Soft Matter and Complex Systems Seminar
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2019-12-06 (Friday)
room 228, IPPT PAN, Pawińskiego 5B at 09:30 
Francois Feuillebois (LIMSI-CNRS, Orsay, France)Effective viscosity of a dilute suspension between parallel slip walls
Uwaga – Seminarium odbędzie się o godz. 9:30 w Instytucie Podstawowych Problemów Techniki PAN w Warszawie przy ul. Pawińskiego 5B w sali 228 na drugim piętrze.
François FEUILLEBOIS
LIMSI-CNRS, Orsay, France
Coauthors:
Néjiba GHALYA, Antoine SELLIER, Maria L. EKIEL-JEZEWSKA
The energy cost for transporting suspensions in micro-channels may be reduced by using slipping walls. A theoretical model is presented here. Navier's(1823) slip condition is applied on the walls. We consider a suspension of identical solid spherical particles. The suspension is dilute, so that hydrodynamic interactions between particles are neglected. The ambient Poiseuille flow between the parallel slip walls has a high frequency, so that particles are uniformly distributed. The pressure drop for driving particles is derived by using Lorentz reciprocal theorem. The effective viscosity is obtained from the relationship between this pressure drop and the volume flow rate. It involves (as is classical for the viscosity of suspensions) the stresslet on a particle, that is the symmetric first moment of stresses on its surface. It also involves the quadrupole, second moment of stresses. These quantities are calculated from analytical solutions of the Stokes equations for the flow around a spherical particle near a slip wall, using the bispherical coordinates technique. Then two models are used, and validated, to describe the hydrodynamic interactions with both walls: the nearest wall and superposed walls approximations. The effective viscosity of the suspension is found to be very sensitive to the slip length on the walls. For instance for a gap width of two diameters, the contribution of particles to the effective viscosity (described by the 'intrinsic' viscosity [mu]) is divided by 3 when the slip length on walls is increased from 0 to the size of a particle radius. It is divided by 5 as compared with Einstein's (1905) result for an unbounded suspension. A handy fitting formula for [mu] is provided. Finally, outlooks for experiments are proposed.
François FEUILLEBOIS
LIMSI-CNRS, Orsay, France
Coauthors:
Néjiba GHALYA, Antoine SELLIER, Maria L. EKIEL-JEZEWSKA
The energy cost for transporting suspensions in micro-channels may be reduced by using slipping walls. A theoretical model is presented here. Navier's(1823) slip condition is applied on the walls. We consider a suspension of identical solid spherical particles. The suspension is dilute, so that hydrodynamic interactions between particles are neglected. The ambient Poiseuille flow between the parallel slip walls has a high frequency, so that particles are uniformly distributed. The pressure drop for driving particles is derived by using Lorentz reciprocal theorem. The effective viscosity is obtained from the relationship between this pressure drop and the volume flow rate. It involves (as is classical for the viscosity of suspensions) the stresslet on a particle, that is the symmetric first moment of stresses on its surface. It also involves the quadrupole, second moment of stresses. These quantities are calculated from analytical solutions of the Stokes equations for the flow around a spherical particle near a slip wall, using the bispherical coordinates technique. Then two models are used, and validated, to describe the hydrodynamic interactions with both walls: the nearest wall and superposed walls approximations. The effective viscosity of the suspension is found to be very sensitive to the slip length on the walls. For instance for a gap width of two diameters, the contribution of particles to the effective viscosity (described by the 'intrinsic' viscosity [mu]) is divided by 3 when the slip length on walls is increased from 0 to the size of a particle radius. It is divided by 5 as compared with Einstein's (1905) result for an unbounded suspension. A handy fitting formula for [mu] is provided. Finally, outlooks for experiments are proposed.
Please note that the Seminar will take place at the Institute of Fundamental Technological Research, Pawinskiego St. 5b, room 228,second floor.
François FEUILLEBOIS
LIMSI-CNRS, Orsay, France
Coauthors:
Néjiba GHALYA, Antoine SELLIER, Maria L. EKIEL-JEZEWSKA
The energy cost for transporting suspensions in micro-channels may be reduced by using slipping walls. A theoretical model is presented here. Navier's(1823) slip condition is applied on the walls. We consider a suspension of identical solid spherical particles. The suspension is dilute, so that hydrodynamic interactions between particles are neglected. The ambient Poiseuille flow between the parallel slip walls has a high frequency, so that particles are uniformly distributed. The pressure drop for driving particles is derived by using Lorentz reciprocal theorem. The effective viscosity is obtained from the relationship between this pressure drop and the volume flow rate. It involves (as is classical for the viscosity of suspensions) the stresslet on a particle, that is the symmetric first moment of stresses on its surface. It also involves the quadrupole, second moment of stresses. These quantities are calculated from analytical solutions of the Stokes equations for the flow around a spherical particle near a slip wall, using the bispherical coordinates technique. Then two models are used, and validated, to describe the hydrodynamic interactions with both walls: the nearest wall and superposed walls approximations. The effective viscosity of the suspension is found to be very sensitive to the slip length on the walls. For instance for a gap width of two diameters, the contribution of particles to the effective viscosity (described by the 'intrinsic' viscosity [mu]) is divided by 3 when the slip length on walls is increased from 0 to the size of a particle radius. It is divided by 5 as compared with Einstein's (1905) result for an unbounded suspension. A handy fitting formula for [mu] is provided. Finally, outlooks for experiments are proposed.
2019-11-29 (Friday)
room 1.40, Pasteura 5 at 09:30
Michał Bogdan (University of Cambridge)Fingering instabilities in tissue invasion: an active fluid model
Metastatic tumours often invade healthy neighbouring tissues by forming multicellular finger-like protrusions emerging from the cancer mass. To understand the mechanical context behind this phenomenon, we here develop a minimalist fluid model of a self-propelled, growing biological tissue. The theory involves only four mechanical parameters and remains analytically trackable in various settings. As an application of the model, we study the evolution of a two-dimensional circular droplet made of our active and expanding fluid, and embedded in a passive non-growing tissue. This system could be used to model the evolution of a carcinoma in an epithelial layer. We find that our description can explain the propensity of tumour tissues to fingering instabilities, as conditioned by the magnitude of active traction and the growth kinetics. We are also able to derive predictions for the tumour size at the onset of metastasis, and for the number of subsequent invasive fingers. Our active fluid model may help describe a wider range of biological processes, including wound healing and developmental patterning.
2019-11-22 (Friday)
room 1.40, Pasteura 5 at 09:30
Mateusz Wiliński (Los Alamos National Laboratory)Scalable learning of Independent Cascade model from partial observations
Modelling spreading processes and diffusion on networks is one of the most popular problems approached by researchers dealing with complex systems. The reason for that may be a growing number of phenomenon, which can be described with such models. Epidemics, fake news or cascading failures in power grids, to name only a few. In general, using these models in empirical setting is difficult because the spreading or transmission probabilities are not known. One way to estimate them is to use actual cascades and reverse engineer their values by maximizing their likelihood. Unfortunately, in reality we are often able to observe only a fraction of the network, which makes this task computationally inefficient. We propose a novel efficient algorithm as a solution to this problem. Our approach is based on dynamic message passing and it allows for scalable computations, suited for large real-world networks. We present application of our method to the Independent Cascade model, but it can easily be generalized to other models.
2019-11-15 (Friday)
room 1.40, Pasteura 5 at 09:30
Rafał Ołdziejewski (CFT PAN)Local vs. non-local: battles over quantum dipolar systems in one dimension
In modern experiments with ultracold atoms, systems with a precise number of particles, finely tuneable interactions, and confined in any geometry are studied routinely. New twist to the field has been recently introduced by the long-range dipole-dipole interactions between atoms with a high magnetic moment. In particular, a celebrated roton, best known from the superfluid helium, and exotic quantum droplets can be found in such systems in lower dimensions. I will briefly discuss these intriguing quantum states emerging due to an interplay between short-range and long-range interactions.
2019-10-25 (Friday)
room 1.40, Pasteura 5 at 09:30
Prof. Alina Ciach (Institute of Physical Chemistry, PAS)Density Functional Theory for Systems with Competing Interactions
Density functional theory (DFT) for systems with competinginteractions leading to self-assembly into clusters, networks orlayers is constructed. The contribution to the grand thermodynamicpotential associated with mesoscopic fluctuations is explicitly takeninto account. The expression for this contribution is obtained by themethods known from the Brazovskii field theory. In addition, wedevelop a simplified version of the theory valid for weakly orderedphases, i.e. for the high -T part of the phase diagram. The simplifiedtheory is verified by a comparison with the results of simulations fora particular version of the short-range attraction long-rangerepulsion (SALR) interaction potential. The physical interpretation ofthe fluctuation-contribution to the grand potential is discussed.
2019-10-18 (Friday)
room 1.40, Pasteura 5 at 09:30
Miłosz Panfil (IFT UW)1d quantum liquids and generalised hydrodynamics, part II
The generalised hydrodynamics is a new approach to study strongly correlated quantum matter. It’s an effective theory that captures the low-energy dynamics and extends (generalises) the standard hydrodynamics in the presence of a large number of local conservation laws. It provides insights into equilibration and thermalisation at the quantum level. The presentation will start with a gentle introduction to this theory. In the second part, I will discuss our recent results that on one hand explore the dynamics predicted by GHD and on the other provide some microscopic validations of the theory. Generalised hydrodynamics is an excellent example of how to construct an effective macroscopic theory based on microscopic intuitions.
2019-10-11 (Friday)
room 1.40, Pasteura 5 at 09:30
Adolfo Poma (IPPT PAN)Study of the large-scale conformational changes in proteins by Martini force field
The application of coarse-grained (CG) models in biophysics is essentially important due to the large length and time scales involved in the description of several biological processes. The versatile Martini force field allows the CG representation of proteins, sugars and lipids and hence it has been employed in biophysical simulations. The Martini protein model is based on the well-known elastic network (EN) model which maintains the secondary structure of the protein. Such model is inneficient for the sampling of large conformational changes in proteins due to the nature of unbreakable harmonic bonds associated with EN model. To overcome such limitation we have introduced a new description based on a different structure-based CG model. Our model shows a very good agreement with all-atom MD and former Martini protein simulations. Moreover, we show how our model captures the large-scale motion related to the catalytic activity in Man5B protein complex and it results in a smaller number of interactions compared to former Martini protein model description.
2019-10-04 (Friday)
room 1.40, Pasteura 5 at 09:30
Miłosz Panfil (IFT UW)1d quantum liquids and generalised hydrodynamics
The generalised hydrodynamics is a new approach to study strongly correlated quantum matter. It’s an effective theory that captures the low-energy dynamics and extends (generalises) the standard hydrodynamics in the presence of a large number of local conservation laws. It provides insights into equilibration and thermalisation at the quantum level. The presentation will start with a gentle introduction to this theory. In the second part, I will discuss our recent results that on one hand explore the dynamics predicted by GHD and on the other provide some microscopic validations of the theory. Generalised hydrodynamics is an excellent example of how to construct an effective macroscopic theory based on microscopic intuitions.