Oral Presentations

Co-authors: Barbara P. Klepka (Laboratory of Biological Physics, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland), Agnieszka Michaś (Laboratory of Biological Physics, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland), Tomasz Wojciechowski (International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences, Warsaw, Poland)

Abstract: Biomineralization plays a critical role in many organisms, yet the molecular mechanisms controlling mineral formation remain incompletely understood. Non-classical crystallization pathways are proposed to involve transient liquid phases of calcium carbonate stabilized by polymers, such as acid-rich proteins secreted into the skeletal organic matrix. However, direct evidence for protein-containing liquid phases has been lacking. Here, we demonstrate that highly charged acid-rich proteins regulate calcium carbonate nucleation and growth through liquid–liquid phase separation (LLPS). Using AGARP, the first acid-rich protein cloned from Acropora millepora, as a model, we show that LLPS occurs under physiologically relevant, crowded conditions forming liquid protein-calcium condensates (LPCC) that act as crystallization precursors. AGARP remains intrinsically disordered upon counter-ion binding, highlighting charge mediated interactions as key drivers. These findings introduce LPCCs as biologically relevant intermediates preceding and putative effective calcium export vesicles. This mechanism offers a new molecular-level conceptual framework, bridging the fields of phase separation and biomineralization, and suggests strategies for bioinspired materials design leveraging protein phase behavior.

Acknowledgments: The work was supported by the grant from the National Science Centre of Poland Sonata-Bis 2016/22/E/NZ1/00656 to A.N., and performed in the NanoFun laboratories co-financed by ERDF within the POIG.02.02.00-00-025/09 Project.

Co-authors: Grzegorz Wieczorek

Abstract: Biopolymers, including proteins and RNA, are regarded as condensed matter systems and can be effectively investigated using physical models within molecular simulations. These simulation techniques provide detailed insight into the collective behaviour and organisation of biomolecules (Niedzialek & Wieczorek, ACS Catalysis, 2025). This presentation will illustrate this methodological approach through selected case studies of successful protein-protein and RNA-RNA self-assembly modelling. Additionally, an overview of the principal challenges facing biomolecular modelling will be provided, with emphasis on the limitations inherent to existing theoretical frameworks and computational methodologies. 

Acknowledgments: This work was supported by research grant OPUS 26 no. 2023/51/B/NZ1/01552 funded by the National Science Centre. We gratefully acknowledge Polish high-performance computing infrastructure PLGrid (HPC Center: ACK Cyfronet AGH) for providing computer facilities and support within computational grant no. PLG/2024/017439.

Co-authors: Barbara Klepka (Institute of Physics, Polish Academy of Sciences), Radost Waszkiewicz (Institute of Physics, Polish Academy of Sciences, University of Potsdam), Michał Wojciechowski (Institute of Physics, Polish Academy of Sciences), Agnieszka Michaś (Institute of Physics, Polish Academy of Sciences), and Anna Niedźwiecka (Institute of Physics, Polish Academy of Sciences)

Abstract: Computational tools that rapidly predict protein structure from sequence alone rely on the vast data in the Protein Data Bank. However, deposited structures represent only a small subset of proteins: those with stable, well-defined folds. Predicting the dynamic conformations of intrinsically disordered proteins (IDPs) requires extrapolation beyond AlphaFold’s current capabilities and demands more information than sequence alone can provide.

Since structural experimental data on IDPs remain sparse, and their conformations are highly sensitive to environmental factors, making model development and evaluation challenging. Because of their extended conformations, direct numerical simulation (both all-atom and coarse-grained) can be prohibitively expensive. Our recent work [1] shows that many phenomenological models of average molecular size overfit, particularly when predicting hydrodynamic size.

By selecting an extremely charged, large, intrinsically disordered protein, AGARP, and using precise diffusion measurements, we show that the extent of ionic-strength influence on protein conformation aligns with classical polymeric models in the case of monovalent salts but shows additional chelating-like effects in the presence of divalent salts [2].

Acknowledgments: The work was supported by the grant from the National Science Centre of Poland Sonata-Bis 2016/22/E/NZ1/00656 to A.N., and performed in the NanoFun laboratories co-financed by ERDF within the POIG.02.02.00-00-025/09 Project.

Co-authors: Jolanta Wołowicz (Institute of Fundamental Technological Research, Polish Academy of Sciences)

Abstract: Epithelial cells are tightly packed and, geometrically, are simple prisms. Their bases are most often pentagons, hexagons, and heptagons. Lewis's research (1928) indicates that hexagons constitute about 50 percent, with pentagons and heptagons accounting for the remaining 49 percent. Heptagons are most frequently subdivided, followed by hexagons. After the subdivision of a heptagon, a new hexagon appears. The subdivision of one hexagon results in the change of two, neighbours creating two pentagons and two heptagons. We present a system of two differential equations and show that, under an initial condition consistent with Lewis's law, the proportion of polygons in the tissue does not change. 

Acknowledgments: 

Co-authors: Bartosz Różycki (Institute of Physics, Polish Academy of Sciences)

Abstract: Intrinsically disordered proteins (IDPs) lack a well-defined, stable structure across extended regions of their polypeptide chains under physiological conditions. They participate in numerous cellular processes, including signaling, cell-cycle regulation, and translation initiation. IDPs are also key constituents of biomolecular condensates (BCs), which arise through liquid–liquid phase separation (LLPS) within cells [1]. While BCs in the cytosol and nucleus have been extensively studied, their behavior at cellular membranes remains poorly understood.

Galectin-3 is an intrinsically mixed-folded protein, composed of a disordered N-terminal domain (NTD) and a structured carbohydrate recognition domain (CRD). It plays essential roles in immune responses, cell motility, and signaling. Galectin-3 undergoes LLPS, driven in part by interactions between its aromatic residues, leading to the formation of biomolecular condensates (BCs) [2]. Recent studies have demonstrated that such BCs can assemble at lipid membranes enriched in glycosphingolipids, providing a physical basis for its role in membrane remodeling and clathrin-independent  endocytosis [3].

Numerous computational models have been developed to study the LLPS of IDPs [4]. In this work, we employ dissipative particle dynamics (DPD) simulations, which are well-suited for capturing the mesoscale interactions between BCs of IDPs with membrane. We investigate how polymer models of galectin-3 sense, generate, and respond to membrane curvature. Our results suggest a general mechanism by which BCs detect curvature, with potential implications for curvature-sensitive processes such as endocytosis [5].

Acknowledgments: National Science Centre of Poland via grant number 2020/39/B/NZ1/00377 and LUMI supercomputer, Finland

Co-authors: Agnieszka. M. Słowicka (Institute of Fundamental Technological Research, Polish Academy of Sciences), Nan Xue (Princeton University, Cornell University), Lujia Liu (Institute of Fundamental Technological Research, Polish Academy of Sciences, and Beihang University), Janine K. Nunes (Princeton University), Paweł Sznajder (Institute of Fundamental Technological Research, Polish Academy of Sciences), Howard A. Stone (Princeton University)

Abstract: It will be demonstrated numerically and experimentally that highly elastic fibers in shear flow tend to form elongated double helices, performing Jeffery orbits close to the vorticity direction, New J. Phys. 26, 073011 (2024). 

Acknowledgments:

Co-authors: Marta Wacławczyk (University of Warsaw), Szymon Malinowski (University of Warsaw)

Abstract: Kolmogorov’s theory, one of the most popular and verified theories of turbulence, assumes that under equilibrium, there is a certain, self-similar form of energy spectrum. However, in order to extend this theory to decaying or developing states, some nonequilibrium has to be assumed.

The notion of equilibrium in turbulence is connected to the balance between the incoming energy flux at large scales and the outgoing energy flux at small scales. The former is associated with large structures, formed most commonly by shear or convection. The latter is connected to the dissipation of energy into heat. In the atmosphere, and especially in and around clouds, an imbalance is expected. Studying this imbalance is important for the development of more accurate turbulence models for weather and climate models.

The nonequilibrium correction to the energy spectrum has been studied using simulations, as well as laboratory or atmospheric flows. However, in terms of passive scalar spectrum, no such verifications have been performed. In this presentation, we propose a new relation regarding the nonequilibrium of passive scalar turbulence, in this case temperature, as well as its verification using airborne measurements. The measurements were performed during the Methane-to-Go campaign in 2022 using the Ultra Fast Thermometer. 

Acknowledgments:

Co-authors: Piotr Szymczak (University of Warsaw)

Abstract: Dissolution in porous media produces a range of patterns due to the interplay among fluid flow, reactant transport, chemical reactions, and evolving medium properties. Reactant transport proceeds via advection and diffusion, with morphology largely controlled by their relative strength: advection promotes instabilities whereas diffusion exerts a stabilizing effect [1]. Pore network models provide an efficient framework to simulate dissolution [2], but they often treat axial transport along pores as advection-dominated, an assumption frequently violated in natural and industrial systems such as groundwater flows or catalytic reactors. Here we introduce a method to incorporate axial diffusion into pore network models, enabling continuous control of the relative importance of advection, diffusion, and reaction. We explore a broad spectrum of emergent structures and classify them by their channeling properties and morphology [3]. We further examine how porosity influences pattern selection and benchmark our simulations against laboratory experiments.

Acknowledgments:

Co-authors: Radost Waszkiewicz (Institute of Physics, Polish Academy of Sciences, and University of Potsdam), Jonasz Słomka (ETH Zürich), Maciej Lisicki (University of Warsaw)

Abstract: Sinking marine snow particles, composed primarily of organic matter, control the global export of photosynthetically fixed carbon from the ocean surface to depth. The fate of sedimenting marine snow particles is in part regulated by their encounters with suspended, micron-sized objects, which leads to mass accretion by the particles and potentially alters their buoyancy, and with bacteria that can colonize the particles and degrade them. Their collision rates are typically calculated using two types of models focusing either on direct (ballistic) interception with a finite interaction range, or advective-diffusive capture with a zero interaction range. Since the range of applicability of the two models is unknown, and many relevant marine encounter scenarios span across both regimes, quantifying such encounters remains challenging, because the two models yield asymptotically different predictions at high Péclet numbers. Here, we reconcile the two limiting approaches by quantifying the encounters in the general case using a combination of theoretical analysis and numerical simulations. Solving the advection-diffusion equation in Stokes flow around a sphere to model mass transfer to a large sinking particle by small yet finite-sized objects, we determine a new formula for the Sherwood number as a function of two dimensionless parameters: the Péclet number and the ratio of small to large particle sizes. We find that diffusion can play a significant role in generating encounters even at high Pe. At Pe as high as 10^6, the direct interception model underestimates the encounter rate by up to two orders of magnitude. This overlooked contribution of diffusion to encounters suggests that important processes affecting the fate of marine snow, such as colonization by bacteria and plankton or accretion of neutrally buoyant gels, may proceed at a rate much faster than previously thought.  

Acknowledgments:

Co-authors: Paweł Dłotko (Dioscuri Centre for Topological Data Analysis, Institute of Mathematics,  Polish Academy of Sciences)

Abstract: Open-cell foams and closed-cell foams are two idealized ends of a spectrum of metal porous materials (with the former category sometimes referred to as sponges). In the former, the air forms an interconnected continuous network intertwined with the structural one. In the latter, the gas intermixed with the structure forms distinct, unconnected cells, completely separated from each other by the structure’s elements. However, many protocols used to prepare porous metal materials result in structures intermediate in terms of properties between the two categories, while industry practice regarding technical specifications of porous metal materials is largely limited to a statement of the proportion of the number of open and closed cells based on the gas displacement method. We are proposing the usage of concepts defined in topology (Betti numbers) to quantify the properties of the intermediate porous metallic structures and to define a precise "cell-openness score" τ measuring how close a given structure is to an ideal open cell. We explore the usefulness of the proposed score on the toy example of structures based on Voronoi tesselations of space around point clouds. 

Acknowledgments: National Science Centre, Max Planck Society, Polish Ministry of Science and Higher Education, German Federal Ministry of Education and Research

Co-authors: - 

Abstract: The interior of living cells is a crowded, dynamic environment where molecular mobility is governed majorly by diffusion. Using fluorescence correlation spectroscopy (FCS) and custom physical models of intracellular viscosity, we investigate how cytoplasmic properties regulate fundamental biological processes such as protein translation, stress response, and therapeutic delivery. We show that intracellular diffusion is strongly length-scale dependent and can be modulated by environmental conditions, such as starvation or exposure to transfection agents. These changes influence key processes—from halting ribosomal activity to altering the fate of mRNA vaccines—revealing how cells exploit physical constraints to adapt, survive, or initiate programmed death. Our work builds a quantitative framework for understanding the intracellular landscape and paves the way for optimizing drug delivery and bioanalytical strategies in complex cellular environments. 

Acknowledgments: NCN, OPUS27, 2024/53/B/ST4/01213

Co-authors: Aneta Magiera (Institute of Physical Chemistry, Polish Academy of Sciences), Marta Pilz (Institute of Physical Chemistry, Polish Academy of Sciences), Karina Kwapiszewska (Institute of Physical Chemistry, Polish Academy of Sciences), Robert Hołyst (Institute of Physical Chemistry, Polish Academy of Sciences)

Abstract: Apoptosis plays a crucial role in maintaining tissue homeostasis. In this study, we investigated how intracellular transport is altered during apoptosis using Fluorescence Correlation Spectroscopy (FCS). To track changes in intracellular mobility, we employed fluorescent tracers of varying sizes (radii ranging from 1.3 to 20 nm) and monitored them until individual cells reached the point of death, as determined by morphological evaluation. By analyzing diffusion coefficients, we assessed the nanoviscosity of apoptotic cells across multiple length scales. Our findings revealed that apoptosis is accompanied by a sudden 2- to 3-fold increase in nanoviscosity. This increase primarily stems from water efflux and cell volume reduction, with caspase activity playing a lesser role. Importantly, elevated nanoviscosity alone does not trigger apoptosis; however, a sharp rise in nanoviscosity consistently coincides with it. These results demonstrate that a sudden deceleration of intracellular transport at multiple scales is a hallmark of cell death. 

Acknowledgments: National Science Centre, Poland, grant number [2019/33/B/ST4/00557], Warsaw Doctoral School in Natural and BioMedical Sciences (Warsaw-4-PhD Doctoral School)

Co-authors: Pragyesh Dixit (Institute of Physical Chemistry, Polish Academy of Sciences), Bartłomiej Wacław (Institute of Physical Chemistry, Polish Academy of Sciences), Karolina Drabik (Institute of Physical Chemistry, Polish Academy of Sciences), Arne Traulsen (Max Planck Institute for Evolutionary Biology), Saumil Shah (Max Planck Institute for Evolutionary Biology)

Abstract: Most cancers are genetically and phenotypically heterogeneous. This includes subpopulations of cells with different levels of sensitivity to chemotherapy, which may lead to treatment failure as the more resistant cells can survive drug treatment and continue to proliferate. While the genetic basis of resistance to many drugs is relatively well characterised, non-genetic factors are much less understood. Here we investigate the role of non-genetic, phenotypic heterogeneity in the response of glioblastoma cancer cells to the drug temozolomide (TMZ) often used to treat this type of cancer. Using a combination of live imaging, machine-learning image analysis and agent-based modelling, we show that even if all cells share the same genetic background, individual cells respond differently to TMZ. We quantitatively characterise this response by measuring the doubling time, lifespan, cell cycle phase, area and motility of cells, and determine how these quantities correlate with each other as well as between the mother and daughter cell. We also show that these responses do not correlate with the cellular level of the enzyme MGMT which has been implicated in the response to TMZ. 

Acknowledgments: The research leading to these results has received funding from the Norwegian Financial Mechanism 2014-2021 under the NCN POLS grant no. 2020/37/K/NZ2/03737

Co-authors: Maja K. Cieplak-Rotowska (University of Warsaw), Zuzanna Staszałek (Institute of Physics, Polish Academy of Sciences), Marc R. Fabian (McGill University), Nahum Sonenberg (McGill University), Michał Dadlez (Institute of Biochemistry and Biophysics, Polish Academy of Sciences), Anna Niedźwiecka (Institute of Physics, Polish Academy of Sciences)

Abstract: GW182 is a fuzzy, intrinsically disordered protein that plays a key role in degrading mRNA during post-transcriptional microRNA-mediated gene silencing [1]. Its N-terminal Ago-binding domain (ABD) interacts with the Argonaute (Ago) protein, which is a core component of the miRNA-induced silencing complex (miRISC), while the C-terminal silencing domain (SD) recruits the CCR4-NOT deadenylase complex to targeted transcripts. CCR4-NOT also functions in a different silencing mechanism controlled by tristetraprolin (TTP) [2], another intrinsically disordered RNA-binding protein that targets AU-rich elements in the 3′ untranslated regions of cytokine mRNAs. Although GW182 and TTP engage CCR4-NOT to degrade mRNA through distinct mechanisms, their overlap raises the question of whether the pathways converge [3] or compete. 

To explore this, we conducted biophysical studies showing that the GW182 SD is capable of driving liquid-liquid phase separation (LLPS). Phase diagrams reveal temperature-sensitive LLPS behaviour dependent on π–π interactions between tryptophan residues. Moreover, our results show that the GW182 SD forms multiprotein condensates with the CNOT1 subunit of CCR4-NOT, suggesting a host–client interaction [4]. Notably, the presence of TTP as a third component disrupts condensate formation. This indicates direct competition between GW182 SD and TTP for binding to the same CNOT1 region. These findings reveal a potential molecular cross-talk between the two post-transcriptional silencing pathways.

Acknowledgments: This work was supported by the NCN grants no. UMO-2016/22/E/NZ1/00656 to A. N. and no. UMO-2023/49/B/NZ1/04320 to M. K. B. The studies were performed in the NanoFun laboratories co-financed by ERDF within the POIG.02.02.00-00-025/09 project.

Co-authors: Margot Young (University of Pennsylvania), Albane Théry (University of Pennsylvania), Talia Becker Calazans (University of Pennsylvania),  Arnold J.T.M. Mathijssen (University of Pennsylvania), Maciej Lisicki (University of Warsaw)

Abstract: Many microorganisms inhabit a world of low Reynolds number, where movement through fluid is dominated by viscosity. To swim or feed, they rely on mechanisms like flagellar beating or metachronal waves of cilia. These actions are often modeled using steady Stokes flow. Yet even without inertia, microorganisms can break the constraints of the “scallop theorem” by carefully tuning the timing and location of the forces they exert. A conceptual model that captures this is the blinking Stokeslet, which mimics alternating power and recovery strokes by periodically switching the position of a point force. This setup can generate chaotic flows, enhancing transport and mixing—a strategy likely used by filter-feeding ciliates. But what happens when these oscillations become fast enough for unsteady effects to matter? Understanding this transition may shed light on how microorganisms exploit time-dependent flows to optimize feeding and transport. Comparing steady and unsteady Stokes regimes in such models helps reveal the limits of quasi-steady assumptions and the potential biological advantages of exploiting unsteady flow dynamics. 

Acknowledgments: National Science Centre of Poland grant no. 2023/50/ST3/00465 to ML

Co-authors: Mikołaj Miszczak (Warsaw University of Technology)

Abstract: Jet-in-a-crossflow is commonly encountered in natural and engineering systems, such as combustion chambers, volcanic eruptions, or film cooling. We characterise the first oscillatory bifurcation in JICF from a stationary regime dominated by a pair of counter-rotating vortices to periodic hairpin shedding, using both flow visualisation and PIV measurements. Dominant frequency associated with hairpin shedding remains approximately constant for the investigated velocity ratio range. Based on the observed Strouhal number, we propose a threshold that differentiates the low-velocity-regime (with dynamics dominated by downstream recirculation zone) and high-velocity-ratio regime (in which the upstream recirculation zone dominates, see e.g. Mahesh, ARFM, 2013; Karagozian, PoF, 2014;  Shoji et al., PRF, 2020). We determine the spatial support of the global mode and track its global spatial peak to characterise the amplitudę of hairpin shedding oscillations. We show that the hairpins are generated by a Hopf bifurcation. Using the Landau model, we determine the critical value of the velocity ratio for hairpin shedding. We also characterise how its value depends on the crossflow Reynolds number. Finally, we investigate the effect of the jet diameter. Our findings apply to film cooling to detect the optimal range of velocity ratio to minimise the mixing rate.

Acknowledgments: This research was funded by National Science Center (Poland) within the OPUS-21 project (2021/41/B/ST8/03142). L.K. was partially supported by the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska- Curie grant agreement number 754411.

Co-authors: Jeffrey C. Everts (University of Warsaw), Bogdan Cichocki (University of Warsaw)

Abstract: The creeping flow regime is a branch of hydrodynamics applicable to a multitude of physical systems, ranging from microfluidics in lab-on-chip technologies to the movement of E. coli bacteria. The foundation of creeping flow models are the Stokes equations, which due to their linear nature present an additional advantage; all boundary value problems follow the superposition principle of fundamental solutions. In this work we consider the Stokes flow in an anisotropic fluid. Symmetry properties of the fluid, such as different types of anisotropy, enter the model through the viscosity tensor. The mathematical structure of this quantity and its relation to the characteristics of the physical systems it describes, play, therefore, an important role. We focus on two examples of anisotropic systems with different symmetry properties. The first case is a nematic phase consisting of aligned uniaxial particles, that we model using three shear viscosities. We find a closed form of the fundamental solution for such a fluid, which has not been explicitly found before. The second example is a system of self-spinning particles, also referred to as a chiral active fluid, with a spatially homogeneous spin angular momentum density, which we describe using two types of viscosity: an isotropic shear viscosity and an odd viscosity that couples only to the strain rate. We present the fundamental solution for such a system and from it we construct the exact velocity field and pressure profile around a translating sphere, assuming no-slip boundary conditions. The velocity profile has been previously obtained only for a particular direction of the translational velocity of the sphere; we extend this result by deriving the general form. Analysis of the exact velocity field reveals chiral axial flows, that are not found in isotropic systems or the aforementioned nematic systems.

Acknowledgments: L. M. and J. C. E. acknowledge funding from the National Science Centre, Poland, within the SONATA BIS grant no. 2023/50/E/ST3/00452.

Co-authors: Michał Klamka (Warsaw University of Technology)

Abstract: The interaction of liquid droplets with solid surfaces is a phenomenon of fundamental importance across numerous industrial processes, including spray coating, spray cooling, and cleaning applications. While thermal effects, such as the well-documented Leidenfrost effect, can induce droplet levitation via a vapor cushion, analogous non-wettable behavior can be achieved at ambient temperatures through hydrodynamic means. A moving surface submerged in a fluid generates a boundary layer capable of preventing direct contact between an impacting droplet and the surface itself. This hydrodynamic levitation has been observed in both low and high-velocity flow regimes.

This investigation presents a comprehensive experimental and computational fluid dynamics (CFD) analysis of the interaction between a liquid droplet and the boundary layer generated by a vertically rotating flat disk. The primary experimental objective was to determine the feasibility of achieving stable droplet levitation within both laminar and turbulent boundary layers. Furthermore, the study aimed to define the operational limits of this levitation, specifically by identifying the critical impact velocity beyond which a free-falling droplet penetrates the boundary layer and makes contact with the disk surface.

The computational portion of this work focuses on elucidating the complex flow field surrounding a levitating droplet. We analyze the mutual interaction between the primary rotating disk flow and the stationary droplet, quantifying the modifications to the base flow caused by the droplet's presence and the resultant aerodynamic forces. A key aspect of this analysis is explaining the origin of observed droplet shape oscillations during levitation by examining flow instabilities and their subsequent effect on the pressure distribution across the droplet's surface. This dual approach provides a detailed understanding of the underlying physics governing hydrodynamic droplet levitation. 

Acknowledgments: This research was carried out with the support of the Interdisciplinary Centre for Mathematical and Computational Modelling University of Warsaw (ICM UW) under computational allocation no G100-2222. Research was funded by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme.

Co-authors: Tomasz Bobiński (Warsaw University of Technology)

Abstract: This work presents a dynamic method for determining contact angle hysteresis (CAH) by placing droplets on a harmonically oscillating substrate, providing new kinetic insights into surface wettability. We designed a custom experimental setup featuring a lightweight, 3D-printed motion carriage actuated by a high-performance linear motor capable of sinusoidal oscillations with accelerations up to 9g, and equipped with a high-speed optical system for millisecond-scale imaging and analysis. Silicon wafers were used as substrates with Glaco superhydrophobic surface treatment, and deionised water was chosen as the working fluid due to its well-characterized and reproducible physicochemical properties, ensuring comparability and minimizing variability. The integrated imaging and analysis approach, including precise droplet deposition and a robust MATLAB processing pipeline, enabled accurate measurement of contact angle dynamics and improved uncertainty quantification. Results show this oscillation-based method effectively probes the thresholds required for depinning, advances the study of dynamic droplet mobility, and facilitates detection of local surface heterogeneities, outperforming conventional static and quasi-static CAH measurement techniques. 

Acknowledgments:

Co-authors: Solomon Adera (University of Michigan)

Abstract: Comprehending the behavior of water droplets as they encounter microengineered surfaces is essential for numerous technological applications, yet direct observation of wetting dynamics and air capture processes during droplet collision has posed significant difficulties. In this work, we employ Brewster angle methodology to examine the development of air bubbles and wetting phenomena of a colliding droplet on a silicon micropillar substrate. Through adjusting the incidence angle of p-polarized 532 nm laser light to match the Brewster angle of specific interfaces, we eliminate undesirable reflections and obtain high-contrast interface-selective visualization. This methodology enables direct observation of the genesis and progression of both central and edge-trapped air bubbles, the structure of the advancing wetting boundary, and the thickness distribution of the spreading liquid film. Our findings reveal that low pillars (2 μm) reliably capture a central air bubble, whereas tall pillars (20 μm) result in suspended bubbles atop pillar structures and the development of peripheral air rings. The configuration and geometry of pillars influence the morphology of captured air and the directional properties of the wetted region. Through interferometric fringe examination, we quantitatively determined the local thickness of the expanding liquid film, with findings aligning with theoretical models based on Fresnel reflectance calculations. This investigation establishes that Brewster angle visualization serves as an effective and non-intrusive method for examining concealed interfacial phenomena during rapid wetting transformations. Our findings contribute fresh insights into the fluid-solid interactions occurring when a droplet strikes a microengineered substrate. 

Acknowledgments: M.R. was supported by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) program. This work was supported by the National Science Foundation CAREER Award Number NSF/CBET-2338362