SPP 2171 Spring Conference: Dynamic Wetting of Flexible, Adaptive, and Switchable Substrates

In 2011, Quere, Aizenberg, and co-workers independently introduced the innovative concept of infusing a solid surface with a thin oil film to eliminate contact-line pinning [1-3]. The invention of this new material class—lubricated surfaces—has since revolutionized the design of super-repellent surfaces, with important anti-fouling and heat transfer applications.
In this talk, I will explore...

Liquid diodes are surface structures that enable the flow of liquids in one direction but prohibit flow in the reverse direction[1]. When these diodes are made from flexible materials, their structure can deform under external compressive forces, and, in return, their diodic nature can break. In a previous study by Sammartino et al.[2], it was demonstrated that reverse flow in liquid diodes...

We are going to present a numerical method which combines the fluid-structure-interaction solver for simulating the wetting of solid substrates, developed in the first funding period of the priority programme with a solver for heat transfer and evaporation. This solver is based on the highly accurate Extended Discontinuous Galerkin discretization, to achieve a highly accurate spatial...

Droplets resting on flexible sheets deform into lens-like shapes, offer-
ing promising applications in areas like tunable liquid lenses. We have
extended and employ our fully three-dimensional Boundary Element
Method (BEM) simulation framework [1] to investigate dynamic wet-
ting on thin flexible sheets. Our study focuses on the intricate interplay
between the mechanical properties of the...

Soft materials are ubiquitous in technological applications that require deformability, for instance, in flexible, water-repellent coatings. However, the wetting properties of pre-strained soft materials are only beginning to be explored. Here we study the sliding dynamics of droplets on pre-strained soft silicone gels, both in tension and in compression. Intriguingly, in compression we find a...

Bacteria invade surfaces by forming dense colonies encased in a polymer matrix. Successful settle-
ment of founder bacteria, early microcolony development and later macroscopic spreading of these
biofilms on surfaces rely on complex physical mechanisms. Recent data show that on soft hydrogels,
substrate rigidity is an important determinant for biofilm initiation and spreading, through...

Hydrophilic polymer brushes are widely utilized in biomedical applications to prevent non-specific interactions with biological fluids. While the chemical structure of the coating plays an important role in the non-fouling character and bio-functionality, the physical state of polymer chains determines the coating's properties. However, the actual physical state of the polymer chains...

Synthesis, polymerization and characterization of dye-labeled polymethacrylate brushes

Jonah Decker1, Sören Steup2, Sergey I. Druzhinin1, Heiko Ihmels2, Holger Schönherr1*

1 Physical Chemistry I, Department of Chemistry and Biology and Research Center of Micro and Nanochemistry and (Bio)Technology (Cμ),...

Vesicles on substrates play a crucial role in various biological transport processes, including neurotransmitter release at the synapse, transport between cells, and drug delivery using synthetic vesicles. The adaptive adhesion of the vesicles to a biological substrate is crucial for all these processes. Although in shape similar to droplet wetting, vesicle adhesion is fundamentally different...

Conducting polymers (CPs) are used in a wide range of electrochemical devices, such as organic field effect transistors, batteries, actuators, and chemical-biological sensors. Compared to other conducting materials, CPs offer advantages such as lightweight, low cost, non-toxicity, flexibility, ease of processing, low-voltage operation (around 1 volt), and low power consumption....

We studied the wetting of diblock copolymer brushes by water. Goal of this work is to understand how the thickness of the bottom and top copolymer block affect the wetting behavior, respectively. For the synthesis of diblock copolymer brushes we grafted the bottom block, a hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) brush from a silanized silicon wafer. Than we grafted the top block,...

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 surface wettability. In this project we will investigate...

Frictional forces resisting droplet motion often appear to be separate to surface wettability and liquid adhesion. Here I will show how equilibrium surface wettability, representing normal adhesion, combines with hysteresis, representing surface heterogeneity, to produce static and kinetic contact line friction. I will show how an Amontons-like liquid-on-solid law provides a unified view for...

Droplets impacting at low Weber numbers on sufficiently smooth solid substrates will bounce on a thin layer of entrained air, or will eventually contact the substrate. When and how this happens is influenced by the substrate properties, e.g., by charging or by elasticity. Here, we consider smooth glass slides covered by thin oil films of different thicknesses and viscosity. We investigate this...

We present a mesoscopic thin-film model in gradient dynamics form for binary liquid mixtures on solid substrates incorporating interface tension-induced flow, and volatility in a narrow gap. Thereby, we use and expand models established in [1–4] by accounting for the two substances in each of two bulk phases - liquid and gas - and for the enrichment of one component at the liquid-gas...

This talk focuses on the experimental, theoretical, and computational study of the dynamics of a thin silicon oil drop driven by the forcing resulting from applied surface acoustic wave (SAW) applied to the underlying substrate. Our experiments consider a drop spreading either on a flat substrate or on a substrate with a superimposed topology and serve as a test for the theoretical model and...

It is well-established that the receding contact angle (CA) plays a vital role in various applications where repelling liquids, preventing fouling, promoting self-cleaning, and drop friction are essential requirements and that it can also provide very relevant information for coating and bonding processes. However, measurements of the receding CA are typically time-consuming, challenging, and...

Polymers are known to respond to various stimuli, including liquid and gas phases. When partially wetted, they are exposed to both liquid and gas phases. The surrounding gas phase can influence the amount of swelling around the contact line or even change the wetting behavior depending on the gas used. While there are many studies of the brush response in the liquid phase, there are few that...

Functional polymer brush coatings have significant potential for a wide range of industrial applications due to their responsiveness to environmental stimuli, which allows for precise tuning of surface properties. Polymer brushes can swell or collapse in response to external stimuli such as temperature changes or variations in the chemical composition of the surrounding medium, leading to...

Wettability can be applied in many applications such as microfluidic devices, oil/water separation, water energy harvesting, etc. There are two known approaches to modify the wettability properties of different materials: changing their topography or chemical modification of the surface The topography of materials also can be modified by applying external stimuli such as temperature, magnetic...

Solar panels are becoming an increasingly efficient and economical source of green energy. However, they rarely operate at optimum efficiency due to the accumulation of dust, pollen and dirt, which reduces the amount of sunlight that can be absorbed. In addition, harsh outdoor conditions can cause irreversible degradation of solar panels. We want to improve the ease of cleaning while...

Wettability gradients are particularly advantageous for controlled liquid imbibition and transfer, and they can be achieved through compositional variations in surface energy or topographical alteration of the surface, or both. Common methods to create wettability gradients including chemical modification of surfaces or the roughness patterning through etching and/or lithography are limited in...

Using porous materials for fluid management has been practised for thousands of years, e.g. water storage and release or filtration purposes. Thereby, the chemical nature of the porous material and the pore geometries determine the capacity of a porous material to take up a specific fluid through spontaneous imbibition. This talk addresses how geometric and chemical features impact multiphase...

We revisit the classical problem of liquid imbibition in a single pore with spatially varying wettability. Starting from the Lucas-Washburn equation, we derive analytical solutions for the imbibition time (crossing time) in systems where wettability alternates between two materials. For ordered arrangements, we demonstrate that the imbibition speed depends non-trivially on the spatial...

Polymer brushes with infused lubricants show significant potential in reducing drop adhesion, yet the depletion of lubricant hinders its durability and the underlying mechanisms remain poorly understood. Our study focuses on observing and analyzing drop-induced lubricant depletion process on Poly Lauryl Methacrylate (PLMA) brushes with dimple and wetting ridge forming. We achieved...

The accumulation of dust on surfaces is a well-known phenomenon in everyday life, for example on windows and solar panels. Both manual cleaning and self-cleaning of hydrophobic surfaces with water can restore the properties of soiled surfaces. However, it is not yet understood how particles are removed by a water drop and which forces play a role for successfully removing the particles. We...

Water drops spontaneously become electrically charged when moving on different surfaces, such as plant leaves, building walls, window glass, and plastic. This process is called contact or slide electrification. The electric potential of water drops charged in this way can even exceed 1 kV. A vital question as yet unanswered is whether the charge in water drops generates corrosion. Here, we...

The understanding of wetting phenomena plays a crucial role in many daily processes. For example, dirt repelling glasses can be achieved by a hydrophobic coating. Typically, the wetting properties of such a coating are characterized by measurements of the advancing and receding contact angles by sessile drop goniometry [1]. However, this method provides only information on the selected areas...

Wetting phenomena are of great importance across various scientific disciplines, and as a fundamental research area, the wettability dynamics of adaptive substrates, such as polyelectrolyte multilayers (PEM), have gained significant attention. Notably, an interesting property of polyelectrolyte (PE) substrates is their propensity to swell in a liquid environment. However, the complexities such...

Block copolymer gradient brushes: A detailed look at the synthesis and characterization

Jonah Decker1, Sören Steup2, Sergey I. Druzhinin1, Heiko Ihmels2, Holger Schönherr1*

1 Physical Chemistry I, Department of Chemistry and Biology and Research Center of Micro and Nanochemistry and (Bio)Technology (Cμ),...

Nanoscale poly(dimethylsiloxane) (PDMS) layers prepared by grafting the polymer chains on a smooth solid surfaces are typical example for the so called slippery covalently-attached liquid surfaces (SCALS). Such surfaces exhibit very low contact angle hysteresis (CAH) and low friction for sliding water drops [1-2]. These properties are commonly attributed to the liquid-like mobility of the...

Wetting is generally regarded as an inherent property of material surfaces. Wetting imperfections contribute to contact angle hysteresis, influencing the overall wetting dynamics. Existing techniques like contact angle goniometry are not sufficient for accurately quantifying variations in wetting behavior due to temporal and local resolution limitations. A direct quantitative measurement of...

High-speed side-view videos of sliding drops enable researchers to investigate drop dynamics and surface properties. However, understanding the physics of sliding requires knowledge of the drop width, which necessitates a front-view perspective. The drop’s width is a crucial parameter due to its association with the friction force. Incorporating extra cameras or mirrors to monitor changes in...

The interaction of liquid droplets with lubricated substrates is rich with interesting physics combining thermodynamics, polymer science, and fluid dynamics, and has the potential for various industrial applications. In previous work, we showed the existence of a cloaking transition where the lubricant covers the surface of the droplet at equilibrium. Here, we describe the kinetics of the...

The advancement of renewable energies is crucial in an age in which we are confronted with growing climate and environmental challenges. Solar energy is considered a highly promising, efficient and environmentally friendly source of energy. However, solar panels lose a significant amount of efficiency due to soiling [1]. Thus, a lot of water (more than 35 billion liters per year) is required...

We designed and fabricate stimulus responsive polymer brushes which can switch the chain conformation reversibly in response to addition/removal of small amount of Cd2+ions. Design of the polymer structure was inspired by plant protein (phytochelatin) that selectively detect and capture Cd2+ ions. As our polymer realized an excellent affinity to Cd2+ (KD ~ 10-9 M), the brush conformation and...

Synthesis and investigation of Nile Red- and aurone-labeled methacrylate derivatives as novel solvatochromic probes for polymer analysis

Sören Steup1, Jonah Decker2, Sergey I. Druzhinin2, Daria V. Berdnikova1, Holger Schönherr2, Heiko Ihmels1*

1 Organic Chemistry II, Department of Chemistry and Biology and Research Center of Micro and Nanochemistry and (Bio)Technology (Cμ), University...

Nanopores are a key component in various technologies from oil production, separation and sensing, to drug delivery or catalysis and energy conversion. In contrast to technological pores, biological pores and channels demonstrate highly precise transport being directed, highly selective, and gated. A key factor to this performance is their nanoscale structure and local control on charge...

Biological cells use membranes and condensates (liquid-like droplets) to compartmentalize their interior. As every structure within a cell is either enclosed by a membrane or by a liquid interface it is fundamental to understand what happens if these two come into contact. Recent studies suggest that membrane-droplet interactions are involved in various key biological processes. As...

We designed and fabricate stimulus responsive polymer brushes which can switch the chain conformation reversibly in response to addition/removal of small amount of Cd2+ions. Design of the polymer structure was inspired by plant protein (phytochelatin) that selectively detect and capture Cd2+ ions. As our polymer realized an excellent affinity to Cd2+ (KD ~ 10-9 M), the brush conformation and...

The shapes of liquid polystyrene (PS) droplets on viscoelastic polydimethylsiloxane (PDMS) substrates are investigated experimentally using atomic force microscopy for a range of droplet sizes and substrate elasticities. These shapes, which comprise the PS-air, PS-PDMS, and PDMS-air interfaces as well as the three-phase contact line, are compared to theoretical predictions using axisymmetric...

Electrostatic charge separation at receding contact lines, called slide electrification, has been extensively studies in recent years. Resulting electrostatic interactions fundamentally contribute to contact angle hysteresis and can significantly slow down the movement of sliding drops. Yet, the relevance of electrostatic effects in dynamic wetting beyond single drops has largely remained...

Water dewetting generates static electricity. We studied the charging of polymer slides and metal electrode supported polymer films withdrawn vertically from a pool of aqueous solutions. For pure water, charging was negative and surface charge densities increased with the speed of dewetting, which we explain by the entrainment of nanometer-sized water droplets charged by unbalanced adsorbed...

Surface binding and surface phase separation of cytosolic scaffold proteins on lipid membranes are involved in many cellular processes, such as cell signaling, cell adhesion, and cortex regulation. However, the interplay between surface binding and surface phase separation is poorly understood. In this work, we study this interplay by deriving a general thermodynamic model and applying it to...

Previous studies have shown that a pure architecture change of polymer brushes from linear to un-concatenated cyclic chains without a change of effective grafting density, can only lead to trivial changes of swelling properties of brushes in solutions. But it remains unknown whether this conclusion is valid for the swelling characteristics of brushes in vapor phase, because vapors often...

Wetting of morphologically complex substrates is omnipresent, since a rare surface is perfectly physically homogeneous. Surface defects, roughness, porosity and/or peculiarities of the upper-layer structure of a surface dictated by the industrial needs often result in the different wetting and evaporation dynamics, when compared to that on a smooth surface. In order to better predict and...

Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Like in triboelectricity, moving drops can separate electric charges. This phenomenon is called slide or contact electrification. Typically, water drops sliding down hydrophobic surfaces spontaneously acquire a positive charge while they deposit...

The evaporation of a water droplet on a solid surface is ubiquitous in both natural and industrial settings. However, the mechanism by which evaporating droplets deposit charges on the interface, driven by the movement of the three-phase contact line, remains poorly understood. In this study, we investigate the nanoscale charge deposition of evaporating droplets and subsequent charge decay...

Charge separation at the moving interface of liquid-solid has been observed and studied through the years [1]. Water droplets impinging on a solid substrate [2,3], coalescence induced droplet jumping [4] and slide electrification [1,5] are among the phenomena which lead to voltage generation. It is shown that sliding water drops on hydrophobic insulating surfaces leads to charge separation at...

Droplet motion on surfaces plays an important role in biological systems and technological applications [1–3]. Most natural and artificial superhydrophobic surfaces have irregular or stochastic roughness [4–6], making them challenging to model. However, all existing studies on wetting have been done for well-defined surfaces, such as micropillar arrays with a regular structure [7–11]. Here, I...