Soft Matter and Complex Systems Seminar
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2021-06-11 (Friday)
join us at 09:30 
Antoni Wrzos, Kamil Gmiterek, Dominik Suwała, Stanisław Żukowski, Marcin Pruszczyk (IFT UW)Students' Talks
On Friday 11 June 2021 at 9:30 AM we are hosting the final seminar of this term, during which we will hear short presentations by:
Antoni Wrzos
Selected ideas in modelling of blood vessels
The presentation is covering some general ideas connected with modelling of blood vessels growth. A bit more will be said about cellular Potts model (CMP) and finite element method (FEM), based on an article written by Merks et al., 2014. Also the talk will touch results from their model and some of its limitations.
Kamil Gmiterek
The rigged Hilbert space and resonances in QM
In QM Dirac's bra-ket formalism is fully implemented by the rigged Hilbert spaces rather than just by the Hilbert space. To describe the full spectrum of a quantum system (bound, scattering and resonant parts) one have to enlarge the Hilbert space to a rigged Hilbert space (Gelfand triplet). Using mathematical tools from the theory of distributions one can provide an appropriate meaning of objects like eg. the delta function.
Dominik Suwała
A tour on CFD algorithms: SIMPLE
I this talk I will present one of the widely used computational fluid dynamics (CDF) algorithm: SIMPLE (Semi-Implicit Method for Pressure Linked Equations). I will start from the Navier-Stokes equations and go through the SIMPLE solution process while covering topics like different discretization schemes and pressure correction methods. Finally I will give some comments on the SIMPLE-based algorithms SIMPLER and SIMPLEC.
Stanisław Żukowski
Hydrodynamical instabilities - why does water spill out of a bucket?
The question from the title may sound trivial, however, I will present some examples of home experiments, where one observes water in the jar positioned upside down, which remains stable and does not spill. In my short talk I will try to answer what differs the bucket and the jar, which will lead us to a description of so-called Rayleigh-Taylor instability.
Marcin Pruszczyk
One-dimensional Bose gas with attractive boundary conditions
In one-dimensional ideal Bose gas with the so-called attractive boundary conditions Bose-Einstein condensation is observed. It is a phase transition of second order in regards to the Ehrenfest classification. The phase containing the condensate is not homogeneous - the condensate is localised in the vicinity of point-like walls confining the gas.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
Antoni Wrzos
Selected ideas in modelling of blood vessels
The presentation is covering some general ideas connected with modelling of blood vessels growth. A bit more will be said about cellular Potts model (CMP) and finite element method (FEM), based on an article written by Merks et al., 2014. Also the talk will touch results from their model and some of its limitations.
Kamil Gmiterek
The rigged Hilbert space and resonances in QM
In QM Dirac's bra-ket formalism is fully implemented by the rigged Hilbert spaces rather than just by the Hilbert space. To describe the full spectrum of a quantum system (bound, scattering and resonant parts) one have to enlarge the Hilbert space to a rigged Hilbert space (Gelfand triplet). Using mathematical tools from the theory of distributions one can provide an appropriate meaning of objects like eg. the delta function.
Dominik Suwała
A tour on CFD algorithms: SIMPLE
I this talk I will present one of the widely used computational fluid dynamics (CDF) algorithm: SIMPLE (Semi-Implicit Method for Pressure Linked Equations). I will start from the Navier-Stokes equations and go through the SIMPLE solution process while covering topics like different discretization schemes and pressure correction methods. Finally I will give some comments on the SIMPLE-based algorithms SIMPLER and SIMPLEC.
Stanisław Żukowski
Hydrodynamical instabilities - why does water spill out of a bucket?
The question from the title may sound trivial, however, I will present some examples of home experiments, where one observes water in the jar positioned upside down, which remains stable and does not spill. In my short talk I will try to answer what differs the bucket and the jar, which will lead us to a description of so-called Rayleigh-Taylor instability.
Marcin Pruszczyk
One-dimensional Bose gas with attractive boundary conditions
In one-dimensional ideal Bose gas with the so-called attractive boundary conditions Bose-Einstein condensation is observed. It is a phase transition of second order in regards to the Ehrenfest classification. The phase containing the condensate is not homogeneous - the condensate is localised in the vicinity of point-like walls confining the gas.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-05-28 (Friday)
join us at 09:30
Lyndon Koens (Macquarie University, Melbourne)Generalising geometric swimming for viscous flow
The geometric phase techniques for swimming in viscous flows express the net displacement of a swimmer as a path integral of a field in configuration space. This representation can be transformed into an area integral for simple swimmers using the Stokes theorem. Since this transformation applies for any loop, the integrand of this area integral can be used to help design these swimmers. However, the extension of this Stokes theorem technique to more complicated swimmers is hampered by problems with variables that do not commute and by how to visualise and understand the higher-dimensional spaces. In this talk I will introduce you to the general geometric swimming process and then develop novel methods to approach non-commuting variables and the visualisation of high dimensional spaces. In do so we will discuss the implications for our understanding of microscopic swimming and outline how it could be used to help with the design of swimmers.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-05-21 (Friday)
join us at 09:30
Leszek Czechowski (IGF UW)Some problems of gravity flows
Gravitational forces are one of the most important causes of mass motion considered in Earth and Planetary Sciences. Even on comets with a very small force of gravity, their effects have been observed. This seminar discusses various geological phenomena caused by the force of gravity.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-05-14 (Friday)
join us at 09:30
Ahmad Ababaei (Institute of Meteorology and Water Management – National Research Institute)The impact of lubrication forces and short-range non-continuum molecular effects on the collision statistics of cloud droplets in homogeneous isotropic turbulence
The dynamics of inertial particles in homogeneous isotropic turbulence is investigated, under one-way momentum coupling, using a new computational approach. The method incorporates the long-range many-body aerodynamic interactions with the short-range lubrication forces and the non-continuum molecular effects.
The implementation couples Hybrid Direct Numerical Simulations (HDNS) with the analytical solutions of two rigid spheres moving in an unbounded fluid. Merging of these two approaches allows us to jointly represent two important effects usually neglected in the previous studies: (i) HDNS accurately captures the effect of many-body interactions among widely separated droplets, while (ii) the analytical solution for two interacting rigid spheres in low-Reynolds-number flows allows us to represent the lubrication effects. In case of very short separation distances between two droplets, the non-continuum correction to the lubrication forces is additionally considered.
Concerning the velocity field seen by the particles, the algorithm switches from the flow solution in terms of HDNS to analytical formulae when the separation distance between particles becomes comparable to their average radius. Switching to the analytical solutions is made considering that the standard HDNS is unable to correctly represent the short-range interactions, since this method is based on the superposition of the Stokes solutions for single spheres.
The results show that for the turbulent kinetic energy dissipation rates typical of atmospheric clouds, the radial relative velocities (RRV) of the droplets increase, and the radial distribution function (RDF) decreases in the near-contact region if the lubricative forces are taken into account. These changes are more pronounced when the effect of gravity is considered. Away from the contact region, however, there is not much change in RRVs and RDFs. For turbulent clouds with lower dissipation rates lubrication forces significantly enhance the average RRV in the limit of low Stokes number. This enhancement, however, is statistically insignificant because the number of particle pairs at close proximity is very small. The effect of mass loading on the collision statistics is also investigated, demonstrating an increase in RRV and a reduction in RDF with the droplet concentration.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The implementation couples Hybrid Direct Numerical Simulations (HDNS) with the analytical solutions of two rigid spheres moving in an unbounded fluid. Merging of these two approaches allows us to jointly represent two important effects usually neglected in the previous studies: (i) HDNS accurately captures the effect of many-body interactions among widely separated droplets, while (ii) the analytical solution for two interacting rigid spheres in low-Reynolds-number flows allows us to represent the lubrication effects. In case of very short separation distances between two droplets, the non-continuum correction to the lubrication forces is additionally considered.
Concerning the velocity field seen by the particles, the algorithm switches from the flow solution in terms of HDNS to analytical formulae when the separation distance between particles becomes comparable to their average radius. Switching to the analytical solutions is made considering that the standard HDNS is unable to correctly represent the short-range interactions, since this method is based on the superposition of the Stokes solutions for single spheres.
The results show that for the turbulent kinetic energy dissipation rates typical of atmospheric clouds, the radial relative velocities (RRV) of the droplets increase, and the radial distribution function (RDF) decreases in the near-contact region if the lubricative forces are taken into account. These changes are more pronounced when the effect of gravity is considered. Away from the contact region, however, there is not much change in RRVs and RDFs. For turbulent clouds with lower dissipation rates lubrication forces significantly enhance the average RRV in the limit of low Stokes number. This enhancement, however, is statistically insignificant because the number of particle pairs at close proximity is very small. The effect of mass loading on the collision statistics is also investigated, demonstrating an increase in RRV and a reduction in RDF with the droplet concentration.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-04-30 (Friday)
join us at 09:30
Daniel Matoz-Fernandez (IFT UW)Growth, form, and active mechanics in biology
Despite significant progress in understanding the behavior of active fluids, much less is known about how activity affects the behavior of solid and viscoelastic materials, such as epithelial tissues or bacterial biofilms. In this talk, I will show how activity in the form of growth and chemical signaling can drive the system to develop a wide variety of shapes due to competition with viscous relaxation. The latter is of particular interest in developing a physical understanding of morphogenesis, where the embryo has to undergo a series of carefully orchestrated shape changes to establish the functioning organism.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-04-23 (Friday)
join us at 09:30
Joanna Piotrowska (Kavli Institute for Cosmology, University of Cambridge)Galactic death in simulations and observations: evidence for the role of supermassive black hole feedback
Understanding the physical mechanisms responsible for shutting down star formation in galaxies is one of the most important questions in the field of extragalactic astronomy. In this work we investigate how star formation is ceased (or ‘quenched’) in local, massive, central galaxies by comparing the Universe observed by the Sloan Digital Sky Survey (SDSS) with three state-of-the-art cosmological simulation suites – EAGLE, Illustris and TNG. Being aware of the complex and non-linear nature of quenching we combine machine learning techniques with a partial correlation analysis to determine which galactic property is the most relevant for ceasing star formation. Our results show that the supermassive black hole mass is the most powerful parameter in determining whether a galaxy is actively forming stars or not – a statement we find true for all three implementations of black hole interaction with its surrounding gas in the simulations. This prediction is met overwhelmingly well in the SDSS, where we infer black holes masses from empirical calibrations, using stellar velocity dispersions in an impressive sample of ~230 000 local galaxies.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-04-16 (Friday)
room 1.40, Pasteura 5 at 09:30
Debasish Das (University of Strathclyde, Glasgow)Electrohydrodynamics of drops in strong electric fields
Electrohydrodynamics of drops is a classic fluid mechanical problem where deformations and microscale flows are generated by application of an external electric field. In weak fields, electric stresses acting on the drop surface drive quadrupolar flows inside and outside and cause the drop to adopt a steady axisymmetric shape. This phenomenon is best explained by the leaky-dielectric model under the premise that a net surface charge is present at the interface while the bulk fluids are electroneutral. In the case of dielectric drops, increasing the electric field beyond a critical value can cause the drop to start rotating spontaneously and assume a steady tilted shape. This symmetry-breaking phenomenon, called Quincke rotation, arises due to the action of the interfacial electric torque countering the viscous torque on the drop, giving rise to steady rotation in sufficiently strong fields. Here, we present a small-deformation theory for the electrohydrodynamics of dielectric drops for the complete Melcher–Taylor leaky-dielectric model in three dimensions. Our theory is valid in the limits of strong capillary forces and highly viscous drops and is able to capture the transition to Quincke rotation. A coupled set of nonlinear ordinary differential equations for the induced dipole moments and shape functions are derived whose solution matches well with experimental results in the appropriate small-deformation regime. Retention of both the straining and rotational components of the flow in the governing equation for charge transport enables us to perform a linear stability analysis and derive a criterion for the applied electric field strength that must be overcome for the onset of Quincke rotation of a viscous drop.
Articles:
1) D. Das and D. Saintillan, "A three-dimensional small-deformation theory for electrohydrodynamics of dielectric drops" Journal of Fluid Mechanics, 914, A22 (2021).
2) D. Das and D. Saintillan, "A nonlinear small-deformation theory for transient droplet electrohydrodynamics", Journal of Fluid Mechanics 810, 225-253 (2017).
3) D. Das and D. Saintillan, "Electrohydrodynamics of viscous drops in strong electric fields: Numerical simulations", Journal of Fluid Mechanics 829, 127-152 (2017).
4) D. Das and E. Lauga, "Active particles powered by Quincke rotation in a bulk fluid", Physical Review Letters 122, 194503 (2019).
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
Articles:
1) D. Das and D. Saintillan, "A three-dimensional small-deformation theory for electrohydrodynamics of dielectric drops" Journal of Fluid Mechanics, 914, A22 (2021).
2) D. Das and D. Saintillan, "A nonlinear small-deformation theory for transient droplet electrohydrodynamics", Journal of Fluid Mechanics 810, 225-253 (2017).
3) D. Das and D. Saintillan, "Electrohydrodynamics of viscous drops in strong electric fields: Numerical simulations", Journal of Fluid Mechanics 829, 127-152 (2017).
4) D. Das and E. Lauga, "Active particles powered by Quincke rotation in a bulk fluid", Physical Review Letters 122, 194503 (2019).
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-04-09 (Friday)
room 1.40, Pasteura 5 at 09:30
Klaudia Dradrach (FUW)Photoresponsive materials as actuators in microfluidic chips
Light in microfluidic setups is often used to monitor the flows in the micro-channels and to read out information from the microfluidic chips. But it can be also a clean and powerful energy source for machining. My aim is to use light to control the flows using photoresponsive materials that change their properties in response to light to fabricate active microfluidic components. Focused light beams could open and close miniature valves built directly into the structure of the chip, and peristaltic light-driven pumps could drive the liquid through the channels. Among all photoresponsive materials, my interest focuses mainly on liquid crystal elastomers (LCE), liquid crystal gels (LCG) and liquid crystal networks (LCN). They exhibit fast and reversible shape changes that can be controlled by light. These materials have already been used in e.g. light-driven microrobots. In my presentation I would like to discuss ideas related to designing, testing and optimization of microfluidic active components and its integration into microfluidic setups.
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
The seminar will be held on Zoom
https://us02web.zoom.us/j/82784273907
2021-03-26 (Friday)
room 1.40, Pasteura 5 at 09:30
Alina Ciach (Institute of Physical Chemistry, PAS, Warszawa)Systems with competing interactions near a confining wall
In the first part of the talk, simulation results for the structure near the boundary, and for the adsorption isotherms will be presented and discussed for model systems with short-range attraction and long-range repulsion (SALR). In particular, we will show that in the SALR systems an anomalous decrease of the adsorption for increasing chemical potential begins when the clusters start to dominate over single particles in the bulk.
In the second part of the talk, Euler-Lagrange (E-L) equations and boundary conditions (BC) for the density profile and correlation functions near the boundary will be derived from the classical DFT. The E-L equations allow for various approximations. The approximate equations corresponding to the generic model of self-assembly can be solved easily, and analytical expressions for the density and correlation functions will be shown. The BC represents in particular the effect of missing neighbors. Because in the SALR system the long-range repulsion by the fluid particles is not compensated near the boundary, an effective attraction to a hard wall is generated. The correlation function shows an increase of periodic ordering in directions parallel to the wall when the wall is approached.
The seminar will be conducted on Zoom
https://us02web.zoom.us/j/82784273907
In the second part of the talk, Euler-Lagrange (E-L) equations and boundary conditions (BC) for the density profile and correlation functions near the boundary will be derived from the classical DFT. The E-L equations allow for various approximations. The approximate equations corresponding to the generic model of self-assembly can be solved easily, and analytical expressions for the density and correlation functions will be shown. The BC represents in particular the effect of missing neighbors. Because in the SALR system the long-range repulsion by the fluid particles is not compensated near the boundary, an effective attraction to a hard wall is generated. The correlation function shows an increase of periodic ordering in directions parallel to the wall when the wall is approached.
The seminar will be conducted on Zoom
https://us02web.zoom.us/j/82784273907
2021-03-19 (Friday)
room 1.40, Pasteura 5 at 09:30
Łukasz Gładczuk (University of Oxford & Diamond Light Source)Likelihood ratio testing with applications to MgO-based magnetic tunnel junctions experimental data
Heterostructures composed of ferromagnetic layers that are mutually interacting through a non-magnetic spacer are at the core of magnetic sensor and memory devices. Such devices can exhibit different types of coupling between magnetic layers, such as: exchange coupling and spin-current mediated coupling. We used the technique of x-ray detected ferromagnetic resonance (XFMR) to study the dynamics of a Co/MgO/Permalloy magnetic tunnel junction (MTJ). The experimental results were quantitatively compared to the Landau-Lifshitz-Gilbert-Slonczewski model where an in-depth statistical analysis based on a likelihood ratio test was employed to determine the presence of spin-current mediated coupling between the two magnetic layers
Phys. Rev. B 103 064416
The seminar will be conducted on Zoom
https://us02web.zoom.us/j/82784273907
Phys. Rev. B 103 064416
The seminar will be conducted on Zoom
https://us02web.zoom.us/j/82784273907