2023, Essink, Martin H., van Limbeek, Michiel A. J., Pandey, Anupam, Karpitschka, Stefan, Snoeijer, Jacco H.

An elastic material that experiences strong compression parallel to its free surface can exhibit sharp surface folds. Such creases arise due to an instability where a self-contacting fold appears on the surface, often observed in growing tissues or swelling gels. Self-adhesion of the contact is known to affect the bifurcation behavior and morphology of these structures, yet a quantitative description remains elusive. From numerical simulations and an energy analysis we resolve how adhesion quantitatively affects both morphology and bifurcation behavior. It is found that a reduced energy is able to accurately describe the bifurcation, in terms of an effective scaling that collapses the data very well. The model accurately describes how adhesion hinders crease nucleation. Furthermore, we show that the free surface profiles in the presence of surface tension exhibit self-similarity, and can be collapsed onto a universal curve.

2022, Mokbel, Dominic, Aland, Sebastian, Karpitschka, Stefan

The capillary traction of a liquid contact line causes highly localized deformations in soft solids, tremendously slowing down wetting and dewetting dynamics by viscoelastic braking. Enforcing nonetheless large velocities leads to the so-called stick-slip instability, during which the contact line periodically depins from its own wetting ridge. The mechanism of this periodic motion and, especially, the role of the dynamics in the fluid have remained elusive, partly because a theoretical description of the unsteady soft wetting problem is not available so far. Here we present the first numerical simulations of the full unsteady soft wetting problem, with a full coupling between the liquid and the solid dynamics. We observe three regimes of soft wetting dynamics: steady viscoelastic braking at slow speeds, stick-slip motion at intermediate speeds, followed by a region of viscoelastic braking where stick-slip is suppressed by liquid damping, which ultimately gives way to classical wetting dynamics, dominated by liquid dissipation.

2022, Ramírez-Soto, Olinka, Karpitschka, Stefan

The Marangoni contraction of sessile droplets occurs when a binary mixture of volatile liquids is placed on a high-energy surface. Although the surface is wetted completely by the mixture and its components, a quasistationary nonvanishing contact angle is observed. This seeming contradiction is caused by Marangoni flows that are driven by evaporative depletion of the volatile component near the edge of the droplet. Here, we show that the composition of such droplets is governed by Taylor dispersion, a consequence of diffusion and strong internal shear flow. We demonstrate that Taylor dispersion naturally arises in a self-consistent long-wave expansion for volatile liquid mixtures. Coupled to diffusion-limited evaporation, this model quantitatively reproduces not only the apparent shape of Marangoni-contracted droplets, but also their internal flows.

2021, van Limbeek, Michiel A. J., Essink, Martin H., Pandey, Anupam, Snoeijer, Jacco H., Karpitschka, Stefan

The compression of soft elastic matter and biological tissue can lead to creasing, an instability where a surface folds sharply into periodic self-contacts. Intriguingly, the unfolding of the surface upon releasing the strain is usually not perfect: small scars remain that serve as nuclei for creases during repeated compressions. Here we present creasing experiments with sticky polymer surfaces, using confocal microscopy, which resolve the contact line region where folding and unfolding occurs. It is found that surface tension induces a second fold, at the edge of the self-contact, which leads to a singular elastic stress and self-similar crease morphologies. However, these profiles exhibit an intrinsic folding-unfolding asymmetry that is caused by contact line pinning, in a way that resembles wetting of liquids on imperfect solids. Contact line pinning is therefore a key element of creasing: it inhibits complete unfolding and gives soft surfaces a folding memory.

2023, Abbaspour, Leila, Malek, Ali, Karpitschka, Stefan, Klumpp, Stefan

Active matter systems provide fascinating examples of pattern formation and collective motility without counterpart in equilibrium systems. Here, we employ Brownian dynamics simulations to study the collective motion and self-organization in systems of self-propelled semiflexible filaments, inspired by the gliding motility of filamentous Cyanobacteria. Specifically, we investigate the influence of stochastic direction reversals on the patterns. We explore pattern formation and dynamics by modulating three relevant physical parameters, the bending stiffness, the activity, and the reversal rate. In the absence of reversals, our results show rich dynamical behavior including spiral formation and collective motion of aligned clusters of various sizes, depending on the bending stiffness and self-propulsion force. The presence of reversals diminishes spiral formation and reduces the sizes of clusters or suppresses clustering entirely. This homogenizing effect of direction reversals can be understood as reversals providing an additional mechanism to either unwind spirals or to resolve clusters.

2022, Chao, Youchuang, Ramírez-Soto, Olinka, Bahr, Christian, Karpitschka, Stefan

The interplay between phase separation and wetting of multicomponent mixtures is ubiquitous in nature and technology and recently gained significant attention across scientific disciplines, due to the discovery of biomolecular condensates. It is well understood that sessile droplets, undergoing phase separation in a static wetting configuration, exhibit microdroplet nucleation at their contact lines, forming an oil ring during later stages. However, very little is known about the dynamic counterpart, when phase separation occurs in a nonequilibrium wetting configuration, i.e., spreading droplets. Here we show that liquid–liquid phase separation strongly couples to the spreading motion of three-phase contact lines. Thus, the classical Cox–Voinov law is not applicable anymore, because phase separation adds an active spreading force beyond the capillary driving. Intriguingly, we observe that spreading starts well before any visible nucleation of microdroplets in the main droplet. Using high-speed ellipsometry, we further demonstrate that the evaporation-induced enrichment, together with surface forces, causes an even earlier nucleation in the wetting precursor film around the droplet, initiating the observed wetting transition. We expect our findings to improve the fundamental understanding of phase separation processes that involve dynamical contact lines and/or surface forces, with implications in a wide range of applications, from oil recovery or inkjet printing to material synthesis and biomolecular condensates.

2022, Khattak, Hamza K., Karpitschka, Stefan, Snoeijer, Jacco H., Dalnoki-Veress, Kari

When a droplet is placed on a soft surface, surface tension deforms the substrate, creating a capillary ridge. We study how the motion of the ridge dissipates energy in microscopic droplets. Using a micropipette based method, we are able to simultaneously image and measure forces on a microscopic droplet moving at a constant speed along a soft film supported on a rigid substrate. Changing the thickness of the thin film tunes the effective stiffness of the substrate. Thus we can control the ridge size without altering the surface chemistry. We find that the dissipation depends strongly on the film thickness, decreasing monotonically as effective stiffness increases. This monotonic trend is beyond the realm of small deformation theory, but can be explained with a simple scaling analysis.

2023, Jeon, Hansol, Chao, Youchuang, Karpitschka, Stefan

The surface mechanics of soft solids are important in many natural and technological applications. In this context, static and dynamic wetting of soft polymer gels has emerged as a versatile model system. Recent experimental observations have sparked controversial discussions of the underlying theoretical description, ranging from concentrated elastic forces over strain-dependent solid surface tensions to poroelastic deformations or the capillary extraction of liquid components in the gel. Here we present measurements of the shapes of moving wetting ridges with high spatiotemporal resolution, combining distinct wetting phases (water, FC-70, air) on different ultrasoft PDMS gels (∼100Pa). Comparing our experimental results to the asymptotic behavior of linear viscoelastocapillary theory in the vicinity of the ridge, we separate reliable measurements from potential resolution artifacts. Remarkably, we find that the commonly used elastocapillary scaling fails to collapse the ridge shapes, but, for small normal forces, yields a viable prediction of the dynamic ridge angles. We demonstrate that neither of the debated theoretical models delivers a quantitative description, while the capillary extraction of an oil skirt appears to be the most promising.

2022, Baumgartner, Dieter A., Shiri, Samira, Sinha, Shayandev, Karpitschka, Stefan, Cira, Nate J.

When a droplet comes in contact with a completely wetting surface, the liquid typically spreads until it covers the entire substrate. However, nonuniform evaporation of a multicomponent droplet can generate surface tension gradients that alter this behavior. Here, we explore the rich dynamics of fully miscible, three-component droplets composed of water, ethanol, and propylene glycol on completely wetting glass substrates. These droplets initially spread rapidly but then stop and contract. We experimentally and theoretically investigate this behavior throughout the ternary parameter space at different relative humidities. Evaporation changes the composition of the droplet over space and time, resulting in a reversal of Marangoni flows that ultimately determines the dynamic droplet shape. We illustrate the utility of such dynamics by collecting, aggregating, and removing contaminants from a 4-cm² area using a single µL-scale droplet.

2021, Molina, Anton, Kumar, Shailabh, Karpitschka, Stefan, Prakash, Manu

Geometry in materials is a key concept which can determine material behavior in ordering, frustration, and fragmentation. More specifically, the behavior of interacting degrees of freedom subject to arbitrary geometric constraints has the potential to be used for engineering materials with exotic phase behavior. While advances in lithography have allowed for an experimental exploration of geometry on ordering that has no precedent in nature, many of these methods are low throughput or the underlying dynamics remain difficult to observe directly. Here, we introduce an experimental system that enables the study of interacting many-body dynamics by exploiting the physics of multidroplet evaporation subject to two-dimensional spatial constraints. We find that a high-energy initial state of this system settles into frustrated, metastable states with relaxation on two timescales. We understand this process using a minimal dynamical model that simulates the overdamped dynamics of motile droplets by identifying the force exerted on a given droplet as being proportional to the two-dimensional vapor gradients established by its neighbors. Finally, we demonstrate the flexibility of this platform by presenting experimental realizations of droplet−lattice systems representing different spin degrees of freedom and lattice geometries. Our platform enables a rapid and low-cost means to directly visualize dynamics associated with complex many-body systems interacting via long-range interactions. More generally, this platform opens up the rich design space between geometry and interactions for rapid exploration with minimal resources.