Publikationen

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.

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.

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.

Shashi Thutupalli, Jean-Baptiste Fleury, Ulf D. Schiller, Gerhard Gompper, Stephan Herminghaus, and Ralf Seemann:
"Hydrodynamics Mediated Collective Motions in Populations of Microdroplets"
In: Alexander S. Mikhailov and Gerhard Ertl (Eds.):
"Engineering of Chemical Complexity II"
World Scientific, Singapore
ISBN 978-981-4616-12-6
[Article URL], [BibTeX], [Abstract]

@incollection {thutupalli-ecc-2014,
  title = {Hydrodynamics Mediated Collective Motions in Populations of Microdroplets},
  author = {Thutupalli, Shashi AND Fleury, Jean-Baptiste AND Schiller, Ulf D. AND Gompper, Gerhard AND Herminghaus, Stephan AND Seemann, Ralf},
  editor = {Mikhailov, Alexander S. AND Ertl, Gerhard},
  booktitle = {Engineering of Chemical Complexity II},
  pages = {125--148},
  year = {2014},
  place ={Singapore},
  isbn = {978-981-4616-12-6},
  doi = {10.1142/9789814616133_0008},
  publisher = {World Scientific}
}

The following sections are included:
  Introduction
  Experimental Techniques
  Self-Propelled Squirming Droplets
  Oscillations of Passive Droplets
  Conclusions
  References

Jitesh Barman, Digendranath Swain, Bruce M. Law, Ralf Seemann, Stephan Herminghaus, and Krishnacharya Khare:
"Electrowetting Actuated Microfluidic Transport in Surface Grooves with Triangular Cross Section"
Langmuir 31 (2014) 1231
[Journal URL], [BibTeX], [Abstract]

@article {barman-lang-2014,
  title = {Electrowetting Actuated Microfluidic Transport in Surface Grooves with Triangular Cross Section},
  author = {Barman, Jitesh AND Swain, Digendranath AND Law, Bruce M. AND Seemann, Ralf AND Herminghaus, Stephan AND Khare, Krishnacharya},
  journal = {Langmuir},
  volume = {31},
  issue = {3},
  year = {2014},
  pages = {1231--1236},
  doi = {10.1021/la504354a},
  publisher = {American Chemical Society},
}

Liquids show different static wetting morphologies in open triangular grooves depending upon the wedge angle (ψ) of the groove and the liquid contact angle (θ) with the substrate. Switching between different morphologies can be achieved either by varying the contact angle of the liquid or by changing the wedge angle of the groove. In the present work we manipulate the apparent contact angle of a liquid by electrowetting to switch between liquid morphologies, from droplet to filament, to achieve microfluidic transport of the liquid into open triangular grooves. The static length of liquid filaments in grooves is analyzed as a function of applied voltage for different applied ac frequencies. The dynamic advancement of the filament lengths in grooves is analyzed as a function of time for different applied voltages for two different liquids: first with contact angle greater than the wedge angle and second with contact angle smaller than the wedge angle. Later an exact electrical model is derived to explain the liquid transport in triangular grooves actuated by electrowetting which includes the precise geometry of the liquid morphology.

Yuji Sasaki, Yoshinori Takikawa, V. S. R. Jampani, Hikaru Hoshikawa, Takafumi Seto, Christian Bahr, Stephan Herminghaus, Yoshiki Hidaka, and Hiroshi Orihara:
"Colloidal caterpillars for cargo transportation"
Soft Matter 10 (2014) 8813
[Journal URL], [BibTeX], [Abstract]
See also the Research News on chemistryworld

@article {sasaki-sm-2014,
  title = {Colloidal caterpillars for cargo transportation},
  author = {Sasaki, Yuji AND Takikawa, Yoshinori AND Jampani, V. S. R. AND Hoshikawa, Hikaru AND Seto, Takafumi AND Bahr, Christian AND Herminghaus, Stephan AND Hidaka, Yoshiki AND Orihara, Hiroshi},
  journal = {Soft Matter},
  volume = {10},
  year = {2014},
  pages = {8813--8820},
  doi = {10.1039/C4SM01354A},
  publisher = {Royal Society of Chemistry},
}

Tunable transport of tiny objects in fluid systems is demanding in diverse fields of science such as drug delivery, active matter far from equilibrium, and lab-on-a-chip applications. Here, we report the directed motion of colloidal particles and self-assembled colloidal chains in a nematic liquid crystal matrix using electrohydrodynamic convection (EHC) rolls. The asymmetric distortion of the molecular orientation around the particles results – for single particles – in a hopping motion from one EHC roll to the next and – for colloidal chains – in a caterpillar-like motion in the direction perpendicular to the roll axes. We demonstrate the use of colloidal chains as microtraction engines for the transport of various types of microcargo.

Anupam Sengupta, Stephan Herminghaus and Christian Bahr:
"Liquid crystal microfluidics: surface, elastic and viscous interactions at microscales"
Liquid Crystals Reviews 2 (2014) 73
[Journal URL], [BibTeX], [Abstract], [Download Manuscript]

@article {sengupta-lcr-2014,
  title = {Liquid crystal microfluidics: surface, elastic and viscous interactions at microscales},
  author = {Sengupta, Anupam AND Herminghaus, Stephan AND Bahr, Christian},
  journal = {Liquid Crystal Reviews},
  volume = {2},
  issue = {2},
  year = {2014},
  pages = {73--110},
  doi = {10.1080/21680396.2014.963716},
  publisher = {Taylor & Francis Group},
}

The hydrodynamic properties of nematic liquid crystals are characterized by a complex mutual coupling between flow, viscosity, and nematic order. While the flow behaviour of nematic bulk samples is well known, corresponding studies in microfluidic settings are still at an early stage. The presence of the four confining channel walls – and in particular the nature of the surface anchoring of the nematic order on the walls – adds new phenomena to the already rich and multifaceted flow behaviour. We present an overview of recent studies focusing on the microfluidics of nematic liquid crystals. Particular topics are the functionalization of the channel walls for defined surface anchoring conditions and the resulting structures of the nematic director field, the controlling and tuning of the flow velocity profile and director field configuration and resulting opto-fluidic applications, and the behaviour of topological defects in the flowing nematic and their application for a guided colloidal transport.

Karthik Peddireddy, V. S. R. Jampani, Stephan Herminghaus, Christian Bahr, Maruša Vitek and
Igor Muševič:
"Lasing and waveguiding in smectic A liquid crystal optical fibers"
Proceedings of the SPIE: Liquid Crystals XVIII 9182 (2014) 91820Y
[Journal URL], [BibTeX], [Abstract]

@article {peddireddy-spie-2014,
  title = {Lasing and waveguiding in smectic A liquid crystal optical fibers},
  author = {Peddireddy, Karthik AND Jampani, V. S. R. AND Herminghaus, Stephan AND Bahr, Christian AND Vitek, Maruša AND Muševič, Igor},
  journal = {Proceedings of the SPIE: Liquid Crystals XVIII},
  volume = {9182},
  year = {2014},
  pages = {91820Y},
  doi = {10.1117/12.2061303},
  publisher = {Society of Photo-Optical Instrumentation Engineers (SPIE)},
}

We demonstrate a new sort of optical fibers, which are self-assembled from a smectic-A liquid crystal. When this liquid crystal is put in contact with water solution of surfactant CTAB, microfibers start spontaneously growing at the liquid crystal-water interface. The fibers are of very uniform diameter and can be several hundreds of micrometers long. They all have a line topological defect in the core of the fiber with a local optical axis pointing from the defect core towards the surface. The ends of the fiber are of perfect spherical shape. By doping the fibers with a fluorescent dye, we demonstrate guiding of light along the fiber. When the fiber is illuminated with pulsed light, which is absorbed by the dye, we observe Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for the realization of soft matter photonic circuits.

Julie Murison, Benoît Semin, Jean-Christophe Baret, Stephan Herminghaus, Matthias Schröter, and Martin Brinkmann:
"Wetting Heterogeneities in Porous Media Control Flow Dissipation"
Phys. Rev. Applied 2 (2014) 034002
[Journal URL], [BibTeX], [Abstract]

@article { murison-pra-2014,
  title = {Wetting Heterogeneities in Porous Media Control Flow Dissipation},
  author = {Murison, Julie AND Semin, Beno{\^i}t AND Baret, Jean-Christophe AND Herminghaus, Stephan AND Schr{\"o}ter, Matthias AND Brinkmann, Martin},
  journal = {Physical Review Applied},
  volume = {2},
  issue = {},
  year = {2014},
  pages = {034002-1--034002-10},
  doi = {10.1103/PhysRevApplied.2.034002},
  publisher = {American Physical Society},
}

Pressure-controlled displacement of an oil-water interface is studied in dense packings of functionalized glass beads with well-defined spatial wettability correlations. An enhanced dissipation is observed if the typical extension ξ of the same-type wetting domains is smaller than the average bead diameter d. Three-dimensional imaging using x-ray microtomography shows that the frequencies n(s) of residual droplet volumes s for different ξ collapse onto the same curve. This indicates that the additional dissipation for small ξ is due to contact line pinning rather than an increase of capillary break-up and coalescence events.

Stephan Herminghaus, Corinna C. Maass, Carsten Krüger, Shashi Thutupalli,
Lucas Goehring, and Christian Bahr:
"Interfacial mechanisms in active emulsions"
Soft Matter 10 (2014) 7008
[Journal URL], [BibTeX], [Abstract]

@article { herminghaus-sm-2014,
  title = {Interfacial mechanisms in active emulsions},
  author = {Herminghaus, Stephan AND Maass, Corinna C. AND Kr{\"u}ger, Carsten AND Thutupalli, Shashi AND Goehring, Lucas AND Bahr, Christian},
  journal = {Soft Matter},
  volume = {10},
  issue = {36},
  year = {2014},
  pages = {7008--7022},
  doi = { 10.1039/C4SM00550C},
  publisher = {The Royal Society of Chemistry},
}

Active emulsions, i.e., emulsions whose droplets perform self-propelled motion, are of tremendous interest for mimicking collective phenomena in biological populations such as phytoplankton and bacterial colonies, but also for experimentally studying rheology, pattern formation, and phase transitions in systems far from thermal equilibrium. For fuelling such systems, molecular processes involving the surfactants which stabilize the emulsions are a straightforward concept. We outline and compare two different types of reactions, one which chemically modifies the surfactant molecules, the other which transfers them into a different colloidal state. While in the first case symmetry breaking follows a standard linear instability, the second case turns out to be more complex. Depending on the dissolution pathway, there is either an intrinsically nonlinear instability, or no symmetry breaking at all (and hence no locomotion).