From Adaptive to Active Wetting Dynamics

Biomolecular condensates are key organizers of cellular biochemistry, yet their interactions with membranes remain underexplored. Given the prevalence of membrane-bound compartments in cells, condensate–membrane encounters are inevitable and can profoundly influence cellular function and architecture. Using giant unilamellar vesicles (10–100 µm) as a minimal model, we investigate how condensates wet, reshape, and remodel membranes [1–3]. We show that condensates can induce striking morphological transformations [4], reorganize lipids [5], trigger endocytosis-like events [6], and even seal damaged membranes by patching pores [7]. Membranes, in turn, actively modulate these interactions: their lipid composition and packing determine condensate affinity and wetting behavior [8]. Notably, we find that affinity is governed not by the hydrophobicity of condensates but by the permittivity contrast between them and the dilute phase around them [9]. Together, these results reveal how fundamental physicochemical principles—independent of active processes or scaffolding proteins—can drive membrane interactions and remodeling. Our findings provide insight into the role of condensate-membrane interactions in cellular organization and offer design principles for engineering artificial cells. References: 1. Mangiarotti & Dimova, Annu. Rev. Biophys. 53:319, 2024. 2. Dimova & Lipowsky, Adv. Mater. Interfaces 4:1600451, 2017. 3. Liu et al., Front. Chem. 7:213, 2019. 4. Mangiarotti et al., Nature Commun. 14:2809, 2023. 5. Mangiarotti et al., Nature Commun. 14:6081, 2023. 6. Mangiarotti, Aleksanyan et al., Adv. Sci. 11:2309864, 2024. 7. Bussi et al., Nature 623:1062, 2023. 8. Mangiarotti et al., Nature Commun. 16:2756, 2025. 9. Sabri et al., bioRxiv, doi:10.1101/2025.03.09.642144 (2025).

In recent years, it has become apparent that water droplets sliding over hydrophobic surfaces spontaneously deposit negative charges. Consequently, the drops themselves accumulate a positive charge. This phenomenon, known as slide or contact electrification, is surprising because the energy of a charge at the solid–air interface is much higher than at the solid–water interface. The underlying process is not yet clear. The presentation will cover the current state of knowledge, including the consequences of slide electrification for the dynamics of drop sliding and contact angles will be discussed. Contact angles will also be discussed.

Wetting — where fluid-fluid interfaces move across surfaces — is a rich, well-studied topic, showcasing a huge range of important capillary phenomena. At the heart of these are concepts such as Laplace pressure, contact angles, menisci, and surface energies. Interestingly, all of these concepts and many similar phenomena also arise in solidification, where solid-liquid interfaces move across surfaces. Solidification is so analogous to wetting that one might call it a sibling topic. However, it is far less studied. Here, I will explain why solidification contact lines are important to understand for many freezing processes, ranging from cryopreservation to freezing-induced damage in soils. Firstly, I will showcase the similarities (and some important differences) between wetting and solidification by showing how particles suspended in a liquid are pushed across a surface by wetting and solidification contact lines. I will show how we can use this process to precisely position microparticles on templated solid surfaces. Secondly, I will show how solid/liquid interfaces deform soft surfaces, via the mechanism of cryosuction. This is a key mechanism causing damage during freezing. Finally, I will explain some outstanding problems in the field, which I believe would benefit from insights from the wetting community.

Droplets allow biological cells to control the spatial organization of their liquid-like cytosol. In particular, cells use active processes to drive chemical reactions, which affects droplets. Moreover, cells possess many solid-like structures, like the cytoskeleton and lipid membranes. In this talk, I will discuss how chemically active droplets wet such solid-like structures. I will discuss the role of activity in heterogeneous nucleation, the deformation of sessile droplets, and the repulsion by liquid bridges. These examples demonstrate the rich dynamics of chemically active droplets in complex environments.

Biomolecular condensates are liquid droplets of various compositional and functional identities formed by key proteins and nucleic acids inside the nucleosol and cytosol. The cytosol in particular is crossed by cytoskeletal filaments, filled with membrane-enclosed compartments, and densely packed with various biomolecules. Biomolecular condensates form and function in this active, crowded, and complex environment. These condensates interact with cytoskeletal filaments through wetting interactions, tuned by the biochemistry of the condensate and protein interactions. Cytoskeletal filaments, on the other hand, are dynamic biopolymers composed of molecular building blocks. Different condensates show characteristic wetting behavior towards cytoskeletal filaments. On the molecular scale, these interactions translate to distinct partitioning of the respective cytoskeletal building blocks into the condensates. Combining in vivo and in vitro experiments of two different biomolecular condensates with simple physical models reveals how different wetting interactions towards cytoskeletal filaments (here actin and microtubules) lead to varying physiological function: Stress granules, bulk cytosolic condensates, show neutral wetting interactions towards cytoskeletal filaments and their molecular building blocks, leading to accumulation of filaments at the condensate interface and coupling of condensate motion and localization to cytoskeletal flow and structure. Recently, we identified a novel condensate formed by the cytosolic domain of the neuronal membrane protein PLPPR3. PLPPR3 condensates show strong partitioning of actin monomers mediated by a newly identified actin-affinity domain within PLPPR3. These condensates consequently act as actin sinks, locally boosting actin polymerization kinetics and sustaining filament formation at the expense of actin in the environment through thermodynamic coupling of partitioning and polymerization. Coherent with the pronounced actin partitioning, actin filaments are strongly wetted by PLPPR3 condensates in vitro, leading to the formation of actin rings in large condensates and outgrowth of actin filaments from small condensates. In cells, we find that PLPPR3 condensates induce formation of filopodia, finger-like membrane protrusions filled with parallel bundles of actin filaments that are important for cellular motility and neuronal morphogenesis. In this way, PLPPR3 condensates are potent hubs of actin polymerization directly at and interacting with the plasma membrane. Functional analysis inside neuroblastoma cells affirms the necessity of the actin-affinity domain within PLPPR3 for filopodia formation, directly linking wetting behavior to physiological function.

We present two distinct systems in which wetting dynamics play a crucial role in generating self-sustained oscillations. In the first system, we observed spontaneous and recurrent deposition and dissolution at the surface of a camphor-methanol solution. This oscillation arises from a coupling between temperature-dependent solubility and camphor layer wettability. In the second system, the water level around a camphor disk fixed at the water surface exhibits self-sustained oscillations. Surface-active camphor molecules are released from the disk, and the coupling between the resulting chemical concentration field and the wettability of the disk surface plays a crucial role. These findings highlight the importance of wetting dynamics in generating emergent spatio-temporal behavior.

We study wetting droplets formed of active Brownian particles in contact with a repulsive potential barrier, in a wedge geometry. Our numerical results demonstrate a transition between partially wet and completely wet states, as a function of the barrier height, analogous to the corresponding surface phase transition in passive fluids. We analyse partially wet configurations characterised by a nonzero contact angle $\theta$ between the droplet surface and the barrier including the average density profile and its fluctuations. These findings are compared with two equilibrium systems: a Lennard-Jones fluid and a simple contour model for a liquid–vapour interface. We locate the wetting transition where $\cos(\theta) = 1$, and the neutral state where $\cos(\theta) = 0$. We discuss the implications of these results for possible definitions of surface tensions in active fluids.

Atmospheric water capture has the potential to alleviate water scarcity around the world and works by condensing water vapor onto a cold surface. However, actively cooling a surface requires substantial energy. We report the application of large scale (20  20 cm2) porous polymer coatings with ultra-high total solar reflectance (97%) and emittance (98% in the atmospheric window), which can passively cool 6 ºC below ambient temperature when exposed to the sky, even under direct sun. When the coating cools below the dew point, it harvests water from the atmosphere through condensation of droplets. Thanks to the application of a smooth UV-resistant topcoat promoting water droplet roll-off, 390 mL/m2/day of water could be collected, entirely passively. A longitudinal six-month study demonstrates that the coatings are functional, robust, and suited to long-term outdoor deployment. The minute-by-minute recording of the surface cooling, water capture and weather factors over six months, allow to identify the major factors impacting the surface performance in Sydney, Australia, and a theoretical model extends the water capture prediction to the rest of Australia. These insights will advance cool roof coatings, and advance the provision of sustainable, delocalized and low-cost sources of water from the atmosphere.

Flows along an interface due to a difference in surface tension, known as Marangoni flows, are important in an array of systems from microfluidic devices to plant vasculature. In this work, we investigate a novel Marangoni-driven phenomenon: star-like surface fracture on milk-air interfaces. We show that this phenomenon can be reproduced using a simplified solution of deionized water and $\beta$-lactoglobulin, a milk protein that forms a thin, gel-like layer at a water-air interface. By combining high-speed video microscopy, mechanical measurements of interfacial properties, and analytic theory, we derive a relationship between the number of star points and interface-age-dependent properties of solid protein films on fluid interfaces. Our results provide a new approach to measuring the fracture energy of very thin films and reveals an unexpected role for solid fracture mechanics in understanding of fluid surfaces in the presence of adsorbed species.

The physicochemical properties of polymer solutions in confined spaces often differ significantly from their bulk behavior, particularly when interfacial effects are involved [1, 2]. We have investigated polymer solutions encapsulated within lipid membrane–covered droplets to clarify how membrane wetting affects molecular diffusion and phase separation under confinement. By systematically varying droplet size and polymer composition, we demonstrate that lipid membranes induce characteristic wetting layers that modify both polymer diffusivity [3] and the onset of phase separation [4] compared to bulk systems. These effects are especially pronounced at the cell-size scale (droplet radius R < 30 μm), where confinement and membrane interactions act synergistically. We identify membrane wetting, rather than small volume alone, as the dominant factor governing the physicochemical landscape of confined polymer solutions. Our findings provide new insights into how wetting and confinement regulate phase behavior in soft matter and may help to elucidate the physicochemical basis of intracellular phase transitions, such as liquid–liquid phase separation. In this presentation, I will focus on the impact of membrane wetting in confined droplets [1, 2], and, if time allows, briefly discuss wetting effects at solid–liquid interfaces [5]. [1] M. Yanagisawa, K. Fujiwara, Macromolecules, doi:10.1021/acs.macromol.5c00706 [2] M. Yanagisawa, et al., Langmuir, 38, 11811 (2022) [3] Y. Kanakubo, et al., ACS Mater. Au, 3, 442 (2023) [4] C. Watanabe, ACS Mater. Lett., 4, 1742 (2022) [5] H. Gong, et al., Nature, 636, 92-99 (2024)

It is now possible to create hydrophobic and hydrophilic surfaces for which the contact angle hysteresis for water droplets is as low as 1 degree and which are complementary to slippery superhydrophobic surfaces. Remarkably, it is also possible to have two hydrophobic liquid-like surfaces with almost identical static wetting properties, but with dramatically different dynamic droplet friction. In this talk, I will present theoretical concepts and experiments revisiting the Kawasaki-Furmidge equation for both static and dynamic droplet friction, and I will also revisit the case of strong dilute defects created by combining two ultra-low contact angle hysteresis surface coatings. Within the context of almost perfectly intrinsically hysteresis-free coatings, I will show how geometric shape factors can be determined and a 3D molecular kinetic theory developed, and I will discuss how strength of defects depends on the topography and chemistry of the defect and background surfaces.

Biological cells utilize membranes and liquid-like droplets, known as biomolecular condensates, to structure their interior. The interaction of droplets and membranes, despite being involved in several key biological processes, is so far little understood. Here, we present a first numerical method to simulate the continuum dynamics of droplets interacting with deformable membranes via wetting. The method combines the advantages of the phase-field method for multiphase flow simulation and the arbitrary Lagrangian-Eulerian method for an explicit description of the elastic surface. The model is thermodynamically consistent, coupling bulk hydrodynamics with capillary forces, as well as bending, tension, and stretching of a thin membrane. Its capabilities are illustrated in several two- and three-dimensional axisymmetric scenarios.

Condensates with liquid-like properties form in cells and interact with their environment, including membrane-bound organelles and the cytoskeleton. These interactions can be understood as wetting phenomena on a microscopic scale. In this talk, I will present recent data on condensate wetting in plant embryo cells. Condensates and membranes engage in elastocapillary interactions, where the surface tension of the condensate deforms the membrane and the membrane responds by adopting distinct shapes, ranging from tubular to sheet-like to cup-like, or closed sheet, structures. We complemented cellular observations with reconstitution experiments and Monte Carlo simulations using well-defined material properties, particularly condensate surface tension and membrane availability, which govern the observed morphologies. We find that these shapes are separated by energy barriers and exhibit metastable behavior, which governs the dynamics of shape transitions. These findings indicate that temporal control of condensate surface properties can mediate the morphogenesis of cup-like structures in cells, such as the formation of “bulbs” within plant vacuoles. More broadly, our results illustrate how the interplay of condensates and membranes contributes to intracellular organization through wetting and elastocapillary forces.

Cleavage - the series of rapid cell divisions that follow fertilization - marks the onset of metazoan development and represents a deeply conserved evolutionary process. Across animals, two principal modes exist: complete (holoblastic) and incomplete (meroblastic) cleavage. While holoblastic cleavage resembles conventional cytokinesis both in vitro and in vivo, the mechanisms underlying meroblastic cleavage have remained poorly understood. Using zebrafish embryos as a model, we show that meroblastic cleavage proceeds through a distinct two-step mechanism. The process begins with the assembly and contraction of a large, arc-shaped actomyosin cable. However, this contractile event alone is insufficient to complete division. A second phase, driven by cadherin-mediated membrane adhesion, is required to invaginate the furrow ridge. Strikingly, this transition depends on mechanical uncoupling of the contractile cable from the surrounding cortex. We demonstrate that such uncoupling arises from an active nematic instability, which both enhances contractility along the cable and generates actin depletion zones that relieve lateral connections. Together, these findings reveal that meroblastic cleavage is governed not by a single actomyosin-based event but by a sequential interplay between cytoskeletal contraction and cadherin-dependent adhesion, highlighting a mechanism fundamentally distinct from canonical cytokinesis.

Active fluids encompass a wide range of non-equilibrium fluids, in which the self-propulsion or rotation of their units can give rise to large-scale spontaneous flows. Despite the diversity of active fluids, they are commonly viscoelastic. Therefore, we develop a hydrodynamic model of isotropic active liquids by accounting for their viscoelasticity. Specifically, we incorporate an active stress term into a general viscoelastic liquid model to study the spontaneous flow states and their transitions in two-dimensional channel, annulus, and disk geometries. We have discovered rich spontaneous flow states in a channel as a function of activity and Weissenberg number, including unidirectional flow, travelling wave, and vortex-roll states. The Weissenberg number acts against activity by suppressing the spontaneous flow. In an annulus confinement, we find that a net flow can be generated only if the aspect ratio of the annulus is not too large nor too small, consistent with the experiments of microtubule-based active fluids. In a disk geometry, we observe a periodic chirality-switching of a single vortex flow, resembling the bacteria-based active fluid experiments. As such, our active viscoelastic model offers a unified framework to elucidate diverse active liquids, uncover their connections, and highlight the universality of dynamic active-flow patterns.

Contact angle goniometry is the current industry and academic standard for characterizing the wettability of surfaces. With this method, mm-sized drops are used and only a handful of discrete locations on a surface can be investigated. Furthermore, any wetting features smaller than the diameter of drop remain undetected. Here, we present a novel approach: scanning drop friction force microscopy (sDoFFi). The sDoFFi allows us to characterize cm2 areas by raster-scanning a surface with a drop. It is a quick and reliable technique with an ability to characterize wetting imperfections down to a µm-scale [1]. The friction force of drops is a consequence of differences in the advancing and receding contact angles [2]. In sDoFFI, we use the drop friction force as a property to image and localize the wetting properties (Figure 1). Water drops sliding on hydrophobic surfaces often acquire a net charge and counter-ions are adsorbed at the surface. This phenomena is known as slide electrification. We investigate slide electrification at the onset of drop sliding and at low sliding velocities ≤ 1cm/s. The sDoFFI facilitates simultaneously measurements of the drop discharging current and drop friction force [3],[4]. Kelvin Probe analysis on hydrophobic surfaces complement slide electrification measurements. Interestingly, sliding drops can reach potentials in the kV regime and discharge close to electrical conductors. Discharging of drops leads to an electrochemical process and to a chemical modification of the surface coatings on a nanometer scale which can be further investigated by nano IR measurements [5]. References: [1] C. Hinduja, A. Laroche, S. Shumaly, Y. Wang, D. Vollmer, H.-J. Butt, Rüdiger Berger, Langmuir 38, 14635-14643 (2022). [2] N. Gao, F. Geyer, D.W. Pilat, S. Wooh, D. Vollmer, H.-J. Butt, R. Berger, Nature Physics, 14, 191 - 196 (2018). [3] C. Hinduja, H.-J. Butt, R. Berger, Soft Matter 20, 3349 - 3358 (2024). [4] C. Hinduja, B. Leibauer, R. Chaurasia, N. Knorr, A.D. Ratschow, S. Singh, H.-J. Butt, R. Berger, submitted (2025). [5] Z. Ni, X. Li, A.D. Ratschow, L. Jian, X. Zhou, P. Bista1, D. Cortes, G. Glasser, H. Luo, S. Chen,Y. Wang, K. Amann-Winkel, K. Koynov, R. Berger, H.-J. Butt, submitted (2025).

Capillary imbibition—the spontaneous invasion of a liquid into a pore or channel—plays a central role in applications ranging from microfluidics and porous materials to biological systems and agriculture. A precise understanding of this process requires accounting for both wetting dynamics and interfacial dissipation, particularly in systems involving multiple immiscible fluids and soft or treated surfaces. In this work, we investigate the capillary dynamics of binary fluid mixtures in confined geometries, focusing on the interplay between substrate wettability, dynamic contact angles, and asymmetric viscous dissipation. Using both theoretical modeling and numerical simulations, we analyze a canonical configuration: a lubricant-coated capillary, representative of slippery liquid-infused porous surfaces (SLIPS) and lubricant-impregnated surfaces (LIS). In these systems, a thin lubricant film separates the invading and displaced fluids from direct contact with the solid, fundamentally altering the imbibition dynamics. We demonstrate that the distribution of dissipation across the fluid phases governs the advancement of the fluid front. When dissipation is dominated by the displaced phase, the front progresses diffusively, consistent with the classical Lucas–Washburn law. In contrast, when dissipation is localized in the lubricant layer, a linear, plug-like motion emerges. A reduced-order model is developed to capture these distinct regimes and the crossover between them, grounded in a recently proposed framework for both spontaneous and forced imbibition. This study elucidates how interfacial rheology and substrate affinity conspire to define the dynamical regimes of wetting in complex environments. It provides new insights into the design and optimization of soft and functional surfaces in microfluidic and porous systems.

Wetting of solid surfaces with thin films of simple and complex liquids on solid substrates is not only a ubiquitous phenomenon but also a highly important process in life science, natural science and engineering, related, for example, to the prevention of “dry eyes” with tear films, the cleaning of surface contaminants by detergents, and the spin-coating of organic and inorganic semiconductors. When hard solids are wet by a Newtonian fluid, the energy dissipation during the motion occurs solely by the viscous flow in the liquid phase. In contrast, the wetting with surfactant films and the wetting of soft, deformable surfaces are distinct, because the energy is dissipated viscoelastically. Lipid membranes, the main constituent of cell membranes, are approximately 5 nm-thick viscoelastic bimolecular surfactant films that can wet macroscopic substrate areas of some tens of cm2. In my talk, I introduced our recent experimental data, demonstrating that the addition and removal of a small molecule possessing an analogous structure to the head group of lipids can stop and restart the dissipative membrane spreading on demand.

Shape changes are a key feature of living matter, from the division of individual cells to tissue-scale morphogenesis. They rely on a precise interplay of mechanical force generation, chemical signals, and geometry. In recent years, active surfaces models have proven efficient physical descriptions of thin biological materials such as 2D epithelial tissues or the cellular surface, providing insights into both self-organised and guided morphogenesis. Here, I will discuss our model of the cellular actomyosin cortex as an active viscous layer. With this model, we study the conditions under which a cell is able to divide into two daughter cells and how to induce deformations of a single cell with optogenetic tools.

Suspensions of swimming bacteria like Bacillus subtilis often display homogeneous active turbulence in the swarming regime. Alternatively, biofilms may form from a process resembling motility induced phase separation (MIPS) [1] or large cluster emerge for mutant species with large aspect ratio [2]. Both phenomena are captured in a model for a compressible active polar fluid, where clustering appears either by an instability similar to MIPS caused by decreased swimming velocity at larger densities [3] or by turbulence-induced clustering stemming from a nonlinear advective coupling term [4]. In both situations, arrested coarsening, inverse Ostwald ripening and instabilities of planar interfaces are crucial for the resulting dynamical patterns. More, recently a similar phenomenology has also found to occur in a model describing protein dynamics at multicomponent lipid membranes consisting of two conserved concentration fields that are nonvariationally (or non-reciprocally) coupled [5]. [1] V. M. Worlitzer, A. Jose, I. Grinberg, M. Bär, S. Heidenreich, A. Eldar, G. Ariel and A. Be´er. Biophysical aspects underlying the swarm to biofilm transition. Science Advances, 8(24), 2022. [2] A. Be´er, B. Ilkanaiv, R. Gross, D. B. Kearns, S. Heidenreich, M. Bär and G. Ariel A phase diagram for bacterial swarming. Commun Phys, 3(66), 2020. [3] V. M. Worlitzer, G. Ariel, A. Be'er, H. Stark, M. Bär and S. Heidenreich. Motility-induced clustering and meso-scale turbulence in active polar fluids. New Journal of Physics, 23 033012, 2021. [4] V. M. Worlitzer, G. Ariel, A. Be'er, H. Stark, M. Bär and S. Heidenreich. Turbulence-induced clustering in compressible active fluids. Soft Matter, 2021(17), 10447-10457, 2021. [5] B. Winkler, S. Alonso and M. Bär. A simple model for conserved intracellular protein-lipid dynamics exhibits multiscale pattern formation, traveling domains and arrested coarsening. Preprint. 2025.

Wetting of surfaces by liquids is commonly observed in everyday life and is of paramount importance for many natural and engineering processes. In many cases, it is beneficial to design surfaces that repel liquids: these surfaces can suppress icing and surface fouling, they reduce drag and friction, and contaminants can be removed easily. Interestingly, nature provides several conceptually distinct inspirations to design liquid repellent surfaces. In this talk, I will discuss our recent works to understand complex droplet dynamics on structured, nature-inspired liquid repellent surfaces. In particular, I will focus on three phenomena. First, I will elucidate salient mechanisms of how particles can be captured and removed by liquid droplets, which is key for designing self-cleaning surfaces. Second, I will discuss how droplet dynamics are important for suppressing icing on superhydrophobic surfaces. Third, I will explain distinctive features of droplet dynamics on uniform and patterned lubricant infused surfaces, such as lubricant cloaking and the formation of wetting ridge, and how they give rise to new wetting states and contact line pinning mechanisms.

The apical junctional complex (AJC) in epithelial cells is composed of tight junctions (TJs) and adherens junctions (AJs), which organize into a continuous sub-apical belt that is critical for epithelial tissue integrity. While the molecular components of the AJC have been characterized recently, the physical mechanisms governing their spatial organization and assembly remain unclear. An open question is how size differences in the extracellular domains of AJ receptor proteins—such as the Junctional Adhesion Molecule (JAM-A), E-cadherin (E-CAD), and Nectin-1—contribute to their segregation during junction formation. We combine theory and in vitro constitution to understand receptor-mediated adhesion between a giant unilamellar vesicle (GUV) and a supported lipid bilayer (SLB).  A key finding is that differences in receptor length are sufficient to drive spatial segregation, even in the absence of cis-interactions among receptors. Our results provide new insights into the physical principles underlying AJ assembly, and suggest a new principle of spatial segregation not relying on direct inter-molecular interactions.

Jan Diekmann Institut für Theoretische Physik, Universität Münster, Wilhelm Klemm Str. 9, D-48149 Münster, Germany Uwe Thiele Institut für Theoretische Physik, Universität Münster, Wilhelm Klemm Str. 9, D-48149 Münster, Germany and Center of Nonlinear Science (CeNoS), Universität Münster, Corrensstr. 2, 48149 Münster, Germany We revisit thin-film modeling for partially wetting liquids in the context of adaptive substrates, i.e., brush-covered surfaces [1], and give a phenomenological overview of various dynamics of drops including spreading, sliding, and stick-slip behavior [2, 3]. Thereby, we present systems of simple and of mixed liquids that may imbibe into the brush and evaporate into an ambient gas phase, and either exhibit relaxational dynamics to equilibrium, or are driven out of such. Some of the presented theoretical results are linked to experiments. [1] Smook, van Eck, de Beer. Macromolecules, 2020. doi: 10.1021/acs.macromol.0c02228. [2] Hartmann, Diekmann, Greve, Thiele. Langmuir, 2024. doi: 10.1021/acs.langmuir.3c03313. [3] Greve, Hartmann, Thiele. Soft Matter, 2023. doi: 10.1039/d3sm00104k. [4] Diekmann, Thiele. The European Physical Journal Special Topics. doi: 10.1140/epjs/s11734-024-01169-4.

Classical elastocapillarity describes the stationary balance of surface tension and elasticity and provides a powerful framework for wetting phenomena on soft solids. Recent high-precision experiments reveal systematic discrepancies near the three-phase contact line that remain unexplained within this classical picture. These observations motivate consistent model extensions that incorporate viscoelasticity and poroelasticity while preserving the variational structure of elastocapillarity. I will discuss how such formulations naturally give rise to saddle-point problems, dissipation mechanisms, and finite element discretizations, and how these extensions help to overcome key shortcomings of the classical theory.

I will show how to understand spreading cell clusters as active liquid droplets that wet a substrate. I will discuss applications of our active wetting theory to explain (i) embryo implantation, (ii) collective durotaxis — the migration of cell clusters along gradients of substrate stiffness — as the motion of a droplet on a wettability gradient, and (iii) the asymmetric shapes of migrating cell clusters, with different front and back contact angles.

Morphogenesis, tissue regeneration, and cancer invasion involve dynamic transitions in tissue morphology, often driven by collective cell migration. These transitions can be viewed as active and passive wetting/de-wetting processes of epithelial monolayers. Like other soft matter systems, monolayers extend (wetting) or compress (de-wetting) on substrates depending on the balance of cohesion properties of both: an epithelial monolayer and substrate matrix, as well as their viscoelastic behaviour, and cell-matrix adhesion. Unlike passive materials, multicellular systems actively modulate surface and interfacial tensions and viscoelasticity through actomyosin contractility and remodelling of adhesion contacts during collective cell migration, while passive mechanical effects, such as Poisson’s effect, also contribute, particularly in anisotropic regions. Epithelial monolayers exhibit inhomogeneous distributions of adhesion strength, traction forces, contractility, packing density, cell velocity, and mechanical stress. These spatial variations lead to domains with approximately uniform physical properties, which are inherently unstable and interact with neighbouring domains, generating local forward and backward flows. Each domain has two degrees of freedom: spatial deformation and translational displacement of its center of mass. Domains can undergo uni-axial or biaxial extension/compression: biaxial deformations indicate isotropic domains driven predominantly by active process such as collective cell migration, whereas uni-axial deformations reflect anisotropy and a combination of active (along migration) and passive (perpendicular, Poisson) contributions. This domain-based view highlights how inhomogeneous mechanical properties and multiscale interactions govern wetting and de-wetting in epithelial monolayers, providing insight into the physical principles underlying tissue morphogenesis, repair, and invasion. The principal aims of this theoretical analysis are: (i) to emphasize the main characteristics of isotropic and anisotropic wetting/de-wetting of migrating epithelial collectives as the main factor in mechanical stress generation; (ii) to formulate constitutive models of the viscoelasticity of anisotropic and isotropic domains, (iii) to explore potential cellular strategies to mitigate mechanical stress, while also highlighting the associated costs of these strategies such as: cell jamming, live cell extrusion, and epithelial-to-mesenchymal transition.

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 changes in interfacial properties and enabling specific functionalities. In addition to these external stimuli, intrinsic polymer transitions—such as melting and glass transitions— provide an effective way to modulate polymer brush behavior, offering an additional mechanism to control and actuate brush properties, thus expanding their potential applications. To investigate this concept, we examine the wetting behavior of liquid n-alkanes on oleophilic bottle brushes composed of poly-n-alkyl methacrylate (PnMA). The melting temperature of these polymer brushes can be precisely tuned by adjusting the length of their side chains.Through macroscopic wetting experiments, Atomic Force Microscopy (AFM) adhesion measurements, and vibrational Sum-Frequency Generation (SFG) spectroscopy, we demonstrate that the melting transition of a semicrystalline oleophilic poly-octadecylmethacrylate (P18MA) brush drives a coupled swelling and wetting transition (Fig. 1) when exposed to various liquid alkanes. Notably, the top surface of the P18MA polymer exhibits a slightly higher melting temperature compared to the bulk, allowing for independent control of bulk-driven swelling and surface-driven wetting transitions. These transitions can be activated on demand—either globally through heating or locally using a focused laser beam. This "activation on demand" capability offers a novel approach to precisely control brush behavior, enabling the design of surfaces that dynamically respond to external stimuli. Our findings open up new possibilities for polymer brush-based functional coatings, allowing controlled fluid transport via independently switchable surface barriers and bulk transport layers.

Wetting of liquid droplets on passive surfaces is ubiquitous in our daily lives, and the governing physical laws are well understood. When surfaces become active, however, the governing laws of wetting remain elusive. Here, we propose chemically active wetting as a class of active systems where the surface is active due to a binding process that is maintained away from equilibrium. We derive the corresponding non-equilibrium thermodynamic theory and show that active binding fundamentally changes the wetting behavior, leading to steady, non-equilibrium states with droplet shapes reminiscent of a pancake or a mushroom. The origin of such anomalous shapes can be explained by mapping to electrostatics, where pairs of binding sinks and sources correspond to electrostatic dipoles along the triple line. This is an example of a more general analogy, where localized chemical activity gives rise to a multipole field of the chemical potential. The underlying physics is relevant for cells, where droplet forming proteins can bind to membranes accompanied by the turnover of biological fuels.

Recent years have seen a steady flow of media reports about cases of unethical behavior in academia. Such behavior seems to (a) be surprisingly common, (b) have the potential to cause great damage, and (c) typically remain unsanctioned. In my talk, I will first introduce a number of concepts that are relevant to the discourse on this topic (e.g., power, abuse of power). Then I will discuss some key factors that may explain the emergence and the persistence of patterns of unethical behavior in academia. Some of these factors are properties of unethical actors themselves (e.g., psychopathic traits), some are properties of the people that surround unethical actors (e.g., fear), and some are properties of the organizational setup (e.g., incentives, hierarchies, lack of effective controls). Based on this analysis, I will present some recommendations for reforms of the academic system that may help reduce the frequency and the severity of unethical behavior in academia.

Wie verhält sich ein Flüssigkeitstropfen, der mit einer festen Oberflächen in Kontakt kommt? Benetzungsvorgänge begegnen uns ständig im täglichen Leben – zu den typischen Anwendungen gehören das Beschichten, Drucken, das Verteilen von Herbiziden und Insektiziden. Bei der Anreicherung von Mineralien durch Flotation sowie beim Löten und Schmieren treten sie ebenso auf, wie beim Benetzen von Textilien und Filtern. Dennoch ist die dynamische Benetzung sowohl qualitativ als auch quantitativ nur unzureichend verstanden. Bis heute können wir nicht vorhersagen, ob und wie schnell ein Wassertropfen eine schiefe Ebene hinunterläuft. Um diese Prozesse besser zu verstehen, ist es notwendig, die Physik nahe der Kontaktlinie, an der Flüssigkeit, Festkörper und Gas aufeinandertreffen, zu betrachten. Ein wichtiger Aspekt, der erst in den letzten Jahren entdeckt und systematisch untersucht wurde ist die spontane Aufladung von Wassertropfen. Wassertropfen, die z.B. über ein Kunststoffoberfläche laufen, können leicht Spannungen von mehr als 1 kV erzeugen. In dem Vortrag werden diese neuen Erkenntnisse und ihre praktischen Auswirkung beschrieben. Und es geht darum, welche Herausforderungen noch zu überwinden sind.

Biomolecular condensation creates functional, subcellular compartments with fluid-like properties, typically referred to as condensates or membrane-less organelles. Interestingly, the interaction of condensates with other cellular structures has remained largely unexplored. During the 2019 Wetting Workshop, I proposed that fundamental physiological processes are controlled by the wetting of condensates and discussed the cell-specific challenges of such studies. In my presentation, I will provide a current overview of our understanding of how condensates drive the morphogenesis of membrane-bound organelles. Not only can they influence membrane bending processes, but they can also alter the topology of membranes by cutting membrane necks without relying on active, ATP-consuming protein machines such as ESCRTs. Examples include the formation of intraluminal vesicles in multivesicular bodies, autophagosomes during autophagy, and protein storage vacuoles during plant embryogenesis. Our studies reveal previously unknown wetting phenomena in cells and show how they contribute to intracellular organization.

Slippery liquid-infused porous surfaces (SLIPS), inspired by the slippery properties of the Nepenthes pitcher plant, have been introduced to increase the mobility of liquids by minimising the liquid–solid contact. SLIPS involve infusing a lubricating liquid into a textured surface, creating a thin liquid layer on top of the surface resulting in high drop mobility as well as unique properties such as anti-corrosion, self-cleaning, heat transfer enhancing, anti-fouling and anti-icing, water harvesting properties with broad implication for industry, including biomedical devices, pharmaceuticals, and food packaging. Despite these promising properties of SLIPS, there are several challenges that have remained unaddressed. These obstacles include questions regarding long-term durability, robustness, scalability, and cost-effectiveness. For example, maintaining the stability and duration of the lubricating liquid layer over time and the factors that contribute to this issue such as the amount of lubricant required to have a proper SLIP surface. To maintain optimal performance, the lubricant may need to be replenished and maintained on a continuous basis, increasing operational expenses and complexity. Furthermore, the scalability of SLIPS fabrication techniques, as well as their suitability with large-scale industrial applications, need to be investigated and optimised. Altering the properties of lubricants to decrease loss while maintaining the optimal slippery features in SLIPS is a promising feature to follow. The complex relationship between lubricant properties and retention mechanisms in SLIPS, particularly the role of the lubrication ridge forming around the drop causing depletion of the lubricant over time brings the questions of whether the lubricant ridge can be effectively controlled as well as whether there is an optimal size in relation to the volume of the drop sliding over the SLIPS. In this contribution I will present recent experiments performed in the SMSL laboratory at Northumbria University, elucidating the role of the lubricant ridge in the dynamics of drops on SLIPS, and develop a theoretical understanding of the experimental observations combining finite-element and analytical modeling approaches.

Wetting and evaporation of droplets on micropatterned surfaces play a key role in both natural phenomena and a wide range of technologies, including anti-icing, spray cooling, inkjet printing, and semiconductor manufacturing. On these substrates, droplet dynamics are governed by a subtle interplay between surface chemistry and micro-scale topography. Most existing approaches to control wetting then rely on specific materials or geometrical designs, which often restrict their versatility. By depositing droplets onto a pillar-decorated substrate in a vapor-controlled chamber, we demonstrate that spreading and wicking can be actively tuned, and even temporarily suppressed, using the vapor of a second, low surface-tension liquid. Its localized condensation near the droplet edge generates Marangoni stresses that oppose wetting and regulate the ensuing dynamics. It results in a mixed state in which a macroscopic droplet coexists with a mesoscopic imbibed film; the apparent wetting no longer reflects the underlying pattern and the film acts as a tunable precursor. This quasi-stationary morphology is reproduced by numerical and semi-analytical solutions of the thin-film equation coupled to a composition-evolution equation and diffusion-limited evaporation, with texture wicking represented by an effective height-dependent pressure. Because the effect is reversible and broadly applicable across liquids, vapor conditions, and surface patterns, we further demonstrate that spatiotemporal control of the vapor field can drive droplet transport and even extract liquid from fully imbibed textures, opening routes to controlled coating, cleaning, and drying on textured surfaces.

Effect of the demixing on the equilibrium droplet shape at the three-phase contact line Abstract: 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 sharp-interface models derived through energy minimization. We find that the polystyrene droplets are cloaked by a thin layer of PDMS molecules lowering the effective surface tension. By incorporating this effect into the surface energies in the theoretical model, we show that the global features of the experimental droplet shapes are in excellent quantitative agreement for all droplet sizes on stiffer substrates. However, a systematic local mismatch is observed around the three-phase contact line whose relative importance increases for smaller droplets and softer substrates, which points to a locally larger elastocapillary length. While a negative Shuttleworth effect can theoretically be expected to play a role in this local increase of the elastocapillary length, direct experimental proof could not be attained and there is also no evidence is found in literature for such a negative Shuttleworth effect for PDMS. However, by combining a lift off-technique with atomic force microscopy characterization, a demixing area around each PS droplet could be observed, which results in a locally increased elastocapillary length and that can be explained by the poroelastic nature of PDMS.