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.

  • Summer term 2023

    Date Talks










  • Winter term 2022/23

    Date Talks

    Gradient flows coupling order parameters and mechanics

    Dirk Peschka & Leonie Schmeller
    Weierstrass Institute for Applied Analysis and Stochastics, Berlin

    In this talk, a general framework to couple phase-field evolution will be presented. The approach covers nonlinear viscoelasticity, diffusion, phase-separation and phase-transitions and varous types of nonlinear coupling. For this, different types of energy, dissipation and coupling will be presented. Based on the corresponding gradient system general space and time discretizations are proposed and two main applications will be discussed: i) Relaxation of a simple droplet on a simple viscoelastic substrate and ii) the additional effect of diffusion and phase-separation in the solid.

    See also:
    Schmeller, L., & Peschka, D. (2022). Gradient flows for coupling order parameters and mechanics. WIAS Preprint 2909, 2022 (submitted to SIAP) 


    Stick Slip and Slip Stick is not Slap Stick for a Drop

    Rüdiger Berger
    Max-Planck-Institut für Polymerforschung Mainz

    Wetting imperfections are omnipresent on surfaces. They cause contact angle hysteresis and determine wetting dynamics. Wetting of surfaces depends critically on defects, which alter the shape of the drop. I will present a tool, which we named scanning Drop Friction Force Microscopy (sDoFFI). With sDoFFI we measure the friction force acting on a drop while it is scanned over a surface. I will discuss two scenarios, which we recently studied [1,2]: 

    (i) the receding line of a drop is interacting with a topographic defect. In addition to friction force measurements, we imaged the contact line of the sliding drops by video microscopy. From the deformation of the contact line, we calculate the force acting on a sliding drop using an equation going back to Joanny and de Gennes (J. Chem. Phys., 81 (1984) 554).

    (ii) a drop is scanned across the transition of areas having different contact angles. The sliding of a drop across this transition line follows a characteristic stick-slip motion. We use the variation in force signals, advancing and receding contact line velocity, and advancing and receding contact angles to identify zones of stick and slip motion of the drop.

    Possibly, the sDoFFI is a new tool for 2D characterization of wetting properties, which is applicable to laboratory-based samples but also characterizes biological and commercial surfaces.

    [1] A. Saal, B.B. Straub, H.-J. Butt, R. Berger EPL, 139 (2022) 47001; doi: 10.1209/0295-5075/ac7acf
    [2] C. Hinduja, A. Laroche, S. Shumaly, Y. Wang, D. Vollmer, H.-J. Butt, R. Berger, accepted for publication in Langmuir

    Dewetting morphologies on modelled switchable substrates

    Stefan Zitz
    Roskilde University

    Switchable and adaptive substrates emerged as valuable tools for the control of wetting and actuation of droplet motion.
    In this presentation we show a computational study of the dynamics of an unstable thin liquid film deposited on a "switchable" substrate. Switchable in the sense that we use a space and time varying contact angle, or phrased differently a wettability wave. With a static pattern, all the fluid is drained into droplets located around contact angle minima, whereas for a sufficiently large rate of wettability variation a state consisting of metastable rivulets is observed. A criterion discriminating whether rivulets can be observed or not is identified in terms of a single dimensionless parameter.
    Finally, we show and explain theoretically how the film rupture times, droplet shape and rivulet life time depend on the pattern wavelength and speed. 

    02.12.2022 skipped due to SPP conference

    Modelling Bio-inspired Wetting Phenomena

    Halim Kusumaatmaja
    Durham University

    This talk will consist of two parts. In the first part of the talk, I will focus on our modelling work using the lattice Boltzmann method to study the behaviour of droplets on the so-called liquid infused surfaces (LIS). Inspired by pitcher plants, LIS are typically constructed by infusing rough, textured, or porous materials with a lubricant. The lubricant forms a wetting ridge around a droplet, and I will discuss how understanding the shapes of and the flow dynamics within the ridge is key to describe contact angle, contact angle hysteresis, and viscous dissipation on liquid infused surfaces [1,2]. In addition, I will consider the motion of drops on LIS with texture gradients: when the lubricant is only partially wetting the solid surface, I will show that bi-directional droplet motion can be achieved with the same surface texture [3].

    The second part of the talk concerns with wetting phenomena inside biological cells. There is now burgeoning evidence that liquid droplets composed of RNA and/or proteins form through a phase separation like process, and that they interact with other cellular components [4]. Here, I will focus on the importance of droplet wetting on lipid membranes. As a key example, during seed development of the plant Arabidopsis thaliana, micrometer-sized liquid droplets form within the vacuolar lumen and wet the tonoplast. Distinct tonoplast shapes arise in response to membrane wetting by droplets and I will discuss a simple theoretical model that conceptualises and rationalises these geometries [5]. Conditions of low membrane spontaneous curvature and moderate wettability favor droplet-induced membrane budding, whereas high membrane spontaneous curvature and strong wettability promote a membrane nanotube network that forms exclusively at the droplet surface.

    [1] Semprebon, C.; McHale, G.; Kusumaatmaja, H. Apparent Contact Angle and Contact Angle Hysteresis on Liquid Infused Surfaces, Soft Matter 2017, 13, 101.
    [2] Sadullah, M. S.; Semprebon, C.; Kusumaatmaja, H. Drop dynamics on liquid infused surfaces: the role of the lubricant ridge, Langmuir 2018, 34, 8112.
    [3] Sadullah, M. S.; Launay, G.; Parle, J.; Ledesma-Aguilar, R.; Gizaw, Y.; McHale, G.; Wells G. G.; Kusumaatmaja, H. Bidirectional motion of droplets on gradient liquid infused surfaces, Communications Physics 2020, 3, 166.
    [4] Kusumaatmaja, H.; May, A. I.; Knorr, R. L. Intracellular Wetting Mediates Contact between Liquid Compartments and Membrane-Bound Organelles, Journal of Cell Biology 220, e202103175 (2021)
    [5] Kusumaatmaja, H.; May, A. I.; Feeney, M.; McKenna, J. F.; Mizushima, N.; Frigerio, L.; Knorr, R. L. Wetting of Phase-Separated Droplets on Vacuole Membranes Leads to A Competition between Tonoplast Budding and Nanotube Formation, Proceedings of the National Academy of Sciences 118, e2024109118 (2021)

    10.02.2023 High voltage generated by sliding drops

    Stefan Weber
    Max-Planck-Institut für Polymerforschung

    Slide electrification is a spontaneous charge separation process between an insulating hydrophobic surface and a sliding drop[1]. We recently showed that the electrostatic interaction between charges within the drop and on the substrate has a significant effect on the drop motion [2]. While sliding, the drops generate voltages as high as several kV that can even switch polarity with changing drop rates. To describe the charge separation, we introduce a theory of surface charge adaptation on a timescale of ≈ 100 ms. We conclude a mathematical model that quantitatively agrees with experiments [3]. Our finding of high voltages has implications for energy harvesting while adaptively charging surfaces could open new routes in droplet micro fluidics.

    [1] A. Z. Stetten, D. S. Golovko, S. A. Weber, and H.-J. Butt, Soft matter 15, 8667 (2019).
    [2] X. Li, P. Bista, A. Z. Stetten, H. Bonart, M. T. Schür, S. Hardt, F. Bodziony, H. Marschall, A. Saal, X. Deng, et al., Nature Physics , 1 (2022).




  • Summer term 2022

    Date Talks

    Tatiana Gambaryan-Roisman
    Institute for Technical Thermodynamics, Technical University of Darmstadt

    Spreading, imbibition and evaporation of liquid on porous layers

    Spreading, imbibition and evaporation of drops on porous layers are encountered in numerous applications, including printing, sampling of biological liquids and functionalization of textiles. In addition, using porous coating may lead to significant heat transfer enhancement during drop impact onto hot surfaces, flow boiling in minichannels as well as during evaporation of a single meniscus within a gap. The enhancement of heat transfer can be explained by imbibition of liquid into the porous coating, which leads to increasing of liquid-gas interfacial area and of the cumulative length of contact lines. In this talk, we discuss the influencing factors governing the kinetics of liquid spreading, imbibition and evaporation on substrates with porous coatings, the maximal imbibed area and the drying time. In particular, the effects of the thickness of nanoporous coating, of the drop size and of the substrate temperature are considered. In addition, the influence of deformability of porous coating on imbibition process is discussed.

    Christian Honnigfort
    Westfälische Wilhelms-Universität Münster

    Molecular Kinetics Control the Dynamic Wetting in Photoresponsive Monolayers

    Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. The functionalized surfaces showed reversible E/Z photoswitching causing contact angle changes of up to ∼10°. We monitored these changes on the macroscopic level by recording the contact angle while the monolayer was switched in situ. On the molecular level, time-dependent vibrational sum-frequency generation (SFG) spectroscopy provided information on the kinetic changes within the AAP monolayer and the characteristic times for E/Z switching. Having established the switching times on the molecular scale, we additionally measured the dynamic contact angle and show that the time scales of the substrate and droplet dynamics can be extracted individually. For that, a relaxation model that is solved analytically was established and verified by comparison with simulations of a Lennard–Jones system conducted in the group of Andreas Heuer.


    Hector Gomez
    Purdue University, West Lafayette

    Computational modeling of elasto-capillarity

    Elasto-capillarity involves the deformation of an elastic solid due to the capillary forces at a fluid-fluid interface. In recent years, multiple intriguing experiments involving elasto-capillary phenomena have been reported, including spontaneous migration of droplets on deformable surfaces with stiffness gradients (durotaxis) or strain gradients (tensotaxis), self-wrapping of liquid droplets when placed in contact with elastic membranes and wrinkling patterns arising due to the placement of liquid drops on ultra-thin sheets with low bending stiffness. However, high-fidelity computational modeling of elasto-capillarity has received less attention. We have developed computational models that open new possibilities to better understand elasto-capillary phenomena. In this talk, I will present our computational models, and I will show how we have used them to show that droplet durotaxis can be inverted by altering the wettability of the system and to study soft wetting with compound droplets. 

    Sebastian Burgmann
    Bergische Universität Wuppertal

    Investigation of the incipient motion and flow structure of a sessile drop in shear flow and additional vibrational excitation

    The removal of droplets on surfaces by an (air-) flow is relevant, e.g. for cleaning processes or to prevent corrosion or damage of electronical devices. We present a combined numerical and experimental study about the flow phenomenon of sessile drops in shear flow with additional vibrational excitation. In particular, we investigate the critical velocity that is needed to induce the incipient motion of the drop for pure shear flow and superimposed vertical and horizontal vibrational excitation. In addition, we apply laser-optical measurement techniques and numerical simulation to analyse the flow structure inside and outside of the drop. We report on the finding that the critical velocity can be significantly reduced if the proper excitation frequency is selected, which is correlated to the Eigen-frequencies of the drop. Additionally, we present a coherence of the inner and outer flow structure of the drop. A characteristic flow separation phenomenon occurs that shows features of an aeroelastic process.


    Jonathan Pham
    Chemical and Materials Engineering, University of Kentucky

    Soft wetting on swollen polymer networks

    Soft interfaces are found in a host of applications, from biological situations to adhesives and soft devices. However, understanding the behavior of soft interfaces is an ongoing challenge. For example, wetting, adhesion, and friction are still poorly understood. When materials are sufficiently soft, or the characteristic size scale is small, crosslinked polymeric solids can display liquid-like characteristics, like capillarity. When these crosslinked polymers are infused with a fluid (i.e., swollen gels), the fluid itself can also afford true liquid behavior, creating multi-phase situations that are even more complex. In this talk, we show how combinations of solid and liquid characteristics control the wetting on soft, swollen elastomers. In addition to the polymer elasticity, we demonstrate that surface tension, fluid separation, and osmotic pressure are important considerations. Based on confocal microscopy information, we propose a concept for phase separation and network deformation near the contact line. In addition, our results show that mechanical properties and degree of swelling can play a role in drop pinning and sliding. For soft silicone elastomers, aging leads to re-lubrication at the top surface, generating a more slippery surface over time. Time-permitting, we will demonstrate wetting on high-temperature, uncrosslinked polymers, where melt rheology helps describe the wetting ridge growth.

    Lucia Wesenberg
    Institut für Theoretische Physik, Georg-August-Universität Göttingen

    Adhered vesicles with finite-range membrane-substrate interaction and buoyancy

    Experiments on the adhesion of liquid droplets or vesicles on switchable surfaces often facilitate contact with the substrate by a density difference (buoyancy). But when compared to theoretical expectations, this key experimental characteristic as well as the finite range of the membrane-substrate interaction have often been neglected. Here, we systematically studied the adhesion of axially symmetric vesicles for finite-range membrane-substrate interaction and buoyancy through simulations.
    We investigated the adhesion transition of vesicles in the absence of thermal fluctuations and compare the vesicle adhesion to the wetting of liquid droplets. For downward buoyancy, vesicles merely sediment onto the substrate and there is no mean-field adhesion transition. Whereas for upward buoyancy, adhered vesicles are metastable at best. A proper adhesion transition can only occur for zero buoyancy. Moreover, length scales such as the capillary length, extrapolation length and curvature-decay scale exhibit a pronounced dependence on interaction range and buoyancy and should thus not be used uninformed. Whereas the interaction range and buoyancy significantly modified the adhesion diagram, the local transversality condition — relating contact curvature to adhesion strength and vesicle’s bending rigidity — remains accurate in the presence of moderate buoyancy.

    Lukas Hauer
    Max-Planck-Institute for Polymer Research, Mainz

    Liquid phase separation in dynamic wetting ridges on PDMS

    Droplets sitting on soft substrates form a wetting ridge in the material around the three-phase contact line. While on liquid films the ridge geometry is solely governed by capillarity, on (visco)elastic films elastic contributions add to the ridge geometry. Recently, on (visco)elastic gels a capillary induced phase-separated region of pure liquid was observed. Here, we investigate this phase separation on crosslinked pdms gels with differing amounts of uncrosslinked (free) pdms during dynamic wetting. We let droplets forcefully slide over PDMS gels while monitoring the ridge zone with laser scanning confocal microscopy. Different dyes in the crosslinked and the uncrosslinked pdms enable discrimination between the two phases. We find that phase-separation competes with the motion of the droplet; by tuning the droplets' speed, the phase-separated ridge height ranges from >30 µm (at 5 µm/s) to no phase separation at all for fast speeds.


    Patricia Weisensee
    Washington University, St. Louis

    Dynamics of droplet-meniscus interactions on lubricant-infused surfaces

    Lubricant-infused surfaces (LISs) have aroused significant interest due to their promising potentials in many industrial applications, including drag reduction, anti-fouling coatings, or water harvesting. My lab has studied LISs for many years in the context of dropwise condensation heat transfer. In this presentation I will focus on elucidating some of the fluid dynamic interactions between water droplets and the lubricant oil, rather than the heat transfer itself. During condensation, the thin oil layer constantly re-distributes, forming oil-poor and oil-rich regions. Overlapping oil menisci around water droplets lead to spontaneous and gravity-independent microdroplet propulsion with travel velocities up to 1.1 mm/s for droplets as small as ~10 µm. This high droplet mobility aids in frequently clearing nucleation sites, promoting higher heat transfer rates as compared to solid hydrophobic surfaces. When looking at an individual meniscus surrounding a larger central droplet (or solid sphere), capillary attraction also plays a role, but can be offset by thermocapillary (Marangoni) convection within the meniscus. While relatively larger droplets (larger than the meniscus height) climb the meniscus, as expected, the smallest droplets are transported in the opposite direction, away from the central object. I will show that this size-dependent microdroplet behavior on cooled substrates can be explained by a competition of capillary attraction and viscous drag. We hope that our work can make a contribution towards designing robust (i.e., long-living) and efficient LISs for condensation heat transfer applications by providing a better understanding of the dynamic interplay between water droplets and the lubricant oil.

    Martin Essink
    University of Twente

    Shuttleworth effect and dynamics in soft wetting

    The wetting of droplets on rigid substrates is a well understood problem, with a distinct static solution set by the Young angle. When the substrate is soft, however, the static solution becomes highly dependent on the elastic deformations induced by the droplet. Additionally, the wetting energies can change with the stretching of the substrate surface, known as the Shuttleworth effect. The key features of Soft Wetting are found using Finite Element and Gradient Dynamics simulations. We find how the substrate softness, thickness, and the Shuttleworth effect can change the contact angle of the droplet. Finally, in light of recent experiments, we also investigate the dissipation in the elastic layer. 

    Hansol Jeon
    Max Planck Institute for Dynamics and Self-Organization, Göttingen

    Fast contact lines on soft solids

    When a droplet is resting on a soft surface, the capillary forces deform the surface into a sharp
    wetting ridge. The amplitude of the wetting ridge is determined by elasto-capillary length, but
    the angles by which the interfaces meet at the ridge tip only depend on the balance of surface
    tensions, the so-called Neumann balance. For moving contact lines, dissipation in the wetting
    ridge leads to viscoelastic braking. In recent literature, various effects that could alter Neumann
    balance and viscoelastic braking have been suggested, ranging from free, extractable oligomers
    to point forces emerging from bulk viscoelasticity. We visualize moving wetting ridges at high
    spatio-temporal resolution and determine the tip geometry for various liquids and PDMS
    substrates. We observe an increase of the ridge opening angle at large speeds, even for very
    mild deformations caused by the low surface tension of a fluorinated oil. We also find no
    significant change in ridge rotation and opening angle for gels with different fractions of cross-
    linked and free chains, nor for different bulk rheological properties. These findings highlight the
    need for a non-trivial surface constitutional relation that is different from the bulk.


    Günter Auernhammer
    Leibniz-Institut für Polymerforschung Dresden

    Droplets on soft and responsive substrates

    In this presentation, I will discuss drop dynamics in solid, but sometimes soft, substrates. Sitting drops on soft substrates are influenced by the substrate properties from the nucleation phase to the fully formed drops. For responsive substrates, the substrate properties, and thus the drop behavior depends not only in the current wetting liquid, but also on the wetting history. Finally, I address a few specific features in the merging of immiscible drops. 

    Niloofar Nekoonam
    IMTEK, University of Freiburg

    Photoswitchable soft substrates for droplet behavior evaluation upon on-demand softness alteration

    Softness has shown a significant influence on droplet behavior on soft substrates. Alteration of thickness, crosslinking degree or curing time on different spots of PDMS or hydrogels have been shown to provide softness patterns which have been used for droplet movement or durotaxis experiments. For further studies and applications, softness patterns that can be switched on-demand on the surface or below the droplet are of high interest. Here, we introduce a photoswitchable gel which shows significant reversible changes in softness upon UV irradiation. The material is based on entangled reverse wormlike micellar systems. These systems are often based on volatile solvents and very low elasticity as their modulus is in the range of a few Pascals. Therefore, we have enhanced the viscoelastic behavior of the material via addition of silica particles, while the photoswitchable properties were preserved. The material was employed as a soft substrate with adjustable softness to evaluate the wetting ridge formation depending on the local softness of a substrate. Wetting ridges were visualized using confocal microscopy on substrates with different silica content, and later compared before and after UV exposure to the substrate. The high resolution of the photoswitchable softness allows softening of one side of the droplet for the investigation of droplet behaviour such as asymmetric spreading. 

  • 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

    Setting droplets in motion by dynamic wettability patterns

    Holger Stark
    Technische Universität Berlin

    Photo-switchable substrates provide a unique mechanism to manipulate liquid droplets by creating a light-induced wettability landscape, which changes in space and time. This controls the shape of the droplet contact line and contact angle and ultimately sets the droplet in motion. At low Reynolds number moving droplets might be used in lab-on-a-chip devices for transport and fluid mixing.

    In the talk I start with introducing the boundary-element method, which we use to determine fluid flow within the droplet from the governing Stokes equations. Supplemented by appropriate boundary conditions and the Cox-Voinov law for the contact line, we are able to simulate a moving droplet with arbitrary shape.

    First, I illustrate our method by discussing how a droplet surfs on a moving wettability step or is left behind and also introduce a feedback loop by which the droplet controls its own motion. Second, I discuss droplets on an oscillating wettability profile. For small frequencies the spherical-cap model together with the Cox-Voinov law provides a master curve for the droplet oscillations, while for larger frequencies non-reciprocal shape changes give rise to deviations from the master curve and generate toroidal flow within the droplet. In the end I mention some possible directions to extend our studies.

    Oil-cloak influence on droplet evaporation

    Lukas Hauer
    Max-Planck-Institute for Polymer Research, Mainz

    When oil surrounds a sessile water drop, various wetting configurations can be observed, such as the formation of wetting ridges or cloaking. Cloaking describes the wrapping of the entire water-air interface by the oil. The cloaking layer influences the interfacial energy of the sessile drop and affects phase change interactions between drop and ambient, i.e., condensation and evaporation. The thickness of the cloaking layer is governed by Hamaker interactions and is thought to be in the order of 10 to 100 nm. Here, we investigate cloaking of a water drop by hexadecane and silicone oil. While both like to wrap the water-air interface, only silicone oil cloaks the drop entirely. We utilize confocal laser scanning microscopy to measure the volume of the cloaking layer around a droplet. Further, we conduct macroscopic drop-evaporation experiments in which we observe slower evaporation rates on cloaked drops compared to uncloaked ones.


    Dynamics of liquid droplets on switchable substrates – from microscopic to mesoscopic models

    Moritz Stieneker, Leon Topp, Svetlana Gurevich, Andreas Heuer
    Westfälische Wilhelms-Universität Münster

    In this talk we will theoretically investigate the dynamical properties of a simple liquid on a solid switchable substrate. To understand the non-equilibrium relaxation dynamics of a liquid droplet on a switchable substrate first we study the interplay of different length- and time-scales. We present a method to map the microscopic information, resulting from a molecular dynamics simulation, to a mesoscopic scale, reflected by a thin film mesoscopic model. We analyze the relaxation of a liquid droplet after switching the wettability of the substrate and discuss the cases of the slow and fast periodic switching. Surprisingly, we observe different mapping regimes, depending on the direction of the switching. Interesting phenomena can be also observed when considering an exponential switching process. In general, in that case the wetting dynamics displays non-exponential relaxation effects. The results of an analytical model as well as of model simulations are quantiatively compared with the results of wetting experiments on photoswitchable substrates. Finally, using the molecular dynamics simulations we can extract parameters that we can pass into the equations obtained from the molecular kinetic theory (MKT) of wetting, allowing us to compare our results with those obtained from this theory in case of a single switch as well as for periodic switching of the surface.


    Deformation and contact formation of thin liquid films on a hard solid substrate for slowly impacting drops

    Kirsten Harth
    Otto von Guericke University Magdeburg

    When impacting droplets approach a hard plane substrate slowly, so that the Weber number is below approximately 5, a contact-less rebound will occur due to the entrainment of ambient gas. On slightly deformable and smooth spin-coated liquid films upon a rigid solid, this effect is more robust and may occur until slightly higher Weber numbers. Deformation of the thin film is usually ignored – while it certainly occurs. The deformation amplitude depends on the impact dynamics as well as the thickness and viscosity of the surficial oil layer. At slightly higher impact velocities, i.e. slightly higher Weber numbers, delayed contact formation between the film liquid and the droplet occurs. Depending on the layer’s properties, interestingly, the contact line may be unstable displaying a fingering texture. Instability occurs independently of whether the drop and film liquid differ or not.

    Nanoporous silicon: switchable wetting using a conductive polymer and dynamics of radial imbibition

    Laura Gallardo Domínguez
    Hamburg University of Technology

    Fluid dynamics in nanopores plays a role in many natural and artificial systems. When a droplet gets in contact with a nanoporous substrate three different contributions come into play: spreading, evaporation and imbibition. Here we intend to analytically and experimentally separate these contributions to analyze their impact in the dynamics of the system. In the future, we want to use electrowetting to trigger this interplay in an initially hydrophobic surface. We are also currently exploring the possibility of combining porous silicon with polypyrrol, a conductive polymer that may facilitate the switchability of the system. Preliminar experiments in this direction are presented.

    14.01.2022 skipped

    Variational approach for wetting flows on solid and soft substrates

    Dirk Peschka
    Weierstrass Institute for Applied Analysis and Stochastics, Berlin

    It is well known that different magnitudes of Navier-slip can have a significant influence on the structure formation in dewetting processes. Using interfacial processes as an example, I discuss how different choices in the energetic-variational description for wetting flows lead to a thermodynamically consistent formulation, how this choice is physically motivated, and how finite-element-based discretization methods can be constructed directly from it. As an example, I discuss dewetting flows on solid and soft substrates with a hierarchy of models for different scales. In particular, I address structure formation during dewetting, the influence of contact line dissipation, and modeling of Navier-slip on solid and soft substrates.


    Micro-trenched dynamic, switchable and stimuli responsive surfaces for on-demand trapping and release of drops 

    Anil Rajak
    Chemistry and Physics of Interfaces, Department of Microsystems Engineering, University of Freiburg, Germany 

    On rough hydrophobic surfaces, wetting can be described either by Cassie-Baxter model where a drop sits suspended on the structures or by Wenzel model where the drop is penetrated by the structures. The drop in Wenzel regime sticks to the surface which is very difficult to release and in case of Cassie-Baxter regime, the drop is very difficult to trap. We present a third wetting regime, the so-called High Hysteresis Suspended State (HySS) that can be observed on regular patterned micro-trenches. In this regime, the drop suspends like in Cassie-Baxter regime while exhibiting high hysteresis (up to values >70°), as in Wenzel state. We present a full model based on our theoretical calculations and experimental results for highly controlled wetting transition between these states (induced by aspect ratio, mass of the drop and applied pressure), trapping of drops and releasing them by an addition of small energy (induced by tilt or mass). We use our study to fabricate a novel way to trap and release the drops – a bi-layered piezo-driven dynamic micro-trenched surface. The top layer is a SU-8 membrane with micro-stripe shaped holes and the bottom layer is a Silicon wafer etched with tall micro-ridges. By using piezo-drive, the micro-ridges can move up and down through the micro-stripes. By using same chemistry on both the layers, the drops can stick (trap) on the membranes and roll-off (release) on the ridges. 
    We also look into light-responsive azobenzene group-covered micro-trenches for controlled trapping and release of drops. While it is easy to trap a Cassie-drop to a Wenzel-drop on a surface by switching UV light, it is relatively difficult to release it. In other words, the energy difference between the two isomeric states provided by the light itself is not enough to move the center of mass of the Wenzel-drop to a Cassie.drop. We calculate the potential energy of the drop to compare the energies and explain this theory. 


  • 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

    Date Talks

    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.