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...
The interaction of a viscoelastic substrate with liquid droplets is at small length scales dominated by surface tension forces. Understanding how these forces deform the solid is crucial for the design of new materials, structures, and fabrication techniques.
We present a numerical model to simulate the interaction between two immiscible fluids and an incompressible viscoelastic solid. The...
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 use...
The lattice Boltzmann method is an efficient approach to simulate complex fluids and wetting processes. We present results based on the colour gradient method which is particularly designed for multicomponent fluids and is able to obtain thermodynamically consistent results over wide ranges of viscosities and surface tensions. After demonstrating benchmarks to analytically accessible solutions...
On hydrophobic rough 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 new wetting regime, the...
Living organisms have structured surfaces with specific wettability that allows their efficient adaptation to the environment and improves their survival rate. For example, the rice leaves have micro- and nanoscale structures on their surface that form a superhydrophobic surface for self-cleaning and water repellence [1]. Bioinspiration of these natural surfaces in science can be beneficial...
Droplets are set in motion on substrates with a spatio-temporal wettability pattern as generated, for example, on light-switchable surfaces. To study such cases, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside and on the surface of a droplet and supplement it by the Cox–Voinov law for the dynamics of the contact line. Our...
The design of adaptive interlayers between synthetic substrates and soft biological matter is a cross-disciplinary challenge. In analogy to wetting of simple liquids on a solid substrate, one can tailor the adhesion of a cell model (lipid vesicle) by designing the substrate interactions, and the free energy of contact between the substrate and the biological object influences the shape of the...
Drop impact on solid substrates under normal lab conditions typically suffers from effects of substrate inhomogeneities on, e.g., the evolution of interstitial gas films and contact formation. Some recent experiments thus employ micrometer-thin oil films as surficial layers on a hard substrate, but the effect of the film on a droplet’s spreading and receding dynamics is typically neglected....
The first part of this lecture I discuss how the energy of falling drops impacting onto pre-charged hydrophobic polymer surfaces can be harvested in an external electrical circuit as screening charges redistribute within the drop.
High speed video imaging and simultaneous electrical measurements lead to a physical model of the energy conversion process including a quantitative analysis of...
Polymer surfaces can adapt to liquids in many ways: Liquid can penetrates into polymer layers, polymers can reorient, the liquid induces a chemical reaction and many more [1]. These changes in physical-chemical properties at the interface to the liquid may influence drop shape, velocity, advancing angle and receding contact angle. The aim of our work is to investigate the role of adaptation...
Dewetting dynamics and the emergence of equilibrium droplet morphologies of polystyrene on top of visco-elastic substrates is considered experimentally. Our initially prepared sample consists typically of glassy polystyrene layer with a molecular weight of 18 kg/mol and a thickness of about 100 nm prepared on top of PDMS viscoelastic substrates. Dewetting is activated by heating the sample...
When a liquid droplet rests on a soft surface, capillary forces at the contact line deform the solid into a sharp wetting ridge. The size of the wetting ridge is given by the elasto-capillary length. If the droplet moves, strong viscoelastic dissipation in the soft solid leads to viscoelastic breaking. Recently, it was shown that droplet speeds depend on a pre-stretch of the material, which is...
On this poster we analyse the behavior of a liquid droplet on a switchable substrate theoretically. Therefore we performed molecular dynamics and thin film simulations where we switched the substrate once as well as simulations with periodic switching with different frequencies. Also we developed a mapping between the both models. There we observe different mapping regimes when the surface is...
Spiropyran stands among one of the most interesting molecular switches to induce changes in wetting properties due to the reversible switching between colourless hydrophobic spiropyran to magenta hydrophilic merocyanine by triggers such as light, pH, and metal ions. Spiropyran-induced changes in wetting properties were shown for various purposes and applications, such as antifouling coatings,...
The investigation of the wetting behavior on viscoelastic or elastic substrates is of great interest.
On this poster we present a simple model for liquid drops on fully compressible elastic substrates
and show that, besides the doubletransition of steady drops, also dynamic features are captured quantitatively.
In particular we investigate the visco-elastic braking effect, i.e. the...
We use lattice models of particles with hard core repulsion and a short-ranged attraction to describe fluids. Density functionals for such models are introduced and the construction of a dynamic density functional theory is explained.
Previous work in a simple lattice gas model [1] has shown that droplet evaporation and the "coffee ring effect" can be described even though the dynamics...
Droplets sitting on soft, polymeric surfaces form so-called wetting ridges around their three-phase contact line. The vertical component of the droplet’s surface tension exerts traction on the surface, leading to elastic deformation of the polymer network and accumulation of unbounded polymer chains. When droplets move over the surface, the wetting ridge moves accordingly, yielding visco- and...
Wetting phenomena at macro- and microscale have been studied intensively, and significant advancement in the understanding of both statics and dynamics of wetting has been achieved. That resulted in well-established ways to define contact angles and to predict the evolution of the liquid systems. However, due to the current state of imaging techniques, many questions about wetting at the...
Swalbe.jl is an open source lattice Boltzmann based solver for thin film dynamics [1]. Intended for problems raging from the relaxation or coalescence of droplets to the dewetting of thin films on complex substrates [2]. Features such as thermal fluctuations or spatially resolved contact angles are readily usable and shown to agree with theoretical predictions [3,4]. Switchable substrates...
The flexibility and adaptability of microfluidic networks have become more important as the field is rapidly increasing and more sophisticated fluidic systems are going to be introduced in soft autonomous robotics. A new generation of smart fluidic devices requires autonomous feedback-driven structures which can manipulate droplets, bubbles and fluid flows as well as react to different...
The design of adaptive interlayers between synthetic substrates and soft biological matter is a cross-disciplinary challenge. In analogy to wetting of simple liquids on a solid substrate, one can tailor the adhesion of a cell model (lipid vesicle) by designing the substrate interactions, and the free energy of contact between the substrate and the biological object influences the shape of the...
Smart surfaces that can change their wetting behavior on demand are interesting for applications such as self-cleaning surfaces or tunable lenses. We report on the use of vibrational sum-frequency generation (SFG) to study the molecular structure of photoswitchable surfaces as well as polymer brushes that do show structural adaptation. Results from SFG report on the molecular scale changes and...
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 diarylethenes2 and spiropyranes3 have been explored in a range of photoresponsive coatings which utilize their photoisomerization to induce changes in macroscopic properties such as wettability.4...
Wetting phenomena at macro- and microscale have been studied intensively, and significant advancement in the understanding of both statics and dynamics of wetting has been achieved. That resulted in well-established ways to define contact angles and to predict the evolution of the liquid systems. However, due to the current state of imaging techniques, many questions about wetting at the...
Droplet mobility on surfaces is achieved by polymeric coatings, in form of gels or brushes, both holding some mobile (free, i.e., un-crosslinked or un-grafted) polymer chains. In the presence of droplets, these free chains accumulate around the three-phase contact line, leading to contact lubrication and wetting ridge formation. The investigation of free chains and understanding their...
Substrates with switchable or adjustable wetting properties are desirable for actuators, coatings, filters or biomedical applications. In this work we present a study of two different substrates. Firstly, the possibilities of an electrically conductive polymer, i.e., polypyrrol in combination with porous silicon are explored, where previous studies already showed how an electric field can...
In this talk we will theoretically investigate the dynamical properties of a simple liquid on a switchable substrate. To understand the non-equilibrium relaxation dynamics of a liquid droplet on a switchable substrate 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...
Smart surfaces that can change their wetting behavior on demand are interesting for applications such as self-cleaning surfaces. In order to functionalize aluminum oxide surfaces, we have synthesized arylazopyrazole phosphonic acids (butyl-AAP-C$_{18}$PA) that represent a new class of photoswitchable molecules for these oxide surfaces. Butyl-AAP-C$_{18}$PA monolayers were deposited on...
Liquid droplet manipulation directed by stiffness gradient has been of great interest because of its numerous advantages in applications such as microfluidic systems (1), liquid collection (2), and tissue engineering (3–5). Patterning a stiffness gradient on the surface allows for moving of droplets towards the softer regions or create a well-known durotaxis effect and make them migrate to...
Fabrication of superhydrophobic surfaces induced by ultra-short-pulse lasers is a hotspot of surface studies. We report a way of generating nearly superhydrophobic surfaces on stainless steel (304S15). The method for fabricating this water-repellent surface is to microstructure by irridating with ultra-short-pulses.
Contact angle measurements were used to investigate the wettability of the...
We investigate the dewetting dynamics and the emerging
shape of micron sized liquid polystyrene droplets sitting on a
(visco-) elastic substrate with variable E-module spanning from
about 2 MPa to 1 kPa using different elastomers.
Experimentally, the droplets are images by atomic force microscopy (AFM)
while the deformation of the viscoelastic substrate can be obtained
combining AFM...
Establishing an adhesive contact between two materials requires both that the surface energies favor the creation of interfacial area and that it is possible to form contact area given the geometry and material properties. While elasticity has long been understood to be important in determining the relative “stickiness” between non-conformal surfaces, in recent years capillarity has also...
The process of dewetting allows to probe the dynamic behavior of polymers within thin films as the response to an applied force, generated directly and intrinsically by interfacial forces and forces acting within the film itself. In addition, taking advantage of the fact that on a single sample the process of dewetting is not initiated everywhere at the same time, one can follow the evolution...
When a droplet is sliding on surfaces, adaptation of surfaces leads to changes of the dynamic contact angles [1, 2]. Hereby two adaptation processes play a role: (i) the adaptation of the surface upon bringing it in contact to the drop (wetting) and (ii) the adaptation of the surface after the drop passed and the surface is in contact with air again (dewetting). In order to study both...
Certain polymer surfaces undergo an adaptation process after being exposed to liquids due to side chain movements. This reorientation often leads to a preferential exposure of certain polymer groups on the liquid interface, modifying consequently the observed features in surface experiments such as contact angle measurements [1]. This behavior is even more accentuated in block copolymers,...
In this talk, I will present a quick overview of the topics that have been considered in this project
i. GENERIC Lagrangian-Eulerian formulations of fluid flows,
ii. Gradient flow descriptions of finite strain elasticity with phase fields,
iii. Thin-film descriptions with dynamic contact angle,
with a focus on topic iii.
i) The thermodynamic structure of fluid flows in Lagrangian...
When a liquid droplet rests on a soft surface, capillary forces at the contact line deform the solid into a sharp wetting ridge. The size of the wetting ridge is given by the elasto-capillary length, but the geometry of the ridge tip is solely governed by the balance of surface tensions. If the droplet moves, strong viscoelastic dissipation in the soft solid leads to viscoelastic breaking. For...
The behaviour of drops on rigid and liquid substrates has been investigated extensively in the past and is well understood. However, soft substrates have gained increasing attention recently, since these exhibit elastic properties and can be deformed by liquid on a scale of the elastocapillary length. Soft wetting problems give rise to a wide variety of phenomena that are currently being...
We present a highly accurate extended discontinuous Galerkin method (XDG) for the simulation of multiphase problems with contact lines. Characteristically, singularities are observed at interfaces and three-phase contact lines between distinct phases, e.g. a jump in pressure or surface tensions. This offers a significant obstacle for high order methods, where generally smooth functions are...
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 microfluidic environments. Our designed 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...