We discuss the problem of solid liquid adhesion and in particular we will focus on a tool for direct measurements of the work needed to separate a liquid from a solid. The method mimics a pendant drop that is subjected to a gravitational force that is slowly increasing until the solid−liquid contact area starts to shrink spontaneously. The work of adhesion is then calculated in analogy to...
State-of-the-art contact angle measurements usually involve image analysis of sessile drops. The drops are symmetric and images can be taken at high-resolution. The analysis of videos of drops sliding down a tilted plate is hampered due to the low-resolution of the cutout area where the drop is visible. The challenge is to analyze all video images automatically, while the drops are not...
Fluorescent dyes are widely used in fluorescence microscopy, providing important advantages such as the detection of specific components or changes in the surrounding medium of a component of interest. The variety of commercial fluorescent dyes and their stability make these tools ideal for potential implementation as probes of molecular-scale changes. In particular, we are interested in...
This poster will give insights into current research activities of the Ludwigs team. In our interdisciplinary and international research team of polymer chemists, physical chemists and materials scientists we are developing functional and intelligent polymer materials and devices for electrochemical, pharmaceutical and soft robotics applications. One of our aims is to control and manipulate...
Arylazopyrazoles (AAPs) are a class of photoswitches and offer significant improvements of more established classes of photoswitches such as azobenzenes. In this work, we present the synthesis of novel poly-thiolactone-hydroxyethyl acrylate-copolymer brushes. These brushes can either be generated to cover a glass or silicon substrate homogeneously or can be structured using microcontact...
In 1903, von Schroeder reported that gelatin swelled more in water or salt solutions than in the corresponding saturated vapors.[1] Since these phases are in equilibrium, it is surprising that they display different partitioning behaviour, and hence this result has become known as Schroeder’s paradox. It has since been found to occur in other cross-linked polymer systems as well, and has...
Water drops moving down inclined hydrophobic and insulating surfaces acquire a charge and deposit counter charges onto the solid surface. This charge separation by sliding drops is also called slide electrification. One of the general observations on fluorinated polymers is that sliding drops leave negative charges on surfaces. Our aim here is to determine if charging processes can be...
Droplet spreading and evaporating on complex surfaces is inherent to various natural and industrial processes. While understanding of the wetting dynamics at the macro- and microscale has essentially advanced over the last decades, nanoscale phenomena still leave many questioned unanswered. Since a droplet of a volatile liquid eventually reaches the state when it is comparable to the range of...
Polymer brushes are highly responsive materials that have a broad spectrum of possible applications, therefore an understanding of their interfacial behavior is essential. The degree of swelling of a polymer brush can be influenced by various external stimuli, such as the presence of a solvent. Under good solvent conditions, drop spreading causes changes in the wettability of the brush surface...
Young model describes the wetting behavior of an ideal surface. Recently, Butt et al., presented a model which connects adaptation processes of the surface to dynamic contact angles.[1] In the first phase of the SPP project, we developed an experimental setup which allows measuring adaptation processes. Li et al., used random copolymer surfaces to confirm the adaptation model.[2] Now, in the...
The dynamics of a droplet deposited on a porous substrate is a combination of three phenomena: spreading, imbibition and evaporation. Here we present a study on the interactions of droplets on nanoporous silicon prepared by electrochemical etching as a function of time. The evolution of the droplet volume is analyzed theoretically and experimentally considering the evaporation and the...
For the development of brush-based functional surface-coatings, it is critical to understand their properties, because they will determine their performance and user-experience. Polymer brush solvation and wetting are key parameters in this. In this presentation we will show that brushes can display counter-intuitive wetting properties. We aim to unravel those by combining molecular dynamics...
Among all living species, microbes are the most abundant and diverse form of life on Earth. They have conquered almost all ecosystems of our planet by successfully adapting to their environment. Life on Earth has evolved under the exposure of sunlight and many species are equipped with a photosynthesis machinery enabling them to transform light into chemical energy. Their microhabitats include...
The (de)wetting of e.g. polymer-coated surfaces with various liquids is of fundamental importance in equilibrium and molecular dynamics in confined mesoscale media and is also vital in many materials and applications. In particular, the response of switchable polymer layers to an externally applied thermal, optical, electrical or chemical stimulus causes alternation of local properties, in...
We investigate the behaviour of liquid drops on soft viscoelastic substrates employing mesoscopic and macroscopic models as well as experiments.
First we introduce a simple mesoscopic gradient dynamics model and show that it recovers the known double-transition in contact angles with
increasing softness and that it is well suited to study multi-drop problems like the dependence of drop...
In the last few years a lot of studies have shown that by contact/slide electrification between water droplets and hydrophobic surfaces, it is possible to generate electricity in an environmentally friendly way. In the first applications LEDs could operate in this way[1] The physical processes are still being discussed today. To extend the understanding Stetten et al.[2] have established an...
We present a highly accurate extended discontinuous Galerkin method (XDG) for the simulation of
multiphase problems involving three-phase contact lines on a flexible solid. We will show results of a simulation of a water droplet sitting on a silicone-gel, utilizing Navier slip boundary conditions on the interface and Young's equation at the three-phase contact line. Characteristically,...
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...
Evening talk: report from the recent climate conference COP27.
Soft and wet contact arises in a range of phenomena that spans many length and time
scales, and includes: landslides, aquaplaning of tires, wear of industrial bearings, ageing of
synovial and cartilaginous joints, cell motion in blood vessels or microfluidic devices, and
atomic-force or surface-force rheology. Therein, the coupling between boundary elasticity
and confined viscous flow...
In biological systems activity can manifest itself in different forms; as individual motility or as growth and production processes. I will present two different applications of the thin film equation coupled to active processes.
In both scenarios, the (reductionist's) model equations represent a gradient dynamics derived from a potential energy supplemented by bioactive terms which break the...
Adhesion of biological cells and lipid vesicles shares common features with wetting droplets. The main difference from the droplets of simple liquid is that the deformation is not only driven by tension but also by bending elasticity of membranes. Inspired by proteins, we designed polymer brushes that switch the conformation by complexation with heavy metal ions. The switching of thickness,...
The design of open surface microfluidics that enables orthogonal control of liquid mobility and chemical composition is crucial for devising the next generation of microfluidic platforms that will find use in applications across chemical, environmental, and biomedical fields. To achieve these functionalities, extensive studies have demonstrated stimuli-responsive liquid mobility on open...
We present a a simple model, derived from basic thermodynamic principles, for active polar free-surface droplets to identify a mechanism of motility in the context of cell crawling. Namely, active stresses drive drop motion through spatial variations of polarization strength. This robustly induces parity-symmetry breaking and motility even for liquid ridges (2D drops) and adds to splay- and...
In this study we are interested in Polystyrene liquid droplets in equilibrium on PDMS elastic solid substrates in the limit of the electrocapillary length, and consequently to the ratio between the surface tensions and the elasticity of the substrates. An experimental analysis using Atomic Force Microscopy of the different parameters constituting the shape of micron-sized droplets in both the...
Despite many experimental and theoretical studies, it is still impossible to predict dissipative forces, which act against the gravitational forces of drops sliding down an inclined plane. The dissipative forces, which resist drop motion, can be termed “friction force". In this work, we measured the velocity (U), width (w), length, advancing contact angle (θ_a), and receding contact angle...
Thin liquid films are important for many microfluidic applications such as printing or coating of e.g. printableelectronics or photovoltaic cells where a evenly spread thin film of certain properties is of utmost importance as wellas so called lab-on-a-chip devices. In biophysics stable thin films play an important role in tear film on eyes or thelining of lungs. On a larger...
The dynamics of membranes, shells, and capsules in fluid flow has become an active research area in
computational physics and computational biology. The small thickness of these elastic materials enables
their efficient approximation as a hypersurface, which exhibits an elastic response to in-plane bending and out-of-plane stretching deformations. If such a closed thin shell is filled with...
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 Cox–Voinov friction for the dynamics of the contact line. One...
We consider the morphologies adopted by liquid in fibrous assemblies, and in particular the coupled effects of geometry, elasticity and swelling on the liquid distribution. For favorable solvants that are absorbed by the fibers, the induced swelling strongly affects the liquid distribution, for exemple inducing transient motions or even coalescence of the drops. When the fibers are flexible,...
Cloaking of surface material (e.g. oligomers in PDMS coatings) on contacting droplets is a phenomenon that directly affects static and dynamic wetting. Of the multitude of changes that the system undergoes with cloaking, the change in the effective droplet surface tension is the most conspicuous. The small length scale of the cloak (in the order of nanometers) together with the imposed...
A solid, withdrawn from a wetting liquid bath, entrains a thin liquid film. This simple process, first described by Landau, Levich, and Derjaguin (LLD), is commonly observed in everyday life. It also plays a central role in liquid capture by animals, and is widely used for surface-coating purposes in industry. Motivated by the emerging interest in the mechanics of very soft materials, and in...
Slide electrification is a spontaneous charge separation between a substrate and a sliding drop. Here, we describe this effect in terms of a voltage generated at the three-phase contact line. This voltage is on the order of 0.2-2 kV and moves charges between the drop and the substrate. We model this system by a surface- and drop capacitor connected to the voltage source of the moving...