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Jens Elgeti (Forschungszentrum Jülich, Institute of Complex Systems)

Simulating Sperm and other Swimmers in Confined Geometries

The living world is full of microscopic organisms that swim about actively.With sizes ranging from micrometers to hundreds of micrometers they life in a world of low Reynolds-numbers, with strong consequences for the possible swimming strategies. Additionally, as a biological system, these swimmers experience a lot of noise.

We use advanced particle based simulation techniques to simulate sperm and other micro-swimmers in confined geometries.

The method of Multi Particle Collision dynamics (MPC) provides a fast an reliable tool to simulate fluctuating hydrodynamics in complex geometries. I will present the advantages and disadvantages of MPC based on examples from sperm swimming in confined geometries, generic self propelled particles and the hydrodynamic synchronization of cilia arrays.

Nikolaos Asproulis (Cranfield University, Engineering Physics)

The boundary condition problem in hybrid continuum-atomistic simulations.

Phenomena observed at micro and nanoscale are characterised by their inherent multiscale nature. Accurate numerical modelling of these phenomena is the cornerstone for enhancing the applicability of micro and nanofluidics in the industrial environment. In the talk the main strategies for applying boundary conditions in hybrid atomistic-continuum methods will be reviewed and discussed. Attention will be drawn in the problem of imposing macroscopic conditions on a molecular system and circumventing the main difficulty associated to the disparity between degrees of freedom modelled by the atomistic and continuum models.

Ioannis Zacharoudiou (Imperial College London, Chemical Engineering)

Viscous Fingering & Nematic Liquid Crystals in Confinement.

Here, we focus on two problems lying within the field of soft condensed matter: the viscous fingering or Saffman-Taylor instability and nematic liquid crystals in confinement.

Whenever a low viscosity fluid displaces a high viscosity fluid in a porous medium, for example water pushing oil out of oil reservoirs, the interface between the two fluids is rendered unstable by virtue of the Saffman-Taylor instability. Viscous fingers develop, grow and compete until a single finger spans all the way from inlet to outlet. Using a free energy lattice Boltzmann algorithm, we examine the viscous fingering instability under different wetting situations. We demonstrate that curvature effects in the third dimension, due to the wetting boundary conditions, can lead to a novel suppression of the instability. This shines new light on this iconic instability and may be of particular importance in enhanced oil recovery. Recent experiments in microchannels using colloid-polymer mixtures support our findings.

Next, we study the nematic phase of rodlike fd-virus particles confined to channels with wedge-structured walls. Using laser scanning confocal microscopy we observe a splay-to-bend transition at the single particle level as a function of the wedge opening angle. Lattice Boltzmann simulations reveal the underlying origin of the transition and its dependence on nematic elasticity and wedge geometry. Our combined work provides a simple method to estimate the splay-to-bend elasticity ratios of the virus and offers a way to control the position of defects through the confining boundary conditions.