When and how self-cleaning of superhydrophobic surfaces works
Max Planck Institute for Polymer Research, Mainz
In recent years, a lot of research went into exploring different strategies to achieve long-lasting and highly liquid-repellent surfaces. This research resulted in a greatly improved understanding on how surface properties (hydrophobicity and geometry) and repellency (i.e. contact angles, impalement pressure) of liquid drops correlate. However, there is poor understanding on how surface properties and contamination (size, concentration, hydrophilicity) correlate. Therefore, we developed a method to slowly move a drop over a contaminated surface while imaging the region close to three-phase contact line by laser scanning confocal micrposcopy simultaneously. This enabled us to monitor in slow motion how a drop takes up particulate contamination while rolling over a contaminated surface.1 Using a lateral adhesion force device, we quantified how the friction force evolve during self-cleaning.
Geyer, F.; D'Acunzi, M.; Sharifi, A.; Saal , A.; Gao N.; Kaltbeitzel, A.; Sloot, T.-F.; Berger, R.; Butt, H.-J.; Vollmer D., When and how self-cleaning of superhydrophobic surfaces works, Sci. Adv. 6, eaaw9727 (2020).
How a water drop removes a particle form a hydrophobic surface
Max Planck Institute for Polymer Research, Mainz
When a water drop collides with a particle on a flat hydrophobic substrate, different outcomes are observed, depending on the speed of the collision. At slow speeds (50 μm s−1), the particle remains attached to the drop after the collision whereas at fast speeds (500 μm s−1), the particle enters and exits the drop. We used laser scanning confocal microscopy to image these collision scenarios and to directly measure the force acting on the drop during the collisions. For a drop to successfully remove a particle from the substrate, the maximum capillary force between the drop and the particle has to exceed the resistive force experienced opposing the motion of the particle over the substrate. We present geometrical models for the maximum capillary force that a drop can exert on a particle and compare the predictions to measured forces. Rolling of the particle is explicitly considered. Finally, we discuss the origins of the resistive force acting on the particle when it is pushed or pulled by a water-air interface. In particular, we show that a particle experiences a resistive capillary torque when rolling at an interface.
||Wetting, Imbibition and Switchable Elastocapillarity in Nanoporous Media
Physics and X-ray Analytics of Functional Materials Group
Technische Universität Hamburg, TUHH und Deutsches Elektronen-Synchrotron, DESY
Liquid-infused nanopores play a pivotal role in many natural and technological processes ranging from transport across biomembranes and plant movements to templating processes for nanomaterials and modern concepts of water desalination. Here I will present three experimental studies aimed at the fundamental exploration of wetting and imbibition dynamics in and at nanoporous surfaces (1) as well as studies aimed at employing the coupling of the elasticity of solids with the nano-capillarity of liquids, most prominently water, for the design of adaptive and electrically switchable actuator materials (2,3). To that end opto-fluidic techniques employing nanoporous photonic crystals will be combined with cyclic voltammetry and high-resolution synchrotron-based X-ray analytics.
(1) L.G. Cencha, G. Dittrich, P. Huber, C.L.A. Berlin, and R. Urteaga, Precursor Film Spreading during Liquid Imbibition in Nanoporous Photonic Crystals, Phys. Rev. Lett. 125, 234502 (2020).
(2) M. Brinker, G. Dittrich, C. Richert, P. Lakner, T. Krekeler, T.F. Keller, N. Huber, and P. Huber, Giant Electrochemical Actuation in a Nanoporous Silicon-Polypyrrole Hybrid Material, Science Advances 6, aba1483 (2020).
(3) M. Brinker and P. Huber, Switchable Elastocapillarity in Silicon Nanopores, manuscript.
A unified numerical model for wetting of soft substrates
Fakultät Informatik/Mathematik, Hochschule für Technik und Wirtschaft Dresden
The wetting of deformable elastic structures has been recently shown to comprise a rich variety of new physical phenomena (stick slip motion, durotaxis, etc.) whose fundamental understanding demands for numerical simulation tools. However, there are several numerical challenges to overcome, including the multiphysics and multiscale nature of the problem.
In this contribution I will present a novel unified model which enables a variety of soft wetting phenomena to be examined more closely for the first time. As one special feature I will point out the inclusion of viscosity in the elastic structure, which allows to model complex interactions between viscous fluids (e.g. water droplets in air) and viscoelastic substrates. Among others, this enables the observation of surfing droplets in the simulations.
Wetting of Polyelectrolyte (Multi)layers: Effect of Swelling and Droplet Evaporation
Regine von Klitzing
Institute for Condensed Matter Physics, TU Darmstadt, Germany
Wetting of polymer coating is of specific interest for the adhesion of cells or proteins and the technical control of wetting. Cells or proteins are in a physiological environment, i.e. an aqueous solution and the polymer coatings are often hydrophilic or partially hydrophobized. Therefore, the coatings often swell in water which changes the wettability during the wetting process. This complex process is not well understood. Only a few studies on the water wettability of polyelectrolyte-coated surfaces exist [e.g. 1-4]. Tay et al. found an effect of osmotic pressure on the contact line in wetting studies of charged and uncharged polymeric coatings . That makes the wetting process rather complex, since different time scales but also different length scales come into play. The adaptive surface needs a certain time until the swelling process is finished. The swelling doesn’t stop at the three phase contact line, and the liquid sucks laterally into the region in contact with the gas phase. Hansen and Miotto called that it peripherical thickness . This deforms the surface and the contact line is not well-defined anymore.
We studied the wettability by water of polyelectrolyte mono- and multilayers with different polycations or polyanions as the outermost layer using the sessile drop technique . Measurements in a water-saturated atmosphere and in ambient conditions [40% relative humidity (r.h.)] are made to study the effect of swelling and evaporation on the contact angle. It is found that these effects strongly depend on the outermost layer of the polyelectrolyte coating. For several kinds of polyelectrolytes as outermost layer the polyelectrolyte-coated surface can be equilibrated by pre-swelling in saturated vapor. Depositing a water droplet leads to a fixed contact angle against vapor. For other types polyelectrolytes as the outermost layer the water contact angle also indicates a change in the swelling state when the pre-equilibrated film is directly in contact with liquid water, resulting in a decrease in contact angle with time. The studies show that a highly sophisticated interplay between hydrophobic backbone and charge density determines the wetting behavior, irrespective of the sign of surface charge. The stability of the wetting film on polyelectrolyte surface is analysed by disjoining pressure isotherms . There surface charge seems to be the most dominant factor for stability of the wetting film.
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Simple gradient dynamics model for drops on elastic substrates
Institut für Theoretische Physik, Westfälische Wilhelms-Universität Münster
The investigation of the wetting behavior on viscoelastic or elastic substrates is of great interest. In this talk we present a simple model for steady liquid drops on fully compressible elastic substrates and show that a double transition of contact angles appears under variation of the substrate softness, similar to the one described in . We further discuss whether these angles agree with the Neumann and Young-Laplace conditions in the liquid-liquid and liquid-solid limit respectively and how the transitions depend on droplet size. Finally, we employ a gradient dynamics model in the long-wave limit and show first results of direct time simulations.
 Lubbers, L. A., Weijs, J. H., Botto, L., Das, S., Andreotti, B., and Snoeijer, J. H., (2014). Drops on soft solids: free energy and double transition of contact angles. Journal of fluid mechanics, 747.