Plenary & Keynote speakers
Spotlight on Visionary Speakers at Droplets 2025
We are thrilled to present an exceptional lineup of plenary and keynote speakers for the Droplets 2025 conference.
These distinguished experts will set the stage for groundbreaking discussions, bringing together diverse perspectives on the challenges and opportunities in the field. Join us as they explore cutting-edge solutions, share visionary insights, and drive meaningful progress in addressing one of the most critical issues of our time.
Plenary speakers
Hans-Jürgen Butt
Max-Planck Institute Mainz, Germany
Hans-Jürgen Butt studied physics in Hamburg and Göttingen. He did his PhD at the Max Planck Institute for Biophysics with Ernst Bamberg, Frankfurt, in 1989.
After a postdoc in Santa Barbara and a researcher position back in Frankfurt, he became associate professor at the University Mainz and three years later full professor in Siegen. In 2002 he joined the Max Planck Institute for Polymer Research in Mainz as a director. His research is on the structure and dynamics of soft matter interfaces in particular wetting and surfaces forces.
Spontaneous charging of sliding water drops
Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Like in triboelectricity, moving drops can separate electric charges. This phenomenon is called slide electrification. Typically, water drops sliding down hydrophobic surfaces spontaneously acquire a positive charge while they deposit negative charges on the solid surface. A mechanism leading to charge separation will be proposed. Consequences of charging for drop motion, a possible degradation of the substrate and the deposition of charge molecules.
Petra S. Dittrich
ETH Zürich, Switzerland
Prof. Petra S. Dittrich is Associate Professor for Bioanalytics at the Department of Biosystems Science and Engineering, ETH Zürich, since 2014. She develops microfluidic devices for bioanalytical and diagnostic applications. Her research interests focus currently on single cell analysis, lipid membrane studies and biomedical applications such as blood-vessels-on-chip. She studied Chemistry at Bielefeld University (Germany) with a semester at the Universidad de Salamanca (Spain). She earned her PhD degree at the Max Planck-Institute for Biophysical Chemistry (MPI Göttingen, Germany) in 2003. After a year as postdoctoral fellow at the MPI Göttingen, she was postdoc at the Institute for Analytical Sciences (ISAS Dortmund, Germany). From 2008, she was Assistant Professor at the Department of Chemistry and Applied Biosciences (ETH Zurich).
For research stays, she visited the Cornell University (Ithaca, USA, in 2002) and the University of Tokyo (Japan, in 2005). She received among others awards the prestigious Starting Grant from the European Research Council (ERC, from 2008-2014) as well as the ERC Consolidator Grant (2016-2021).
Droplets-on-a-plate: High-throughput creation, analyses and applications
Droplet microfluidics is a powerful method for high throughput analysis, e.g. for screening reaction conditions, synthesizing particles, or single-cell analysis and other bioanalytical tasks. We employ droplet-based methods on an open platform, which is conceptionally similar to a multi-well plate, however, has a massively increased density of wells and hence, throughput [1]. Thousands of aqueous nL-droplets are deposited on a custom-made glass slide that has a defined hydrophilic-hydrophobic surface pattern. The droplets are covered by fluorinated oil and remain stable for several days. We can employ optical microscopy for droplet assessment as well as MALDI-MS imaging for analysis of the droplet content. In this presentation, I will show recent technological advancements of this platform, where we controlled the size of the droplets, formed chemical gradients and added and removed compounds at a defined time. We utilized the platform for the label-free kinetic analysis of an enzymatic reaction [2]. Further applications include highly-parallel cultivation of microbial growth, under optimum conditions as well as under antibiotic drug treatment. Here, we identify synergistic/antagonistic effects of drugs as well as unnormal cell behaviour including emergence of drug resistance.
References
[1] R. Strutt, B. Xiong, V. F. Abegg, P. S. Dittrich, Lab Chip 24 (2024), 1064-1075.
[2] M. Breitfeld, C. L. Dietsche, M. A. Saucedo-Espinosa, S. F. Berlanda and P. S. Dittrich: Ultrafast formation of microdroplet arrays with chemical gradients for label-free determination of enzymatic reaction kinetics, Small (2025), in print.
Suzie Protière
Institut Jean Le Rond d'Alembert, France
Suzie Protière is a CNRS researcher in Soft Matter Physics at Sorbonne Université (France). She works at the Institut Jean le Rond d’Alembert in Paris. She obtained her PhD in 2007 at Ecole Normale Supérieure de Paris and Université Paris Diderot working on a quantum analog of droplets bouncing at an interface.
Her research generally concerns fluid-structure interactions at the capillary scale. In particular, she is interested in complex materials and fluids, having worked on swellable beams as well as fluid-grains 2D assemblies of complex interfacial rheology and elastocapillary imbibition.
Freezing water and hydrogel drops
It is well known that water expands when it freezes. At the capillary scale, this effect leads to an unusual shape change for a water droplet with the formation of a pointy tip and to a specific dynamics of the freezing front. In the first part of this talk we describe this phenomenon and model both the dynamics and the drop’s final shape. We also discuss the limitations of these models. In the second part of the presentation we compare these findings to the case of a hydrogel drop. Indeed a hydrogel is a material consisting of a polymer matrix which can absorb up to 99% water. This polymer matrix affects the overall shape of such objects during its freezing and thus how it interacts with the ice growth. We find that our hydrogel drops expand solely vertically in the direction of the temperature gradient. We propose a model to predict this shape change by assuming several conditions that we observe experimentally which differ from the freezing of a water drop. Moreover, since we can mold hydrogel freely without any container, into any kind of shape, we can then use our model to predict the final shape of any random hydrogel shape once frozen. Finally, we find that for hydrogels with a low concentration of polymer, water is expelled from the gel onto its surface during the freezing process (see picture below). We will discuss how this phenomenon may be related to the poroelastic nature of the material.
Contributors
Lila Séguy(1), Axel Huerre(2) and Suzie Protière(1)
(1)Institut Jean le Rond ∂’Alembert, CNRS UMR7190 Sorbonne Université, 75005 Paris France
(2)Laboratoire MSC, CNRS Université Paris Cité, 75013 Paris France
Kripa K. Varanasi
MIT, USA
Kripa K. Varanasi is a Professor of Mechanical Engineering at MIT, Cambridge. He leads an interdisciplinary lab focused on understanding many aspects of physico-chemical transport, and biological phenomena. His work has contributed to the development of innovative materials, devices, products, and processes designed to address challenges in areas such as energy, water, agriculture, transportation, medical devices, and consumer products. He is passionate about entrepreneurship, scale-up, and manufacturing and has translated various technologies from lab to market. He has co-founded several companies, including LiquiGlide, Dropwise, Infinite Cooling, Alsym Energy, AgZen, and Coflo Medical. LiquiGlide has been recognized by Time and Forbes magazines as one of the “Best Inventions of the Year.”
Prof. Varanasi has been honored with several awards, including the NSF Career Award, DARPA Young Faculty Award, SME Outstanding Young Manufacturing Engineer Award, ASME Bergles-Rohsenow Heat Transfer Award, ASME Gustus L. Larson Memorial Award, APS Milton van Dyke Award, and MIT Graduate Student Council’s Frank E. Perkins Award for Excellence in Graduate Advising.
Innovations at Interfaces: Energy & Sustainability to Biomedical Technologies
Physico-chemical interactions at interfaces are ubiquitous across multiple industries, including energy, medical, water, agriculture, transportation, and consumer products. In this talk, I will summarize how surface/interface chemistry, morphology, and thermal and electrical properties can be engineered across multiple length scales to achieve significant efficiency enhancements in a wide range of processes. These approaches involve both passive and active manipulation of interfaces. First, I will describe a variety of slippery interfaces that can significantly reduce interfacial friction for efficient dispensing of viscous products, enhance thermal transport in heating and cooling systems, provide anti-icing solutions, and create self-healing barriers for protection against scaling. I will then discuss active strategies, such as engineering charge transfer to alter multiphase flows for applications like water harvesting, anti-dust systems for solar panels, and reducing agricultural runoff to address critical challenges at the energy-water and water-agriculture nexus. Next, I will highlight our efforts in decarbonization, focusing on CO₂ capture and conversion and hydrogen electrolyzers. These efforts include enhancing electrochemical and biological methods for CO₂ capture and conversion. Specifically, I will discuss recent advancements we have made in CO₂ capture from point sources and direct air capture (DAC), marine CO₂ removal and conversion of CO₂ conversion to valuable products. On hydrogen electrolyzers, we will discuss the role of electrochemical bubbles and approaches to optimize catalyst distribution for enhanced performance.
In parallel, I will discuss ongoing research in biomedical technologies, spanning biomanufacturing to ovarian cancer treatment. I will present surface engineering strategies designed to prevent thrombosis and biofilm formation, tailor cell adhesion and protein adsorption, and enhance the biomanufacturing value chain. Inspired by slippery surface technologies, I will introduce a improved methodology for subcutaneous injection of highly viscous biologics, expanding the range of injectable formulations and improving healthcare accessibility. Furthermore, I will describe scalable approaches to protein separation via undersaturated crystallization, promoted through in-situ templating, which can enable continuous biomanufacturing. Finally, I will detail passive and active techniques for enhancing bioreactors by preventing foam buildup. Our non-invasive approach eliminates the need for defoamers, preventing cell death caused by bubble rupture and optimizing reactor space utilization.
Throughout the talk, I will touch on manufacturing and scale-up strategies, robust materials and processes, and entrepreneurial efforts to translate these technologies into impactful products and markets. Interwoven with these discussions, I will share insights from the start-up companies I have co-founded.
Emmanuel Villermaux
Aix-Marseille Université, France
Manuel Villermaux received his PhD from the University of Paris VI, Pierre & Marie Curie, in Grenoble where he was appointed at CNRS, and where he obtained the habilitation form the Joseph Fourier University. He now holds a position of distinguished Professor at Aix Marseille University, and at the Institut Universitaire de France.
His interests are in the mechanics of deformable bodies in the broad sense, from fluids to solids, with a particular taste for mixing and fragmentation.
He is an associate editor of the Comptes Rendus Mecanique and a founder associate editor of Physical Review Fluids. He is an elected Fellow of APS, of Euromech and has been the recipient of the Bronze Medal from CNRS, of the Edmond Brun Prize from the French Academy of Sciences, and of the Stanley Corrsin Award from APS.
Fragmentation with, and without mechanism: Are principles enough?
When it comes to understand how a cohesive object breaks-up, there are two types of temptations: either seek for detailed mechanisms (capillary instabilities for drops, cracks propagation in brittle solids, for example) and their articulation with external constraints, or rely on a general principle, like a conservation principle for instance, to infer the multiplicity of the fragments sizes. Microscopic descriptions often overlook the question of the sizes distribution, and the uncontrolled use of conservation principles leads to notorious mistakes (see §11 in JFM 898, P1, 2020).
I will argue that a well chosen principle, coupled with a geometrical constraint, helps at understanding why a breaking object fragments into many more small pieces than bigger ones (think of a glass dropped on the floor), in a fashion which is sensitive to its initial shape. Examples including exploding liquid shells, crushed brittle materials, plastic debris in the ocean, and remnants from the cavemen industry will be discussed.
Petia M. Vlahovska
Northwestern University, USA
Petia M. Vlahovska is a Professor of Engineering Sciences and Applied Mathematics at Northwestern University. She received a PhD in Chemical Engineering from Yale University (2003) and MS in Chemistry from Sofia University, Bulgaria (1994). Prior to joining Northwestern University, she was faculty at Dartmouth College and Brown University.
Her research encompasses fluid dynamics, membrane biophysics, and soft matter. Prof. Vlahovska is a Fellow of the American Physical Society (2019). She has been awarded is a Guggenheim Fellowship (2024), National Science Foundation CAREER Award (2009), David Crighton Fellowship from the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK (2004), and a Research Fellowship from the Alexander von Humboldt Foundation to visit the Membrane Biophysics Lab at the Max Planck Institute of Colloids and Interfaces (2016).
Dynamics of complex interfaces: biomimetic membranes, charged droplets, and active fluids
Interfaces abound in multiphase materials and living organisms. In this talk, I will overview the research in my group to understand the interrelation between interfacial dynamics and macroscale behavior of soft matter systems. Examples include electrohydrodynamic streaming producing a “Saturnian” drop, thermally-driven curvature fluctuation of lipid bilayers, and amoeba-like crawling of a droplet enclosing active particles.
Keynote Speakers
We are excited to announce that the Droplets 2025 conference will feature an inspiring lineup of keynote speakers who will share their expertise and vision for the future:
Catherine Barentin (Université de Lyon, France)
Wetting of complex fluids
Complex fluids such as emulsions, suspensions, gels, or pastes exhibit interesting mechanical properties intermediate between solid and liquid [1]. Indeed, at short time for visco-elastic fluids or low stress for yield-stress fluids, they respond elastically, and at long time or large stress, they flow like a liquid. This makes them particularly interesting for industry (food, cosmetics, building). Moreover, in many applications such as coating, printing or imbibition, wetting properties of complex fluids play a crucial role.
In this talk, I will present the wetting properties of complex fluids in two situations: the spontaneous spreading of a drop made of a yield-stress fluid onto a solid surface and the forced imbibition of a hydrophilic yield-stress fluid into a hydrophobic porous medium. For simple fluids, these situations are classical and the wetting laws (Young-Dupré, Washburn, Laplace) are well established. Here I will study how the yield stress modifies these laws and highlight the strong impact of the dynamic history and the boundary conditions on the observed phenomena [2,3].
Contributors
Nicolas Bain, Anne-Laure Biance, Manon Bourgade, Loren Jorgensen, Mathieu Leocmach, Grégoire Martouzet, Loïc Vanel
References
[1] Bonn, D., Denn, M. M., Berthier, L., Divoux, T., & Manneville, S. (2017). Yield stress materials in soft condensed matter. Reviews of Modern Physics, 89(3), 035005.
[2] Martouzet, G., Jørgensen, L., Pelet, Y., Biance, A. L., & Barentin, C. (2021). Dynamic arrest during the spreading of a yield stress fluid drop. Physical Review Fluids, 6(4), 044006.
[3] Péméja, J., Géraud, B., Barentin, C., & Le Merrer, M. (2019). Wall slip regimes in jammed suspensions of soft microgels. Physical Review Fluids, 4(3), 033301.
Xu Deng (University of Electronic Science and Technology of China, China)
Multi-Dimensional Manipulation of Solid-Liquid Interaction
Solid/Liquid interaction play important role in many research and application fields. In this presentation, we will introduce a radically new strategy that resolves the bottleneck through the creation of an unexplored gradient in surface charge density (SCD). By leveraging on a facile droplet printing on superamphiphobic surfaces as well as the fundamental understanding of the mechanisms underpinning the creation of the preferential SCD. We will also show that robust superhydrophobicity can be realized by structuring surfaces at two different length scales, with a nanostructure design to provide water repellency and a microstructure design to provide durability. We apply this strategy to various substrates and show that the water repellency of the resulting superhydrophobic surfaces is preserved even after abrasion by sandpaper. This design strategy could also guide the development of other materials that need to retain effective self-cleaning, anti-fouling or heat-transfer abilities in harsh operating environments.
References
[1] W Zhang, D Wang, Z Sun, J Song and X Deng, Chem. Soc. Rev. 50 (2021), 4031–4061.
[2] D Wang, Q Sun, M J Hokkanen, C Zhang, F-Y Lin, Q Liu, S-P Zhu, T Zhou, Q Chang, B He, Q Zhou, L Chen, Z Wang, R H A Ras and X Deng, Nature 582 (2020), 55–59.
[3] J Song, W Zhang, D Wang, Y Fan, C Zhang, D Wang, L Chen, B Miao, J Cui and X Deng, Adv. Mater. 33 (2021), 2007154.
[4] Q Sun, D Wang, Y Li, J Zhang, S Ye, J Cui, L Chen, Z Wang, H-J Butt, D Vollmer and X Deng, Nat. Mater. 18 (2019), 936–941.
[5] J Yang, Y Li, D Wang, Y Fan, Y Ma, F Yu, J Guo, L Chen, Z Wang and X Deng, PNAS 120 (2023), e2305567120.
Burak Eral (TU Delft, Netherlands)
Non-photochemical laser induced nucleation: Transient thermocavition bubbles triggered by light for controlling crystallization from solution
Crystallization abounds in nature and industrial practice. A plethora of indispensable products ranging from agrochemicals and pharmaceuticals to battery materials are produced in crystalline form in industrial practice. Yet, our control over the crystallization process across scales, from molecular to macroscopic, is far from complete. This bottleneck not only hinders our ability to engineer the properties of crystalline products essential for maintaining our quality of life but also hampers progress toward a sustainable circular economy in resource recovery. In recent years, approaches leveraging light fields have emerged as promising alternatives to manipulate crystallization. In this presentation, recent advances in nonphotochemical laser-induced nucleation will be discussed. Particularly, we recently demonstrated that a cavitation bubble initiated by a Nd:YAG laser pulse below breakdown threshold induces crystallization from supersaturated aqueous solutions with supersaturation and laser-energy-dependent nucleation kinetics[1]. Combining high-speed video microscopy and simulations, we argue that a competition between the dissipation of absorbed laser energy as latent and sensible heat dictates the solvent evaporation rate and creates a momentary supersaturation peak at the vapor-liquid interface. The number and morphology of crystals correlate to the characteristics of the simulated supersaturation peak. Inspired by these results, we further explore effect of thermocaviation bubble-bubble interactions to further our control over nucleation from solution.
Contributors
N. Nagalingam(1) , V. Korede(1) ,R. Hartkamp(1) , J. T. Padding(1) , H.B.Eral(1)
(1)TU Delft, Delft, The Netherlands
References
[1] N. Nagalingam et. al. Physical Review Letters 2023 131 (12), 124001, N. Nagalingam et. al. 2024 Scientific Reports 14 (1), 21226
Hanneke Gelderblom (TU Eindhoven, Netherlands)
Active bacterial pattern formation inside evaporating droplets
Bacteria living on surfaces are often confined to droplets. When these droplets evaporate, the motion of the liquid-air interface and the associated internal capillary flow confine the bacteria. Here we study how E. coli bacteria interact with this capillary confinement and agglomerate at the droplet’s contact line. Counter-intuitively, the bacterial activity does not cause the bacterial patterns to homogenize. Instead, a non-trivial self-organization of bacteria arises that causes a persistent local stirring of the liquid. We demonstrate that, depending on the droplet’s ambient conditions, this bacterial collective motion gives rise to an instability in bacterial number density that leads to the formation of a fingering pattern. By a combination of experiments and theoretical modelling, we investigate how the bacterial motility, number density and droplet evaporation rate control the pattern dynamics. Ultimately, our aim is to understand how the interplay between the bacteria and their surrounding liquid governs their dispersal.
Jose Manuel Gordillo (Universidad de Sevilla, Spain)
From drop splashing to Worthington jets
In this talk we show that the splashing of drops impacting over a solid substrate and the ejection of Worthington jets produced as a consequence of the collapse of a cavity can be described using the same theoretical framework. The results of the theory, which will be validated by comparison with experiments and numerical simulations reveal that, in contrast to what it is commonly believed, the jets produced after the bursting of bubbles, after the collapse of high amplitude Faraday waves, as a consequence of the collapse of the cavity produced after a solid impacts a free surface or after a cavitation bubble implodes in close proximity to a wall, emerge as a consequence of a common, purely inertial mechanism.
Anne Juel (University of Manchester, United Kingdom)
Oscillatory dynamics of a Saffman-Taylor finger with a bubble placed at its tip
Pattern formation due to viscous fingering exhibits a fascinating range of complex dynamics. For example, when air displaces a viscous liquid in the narrow gap between two parallel plates – a Hele-Shaw channel – the resulting steadily propagating finger of air can undergo a relatively abrupt transition to disordered front propagation. This motivates an exploration of the system’s nonlinear dynamics, which in turn requires the finite-amplitude perturbation of the finger. In the search for a suitable perturbation, we were inspired by Couder et al.’s [1] seminal work on viscous fingering, where he obtained narrow steadily propagating fingers when a small bubble was aggregated at the tip of the finger. This is because the bubble modifies the curvature of the finger’s tip and, thus, changes the finger’s shape. We have explored the dynamics of this system as a function of the driving flow rate and the size of the bubble at the tip. Fig. 1a shows that there are two regions of steady propagation of the finger (blue and green) separated by a red region where the propagation is unsteady. We explore the steady to oscillatory transitions and interpret the dynamics based on competing physical mechanisms. For modest flow rates, the finger oscillates transversely as described by Couder et al. [1], but as the flow rate increases, the dynamics become disordered (Fig. 1b). For high flow rates, the finger appears to meander randomly about the channel’s centreline for a range bubble sizes which may indicate a transient exploration of weakly unstable states.
References
[1] Y. Couder, N. Gérard and M. Rabaud, Narrow fingers in the Saffman–Taylor instability, Phys. Rev. A 34, 1986.
Djamel Lakehal (Afry, Switzerland)
Computational droplet dynamics and heat transfer: from microfluidics and MedTech applications to large-scale energy systems
Microfluidic flows are physically diverse as they can be associated with free surface motion evolving as falling films, spreading and de-wetting of liquids on solid or liquid substrates, chemical reaction of binary mixtures, microbubbles and droplet motion, phase change or transition. The control of micro-flow systems (using pressure, Marangoni, electro-wetting or acoustics) is central to future technological advances in emerging technologies. Modern CMFD (CFD relying on interface tracking) has proven to be highly effective in predicting small-scale multi-phase flows and can provide virtual information about intricate flow details, regardless of system size, which are otherwise impossible to detect (in time or space) using available visualisation technologies. Here we will discuss several examples of droplet dynamics and heat transfer: from microfluidics and MedTech applications to large-scale energy systems.
Dominique Legendre (IMFT, France)
Droplet lift force peak due to internal 3D flow bifurcation
Spherical bodies such as bubble, solid sphere and droplet moving in shear flow are experiencing a force normal to their relative motion, called lift force. When varying the viscosity ratio between the droplet and the external fluid, we report for droplet Reynolds number of order 100 an unexpected lift force peak at a viscosity ratio of order unity. We reveal thanks to a detailed inspection of the flow field obtained using direct numerical simulation that this behavior is related to an internal 3D flow bifurcation also observed under uniform-flow conditions. In the presence of an ambient shear, this internal 3D bifurcation results in the amplification of the vortex tilting-stretching mechanism responsible for the lift force, making the trailing vortices generated in the droplet wake stronger, and as a consequence an enhanced lift effect.
Contributors
Dominique Legendre, Pengyu Shi, Eric Climent
Elise Lorenceau (Université Grenoble Alpes, France)
Stress distribution upon the impact of droplet
The impact of ordinary raindrops on a solid surface can have deleterious effects and induce erosion of fragile solid such as soils [1] but also alteration of the surface optical properties [2] or even degradation of plant leaves [3].
Yet, at first glance, one would think that the liquid would absorb all the deformation resulting from the impact on a surface as stiff as glass or silicon, thus exerting a moderate stress and leaving the surface unharmed. However, during impact, the forced flow of the contact line generates violent velocity gradients in the triple line trail, and therefore important shear and normal stresses at the surface. The theoretical and experimental determination of these actions is a difficult task, as it requires a knowledge of the unsteady and inhomogeneous velocity field at all times and all scales [4,5]. In this talk, I will discuss the state of the art concerning the stress exerted on a surface by an impacting drop [6] and present how we have used nanoparticules as nano-probes for the shear stress enabling us to quantify the maximum shear stress exerted by the impact. [7]. We also discuss how our work can be used for a better understanding of spray cleaning processes in microelectronics industry.
Contributors
A. Lallart(1),(2),(3), A. Cartellier, P. Garnier, E. Charlaix and E. Lorenceau(1)
(1) Université Grenoble Alpes, CNRS, LIPhy, F-38000 Grenoble, France
(2) Université Grenoble Alpes, Grenoble-INP, CNRS, LEGI, F-38000 Grenoble, France
(3) STMicroelectronics, 850 Rue Jean Monnet, Crolles 38926-Cedex, France
References
[1] M. Fernandez-Raga, et al. Earth Sci Rev 171, (2017) 463.
[2] M. H. Keegan, D. H. Nash, and M. M. Stack, J Phys D: Appl Phys 46, (2013) 383001
[3] A. Roth-Nebelsick, et al. J.Exp. Bio. 73, (2022) 1155
[4]. J.Eggers et al. Phys. Fluids 22 (2010) 6
[5] J. Philippi, P.-Y. Lagree, and A. Antkowiak, J. Fluid Mech. 795, (2016) 96.
[6] X. Cheng, T.-P. Sun, and L. Gordillo, Annu. Rev. Fluid Mech. 54 (2022) 57
[7] A. Lallart et al., J. Fluid Mech. 1000 (2024) A31
Jacco Snoeijer (University of Twente, Netherlands)
Soft Condensation
Breath figures are the complex patterns that form when water vapor condenses into liquid droplets on a surface. The primary question concerning breath figures is how the condensing vapor is allocated between the growth of existing droplets and the nucleation of new ones. Although numerous studies have concentrated on scenarios resulting in highly polydisperse droplet ensembles, we experimentally [1] and theoretically [2] demonstrate that nearly monodisperse patterns can be achieved on defect-free substrates in a diffusion-controlled regime.
Specifically, we investigate the nucleation, growth, and coalescence of droplets on soft substrates consisting of crosslinked polymer networks. We discover that, following a short nucleation phase, the number of droplets remains constant over an extensive range of timescales due to collective effects mediated by the diffusion of vapor. The spatial extent of these diffusive interactions is identified through asymptotic matching, based on which we provide an accurate description of breath figures through a mean-field model (Fig. 1). In the experiments, we observe an algebraic sensitivity of the number of droplets on substrate elasticity, which is unexpected since nucleation occurs at a scale where the polymer network resembles a melt. Breath figures thus offer a macroscopic probe of the microscopic characteristics of the polymer surface, and suggest a surprisingly low-energy pathway for soft nucleation.
Contributors
Jacco H. Snoeijer(1), Ambre Bouillant(1),(2), Christopher Henkel(3), Uwe Thiele(3), Bruno Andreotti,(4)
(1)Physics of Fluids Group, University of Twente, 7500 AE Enschede, Netherlands
(2)MSC, UMR 7057 CNRS, UPC, 10 rue A. Domon et L. Duquet, 75013, Paris, France
(3)Institut für Theoretische Physik, Universität Münster, W.Klemm-Str. 9, 48149 Münster, Germany
(4)LPENS, UMR 8550 CNRS, ENS, UPCit´e, SU, 24 rue Lhomond, 75005 Paris, France
References
[1] A Bouillant, C Henkel, U Thiele, B Andreotti and JH Snoeijer, arXiv:2407.07624.
[2] A Bouillant, JH Snoeijer and B Andreotti, arXiv:2407.07608, to appear in Phys. Rev. Fluids.
Tadd Truscott (King Abdullah University of Science and Technology, Saudi Arabia)
Extremely Large Droplet Formation and Breakup
The aerodynamic breakup of droplets is essential for many applications like fuel atomization, agricultural spraying, fire suppression, and crashing sea waves. While most studies focus on droplet sizes from microns up to the capillary length, we explore the breakup of extremely large droplets. We have designed a novel device that releases a large droplet onto an air jet combined with a moving plate mechanism that impulsively exposes the droplets. The droplet freefall allows us to observe the response when no connected needle is present and to observe the altered behavior when the droplet is in different mode shapes (non-spherical). Observed breakup types include a cascading behavior, forward bagging, and dependence on initial mode shape and Weber number. Through supportive volume of fluid-based DNS, we are able to show that the droplet breakup is greatly altered by the phase and amplitude because of competing forces inside vs. outside of the droplet.
Contributors
Sandip Laxman Dighe, Nilamani Sahoo, Dilip Kumar Maity, Charbel El Khoury, Spencer Stephen Truman, Aqeel Almanashi, Aditya Parik, Randy Ewoldt, Som Dutta
Maja Vuckovac (Aalto University, Finland)
Wetting dynamics on stochastic superhydrophobic surfaces
Understanding droplet mobility on surfaces is crucial for various biological and technological applications [1–3]. Traditionally, wetting dynamics is modelled on regular surfaces like micropillar arrays [4–8], yet many real-world superhydrophobic surfaces exhibit stochastic roughness with irregular characteristics [9–11]. Our study reveals that the wetting dynamics on these stochastic surfaces fundamentally differ from those on regular surfaces [12]. We found that droplet friction in the quasi-static regime (𝑣d ~ μm/s) is significantly higher than in dynamic conditions (𝑣d ~ cm/s to m/s), contrasting with the behavior observed on regular surfaces. Using state-of-the-art imaging and modelling, we demonstrate that at lower velocities, the liquid-air interface has extended adaptation times to surface irregularities (τ ~ s). This adaptation leads to an increased liquid-solid contact fraction. Conversely, at higher velocities, the interface has reduced adaptation time (τ ~ μs). Thus, we introduce the concepts of static and dynamic liquid-solid contact fractions to explain these differences while highlighting the stochasticity levels required to repel droplets of varying sizes effectively. These findings provide important insights that will contribute to advancements in fabrication methods and lead to novel superhydrophobic materials with potential applications in various fields, such as microfluidics, coatings, and biomimetic surfaces.
References
[1] D. Quéré, Annu. Rev. Mater. Res. 2008, 38, 71.
[2] T. M. Schutzius, S. Jung, T. Maitra, G. Graeber, M. Köhme, D. Poulikakos, Nature 2015, 527, 82.
[3] J. C. Bird, R. Dhiman, H.-M. Kwon, K. K. Varanasi, Nature 2013, 503, 385.
[4] M. Reyssat, D. Quéré, J. Phys. Chem. B 2009, 113, 3906.
[5] D. W. Pilat, P. Papadopoulos, D. Schäffel, D. Vollmer, R. Berger, H.-J. Butt, Langmuir 2012, 28, 16812.
[6] C. W. Extrand, Langmuir 2002, 18, 7991.
[7] C. Dorrer, J. Rühe, Langmuir 2007, 23, 3179.
[8] H.-J. Butt, N. Gao, P. Papadopoulos, W. Steffen, M. Kappl, R. Berger, Langmuir 2017, 33, 107.
[9] H. J. Ensikat, P. Ditsche-Kuru, C. Neinhuis, W. Barthlott, Beilstein J. Nanotechnol. 2011, 2, 152.
[10] W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, H. F. Bohn, Adv. Mater. 2010, 22, 2325.
[11] L. Wang, Soft Matter 2023, 19, 1058.
[12] Vuckovac, M. et al., in preparation.