Soft Matter and Complex Systems Seminar
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2025-02-28 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30 
Magdalena Kopczyńska, Victoria Vasileuskaya, Laura Meissner (FUW)Student Talks
Talks Schedule
28 February 2024
- Magdalena Kopczyńska: Photoluminescence of CrPS₄ and its dependence on the magnetic field parallel to the flake plane
- Victoria Vasileuskaya: Topological phase transition, from cyclic to tree structures in evolving transport networks
- Laura Meissner: Stokes flow around a sphere with odd viscosity
2025-01-17 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Tomasz Bobiński (Faculty of Power and Aeronautical Engineering, Warsaw University of Technology)Cloaking defects in a water waveguide system
Controlling surface water wave propagation is crucial for wave manipulation and cloaking technologies. By leveraging the invariance of shallow water equations under coordinate transformations, objects can be rendered invisible to incident waves. Traditional transformations often require spatially anisotropic bathymetries, which typically violate the assumptions of the depth-averaged models describing the propagation of water waves. We demonstrate that conformal mapping, which provides smoothly varying bathymetry, can be effectively applied to water waveguide systems with defects in the form of local variations in the waveguide wall shape. Our approach successfully cloaks these defects across a broad range of frequencies, including regimes where dispersive effects are significant. Despite the inherent dispersive nature of water waves, forward scattering remains weak, ensuring robust cloaking performance. Experimental results validate the broadband capabilities of this method.
Based on the results obtained in the case of the meandering waveguide, we present a novel technique to render objects invisible to incident waves in a water waveguide system with parallel walls at low frequencies. The invisibility of a waveguide defect, specifically a vertical surface-piercing circular cylinder, is achieved through local deformations of the waveguide walls in the immediate vicinity of the defect. Our method results in a reflection coefficient that is at least 20 times lower than in the case of a parallel waveguide. The effect is observed over a broad frequency range. Experimental results confirm the high efficiency of our approach, showing that backscattered energy is reduced by a factor of 100 to 5000 compared to the reference case within the considered frequency range.
Based on the results obtained in the case of the meandering waveguide, we present a novel technique to render objects invisible to incident waves in a water waveguide system with parallel walls at low frequencies. The invisibility of a waveguide defect, specifically a vertical surface-piercing circular cylinder, is achieved through local deformations of the waveguide walls in the immediate vicinity of the defect. Our method results in a reflection coefficient that is at least 20 times lower than in the case of a parallel waveguide. The effect is observed over a broad frequency range. Experimental results confirm the high efficiency of our approach, showing that backscattered energy is reduced by a factor of 100 to 5000 compared to the reference case within the considered frequency range.
2025-01-10 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Tetuko Kurniawan (IPPT PAN)Formation of Droplets in Microfluidic Cross-Junctions and Their Application as Cell Incubators
Droplet microfluidics is an innovative technique in biomedical research that leverages the creation of small, isolated compartments formed by immiscible fluids, typically a water-in-oil system, within a network of microscale channels on a chip. In this talk, I will explore droplet formation in the very low capillary number regime. Interestingly, droplet formation in this regime diverges from the well-known squeezing mechanism, as evidenced by a significant increase in droplet size and neck length before pinch-off with respect to the capillary number. A generalized scaling law was developed to predict droplet volume in microfluidic cross-junctions and was validated using experimental data from devices with varying cross-sectional geometries. These findings deepen our understanding of droplet formation mechanics in the very low capillary number range. Additionally, I will discuss the practical applications of droplet microfluidics as micro-sized incubators for cell culture. Effective strategies for reducing the loading time of cell-containing media and minimizing droplet liquid mass transport through the permeable PDMS material will be presented, resulting in the ability to sustain the viability of most cells for over 24 hours.
2024-12-20 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Dimitrius Khaladj (Lawrence Berkeley National Lab, USA)Electrochemical lithium separation from natural brines using non-equilibrium graphene-oxide liquid crystal
Conventional methods for lithium extraction via evaporation pools or hard rock mining are environmentally consequential both locally and globally. To meet the demand for mineral components for lithium batteries while lessening the environmental impact, new technologies for 'direct' lithium extraction aim to selectively capture lithium from aqueous solution among a mélange of competing ionic species. In this work, we present a new concept for direct lithium extraction based on far-from-equilibrium transport of lithium through self-assembled percolating colloidal graphene oxide (GO) driven by AC Electric fields. We report that these self-assembled GO networks enhance ionic conductivity, actively transporting and selecting lithium without requiring extensive pre-treatment. We demonstrate that lithium transport far-from-equilibrium can be enhanced relative to competing cations due to its weaker specific interactions with the percolating GO network, resulting in high mobility. This runs counter to many conventional approaches to lithium extraction, which rely on high lithium selectivity but are implicitly limited by low binding/exchange turnover rates. We propose that enhancing lithium transport, rather than binding selectivity, may be key to high-rate direct lithium extraction from brine sources.
2024-12-13 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Reinier van Buel (IFT UW)Active and passive control of elastic turbulence at low Reynolds numbers
Elastic turbulence, a chaotic flow state occurring in viscoelastic fluids at negligible Reynolds numbers, enhances fluid mixing on small scales, making it valuable for applications like lab-on-a-chip devices. Here, we show the influence of active and passive control on elastic turbulence using numerical simulations of the Oldroyd-B model in OpenFOAM®. In a two-dimensional Taylor-Couette geometry, the onset of elastic turbulence is characterised by a critical Weissenberg number, marking a transition from laminar to turbulent flow. Flow resistance and secondary-flow strength, serving as order parameters, increase with turbulence, correlating strongly with enhanced fluid mixing. Power-law scaling of velocity fluctuations aligns with experimental results, confirming the turbulent nature of the flow. Active control, implemented through imposed shear-rate modulations, shows promise for managing turbulence. Slow modulations induce complex behaviours, while fast modulations suppress turbulence. The state diagram of Weissenberg and Deborah numbers highlights the transition to turbulence and suggests an effective critical Weissenberg number to approximate the transition line. In three-dimensional von Kármán flow, elastic turbulence exhibits a subcritical transition, marked by bistable behaviour and hysteresis. Flow resistance and order parameters increase significantly, confirming turbulence. Active control applied here reduces turbulence and relaminarises the flow, mirroring results from the Taylor-Couette system. These findings underscore the potential of controlling elastic turbulence to optimise fluid mixing and stability in viscoelastic systems.
2024-11-29 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Wojciech Góźdź (Institute of Physical Chemistry, PAS)Influence of obstacles on the collective motion of self-propelled particles forming traveling bands
The influence of regularly distributed disk-like obstacles on the motionof self-propelled particles is investigated within the framework of the Vicsek model. We focus on systems with a large number of self-propelled particles that form ordered structures such as traveling bands. The obstacles are arranged in a square lattice. We investigate the influence of their size and their separation on the formation and stability of ordered patterns of moving particles. We have discovered new structures stabilized by different arrangements of obstacles.
2024-11-22 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Stanisław Gepner (Warsaw University of Technology)Is the Laminar-Turbulent Edge Crowded? Exploring Multiple Local Attractors in the Edge of Square-Duct Flow
In this work, we present the first streamwise-localized invariant solution for turbulent square duct flow in the moderate Reynolds number range. Through heuristic analysis, we demonstrate that during specific periods within the turbulent time evolution, the flow state approaches the identified localized solution. This finding indicates that the localized solution is embedded within the turbulent attractor, making it the first localized solution identified for square duct flow and a the potential building block of turbulence in this configuration.
We obtain this solution through a bisection process applied within the symmetric subspace of the full state space, which enables the tracking of edge state solutions. Edge states are characterized by a single unstable direction, or a co-dimension one stable manifold, within the symmetric subspace. In the context of the full state space, these solutions are embedded within the turbulent attractor. As relative attractors on the edge of the laminar and turbulent basins, edge states play a significant role in governing the laminar-turbulent transition process. This characteristic makes them particularly interesting for turbulence control applications. In addition to the bisection method, we use Newton-Krylov GMRES-based iterations to converge to invariant solutions. To analyze stability, we apply an Arnoldi-based eigenvalue solver, and an arc-length continuation to track bifurcations. Stability analysis reveals that both branches of our localized solution are unstable in at least one direction. This instability suggests the presence of additional structures that may connect to the branches of the identified solution, indicating that the edge subspace (a co-dimension one subspace of the full space) contains multiple local attractors. Each of these local edge states would have stable manifolds that locally separate initial conditions, leading either toward the laminar attractor, a transient non-laminar excursion or, if it exists, a turbulent attractor. In our ongoing work, we identify and analyze a series of solutions on the edge. We study the positions and potential connections between the lower and upper branches of the identified solutions. By disturbing either the lower or upper branch in the unstable direction, we observe that the system tends either to laminarize smoothly or to experience a transient turbulent excursion. This behavior confirms that both solution branches reside on the edge and that the bifurcation responsible for their creation also lies on the edge. Additionally, we identify a potential heteroclinic connection between these states, which further enriches our understanding of the dynamics governing laminar-turbulent transition in square duct flow.
We obtain this solution through a bisection process applied within the symmetric subspace of the full state space, which enables the tracking of edge state solutions. Edge states are characterized by a single unstable direction, or a co-dimension one stable manifold, within the symmetric subspace. In the context of the full state space, these solutions are embedded within the turbulent attractor. As relative attractors on the edge of the laminar and turbulent basins, edge states play a significant role in governing the laminar-turbulent transition process. This characteristic makes them particularly interesting for turbulence control applications. In addition to the bisection method, we use Newton-Krylov GMRES-based iterations to converge to invariant solutions. To analyze stability, we apply an Arnoldi-based eigenvalue solver, and an arc-length continuation to track bifurcations. Stability analysis reveals that both branches of our localized solution are unstable in at least one direction. This instability suggests the presence of additional structures that may connect to the branches of the identified solution, indicating that the edge subspace (a co-dimension one subspace of the full space) contains multiple local attractors. Each of these local edge states would have stable manifolds that locally separate initial conditions, leading either toward the laminar attractor, a transient non-laminar excursion or, if it exists, a turbulent attractor. In our ongoing work, we identify and analyze a series of solutions on the edge. We study the positions and potential connections between the lower and upper branches of the identified solutions. By disturbing either the lower or upper branch in the unstable direction, we observe that the system tends either to laminarize smoothly or to experience a transient turbulent excursion. This behavior confirms that both solution branches reside on the edge and that the bifurcation responsible for their creation also lies on the edge. Additionally, we identify a potential heteroclinic connection between these states, which further enriches our understanding of the dynamics governing laminar-turbulent transition in square duct flow.
2024-11-15 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Alina Ciach (IChF PAN)Anomalous underscreening in concentrated ionic systems
Concentrated ionic systems can find practical applications in energystorage devices, and in living cells the density of ions is large.Classical theories developed for dilute electrolytes, however, are notvalid when the average distance between the ions becomes comparable withtheir diameters. Different experimental techniques, approximate theoriesand simulations give contradictory results for the distribution of theions and for screening of charged objects, and a commonly acceptedtheory is still to be developed.
I will very briefly present the experimental and simulation results.Next I'll discuss major differences between dilute and concentratedionic systems, and introduce the mesoscopic approach for ionic systemswith any density. In the theory, the finite size of the ions and thevariance of the local charge are taken into account.The correlationfunctions obtained within the theory will be compared with experiments.The remaining open questions will be discussed.
I will very briefly present the experimental and simulation results.Next I'll discuss major differences between dilute and concentratedionic systems, and introduce the mesoscopic approach for ionic systemswith any density. In the theory, the finite size of the ions and thevariance of the local charge are taken into account.The correlationfunctions obtained within the theory will be compared with experiments.The remaining open questions will be discussed.
2024-11-08 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Tony Ladd (University of Florida at Gainesville)Using molecular flexibility to purify DNA from a cell lysate
DNA is a semi-flexible polyelectrolyte with a persistence length of about 50 nm. At equilibrium, entropic forces cause the individual molecules to form compact spherical coils, which can be easily stretched by a weak shear flow into a cigar-like conformation that is not, on average, aligned with the flow direction. Because the electrophorectic mobility of the sheared polyelectrolyte is no longer isotropic, DNA can be driven to the walls of a confining channel by an electric field that pushes the DNA in the opposite direction to the flow. The migration velocity is sufficient to keep the DNA in a thin layer next to the wall. With a suitable choice of field strengths, the DNA can be driven against the flow (since it is next to the channel walls), while all the other components of a cell lysate are flushed in the opposite direection by the flow. This is a much stronger separation than in typical microfluidic processes, which rely on mobility contrast between the species. A simple microfludic device, assembled from acrylic sheets for less than $1, can provide a chemical-free purification of DNA. In this talk I will outline the physics underpinning the separation and describe experiments that purify DNA in sufficient quantities (up to 40 ng) for PCR amplification and gel electrophoresis.
2024-10-25 (Piątek)
Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 09:30
Łukasz Klotz (Warsaw University of Technology)Influence of porous material on the flow behind backward-facing step - experimental study
We investigate effect of porous insert located upstream of the separation edge of backward facing step (BFS) in early transitional regime as a function of Reynolds number. This is an example of hydrodynamic system that is a combination of separated shear flow with large amplification potential and porous materials known for efficient flow destabilisation. Spectral analysis reveals that dynamics of backward-facing step is dominated by spectral modes that remain globally coherent along the streamwise direction. We detect two branches of characteristic frequencies in the flow and with Hilbert transform we characterise their spatial support. For low Reynolds numbers, the dynamics of the flow is dominated by lower frequency, whereas for sufficiently large Reynolds numbers cross-over to higher frequencies is observed. Increasing permeability of the porous insert results in decrease in Reynolds number value, at which frequency cross-over occurs. By comparing normalized frequencies on each branch with local stability analysis, we attribute Kelvin-Helmholtz and Tollmien-Schlichting instabilities to upper and lower frequency branches, respectively. Finally, our results show that porous inserts enhance Kelvin-Helmholtz instability and promote transition to oscillator type dynamics. Specifically, the amplitude of vortical (BFS) structures associated with higher frequency branch follows Landau model prediction for all investigated porous inserts.
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