droplet—rather, relaxation scales with the radius squared. Thus, the energy dissipation depends on the contact area of the droplet, rather than the contact line.

Sabrina Haefner, Michael Benzaquen, Oliver Bäumchen, Thomas Salez, Robert Peters, Joshua D. McGraw, Karin Jacobs, Elie Raphaël, and Kari Dalnoki-Veress:
"Influence of slip on the Plateau-Rayleigh instability on a fibre"
Nat. Commun. 6 (2015) 7409
[Journal URL], [BibTeX], [Abstract]

@article { hefner-natcom-2015,
  title = {Influence of slip on the Plateau-Rayleigh instability on a fibre},
  author = {Haefner, Sabrina AND Benzaquen, Michael AND B{\"a}umchen, Oliver AND Salez,Thomas AND Peters, Robert AND McGraw, Joshua D. AND Jacobs, Karin AND Rapha{\"e}l, Elie AND Dalnoki-Veress, Kari},
  journal = {Nature Communications},
  volume = {6},
  year = {2015},
  pages = {7409-1--7409-6},
  doi = {10.1038/ncomms8409},
  publisher = {Nature Publishing Group},
}

The Plateau-Rayleigh instability of a liquid column underlies a variety of fascinating phenomena that can be observed in everyday life. In contrast to the case of a free liquid cylinder, describing the evolution of a liquid layer on a solid fibre requires consideration of the solid–liquid interface. Here we revisit the Plateau-Rayleigh instability of a liquid coating a fibre by varying the hydrodynamic boundary condition at the fibre-liquid interface, from no slip to slip. Although the wavelength is not sensitive to the solid–liquid interface, we find that the growth rate of the undulations strongly depends on the hydrodynamic boundary condition. The experiments are in excellent agreement with a new thin-film theory incorporating slip, thus providing an original, quantitative and robust tool to measure slip lengths.

Riëlle de Ruiter, Ciro Semprebon, Mathijs van Gorcum, Michèl H. G. Duits, Martin Brinkmann, and Frieder Mugele:
"Stability Limits of Capillary Bridges: How Contact Angle Hysteresis Affects Morphology Transitions of Liquid Microstructures"
Phys. Rev. Lett. 114 (2015) 234501
[Journal URL], [BibTeX], [Abstract]

@article { ruiter-prl-2015,
  title = {Stability Limits of Capillary Bridges: How Contact Angle Hysteresis Affects Morphology Transitions of Liquid Microstructures},
  author = {de Ruiter, Ri{\"e}lle AND Semprebon, Ciro AND van Gorcum, Mathijs AND Duits, Mich{\`e}l H. G. AND Brinkmann, Martin AND Mugele, Frieder},
  journal = {Physical Review Letters},
  volume = {114},
  year = {2015},
  pages = {234501-1--234501-5},
  doi = {10.1103/PhysRevLett.114.234501},
  publisher = {American Physical Society}
}

The equilibrium shape of a drop in contact with solid surfaces can undergo continuous or discontinuous transitions upon changes in either drop volume or surface energies. In many instances, such transitions involve the motion of the three-phase contact line and are thus sensitive to contact angle hysteresis. Using a combination of electrowetting-based experiments and numerical calculations, we demonstrate for a generic sphere-plate confinement geometry how contact angle hysteresis affects the mechanical stability of competing axisymmetric and nonaxisymmetric drop conformations and qualitatively changes the character of transitions between them.

Seyed Habibolla Ebrahimnazhad Rahbari, Martin Brinkmann and Jürgen Vollmer:
"Arrest stress of uniformly sheared wet granular matter"
Physical Review E 91 (2015) 062201
[Journal URL], [BibTeX], [Abstract]

@article {Rahbari-pre-2015,
  title = {Arrest stress of uniformly sheared wet granular matter},
  author = {Ebrahimnazhad Rahbari, Seyed Habibolla AND Brinkmann, Martin AND Vollmer, J\{"u}rgen},
  journal = {Physical Review E},
  volume = {91},
  issue = {},
  year = {2015},
  pages = {062201-1 -- 062201-6},
  doi = {10.1103/PhysRevE.91.062201},
  publisher = {American Physical Society},
}

We conduct extensive independent numerical experiments considering frictionless disks without internal degrees of freedom (rotation, etc.) in two dimensions. We report here that for a large range of the packing fractions below random-close packing, all components of the stress tensor of wet granular materials remain finite in the limit of zero shear rate. This is direct evidence for a fluid-to-solid arrest transition. The offset value of the shear stress characterizes plastic deformation of the arrested state which corresponds to dynamic yield stress of the system. Based on an analytical line of argument, we propose that the mean number of capillary bridges per particle, ν, follows a nontrivial dependence on the packing fraction, ϕ, and the capillary energy, ɛ. Most noticeably, we show that ν is a generic and universal quantity which does not depend on the driving protocol. Using this universal quantity, we calculate the arrest stress, σa, analytically based on a balance of the energy injection rate due to the external force driving the flow and the dissipation rate accounting for the rupture of capillary bridges. The resulting prediction of σa is a nonlinear function of the packing fraction, ϕ, and the capillary energy, ɛ. This formula provides an excellent, parameter-free prediction of the numerical data. Corrections to the theory for small and large packing fractions are connected to the emergence of shear bands and of contributions to the stress from repulsive particle interactions, respectively.

Michael Hein, Jean Baptiste Fleury and Ralf Seemann:
"Coexistence of different droplet generating instabilities: new breakup regimes of a liquid filament"
Soft Matter 11 (2015) 5246
[Journal URL], [BibTeX], [Abstract]

@article {hein-sm-2015,
  title = {Coexistence of different droplet generating instabilities: new breakup regimes of a liquid filament},
  author = {Hein, Michael AND Fleury, Jean Baptiste AND Seemann, Ralf },
  journal = {Soft Matter},
  volume = {11},
  issue = {26},
  year = {2015},
  pages = {5246--5252},
  doi = {10.1039/C5SM00736D },
  publisher = {Royal Society of Chemistry},
}

The coexistence of multiple droplet breakup instabilities in a Step-emulsification geometry is studied. A liquid filament, which is confined in one dimension by channel walls and surrounded by a co-flowing immiscible continuous phase, decays into droplets when subject to a sudden release of confinement. Depending on the filament aspect ratio and liquid flow rates, an unexpectedly rich variety of droplet breakup regimes is found. All of these breakup regimes are composed of two basic instabilities, i.e. a step- and a jet-instability, that coexist in various combinations on the same filament. Surprisingly, even an asymmetric breakup regime is found, producing droplet families of significantly different diameters, while the filament is subject to a fully symmetric flow field. We suggest key physical principles explaining the spontaneous symmetry breaking and the transitions between individual droplet breakup regimes. The particular ability to produce distinct droplet families from a single filament is demonstrated to allow for simultaneous concentration and encapsulation of particles into one droplet family while excess bulk liquid is released into another family of droplets.

Michael Hein, Michael Moskopp and Ralf Seemann:
"Flow field induced particle accumulation inside droplets in rectangular channels"
Lab Chip 15 (2015) 2879
[Journal URL], [BibTeX], [Abstract]

@article {hein-labchip-2015,
  title = {Flow field induced particle accumulation inside droplets in rectangular channels},
  author = {Hein, Michael AND Moskopp, Michael AND Seemann, Ralf},
  journal = {Lab on a Chip},
  volume = {15},
  issue = {},
  year = {2015},
  pages = {2879--2886},
  doi = {10.1039/C5LC00420A},
  publisher = {Royal Society of Chemistry},
}

Particle concentration is a basic operation needed to perform washing steps or to improve subsequent analysis in many (bio)-chemical assays. In this article we present field free, hydrodynamic accumulation of particles and cells in droplets within rectangular micro-channels. Depending on droplet velocity, particles either accumulate at the rear of the droplet or are dispersed over the entire droplet cross-section. We show that the observed particle accumulation behavior can be understood by a coupling of particle sedimentation to the internal flow field of the droplet. The changing accumulation patterns are explained by a qualitative change of the internal flow field. The topological change of the internal flow field, however, is explained by the evolution of the droplet shape with increasing droplet velocity altering the friction with the channel walls. In addition, we demonstrate that accumulated particles can be concentrated, removing excess dispersed phase by splitting the droplet at a simple channel junction.

Lucas Goehring, Akio Nakahara, Tapati Dutta, So Kitsunezaki, and Sujata Tarafdar:
"Desiccation Cracks and their Patterns: Formation and Modelling in Science and Nature"
WILEY-VCH, Weinheim, Germany (2015)
ISBN: 978-3-527-41213-6
[Book URL], [BibTeX], [Description]

@book {geohring-2015,
  booktitle = {Desiccation Cracks and their Patterns: Formation and Modelling in Science and Nature},
  author = {Goehring, Lucas AND Nakahara, Akio AND Dutta, Tapati AND Kitsunezaki, So AND Tarafdar, Sujata},
  year = {2015},
  isbn = {978-3-527-41213-6},
  publisher = {WILEY-VCH, Weinheim, Germany},
}

Bringing together basic ideas, classical theories, recent experimental and theoretical aspects, this book explains desiccation cracks from simple, easily-comprehensible cases to more complex, applied situations.
The ideal team of authors, combining experimental and theoretical backgrounds, and with experience in both physical and earth sciences, discuss how the study of cracks can lead to the design of crack-resistant materials, as well as how cracks can be grown to generate patterned surfaces at the nano- and micro-scales. Important research and recent developments on tailoring desiccation cracks by different methods are covered, supported by straightforward, yet deep theoretical models. Intended for a broad readership spanning physics, materials science, and engineering to the geosciences, the book also includes additional reading especially for students engaged in pattern formation research.

Kuang-Wu Lee and Marco G. Mazza:
"Stochastic rotation dynamics for nematic liquid crystals"
J. Chem. Phys. 142 (2015) 164110
[Journal URL], [arXiv], [BibTeX], [Abstract]

@article { lee-jcp-2015,
  title = {Stochastic rotation dynamics for nematic liquid crystals},
  author = {Lee, Kuang-Wu AND Mazza, Marco G.},
  journal = {The Journal of Chemical Physics},
  volume = {142},
  year = {2015},
  pages = {164110-1--164110-7},
  doi = {10.1063/1.4919310},
  publisher = {AIP Publishing LLC}
}

We introduce a new mesoscopic model for nematic liquid crystals (LCs). We extend the particle-based stochastic rotation dynamics method, which reproduces the Navier-Stokes equation, to anisotropic fluids by including a simplified Ericksen-Leslie formulation of nematodynamics. We verify the applicability of this hybrid model by studying the equilibrium isotropic-nematic phase transition and nonequilibrium problems, such as the dynamics of topological defects and the rheology of sheared LCs. Our simulation results show that this hybrid model captures many essential aspects of LC physics at the mesoscopic scale, while preserving microscopic thermal fluctuations.

Roman Mani, Ciro Semprebon, Dirk Kadau, Hans J. Herrmann, Martin Brinkmann, and Stephan Herminghaus:
"Role of contact-angle hysteresis for fluid transport in wet granular matter"
Phys. Rev. E 91 (2015) 042204
[Journal URL], [BibTeX], [Abstract]

@article { mani-pre-2015,
  title = {Role of contact-angle hysteresis for fluid transport in wet granular matter},
  author = {Mani, Roman AND Semprebon, Ciro AND Kadau, Dirk AND Herrmann, Hans J. AND Brinkmann, Martin AND Herminghaus, Stephan},
  journal = {Physical Review E},
  volume = {91},
  year = {2015},
  pages = {042204-1--042204-10},
  doi = {10.1103/PhysRevE.91.042204},
  publisher = {American Physical Society}
}

The stability of sand castles is determined by the structure of wet granulates. Experimental data on the size distribution of fluid pockets are ambiguous with regard to their origin. We discovered that contact-angle hysteresis plays a fundamental role in the equilibrium distribution of bridge volumes, and not geometrical disorder as commonly conjectured. This has substantial consequences on the mechanical properties of wet granular beds, including a history-dependent rheology and lowered strength. Our findings are obtained using a model in which the Laplace pressures, bridge volumes, and contact angles are dynamical variables associated with the contact points. While accounting for contact line pinning, we track the temporal evolution of each bridge. We observe a crossover to a power-law decay of the variance of capillary pressures at late times and a saturation of the variance of bridge volumes to a finite value connected to contact line pinning. Large-scale simulations of liquid transport in the bridge network reveal that the equilibration dynamics at early times is well described by a mean-field model. The spread of final bridge volumes can be directly related to the magnitude of contact-angle hysteresis.

Matthias Lessel, Oliver Bäumchen, Mischa Klos, Hendrik Hähl, Renate Fetzer, Michael Paulus, Ralf Seemann, and Karin Jacobs:
"Self-assembled silane monolayers: an efficient step-by-step recipe for high-quality, low energy surfaces"
Surf. Interface Anal. 47 (2015) 557
[Journal URL], [BibTeX], [Abstract]

@article {lessel-sia-2015,
  title = {Self-assembled silane monolayers: an efficient step-by-step recipe for high-quality, low energy surfaces},
  author = {Lessel, Matthias AND B{\"a}umchen, Oliver AND Klos, Mischa AND H{\"a}hl, Hendrik AND Fetzer, Renate AND Paulus, Michael AND Seemann, Ralf AND Jacobs, Karin},
  journal = {Surface and Interface Analysis},
  volume = {47},
  issue = {5},
  year = {2015},
  pages = {557--564},
  doi = {10.1002/sia.5729},
  publisher = {Wiley Online Library},
}

Organosilane self-assembled monolayers (SAMs) are commonly used for modifying a wide range of substrates. Depending on the end group, highly hydrophobic or hydrophilic surfaces can be achieved. Silanization bases on the adsorption, self-assembly and covalent binding of silane molecules onto surfaces and results in a densely packed, SAM. Following wet chemical routines, the quality of the monolayer is often variable and, therefore, unsatisfactory. The process of self-assembly is not only affected by the chemicals involved and their purity but is also extremely sensitive to ambient parameters such as humidity or temperature and to contaminants. Here, a reliable and efficient wet-chemical recipe is presented for the preparation of ultra-smooth, highly ordered alkyl-terminated silane SAMs on Si wafers. The resulting surfaces are characterized by means of atomic force microscopy, X-ray reflectometry and contact angle measurements.

Fabian M. Schaller, Max Neudecker, Mohammad Saadatfar, Gary Delaney, Gerd E. Schröder-Turk, and Matthias Schröter:
"Local origin of global contact numbers in frictional ellipsoid packings"
Phys. Rev. Lett. 114 (2015) 158001
[Journal URL], [arXiv], [BibTeX], [Abstract]

@article { schaller-prl-2015,
  title = {Local origin of global contact numbers in frictional ellipsoid packings},
  author = {Schaller, Fabian M. AND Neudecker, Max AND Saadatfar, Mohammad AND Delaney, Gary AND Schr{\"o}der-Turk, Gerd E. AND Schr{\"o}ter, Matthias},
  journal = {Physical Review Letters},
  volume = {114},
  year = {2015},
  pages = {158001-1--158001-5},
  doi = {10.1103/PhysRevLett.114.158001},
  publisher = {American Physical Society}
}

In particulate soft matter systems the average number of contacts Z of a particle is an important predictor of the mechanical properties of the system. Using x-ray tomography, we analyze packings of frictional, oblate ellipsoids of various aspect ratios α, prepared at different global volume fractions ϕg. We find that Z is a monotonically increasing function of ϕg for all α. We demonstrate that this functional dependence can be explained by a local analysis where each particle is described by its local volume fraction ϕl computed from a Voronoi tessellation. Z can be expressed as an integral over all values of ϕl: Z(ϕg,α,X)=∫Zl(ϕl,α,X)P(ϕl|ϕg)dϕl. The local contact number function Zl(ϕl,α,X) describes the relevant physics in term of locally defined variables only, including possible higher order terms X. The conditional probability P(ϕl|ϕg) to find a specific value of ϕl given a global packing fraction ϕg is found to be independent of α and X. Our results demonstrate that for frictional particles a local approach is not only a theoretical requirement but also feasible.

Yuji Sasaki, Hikaru Hoshikawa, Takafumi Seto, Fumiaki Kobayashi, V. S. R. Jampani, Stephan Herminghaus, Christian Bahr, and Hiroshi Orihara:
"Direct Visualization of Spatiotemporal Structure of Self-Assembled Colloidal Particles in Electrohydrodynamic Flow of a Nematic Liquid Crystal"
Langmuir 31 (2015) 3815
[Journal URL], [BibTeX], [Abstract]

@article { sasaki-langmuir-2015,
  title = {Direct Visualization of Spatiotemporal Structure of Self-Assembled Colloidal Particles in Electrohydrodynamic Flow of a Nematic Liquid Crystal},
  author = {Sasaki, Yuji AND Hoshikawa, Hikaru AND Seto, Takafumi AND Kobayashi, Fumiaki AND Jampani, V. S. R. AND Herminghaus, Stephan AND Bahr, Christian AND Orihara, Hiroshi},
  journal = {Langmuir},
  volume = {31},
  issue = {13},
  year = {2015},
  pages = {3815--3819},
  doi = {10.1021/acs.langmuir.5b00450},
  publisher = {American Chemical Society},
}

Characterization of spatiotemporal dynamics is of vital importance to soft matter systems far from equilibrium. Using a confocal laser scanning microscopy, we directly reveal three-dimensional motion of surface-modified particles in the electrohydrodynamic convection of a nematic liquid crystal. Particularly, visualizing a caterpillar-like motion of a selfassembled colloidal chain demonstrates the mechanism of the persistent transport enabled by the elastic, electric, and hydrodynamic contributions. We also precisely show how the particles’ trajectory is spatially modified by simply changing the surface boundary condition.