Colloquium of the SPP 2171

The colloquium of the SPP 2171 takes place biweekly on Fridays at 13:00-14:30. See below for the list of scheduled sessions. The sessions will be held online, access information is provided via the SPP 2171 mailing list.

  • Winter term 2021/2022

    Date Talks

    Engineering and Processing materials with special wetting properties

    Dorothea Helmer
    IMTEK, University of Freiburg

    Novel materials and processes are key for the development of tailored structures and surfaces. Materials with special wetting properties such as super repellent materials or photo-responsive materials are especially interesting for numerous applications like surface-cleaning, filtration and wetting studies. Additionally, novel 3D printing processes enable easy generation and structuring of such complex materials. This way, soft and hard responsive materials based on polymers and gels can be fabricated, and novel printing technologies that produce high-resolution materials can be engineered. With a toolbox of material development, processing and the inherent responsive material properties, various possibilities for new applications in engineering, chemistry and biology emerge.

    Swelling of partially saturated hydrophobic polymer brush layers

    Özlem Kap
    University of Twente

    Polymer brushes are highly responsive materials that ensure their application areas to be in a broad spectrum while also making it necessary to understand their interfacial behaviors. The spreading of a drop results in a finite contact angle on polymer brushes under good solvent conditions.  A halo around the macroscopic contact line forms from the early stage till the steady-state of the spreading process on hydrophobic polymer brushes. The width of the halo varies, depending on the thickness of the polymer brushes as well as the vapor pressure of the solvent. In a saturated vapor, the adsorption into the polymer brush layer is faster for a low vapor pressure solvent, in comparison to a high vapor pressure solvent. Moreover, the solvent expands over a wider distance with a thicker brush, while the width is relatively confined for a thinner polymer brush layer. Understanding the spreading dynamics of a drop on complex surfaces is crucial to reveal the mechanism of the halo formation and the wetting phenomena.

    05.11.2021 skipped due to SPP conference
    19.11.2021 Holger Stark
    Technische Universität Berlin

    Lukas Hauer
    Max-Planck-Institute for Polymer Research, Mainz
    Svetlana Gurevich
    Westfälische Wilhelms-Universität Münster

    Andreas Heuer
    Westfälische Wilhelms-Universität Münster
    17.12.2021 Kirsten Harth
    Otto von Guericke University Magdeburg

    Laura Gallardo Domínguez
    Hamburg University of Technology
    14.01.2022 skipped

    Dirk Peschka
    Weierstrass Institute for Applied Analysis and Stochastics, Berlin

    Anil Rajak
    University of Freiburg

  • Summer term 2021

    Date Talks

    Pinning of soft contact lines causes folding-unfolding asymmetry of sticky self-contacts

    Stefan Karpitschka
    Max-Planck-Institute For Dynamics And Self-organization, Göttingen

    The compression of soft elastic matter and biological tissue can lead to creasing, an instability where a surface folds sharply into periodic self-contacts. Intriguingly, the unfolding of the surface upon releasing the strain is usually not perfect: small scars remain that serve as nuclei for creases during repeated compressions. Here we present creasing experiments with sticky polymer surfaces, using confocal microscopy, which resolve the contact line region where folding and unfolding occurs. It is found that surface tension induces a second fold, at the edge of the self-contact, which leads to a singular elastic stress and self-similar crease morphologies. However, these profiles exhibit an intrinsic folding-unfolding asymmetry that is caused by contact line pinning, in a way that resembles wetting of liquids on imperfect solids. The pinning behavior of elastic contact lines is therefore a key element of creasing: it inhibits complete unfolding and gives soft surfaces a folding memory.


    Counteracting Interfacial Energetics and Superspreaders

    Ellen H.G. Backus
    Max Planck Institute for Polymer Research, Mainz

    The improved wettability of hydrophobic surfaces by surfactants is usually quantified as a decrease of the contact angle θ of a droplet on the surface, where the contact angle θ is given by the three surface tensions involved. Surfactants are known to lower the liquid−vapor surface tension, but what they do to the two other surface tensions is less clear. Interestingly, a number of very common surfactants do not change the wettability of the solid: they give the same contact angle as a simple liquid with the same liquid−vapor surface tension. Surface-specific sum-frequency generation spectroscopy shows that nonetheless surfactants are present at the solid surface. The surfactants therefore change the solid−liquid and solid−vapor surface tensions by the same amount, leading to an unchanged contact angle.
    Moreover, the superspreading surfactant Silwet shows a systematically higher contact angle on a hydrophobic surface than other surfactant solutions of comparable liquid-vapor surface tension. Similar ‘anti-surfactant’ effects are observed for CTAB on hydrophilic substrates. Combining the contact angle experiments with sum-frequency generation spectroscopy, indicates that this effect is due to charge-binding of the surfactant with the substrate.

    Dynamic Wetting of Self-Assembled Monolayers functionalized with Photoresponsive Arylazopyrazoles

    Niklas Arndt
    Westfälische Wilhelms-Universität, Münster

    Light is a particularly attractive external stimulus to modify surface properties since it can be applied with very high local and temporal resolution. Molecular photoswitches such as azobenzenes [1], diarylethenes [2] and spiropyranes [3] have been explored in a range of photoresponsive coatings which utilize their photoisomerization to induce changes in macroscopic properties such as wettability [4]. This results in a substantial and reversible change of wettability [5].

    Current approaches using immobilized photoswitches still suffer from certain drawbacks [1], while in contrast arylazopyrazoles (AAPs) offer significant improvements of photophysical properties. A much more favorable photostationary state (>98 % in both directions), very slow thermal relaxation of the cis-isomer towards the thermodynamically favored trans-isomer and very good fatigue resistance [6]. In the talk we present the synthesis of multiple AAP-silane derivatives and the successful functionalization of glass and silicon surfaces using self-assembled monolayers.

    (1) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in different classes of azobenzene. Chem. Soc. Rev. 2012, 41, 1809–1825.
    (2) Tian, H.; Yang, S. Recent progresses on diarylethene based photochromic switches. Chem. Soc. Rev. 2004, 33, 85–97.
    (3) Klajn, R. Spiropyran-based dynamic materials. Chem. Soc. Rev. 2013, 43, 148–184.
    (4) Xin, B.; Hao, J. Reversibly switchable wettability. Chem. Soc. Rev. 2010, 39, 769–782.
    (5) Groten, J.; Bunte, C.; Rühe, J. Light-induced switching of surfaces at wetting transitions through photoisomerization of polymermonolayers. Langmuir : the ACS journal of surfaces and colloids 2012, 28, 15038–15046.
    (6) Weston, C. E.; Richardson, R. D.; Haycock, P. R.; White, A. J. P.; Fuchter, M. J. Arylazopyrazoles: azoheteroarene photoswitches offering quantitative isomerization and long thermal half-lives. Journal of the American Chemical Society 2014, 136, 11878–11881.

    21.05.2021 Liquid Dewetting From Liquid and Soft Substrates

    Ralf Seemann1, Stefan Bommer1, Roghayeh Shiri1, Khalil Remini1, Sebastian Jachalski2,  Dirk Peschka2, Leonie Schmeller2, Barbara Wagner2
    1Saarland University, Experimental Physics, D-66123 Saarbücken
    2Weierstrass Institute, Mohrenstr. 39, D-10117 Berlin

    PART I: Spinodal Breakup of liquid-liquid bilayers
    The dewetting of liquid polystyrene (PS) from liquid polymethyl-methacrylate (PMMA) is studied. At dewetting temperatures, both polymers can be considered as Newtonian fluids with the same viscosity. Provided that the liquid PS layer is below 10 nm, breakup occurs by spinodal dewetting. Due to the low interfacial tension of the buried interface compared to the PS-air interface and the large mobility, a very short spinodal wavelength develops with a larger amplitude of the buried interface than that of the free PS-air interface. The spinodal patterns of PMMA-PS and PS-air interface are anti-correlated and the observed wavelength is within the range predicted from thin film models.

    PART II: Contact Angles and Dewetting Dynamics in Visco-Elastic Substrates
    When a micron sized droplet is sitting on a (visco-) elastic substrate gravitational forces can be neglected, and its shape is determined by an interplay of capillary and elastic forces. If the elasticity of the substrate is reasonably low, the three-phase contact line is pulled upwards and the droplet/substrate interface is deformed like a circular cap. Theory predicts that the exact contact angle (Neumann triangle) depends on an interplay of droplet size and elasticity of the substrate. Consequently, the contact angle is expected to impact also the dewetting velocity. And even that droplets on soft substrates have been widely studied experimentally in the recent years, only few experiments were conducted for very small droplets and very soft polymers (E < 10 kPa) enabling sufficient resolution to confirm or falsify the theoretical predictions.
    In our study we investigate theoretically and experimentally the dewetting dynamics and rim shapes of circular holes and the shape of sessile droplets in their equilibrium state.
    We explore the dominate physics that determine the equilibrium contact angle and the dewetting rates for these very soft substrates at small scales. Our model system accounts for nonlinear elasticity and also for the possibly of phase separation at the contact line. We derive a weak formulation and set up a 2d code.

    Remodelling of cellular compartments by wetting

    Roland L. Knorr
    Max-Planck-Institute of Colloids and Interfaces, Potsdam

    Compartmentalisation is essential for eukaryotic cell function, allowing the division of metabolic and regulatory processes into membrane-bound, specialised compartments, such as organelles. In recent years, intracellular phase separation has garnered much attention as a non-membrane means of organising components through the formation of droplet-like compartments, which are functionally implicated in both health and disease.
    Here, we investigate the mechanism of wetting-mediated droplet sequestration during autophagy, a highly-conserved degradation system in which membrane sheets expand and bend to isolate portions of the cell interior inside autophagosomes. Further, we demonstrate that wetting also mediates embryonic plant vacuole remodelling. I propose that formation of autophagosomes and vacuole remodelling represent a class of cellular processes that are driven by elastocapillarity.

    Fujioka, Y; […] ; Knorr, R. L. et al. Phase separation organizes the site of autophagosome formation. Nature, 2020. doi
    Agudo-Canalejo, J.; […]; Knorr, R. L. Wetting regulates autophagy of phase separated droplets and the cytosol. Nature, 2021. doi

    MD of capillary wetting in brush-coated channels

    Guido Ritsema van Eck
    Department of Materials Science and Technology of Polymers, University of Twente

    In recent experimental work [1], constant-rate capillary wetting was observed in a microfluidic channel coated with polymer brushes. Diffusive hydration of the polymer coating ahead of the primary liquid front is proposed as an explanation. Using molecular dynamics, I explore the microscopic wetting behaviour of brush-coated channels and attempt to test this hypothesis.

    [1]: Li, Westerbeek et al., (preprint) doi


    A high order extended discontinuous Galerkin solver for moving contact lines on a flexible substrate

    Florian Kummer
    Chair of Fluid Dynamics, Technical University of Darmstadt

    Within the framework of SPP 2171, a highly accurate numerical method for the simulation of three-phase contact lines on flexible substrates was developed, which is based on the discontinuous Galerkin method (DG).

    The presentation will be split into two parts: First, a general introduction into DG methods will be given in order to motivate its use and illustrate its potential. We further present the extension of the DG method (extended discontinuous Galerkin, XDG) for the simulation of multiphase flows. There, the spatial approximation space can represent jumps in pressure and velocity gradient “infinitely sharp”, which yields a highly accurate numerical solution of the multiphase flow.

    The second part will be more focused on the special developments regarding the SPP 2171: We are going to discuss the extension of the multiphase solver for contact lines and further present the introduction of a second interface to track the fluid and the elastic substrate.

    Statics and Dynamics of Nanowetting

    Nikolai Kubochkin
    Institute for Technical Thermodynamics, Technical University of Darmstadt

    Wetting at nanoscale leaves a plethora of questions unresolved. In the present talk, we discuss the behavior of contact angles for nanosized droplet as well as alteration of wetting dynamics induced by intermolecular forces. We discuss the flow in corner geometries under the action of capillarity and intermolecular forces. We also present a model for the wetting of an elastic half-space by nanodroplets and compare the results with those obtained within a common macroscopic approach.

    02.07.2021 skipped due to  PhD workshop

    Soft elastic microfluidics: From high-throughput trapping of nano-objects to
    adaptive transport through coupled flow paths

    Thomas Pfohl
    Institute of Physics, University of Freiburg

    Soft materials and their ability to dynamic elastic deformations on the nano-, micro- and mesoscale impacted by fluid flow and capillarity (‘elasto-hydrodynamics’) are of broad interest to a wide variety of exciting new applications, such as in bioanalytics, medical devices, flexible electronics, and soft robotics. In recent experiments, we developed polydimethylsiloxane (PDMS)-based geometrical-induced electrostatic (GIE) nano-trapping devices capable of controlling the height of the nanofluidic channels by an elasto-hydrodynamic coupling with an implemented fluid-controllable microchannel. This combined micro- and nanofluidic setup enables fast and flexible tuning of the potential depth and trap stiffness as well as active trapping and release of individual nano-objects during experiments, paving the way toward high-throughput manipulation of nanoparticles and biomolecules.
    In our project within the framework of SPP 2171, we are studying and analyzing the transport of fluids and bubbles/droplets in channels interfaced by flexible membranes in a microfluidic environment. This
    microfluidic setup allows for a defined flow-control within the microchannels as well as for the opportunity to analyze fluid transport within the channels and cross-correlations between the channels linked by the elastic membranes. Moreover, initiating and modifying instabilities of the membranes and the feedback in the transport channels, the coupling and cross-communication of the flowing materials and information transport will be amplified and specifically shaped. This membrane-initiated crosstalk will be used to move, adapt, govern, shift and stop specific flows and bubble/droplet motions within fluid transport routes and furthermore to introduce flow patterns with adaption and self-regulation capabilities as well as on the long term logical links and operations within fluid transport networks.

    Dynamic wetting and dewetting processes on adaptive surfaces

    Xiaomei Li
    Max-Planck-Institute for Polymer Research, Mainz

    Wetting and dewetting, i.e., the respective advancing and receding motion, of liquids on surfaces play a key role in coating, printing, spreading of herbicides and insecticides and the fogging of glass surfaces. With an increasing number of technical fluids, the control of wetting and dewetting becomes more and more important. In order to understand dynamic wetting and dewetting processes on adaptive surfaces, we built up a tilted-plate setup to measure velocity dependent dynamic contact angles. I will present this setup and discuss measurements on adaptive surfaces, which we have performed last year. In particular, I present work on thin films made from random copolymers (PS/PAA and P(MMA-co-HQSEA)) and from polymer brushes (PNIPAM and PS/PAA). By comparing the experimental dynamic contact angle with predicted dynamic contact angle calculated by Butt’s adaptive theory, we estimated the adaption kinetics of surfaces.

  • Winter term 2020/2021


    Droplets versus vesicles: What is their shape on a surface?

    Marcus Müller
    Institut für Theoretische Physik, Georg-August-Universität Göttingen

    Vesicles are lipid membranes that separate in inside drop of fluid from an
    outside. Similar to an interface Hamiltonian one can describe them by the free
    energy associated with the configuration of the membrane surface -- the
    Helfrich Hamiltonian. Since membranes self-assemble, the tension of the
    membrane is vanishingly small and the free energy is dominated by the
    curvature term, instead of the surface-area term. Minimizing the free energy
    of the vesicle in contact with a surface, one obtains the shape of an
    adsorbed vesicle and derives the analog of Young's equation.

    The talk will discuss the shape of vesicles on surfaces and illustrate
    simulation techniques to describe the interaction of vesicles with a
    switchable polymer brush. We will pay particular attention to the curvature at
    the rim of the adsorption zone and the fluctuations of an adsorbed membrane.

    Memory effects in polymer brushes showing co-nonsolvency effects

    Simon Schubotz
    Leibniz-Institut für Polymerforschung Dresden

    Some polymer brushes show a co-nonsolvency effect: They collapse in a mixture of two good solvents at some specific mixing ratio. Previous studies focused on the response of brushes which are entirely covered by a liquid. Here, we concentrate on partial wetting of co-nonsolvent polymer brushes, i.e., on the dynamics of a three-phase contact line moving over such brushes. We demonstrate that the wetting behavior depends on the wetting history of the polymer brush.


    When and how self-cleaning of superhydrophobic surfaces works

    Doris Vollmer
    Max Planck Institute for Polymer Research, Mainz

    In recent years, a lot of research went into exploring different strategies to achieve long-lasting and highly liquid-repellent surfaces. This research resulted in a greatly improved understanding on how surface properties (hydrophobicity and geometry) and repellency (i.e. contact angles, impalement pressure) of liquid drops correlate. However, there is poor understanding on how surface properties and contamination (size, concentration, hydrophilicity) correlate. Therefore, we developed a method to slowly move a drop over a contaminated surface while imaging the region close to three-phase contact line by laser scanning confocal micrposcopy simultaneously. This enabled us to monitor in slow motion how a drop takes up particulate contamination while rolling over a contaminated surface.1 Using a lateral adhesion force device, we quantified how the friction force evolve during self-cleaning.

    Geyer, F.;  D'Acunzi, M.;  Sharifi, A.;  Saal , A.; Gao N.; Kaltbeitzel, A.; Sloot, T.-F.;  Berger, R.; Butt, H.-J.; Vollmer D., When and how self-cleaning of superhydrophobic surfaces works, Sci. Adv. 6, eaaw9727 (2020).

    How a water drop removes a particle form a hydrophobic surface

    Abhinav Naga
    Max Planck Institute for Polymer Research, Mainz

    When a water drop collides with a particle on a flat hydrophobic substrate, different outcomes are observed, depending on the speed of the collision.  At slow speeds (50 μm s−1), the particle remains attached to the drop after the collision whereas at fast speeds (500 μm s−1), the particle enters and exits the drop. We used laser scanning confocal microscopy to image these collision scenarios and to directly measure the force acting on the drop during the collisions. For a drop to successfully remove a particle from the substrate, the maximum capillary force between the drop and the particle has to exceed the resistive force experienced opposing the motion of the particle over the substrate. We present geometrical models for the maximum capillary force that a drop can exert on a particle and compare the predictions to measured forces. Rolling of the particle is explicitly considered. Finally, we discuss the origins of the resistive force acting on the particle when it is pushed or pulled by a water-air interface. In particular, we show that a particle experiences a resistive capillary torque when rolling at an interface.

    26.02.2021 Wetting, Imbibition and Switchable Elastocapillarity in Nanoporous Media

    Patrick Huber
    Physics and X-ray Analytics of Functional Materials Group
    Technische Universität Hamburg, TUHH und Deutsches Elektronen-Synchrotron, DESY

    Liquid-infused nanopores play a pivotal role in many natural and technological processes ranging from transport across biomembranes and plant movements to templating processes for nanomaterials and modern concepts of water desalination. Here I will present three experimental studies aimed at the fundamental exploration of wetting and imbibition dynamics in and at nanoporous surfaces (1) as well as studies aimed at employing the coupling of the elasticity of solids with the nano-capillarity of liquids, most prominently water, for the design of adaptive and electrically switchable actuator materials (2,3). To that end opto-fluidic techniques employing nanoporous photonic crystals will be combined with cyclic voltammetry and high-resolution synchrotron-based X-ray analytics.

    (1)   L.G. Cencha, G. Dittrich, P. Huber, C.L.A. Berlin, and R. Urteaga, Precursor Film Spreading during Liquid Imbibition in Nanoporous Photonic Crystals, Phys. Rev. Lett. 125, 234502 (2020).

    (2)   M. Brinker, G. Dittrich, C. Richert, P. Lakner, T. Krekeler, T.F. Keller, N. Huber, and P. Huber, Giant Electrochemical Actuation in a Nanoporous Silicon-Polypyrrole Hybrid Material, Science Advances 6, aba1483 (2020).

    (3)   M. Brinker and P. Huber, Switchable Elastocapillarity in Silicon Nanopores, manuscript.

    A unified numerical model for wetting of soft substrates

    Dominik Mokbel
    Fakultät Informatik/Mathematik, Hochschule für Technik und Wirtschaft Dresden

    The wetting of deformable elastic structures has been recently shown to comprise a rich variety of new physical phenomena (stick slip motion, durotaxis, etc.) whose fundamental understanding demands for numerical simulation tools. However, there are several numerical challenges to overcome, including the multiphysics and multiscale nature of the problem.
    In this contribution I will present a novel unified model which enables a variety of soft wetting phenomena to be examined more closely for the first time. As one special feature I will point out the inclusion of viscosity in the elastic structure, which allows to model complex interactions between viscous fluids (e.g. water droplets in air) and viscoelastic substrates. Among others, this enables the observation of surfing droplets in the simulations.


    Wetting of Polyelectrolyte (Multi)layers: Effect of Swelling and Droplet Evaporation

    Regine von Klitzing
    Institute for Condensed Matter Physics, TU Darmstadt, Germany

    Wetting of polymer coating is of specific interest for the adhesion of cells or proteins and the technical control of wetting. Cells or proteins are in a physiological environment, i.e. an aqueous solution and the polymer coatings are often hydrophilic or partially hydrophobized. Therefore, the coatings often swell in water which changes the wettability during the wetting process. This complex process is not well understood. Only a few studies on the water wettability of polyelectrolyte-coated surfaces exist [e.g. 1-4]. Tay et al. found an effect of osmotic pressure on the contact line in wetting studies of charged and uncharged polymeric coatings [5]. That makes the wetting process rather complex, since different time scales but also different length scales come into play. The adaptive surface needs a certain time until the swelling process is finished. The swelling doesn’t stop at the three phase contact line, and the liquid sucks laterally into the region in contact with the gas phase. Hansen and Miotto called that it peripherical thickness [6]. This deforms the surface and the contact line is not well-defined anymore.

    We studied the wettability by water of polyelectrolyte mono- and multilayers with different polycations or polyanions as the outermost layer using the sessile drop technique [3]. Measurements in a water-saturated atmosphere and in ambient conditions [40% relative humidity (r.h.)] are made to study the effect of swelling and evaporation on the contact angle. It is found that these effects strongly depend on the outermost layer of the polyelectrolyte coating. For several kinds of polyelectrolytes as outermost layer the polyelectrolyte-coated surface can be equilibrated by pre-swelling in saturated vapor. Depositing a water droplet leads to a fixed contact angle against vapor. For other types polyelectrolytes as the outermost layer the water contact angle also indicates a change in the swelling state when the pre-equilibrated film is directly in contact with liquid water, resulting in a decrease in contact angle with time. The studies show that a highly sophisticated interplay between hydrophobic backbone and charge density determines the wetting behavior, irrespective of the sign of surface charge. The stability of the wetting film on polyelectrolyte surface is analysed by disjoining pressure isotherms [7]. There surface charge seems to be the most dominant factor for stability of the wetting film.

    [1] Yoo, D., Shiratori, S.S., and Rubner, M.F., Macromolecules 1998, 31, 4309.
    [2] Kolasinska, M. and Warszynski, P. Bioelectrochemistry 2005, 66, 65.
    [3] K. Hänni–Ciunel, G. H. Findenegg, R. v. Klitzing, Soft Materials 1998, 5, 61.
    [4] H.-J. Butt, R. Berger, W. Steffen, S. Vollmer, S. A. L. Weber, Langmuir 2018 34, 112992.
    [5] Tay, A., Lequeux, F., Bendejacq, D., Monteux, C., Soft Matter 2011, 7, 4715.
    [6] Hansen, R. S.; Miotto, M., JACS 1957, 79, 1765.
    [7] K. Ciunel, M. Armelin, G. H. Findenegg, R. v. Klitzing, Langmuir 2005, 21, 4790.

    Simple gradient dynamics model for drops on elastic substrates

    Christopher Henkel
    Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster

    The investigation of the wetting behavior on viscoelastic or elastic substrates is of great interest. In this talk we present a simple model for steady liquid drops on fully compressible elastic substrates and show that a double transition of contact angles appears under variation of the substrate softness, similar to the one described in [1]. We further discuss whether these angles agree with the Neumann and Young-Laplace conditions in the liquid-liquid and liquid-solid limit respectively and how the transitions depend on droplet size. Finally, we employ a gradient dynamics model in the long-wave limit and show first results of direct time simulations.

    [1] Lubbers, L. A., Weijs, J. H., Botto, L., Das, S., Andreotti, B., and Snoeijer, J. H., (2014). Drops on soft solids: free energy and double transition of contact angles. Journal of fluid mechanics, 747.