Efficient and accurate numerical simulation of dynamic wetting of flexible structures

PI: Prof. Dr. Sebastian Aland, Fakultät Mathematik und Informatik, TU Freiberg

Wetting of elastic substrates (i.e. soft wetting) plays a major role in a broad variety of phenomena in nature and technology. In the previous funding period we have developed one of the first numerical frameworks for robust simulation of soft wetting. We now aim to not only use this method to analyze accessible soft wetting phenomena, but to extend the methodology in two directions, which have remained completely unexplored so far. Firstly, we will develop a numerical method to account for an accurate dissipation mechanism at the contact line by including slip and dynamic contact angles. This will result in the first description of elastocapillarity providing realistic dynamics at the microscale. Secondly, we will extend the methodology to thin elastic structures, which are actually involved in the majority of soft wetting phenomena. Representing thin structures by dimensionally reduced (hyper-)surfaces will solve the associated resolution problems.The resulting multiscale model will permit for the first time to simulate soft wetting phenomena at membranes, thin sheets, microstructured surfaces, hair and other thin objects. The numerical models are applied to investigate three exciting phenomena that have been discovered in experiments: (i) the intense interplay of wetting and shape dynamics of biomembranes, (ii) the wetting dynamics at elastic structured surfaces, and (iii) the influence of an adaptive lubrication drop on the contact line motion on oil-absorbing substrates. All these points are addressed in collaboration with experimental and theoretical projects within the SPP to finally establish a deeper understanding of the fundamental physics behind the dynamic wetting of elastic substrates.

Microscopic Mechanisms and Surface Adaptation Effects in Slide-Electrification

Dr. Denis Andrienko, Max Planck Institute for Polymer Research, Mainz
Prof. Dr. Stefan Weber, Max Planck Institute for Polymer Research, Mainz

Slide electrification describes a spontaneous charge separation between a moving liquid droplet and a hydrophobic surface. Depending on substrate material and liquid, the drop accumulates either positive or negative charge with the substrate charging oppositely. Here, the drop charge can be influenced by the contact time between surface and water drop and the time between subsequent drops. Models, that describe the charge generation, therefore include a surface adaptation term, very much akin to wetting adaptation phenomena. The microscopic origin of slide electrification and the surface adaptation effects is still unclear. The mechanism has been rationalized phenomenologically, assuming charge pinning at the receding contact line. Here, electron transfer, adsorption/desorption of ions, or an incomplete reorganization of the electric double layer could all lead to the observed effects. Thus, the first goal of our project is a molecular understanding of slide electrification. We will combine experiments with computer simulations and systematically change the solid-liquid interface by varying the three components that are involved: 1) the substrate, 2) the hydrophobic layer 3) and the liquid (type of liquid, salt concentration, pH). For the substrate, the dielectric permittivity and surface chemistry are the important factors contributing to slide electrification. To investigate the contribution of ionic and electronic charge transfer to the slide electrification process, we will also include carbon-based model surfaces with defined electronic conductivity. To control the grafting density and the charge diffusion at the surface, we will change the molecular weight of the hydrophobic layer and the surface roughness. In addition, we will vary the surface chemistry, e.g. with hydroxide and amine functionalized surfaces. To study the interplay between wetting adaptation and charge adaptation, we will study adaptive polymeric and polyelectrolyte surfaces in cooperation with other groups within this SPP. Thus, we want to clarify the mechanisms behind the voltage that forms at the three-phase contact line and the surface adaptation. The application of slide electrification is electricity generation, a potential source of renewable energy. Currently, the efficiency of energy conversion from kinetic droplet energy to electric power is very low: much less than 0,1 %. Therefore, the second goal of the project is to understand the fundamental limits of the slide electrification efficiency. We want to identify substrate – surface layer – liquid combinations that yield improved conversion efficiencies. This will be achieved by computational pre-screening of surface combinations, aiming at those that maximize the surface voltage and the drop charge. The top performing combinations will be then characterized experimentally.

Impact of co-nonsolvency effects on dynamic wetting

Dr. Günter Auernhammer, Leibniz-Institut für Polymerforschung Dresden e.V.
Dr. Petra Uhlmann, Leibniz-Institut für Polymerforschung Dresden e.V.

The primary goal of this project continues to be the investigation of the dynamic wetting behavior of the polymer brushes showing cononsolvency effects. This proposal for the second funding period (FP2) heavily builds on the results of the first funding period (FP1). For this purpose, we keep the same model system: PNiPAAm brushes prepared by a grafting-to method with water and ethanol as solvent and cosolvent. In FP1 we have identified at couple of important features that suggest a coupling of dynamic wetting behavior and the cononsolvency of PNiPAAm brushes. (i) Intense (not necessarily long) prewetting with water or ethanol changes the wetting properties strongly but reversibly and in a long-time stable way. (ii) Repeated wetting and the composition of the gas phase can change the wetting properties. (iii) The changes of the wetting properties are correlated to the structural changes in the PNiPAAm-brush surface as seen with sum frequency generation spectroscopy.These results posed several important questions that we plan to address in FP2. (a) Which are the parameters of the prewetting procedure that mainly control the static and dynamic wetting properties for a successive wetting? (b) Which storage and (pre)wetting conditions strengthen or reduce the long-time stability of the induced wetting state? (c) Regarding its influence on wetting, how is cononsolvency interrelated to thermo-sensitivity of PNiPAAm? (d) Do polymer brushes exhibiting a cosolvency effect show similar features in their wetting behavior as those with cononsolvency effects? To address these questions, we will use a set of complementary experimental techniques (partially in cooperation with other groups within the SPP), see the Methods section in 2.3. (i) Dynamic wetting experiments in a broad range of well-defined contact lines velocities (from μm/s to tens of cm/s). (ii) Analytic methods like spectroscopic VIS-ellipsometry (brush layer thickness), AFM-based techniques for the mechanical characterization, SFG spectroscopy for structural information. (iii) Systematic variations of the experimental parameters, like the properties of the polymer brush (molar mass, brush thickness, cononsolvency vs. cosolvency effect), the time scales and contact-lines velocities, and the composition of the wetting liquid and gas phase In the combination of these methods, we aim for a fundamental understanding on how the wetting of polymer brushes is interrelated with the cononsolvency of the brush.

Focusing Effect for Sliding Drops Induced by Adaptive Surfaces

PI: Dr. Rüdiger Berger, Max-Planck-Institut für Polymerforschung Mainz

Surface adaptation has been put forward to be one cause for the velocity dependence of advancing and receding contact angles, even at low slide velocity. By considering a dry-to-wet and a wet-to-dry relaxation process, we had proposed a model, which is able to explain these dynamic contact angles (Langmuir 2018, 34, 11292). The aim of our project is to experimentally verify the model and explore, how applicable it is. In the first funding period, we built an inclined plane setup to measure advancing and receding contact angles versus velocity for water drops up to 1 m/s. We synthesized a random poly(styrene/acrylic acid) copolymer PS/AA and demonstrated that for water drops on PS/AA films, advancing-contact-angle-versus-velocity-curves can indeed be fitted with the model. The fit revealed relaxation times < 2 ms of the polymer at the surface. However, we are still missing independent measurements of the dry-to-wet relaxation times, which would be required for full validation of the model. The aim for the second funding period is to investigate polymer surfaces, where we can independently measure the relaxation time of the dry-to-wet and the wet-to-dry transitions. Then, recorded advancing-contact-angle-versus-velocity-dependencies can be validated independently using our model. Therefore, we plan to synthesize three types of polymer brushes: (1) PS and poly(2-vinylpyridine) (PVP) binary brush films, (2) pH-responsive poly(N,N-dimethyl aminoethyl meth¬acrylate) (PDMAEMA) brushes and (3) different block copolymer layers. For series of drops sliding on an adaptive surface we predicted a focusing effect. It is induced by partial adaptation caused by the first drop sliding along the surface. This drop leaves behind a lane of increased surface energy and we expect that subsequent drops will then slide along the same lane. Our aim is to verify this hypothesis.

Wetting on soft absorbing substrates: experiment, simulation and theory

Dr. Sissi de Beer, University of Twente
Prof. Dr. Jacobus Snoeijer, Ph.D, University of Twente
Prof. Dr. Uwe Thiele, Westfälische Wilhelms-Universität Münster

The wetting on soft polymeric substrates offers a richness of phenomena, which, owing to the versatility of polymer networks, constitute a great potential for designing adaptive surfaces. Much progress has been made on elastocapillary phenomena for the situation where liquid drops are not absorbed into the substrate. However, polymer networks can greatly enhance their functionality by liquid and vapor absorption, which can be tuned by the physical chemistry of the substrate and the ambient conditions. To leverage the full potential of soft substrates one thus needs to go beyond non-absorbing systems, involving new mechanisms that come on top of the elastocapillary phenomena studied in Phase 1 for non-absorbing substrates.The follow-up project in Phase 2 aims at obtaining and experimentally validating a coherent multiscale description that predicts the wetting (dynamics) on soft absorbing substrates. Several experimental setups, Molecular Dynamics simulations, nonlinear macroscopic poroelasticity theory, and effective mesoscale gradient dynamics models are developed to investigate the behaviour of drops of simple nonvolatile and volatile liquids on polymer brushes and hydrogel layers. This combination shall allow us to study the multiscale interplay of absorption and large-deformation mechanics near resting and moving contact lines. Dedicated experiments and simulations will provide calibration and validation of all modelling ingredients necessary for the systematic incorporation of new interactions and new modes of mass transfer into our flexible gradient dynamics modelling framework. Subsequently, we will explore how the swelling and spreading behaviour changes when a mixture of simple liquids is used instead of a single one. Since the composition of the absorbed solvent likely differs from the drop composition, nonlinear effects of the droplet composition on brush wetting and miscibility effects can be expected to influence droplet spreading and contact line motion. Finally, we investigate how the intriguing nonequilibrium phenomenon of the stick-slip motion of driven contact lines depends on the properties of absorbing substrates.At all stages, the project will be pursued in close cooperation between theory (Westfälische Wilhelms-Universität Münster), experiment and simulation (University of Twente). The flexible gradient dynamics modelling framework will be readily available to other SPP projects, and, more generally, will provide a toolbox for the study and design of soft absorbing substrates.

Structure-Property Relations and Wetting Dynamics of Organic Thin Films with Photo-Switches

PI: Prof. Dr. Björn Braunschweig, Westfälische Wilhelms-Universität Münster

Smart surfaces that can reversibly change their wetting properties are of great interest for applications such as self-cleaning surfaces, microfluidics or microreactors just to mention a few. In this project, we propose to study dynamic wetting at photoswitchable surfaces that can undergo E/Z conformational changes triggered by light irradiation or by changes in the interfacial electric field (electrode potential). In both cases, macroscopic changes like the dynamic contact angle are coupled to the processes that occur within a single molecular layer at the solid-liquid and the solid-vapor interface, where we propose experiments from which we will gain a more complete molecular picture of the coupled substrate and drop dynamics. These are substantially influenced by the interactions of the wetting fluid with the wetted substrate and an understanding of this interaction on a molecular scale is, thus, of key importance for a description of the resulting wetting dynamics. For that reason, we will explore in this project the molecular origin of the remarkably slow switching dynamics in arylazopyrazole photo-switchable monolayers and will then address the role of surface defects and the slow self-assembly during switching on the apparent wetting dynamics. These experiments are then extended to reveal the molecular mechanism of electro dewetting and its coupling to light using photo-switchable substrates and surfactants which can be also used to tune molecular interaction and dynamics of the drop and the substrate and to remotely control wetting with two orthogonal stimuli.

Modelling of spreading, imbibition and evaporation of liquids on structured or porous deformable substrates

PI: Prof. Dr. Tatiana Gambaryan-Roisman, Technische Universität Darmstadt

Spreading, imbibition and evaporation of liquids on deformable surfaces with roughness, topography or with porous coatings is important for numerous industrial applications. These phenomena are multiscale and cannot be described using the macroscopic approach, which ignores the effect of intermolecular interactions. The project is aimed at development of models for spreading, imbibition and evaporation of liquids on structured or porous deformable substrates and takes into account the effects of the surface forces, described employing the disjoining pressure concept. In the first funding period, the static and dynamic wetting of the droplets with height comparable with the range of surface force action has been studied using a thin film theory-based numerical model. It has been found that if the nanodrop spreads over a deformable solid, the wetting ridge can exhibit a non-monotonic dynamics. The evolution of the ridge shape depends on the parameters of the disjoining pressure isotherm. The capillary-driven flow in corner geometries has been studied. The surface forces have been shown to define the maximal length of rivulets in open wedge channels. Remarkably, the steady rivulets cannot be predicted when the surface forces are ignored. In the next funding period the developed models will be extended and used to describe the liquid spreading, imibibition and evaporation in nanopores, on deformable surfaces with topography and inhomogeneous distribution of mechanical properties, as well as in systems containing thin flexible sheets, lamellas and fibers. The results of these studies will allow elucidating the key phenomena governing imbibition on a scale of the whole porous structure. Homogenization techniques for computation of effective transport coefficients will be applied on the basis of this knowledge. These coefficients will be used for macroscopic-scale simulation of imbibition and evaporation on structured or porous deformable substrates, which are relevant to natural phenomena and technological applications. The results of simulations will be validated by comparison with experimental data.

Substrate and Drop Dynamics During Impact and Coalescence on Soft Adaptive Surfaces

PI: Dr. Kirsten Harth, Technische Hochschule Brandenburg

Droplets impacting on soft materials are frequently encountered ineveryday life, e.g. water drops on skin during showering, dropsimpacting on fresh paint or on plant leaves, or simply drops impacting onto a previously wetted surface. Such systems often consist of a (thin) soft and adaptive surface layer on a stiff substrate. The soft layer, however thin, can crucially alter the behavior of contact lines, and particularly of droplets, on microscopic and macroscopic scales. Contrasting to this, research so farmainly focused on hard surfaces, liquid pools and miscible liquid surface layers (of usually non-microscopic depth). The coupled problem of static shapes of a droplet on visco-elastic substrates has been modeled extensively, and comparatively slow processes of contact line motion have been addressed. Recent research develops more towards studies of the interaction ofsoft substrates with rapidly moving contact lines, as e.g. encountered during drop impact. However, in those studies, the dynamics of the substrate, while it is the actual reason for all changes of the macroscopic contact line and droplet behavior, the actual substrate dynamics is not recorded and not analyzed – measuring the microscopic deformations of thin surface layers during rapid processes requires substantial experimental effort and sophisticated, new techniques. My project deals with two exemplary processes, drop impact on soft flexible layers and drop coalescence on fluid layers of limited depth. The substrates may consist of viscous, or viscoelastic liquids, or of viscoelastic gels. Standard side and top / bottom view high-speed imaging will be combined with a number of non-standard high-speed bottom view techniques,providing access to the substrate's deformations, strains or contact formation processes of the surface with the liquid, as well as possibly entrained microscopic gas layers. My setup and expertise will beavailable to the other participating groups of SPP-2171 throughcollaboration, currently planned for several adaptive substrates. I willcollaborate with theoreticians on analytical and numerical modeling ofmy experiments.

Droplet behaviour on switchable and adaptive soft and hard spiropyran-based materials

PI: Dr. Dorothea Helmer, IMTEK, Albert-Ludwigs-Universität Freiburg

The behaviour of droplets on hard surfaces and the wetting of liquids on liquids have experimentally and theroretically been described in detail. Wetting on hard surfaces (Young Equation), neglects the forces acting on the surface, which are a direct result of the droplet surface energy. On soft surfaces, these forces cause a deformation of the surface by the droplet – and the formation of the “wetting ridge” at the three-phase contact line. The behaviour of the ridge under dynamic conditions is not yet fully understood. The experimental evaluation of the dynamics of the three phase contact line and thus the deformation is of great interest, as in nature most wetting phenomena are characterized by the dynamic of the surface. This project will therefore study the influence of substrate dynamics on the behaviour of droplets on surfaces. For this purpose, different surfaces are generated with soft and hard properties. To achieve switchable surfaces, spiropyran (SP) photoswitches are used, which are integrated into the materials. SP switches under UV light from hydrophobic SP to more hydrophilic merocyanine (MC) form. This way, SP can change the surface energy of substrates, so that a switch between two states of contact angles is possible. The wetting properties of SP-containing hard polymeric materials can be further enhanced by introducing a surface structure. In this project, materials with a bulk micro-/nanoporosity will be produced by polymerization induced phase separation. For the evaluation of soft surfaces, this project uses physical and chemical gels. Physical gels consist of wormlike micelles which show viscoelastic behaviour analogous to polymer gels. Polydimethylsiloxan (PDMS) serves as a classical chemical gel, because it can be conveniently tuned in terms of surface stiffness. The PDMS gels will be coated with artificial “skin” layers of parylene to study the effect of dangling chains and monomer leakage into the wetting fluid and to further understand the influence of these factors on PDMS soft wetting. All hard and soft materials of this projects will be analyzed in terms of their switching and dynamics. Here, the focus will be on high-resolution switching of surfaces to allow for the study of the dynamics of individual small droplets on surface patterns. This Schwerpunktprogramm aims at experimental and theoretical analysis of wetting phenomena on switchable, flexible and adaptive surfaces. This project offers substrates of each of these categories and will make a significant contribution to understanding the dynamics of the three-phase contact line under changing substrate dynamics. Collaborations with experimental groups to improve the properties and dynamics of the surfaces as well as with theoretically oriented groups to utilize experimental data for the improvement and adaption of theoretical models are planned.

Depth-Resolved Analyses of Water Penetration During Dynamic Wetting in Solvatochromic Dye-Gradient Polymer Brushes by Time-Resolved Fluorescence

Prof. Dr. Heiko Ihmels, Universität Siegen
Prof. Dr. Holger Schönherr, Universität Siegen

The wide and detailed knowledge of processes that occur during dynamic (de)wetting of surfaces with liquids is of fundamental importance in chemistry, physics, biology, and engineering, in particular for the optimization, development and application of innovative, materials. Despite the progress gained with sophisticated experimental and theoretical approaches, however, even the most advanced models for dynamic (de)wetting rely on simplifying assumptions because of the lack of complete experimental data. Specifically, under dynamic wetting conditions, the main parameters of polymer softening, swelling and structural reorientation are thus far not adequately studied in terms of spatial and temporal resolution. In order to fill this knowledge gap, we propose an approach that monitors the dynamic wetting processes on dye-labeled polymer materials with precisely localized, tailor-made fluorophores, aiming at the delivery of complementary and significantly enhanced input for further development and refinement of models for dynamic wetting of adaptive surfaces.The proposed strategy is based on polymer brushes with controlled thickness and grafting density, that enable a controlled one-dimensional polymer swelling stimulated by solvent and/or temperature. As a key feature, these materials contain fluorosolvatochromic probes at defined depths as well as at the interfaces of the polymer with the supporting substrate and the wetting liquid. For that purpose, precisely tuned dyes will be developed whose emission energy, lifetime and quantum yields change significantly with the varying microscopic environment, thus providing a static and dynamic fluorimetric readout of the physical-chemical changes of the polymer during (de)wetting processes. As an important feature, the polymer brushes contain a dye x-gradient along which the distinct depth of the polymer-attached probes within the material varies linearly.In this set-up, the fluorimetric monitoring of wetting dynamics is facilitated and should enable a depth resolution with up to sub-10 nm precision by spatially resolved microscopy analysis. The confocal microscopic analysis of the emission energy and, in particular, the emission lifetime will reveal the diffusion dynamics of liquid into the polymer layer as well as the triggered polymer chain relaxation. Thereby, the dye-labeled gradient polymer brushes constitute a novel system that enables the fluorimetric analysis of dynamic wetting processes and further provide complementary data sets for critical tests and refinement of existing theoretical models, namely by linking the internal dynamics and relaxations of adaptive polymers, as obtained in this project with high local control, to physical quantities, such contact angles, velocity, drop dimension acquired versus sliding velocity.

Wetting of structured surfaces with switchable topography and mechanical properties

PI: Prof. Dr. Leonid Ionov, Universität Bayreuth

The interaction of liquids with structured surfaces can have different character - either topography can determine wetting if the surface structures are hard, or topography affects wetting and the droplet shape affects the surface topography if the elastic forces and surface tension are comparable with each other. Depending on the size of surface structures and their elastic modulus, either surfaces tension or elastic force can dominate – this transition from domination of elastic force to counteraction of elastic and surface tension force was studied in the first funding period. In particular, we developed a method for fabrication of surface structures with the shape of lamellae with high aspect ratio from polymers with switchable mechanical properties and shape memory behavior. Surface structures either could or could not adapt to the droplet shape depending on the state of the polymer, and we demonstrated that the wetting properties depend on the deformability of the surface structures. In the second funding period, we will make a step forward and develop new method for fabrication of structured surfaces with actively switchable topography and will investigate how counteraction of elastic force and surface tension will affect switching of topography and switching of wetting behavior. Advanced combination of 3D-printing and electrospinning – melt electrowriting will be used to print complex structures using reversibly swelling and two-way shape memory polymers. We will also develop new methodology for acquiring 3D shape of droplet and will use it for studying of wetting on surfaces with switchable topography. The experimental results obtained in this project will be validated by models developed in cooperation with other members of SPP 2171.

Dynamics of Liquid-Liquid-Elastic Three Phase Lines

PI: Dr. Stefan Karpitschka, Max-Planck-Institut für Dynamik und Selbstorganisation Göttingen

The dynamical wetting of rigid surfaces is governed by a balance of capillary forces and viscous dissipation inside the liquid. On soft surfaces however, spreading dynamics are typically slowed down tremendously by viscoelastic braking. Capillary forces at the three phase line deform the soft solid into a sharp wetting ridge, which is dragged along with the liquid interface. This contributes the dominant part of the overall dissipation, and thus governs the spreading motion. A quantitative understanding of the wetting dynamics on soft surfaces remains elusive because available theories are limited to the regime of linear viscoelasticity, but experiments with liquid-vapor-soft solid contact lines show strong nonlinear behavior. The origin of the experimentally observed nonlinearities lies in the strong tractions that liquid-vapor interfaces exert on soft solids, deforming them to strains of order unity and larger. This may excite a number of non-linear effects, ranging from geometry over variable solid surface tensions, to the extraction of un-crosslinked materials. These factors are discussed controversially in recent literature, and in the proposed research we want to advance, if not settle, this discussion. We will focus on soft polymer gels, the most widely used material in soft wetting studies, and specifically address the consequences of the liquid phase that is present in these gels. Liquid surfaces exert one of the sharpest known line tractions on their substrate, thus generating a sharp fold at the tip of a wetting ridge. As a consequence, the solid pressure diverges at the tip of the wetting ridge, which may well alter the solid-liquid equilibrium in the phase. Pressure-induced extraction of the liquid phase, as is known to occur at the edge of adhesive solid-solid contacts, would be the consequence. We will experimentally study the impact of such an oil skirt, artificially added or provided by the gel substrate, to unravel the self-lubricating properties observed for soft gels.

Dynamic Wetting of Adaptive and Responsive Polyelectrolyte Substrates: A multiscale approach

PI: Prof. Dr. Regine von Klitzing, Technische Universität Darmstadt

Wetting of polymer substrates by aqueous solutions is of specific interest in the context of the adhesion of biologically relevant systems like cells or proteins, but also for the technical control of wetting. Polymer substrates which contain charges or polar groups have a strong tendency to swell in water. Therefore, not only elastic but also adaptive properties of the substrate affect the wetting properties. The global aim of the present project is the understanding of the relation between the adaptation/response of polyelectrolyte substrates and their dynamic wetting properties. In the present project adaptation refers to swelling of the polyelectrolyte substrate caused by the (partial) uptake of the wetting liquid which is water or an aqueous salt solution. One class of studied adaptive substrate is responsive to temperature, which raises the question how a change in temperature affects wetting properties.In this context the project aims to contribute to a better understanding of wetting phenomena on nanoscopic/mesoscopic length scale (10 nm - 1 m) lateral and vertical to the surface close to the three-phase contact line (TPCL). Therefore, AFM techniques will be used. To make a link to wetting phenomena on a macroscopic length scale, typical static and dynamic wetting experiments (i.e. sessile drop, drop on a tilted plate, advancing and receding contact angle measurements) will be also carried out. Surface effects like surface elasticity, surface charge and interfacial molecular rearrangement are assumed to govern hysteresis effects on a nano/mesoscale which affects macroscopic wetting phenomena. Three types of surface deformation might occur on the nanoscale and might contribute to the overall profile of the TPCL: 1) A precursor film in front of the droplet, which is dominated by the disjoining pressure, 2) a rim of swollen substrate which is due to transport of liquid from the droplet into the substrate lateral (and vertical) to the substrate surface and 3) a wetting ridge induced by vertical components of the surface stress.The project aims to disentangle the different surface effects on wetting phenomena.As polyelectrolyte substrates polyelectrolyte multilayers (PEMs) and layers/multilayers of adsorbed PNIAPM microgel are used. Both can be considered as gels. Both types of substrates are complementary with respect to their advantages and allows control of surface charge (sign, density), range of achievable thickness, mechanical and rheological properties, hydrophilic/hydrophilic balance and their response to temperature. Cooperations are planned within the SPP with experimentalists and theoreticians in order to get deeper insight into the link between wetting phenomena on different length scales.

Highly accurate numerical simulation of wetting and dewetting on flexible substrates including Heat transfer

PI: Dr.-Ing. Florian Kummer, Technische Universität Darmstadt

The essence of this project is development of numerical simulation methods. The specific physics to be modeled, respectively simulated, is motivated by the practical experiments planned in the Priority Program 2171 (SPP 2171). In particular, experiments are to be investigated in which wetting or dewetting of flexible surfaces takes place. For this purpose, a corresponding high-precision numerical toolbox for the simulation of three-phase systems, consisting of a flexible solid as well as a liquid and a gaseous phase, was developed in the first funding phase. In the second funding phase of the program, these simulation techniques will be further improved and extended to cover a broader range of experiments. In particular, evaporation effects are now to be taken into account.It is important to note that simulation is a complement to the experiments in SPP 2171 here, as it can act as a magnifying glass, so to speak: it provides insights that experiment cannot. For example, simulations make it possible to determine quantities that are difficult or impossible to measure in experiments due to technical limitations.The complementary simulation of experiments of increasing complexity is planned. At the beginning there will be configurations with simple droplets and liquid bridges. In the further course, the impact behavior of droplets on flexible substrates will be investigated. Evaporation, by heating the substrate, will also be considered. The most challenging simulation, which concludes the project, is a Leidenfrost configuration, i.e. an evaporating droplet which, supported by its own vapor layer, hovers above a heated substrate.The simulation procedure is based on a numerical method for two-phase flows, developed by the applicants. These are mixtures of two immiscible fluids, in this case the printing ink and the ambient air. This setup is now supplemented by a third phase, i.e. a flexible solid. If air, liquid and solid meet, this is referred to as a three-phase contact line. The numerical-mathematical basis for this project is a so-called extended discontinuous Galerkin method, which was specially developed to simulate three-phase flows with contact lines with high accuracy. In particular, the interfaces between air and liquid, as well as the three-phase contact line can be followed with high accuracy. For the given problems, the method has to be combined with a simulation method for solids. Furthermore, the heat transfer as well as the evaporation have to be modeled.

Wetting on Patterned Adaptive Conducting Polymer Surfaces for Microfluidic Applications (PolySurf)

Prof. Dr. Sabine Ludwigs, Universität Stuttgart
Prof. Dr.-Ing. Holger Steeb, Universität Stuttgart

The project “Wetting on Patterned Adaptive Conducting Polymer Surfaces for Microfluidic Applications (PolySurf)”, shall be implemented into current activities of the SPP 2171 on “Dynamic Wetting of Flexible, Adaptive and Switchable Surfaces”. PolySurf is based on the coupled interdisciplinary expertise of Sabine Ludwigs (polymer materials science, electrochemistry) and Holger Steeb (microfluidics, material modelling) and aims at hierarchically patterned soft surfaces which can adapt their wetting properties to externally applied electric fields. In addition to water contact angle measurements the fluid dynamics within microfluidic applications shall be explored. For adaptive surface preparation the class of conducting polymers (CPs) has been identified. CPs are gaining increasing interest for flexible actuator and sensor applications, and the interfaces of the CP polymer surfaces with liquids are extremely relevant in this context. Conducting polymers cannot only change their optical and electronic properties as function of the degree of doping, but also the wetting properties are highly influenced. In the literature extreme changes of the surface properties from superhydrophobic to superhydrophilic are reported between neutral and doped films.Within PolySurf a systematic understanding of the dynamic wetting behavior of the conducting polymer surfaces shall be established. On the materials science side both solution-processable films of poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as well as electropolymerized films from branched monomers shall be prepared. Surface patterning techniques involve imprint lithography and electropolymerisation on structured electrodes and are targeted at hierarchical patterning to increase the surface roughness.Characterization of the conducting polymer surfaces will involve systematic electrochemical studies in aqueous electrolytes alongside morphological and functional property analysis by absorption spectroscopy and conductivity measurements to determine the degree of doping.In terms of wetting characterization first systematic contact angle measurements (sessile droplet, advancing/ receding contact angles) will be performed. The focus lies on water and water/ion electrolytes. Surfaces with no potential and with potential applied (during in-situ electrochemical measurements) shall be monitored in terms of their wetting behavior. As complementary method spectroscopic ellipsometry will be used alongside to identify surface variations, e.g. by swelling, when the surfaces are brought in contact with the water and the aqueous electrolytes. This shall also give information about the adaptivity of the surfaces. Beyond the study of the dynamics of three phase contact lines on surfaces, the effect on evolving wetting properties on multi-phase fluid dynamics within microfluidic applications shall be explored. Ultimately, electrochemically-switchable microfluidic devices are envisioned.

Wetting of bio-inspired, stimulus-responsive polymer surfaces by lipid vesicles

Prof. Dr. Marcus Müller, Georg-August-Universität Göttingen
Prof. Dr. Motomu Tanaka, Ruprecht-Karls-Universität Heidelberg

Analog to liquid drops, the shape of a vesicle is dictated by its enclosed volume, the membrane-substrate interaction (interface potential), and the properties of the interface (membrane) between the interior and exterior. Unique to wetting by vesicles is the importance of the membrane’s intrinsic bending rigidity and the buoyancy of the enclosed liquid.The experimental Tanaka team has established a bio-inspired polymer brush platform, capable of switching the adhesion of vesicles, and has monitored the large-scale vesicle shape by 3D confocal microscopy. This setup will be complemented by a micro-interferometry technique, providing information about the local geometry of the edge of the brush-vesicle contact zone. The theoretical Müller team has implemented a highly coarse-grained particle model of the switchable polymer brush and a triangulated Helfrich-Hamiltonian of the thin lipid membrane in a parallel, GPU-accelerated MD program and has generalized the Helfrich description to include a finite-ranged interface potential and buoyancy. In the new period, they will explicitly include solvents to account for viscous dissipation.Both teams will jointly investigate the dynamic change of vesicle shapes in response to a switch in adhesion and the adaptation of the brush to the contact with a vesicle by time-dependent measurements of the vesicle geometry. Thermodynamic forces (bending energy, adhesion, and buoyancy) and dissipation mechanisms (e.g., dissipation at the contact zone and of the surrounding liquids) exhibit different dependencies on the vesicle size, whose systematic variation will allow us to distinguish between different dissipation mechanisms and aid the comparison between experiment and simulation. Additionally, we will consider how transport of membrane species (e.g., positively charged lipids, binding to -COOH groups of the brush) toward the contact zone influences the dynamics of the vesicle. This line of study will be extended to heterogeneous substrates, where a wettability gradient may result in a pining of the contact zone or a gradual variation may induce a translation and spreading of the vesicle. We will also study time-periodic switches of the wettability and wettability gradients that move over the substrate. The lateral motion of vesicles will give rise to additional dynamics such as e.g. sliding or rolling motion/tank-treading of the vesicle.

Mathematical modeling and simulation of  substrate-flow interaction using generalized gradient flows

PI: Dr. Dirk Peschka, Weierstraß-Institut für Angewandte Analysis und Stochastik Berlin (WIAS)

The goal of the project "Mathematical modeling and simulation of substrate-flow interaction using generalized gradient flows" is the thermodynamically consistent description of dewetting phenomena by gradient structures and corresponding generalizations, so that numerical stability is guaranteed by implementing appropriate models. In particular, phenomena at moving contact lines with dynamic contact angles on corresponding complex substrates (adaptive, flexible, switchable) shall be understood in detail. For this purpose, phase-field models and sharp-interface models for viscoelastic substrates with finite strain and also phase separation in the vicinity of contact lines are now being considered in this follow-up proposal. As a prerequisite, in the first funding period, both abstract fundamental work on the structure of equations and discretization methods were developed, but also work on spinodal dewetting on fluid substrates, comparison with molecular dynamics [4], hierarchies of models with dynamic contact angles, and numerical methods for phase-field models were implemented. Based on this, the work in this project divides into three parts: WP1 "Wetting hydrodynamics on flexible substrates with sharp interfaces and contact angles" : Based on the work in, models and discretization methods for moving contact lines with sharp interfaces and contact lines are investigated, where domains and grids are aligned to the interfaces, thus ensuring high control over the accuracy of the numerical methods. WP2 "Extension of long-wave approximation with sharp-interfaces to flexible substrates": Based on abstract model reduction approaches from, the project explores different ideas for model reduction to thin-films on viscoelastic substrates. Similar to the Galerkin ROM methods, a systematic and extensible reduction of the full continuum equations is anticipated here, also aiming at the possibility of sharp contact lines and their dynamics. WP3 "Phase separation and cloaking at solid/fluid interfaces and contact lines": In order to investigate effects such as dynamics of phase separation and cloaking in the vicinity of contact lines, the equations from coupled to elasticity are systematically extended to effective interfacial thermodynamics and compared with corresponding experiments.Results are compared with experiments and other theoretical approaches in SPP.

Dynamic wetting of self-assembled monolayers and polymer brushes functionalized with photoresponsive arylazopyrazoles

PI: Prof. Dr. Bart Jan Ravoo, Westfälische Wilhelms-Universität Münster

Molecular photoswitches are a unique type of molecules that can be switched reversibly between two isomers (shapes) upon irradiation with light. Since the two isomers of the molecular photoswitch have different properties, incorporation of photoswitches into materials and coatings can result in macroscopic effects such as photoswitchable stability, permeability, or wettability. In this project we will investigate a new type of molecular photoswitch to prepare surfaces with photoresponsive wettability. Due to optimized molecular design, the photoswitch can be self-assembled in molecular monolayers or embedded in polymer brush nanofilms with enhanced dynamic wettability.

Dynamic wetting and dewetting of viscous liquid droplets/films on viscoelastic substrates

Prof. Dr. Ralf Seemann, Universität des Saarlandes
Prof. Dr. Barbara Agnes Wagner, Ph.D., TU Berlin

When a liquid morphology moves over a (visco)elastic substrate, friction forces in the substrate both at the point of largest substrate deformation, i.e. at the three-phase contact line and at the moving interface are key to understanding energy dissipation and shapes of dewetting rims and are expected to impact the mode selection of spinodal dewetting. Unlike liquid substrates, the local deformation of a solid viscoelastic substrate always has a global impact on the state of the substrate and the resulting strains are permanent. This proposal aims at developing mathematical models and numerical algorithms for two-layer systems that couple the hydrodynamic and viscoelastic boundary value problem with appropriate interface conditions including intermolecular interactions that become relevant at the scales of the dewetting experiments, to predict the spinodal mode selection and rupture process and the dynamics and rim shapes of simple liquids dewetting from solid viscoelastic substrates. The elastic properties of the substrate will bridge three orders of magnitude down to about 1 kPa. In particular for the very soft substrates, we expect effects such as demixing that will be captured by the introduction of additional entropic contributions to the possibly non-linear elasticity and by adding a mixing free energy to the total free energy of the soft polymer gel. In parallel, the same questions will be tackled experimentally exploring nanoscopic thin polystyrene films dewetting from polydimethylsiloxane networks imaged by atomic force microcopy. A quantitative comparison of these experimental results as functions of systematically varied viscoelastic properties shall allow for a detailed and fundamental understanding of the underlying physical processes and to confirm or to falsify effects that are still discussed highly controversially in literature. Besides others, this concerns the existence of the so called Shuttleworth effect for the considered experimental system and the potential demixing of cross-linked elastomer matrix from non-cross-linked molecules of the same material, which was not yet observed for stiff substrates and the length scales that will be considered here.

Dynamic wetting on deforming substrates, elastic sheets, and under evaporation: A study with the boundary element method

PI: Prof. Dr. Holger Stark, TU Berlin

Dynamic wetting of flexible and switchable substrates has many fascinating manifestations and potential for applications. Droplets on thin elastic sheets assume the shape of a lens, which is tunable by tension forces applied to the sheet. This effect can be used to construct micro-optical lenses. Controlling the surface deformations of soft substrates in time allows to move micron-sized droplets, an essential process for lab-on-a-chip devices. The evaporation of droplets covered with light-switchable surfactants offers the possibility to position solutes such as ink pigments on a substrate with subdroplet precision, which is relevant for technological advances in printing.In our project we plan to address these topics in order to achieve a fundamental and thorough understanding. We will use the boundary element method (BEM) to determine the flow field within the droplet and phenomenological expressions such as the Cox-Voinov law to describe the contact-line dynamics. Recently, we have implemented our BEM approach for rigid substrates with spatio-temporal wettability patterns in a computer code, which we will extend to deal with the different topics.First, we will study the directed transport of droplets by traveling-wave deformations on the surface of light-responsive materials and determine conditions of optimal transport. Second, we address dynamic wetting on elastic sheets, which we describe with the two-dimensional elastic Skalak model and the Helfrich Hamiltonian for bending. We perform a thorough study of the lens-shaped droplets under varying tension forces and investigate durotaxis, where the droplet moves to softer parts of the sheet. Third, we will apply tension forces, which vary in time and along the sheet edge to explore the possibility of controlled droplet motion, also using electrowetting. We will collaborate here with P. Huber (Hamburg), who can fabricate elastic nanoporous silicon sheets and plans to perform corresponding experiments. Finally, for rigid substrates we look at evaporating droplets, where the surface tension determined by light-switchable surfactants is locally changed with irradiated light. The resulting Marangoni currents induce vortices within the droplet, which trap solutes and ultimately deposit them on the substrate in designed patterns.