*Chair –*Jens Harting

*Anisotropic assembly structures from isotropic colloidal building blocks*

The simplest available colloidal particles are silica or polymer spheres which can be synthesized with great precision and low polydispersity, a prerequisite for the controlled structure formation. However, the isotropic nature of a sphere strongly limits the available structural motifs: from everyday observations, we intuitively know that spheres will generally crystallize in the closest possible packing which, in the simplest case of a packing in two dimensions, forms a monolayer with hexagonal symmetry. The same behavior is commonly observed at the nanoscale where hexagonal symmetry prevails in self-assembled colloidal monolayers composed of spherical colloidal particles.

Here, we demonstrate that the combination of rigid colloidal particles with soft components such as microgels at a liquid interface lead to a surprisingly rich phase diagram of observable structures, including phases with rhomboedric symmetry and chain-like arrangements. We discuss the observed phase diagram in the context of a soft-repulsion potential of the interacting colloidal particles.

*Enhanced diffusion of rotating colloids in non-equilibrium*

Here we employ computer simulations to study a system of colloidal rods and spheres under a strong external field that causes the particles to rotate. The action of the external field is to align the particles to a preferred direction which rotates with a center frequency. We find that the frequency strongly affects the non-equilibrium phase behavior of the system at high field strength leading to a rich non-equilibrium state diagram [1].

We also consider the effect of the field on translational diffusion. In this part, we used the fluctuating Lattice-Boltzmann technique to model full hydrodynamic interactions between the particles. Surprisingly, the effect of hydrodynamics is to strongly enhance the diffusion at low densities. Effectively, the particles act as stirring bars for the system. We qualitatively explain the dependence of the diffusion on the frequency using the analytically known behavior of the single-particle system under this field.

References.

[1] E. Fischermeier, M. Marechal, and K. Mecke, J. Chem. Phys. 141, 194903 (2014).*Solid Capillarity*

*Margination dynamics of drug delivery agents in blood flow*

We then propose a novel mechanism dubbed ultrasound-triggered margination by which oscillating microbubbles can be deliberately pushed to the walls of the blood vessel. This property makes microbubbles the ideal drug delivery agent: flowing far away from the biochemically active walls during transport and directly interacting with the endothelium at the target organ.

**Coffee Break**

*Chair –*Jacco Snoeijer

*Hydrodynamic Swarming of Thermally Active Microdimers*

Thermophoresis refers to the drift motion experienced by particles immersed in a fluid with an intrinsic gradient of temperature.

In thermophoretic swimmers a local temperature gradient is generated due to the existence of a material that can quickly absorb heat from a heating source such as irradiating light. An asymmetric distribution of the heated material on the particle surface translates into a persistent particle motion [1]. The motion direction is determined by the thermophoretic character of the particle, whether thermophobic or thermophilic [2]. Experimentally, these particles have been synthesized as Janus spherical particles being half coated with gold [1] .

Other shapes such as dimer swimmers have already been investigated resulting in a significantly different hydrodynamic behaviour [3,4]. We employ a well-established mesoscale simulation technique that couples multi-particle collision dynamics (MPC) as a coarse-grained description of the fluid with molecular dynamics for an adequate resolution of fluid-colloid interactions [5]. This methodology offers an efficient inclusion of hydrodynamics, thermal fluctuations, and the sustainability of temperature gradients.

We investigate dimeric Janus colloids whose hydrodynamic properties are determined by the size ratio of the beads. Both lateral hydrodynamic attraction and repulsion are possible: In combination with the phoretic effects which can also be attractive or repulsive, the interplay enables the system to exhibit a multitude of different possible behaviors. Of particular interest is the combination of phoretic repulsion with lateral hydrodynamic attraction, leading to a collective swarming behavior in which dimers propel aligned in planar-like, dynamic structures.

References.

[1] Jiang, H. R., Yoshinaga, N. and Sano, M.,Active Motion of a Janus Particle by Self-Thermophoresis in a Defocused Laser Beam, Phys. Rev. Lett. 105, 268302, 2010

[2] Lusebrink, D., Yang, M. and Ripoll, M.,

Thermophoresis of colloids by mesoscale simulations, J. Phys.: Condens. Matter 24, 284132, 2012

[3] Yang, M. and Ripoll, M., Simulations of thermophoretic nanoswimmers, Phys. Rev. E 84, 061401, 2011

[4] Yang, M., Wysocki, A., and Ripoll, M.,

Hydrodynamic simulations of self-phoretic microswimmers, 10, Soft Matter, 6208, 2014

[5] Malevanets, A. and Kapral, R., Solute molecular dynamics in a mesoscale solvent, J. Chem. Phys 16, 7260, 2000

*Concurrent coupling of atomistic and supramolecular water models*

We present adaptive resolution molecular dynamics simulations of aqueous solvents using coarse-grained molecular models that are compatible with the MARTINI force field [1,2]. The solvent molecules change their resolution back and forth between the atomistic and coarse-grained representations according to their positions in the system. The difficulties that arise from coupling to a coarse-grained model with a supramolecular mapping can be successfully circumvented by using bundled water models. We discuss the advantages and limitations of this multiscale approach on several examples, e.g., coupling of atomistic water with polarizable [3] and non-polarizable [4] coarse-grained water models. Next, to overcome the limitations of the bundled water models we intruduce a dynamic clustering algorithm SWINGER [5] that can concurrently make, break, and remake water clusters, consisting of several water molecules. It allows for a seamless coupling between standard atomistic and supramolecular water models in adaptive resolution simulations. SWINGER paves the way for efficient multiscale simulations of biomolecular systems without compromising the accuracy of atomistic water models.

References.

[1] Zavadlav, J., Melo, M. N., Marrink, S. J., Praprotnik, M., Adaptive resolution simulation of an atomistic protein in MARTINI water, J. Chem. Phys. 140, 054114, 2014.[2] Zavadlav, J., Podgornik, R., Melo, M. N., Marrink, S. J., Praprotnik, M., Adaptive resolution simulation of an atomistic DNA molecule in MARTINI salt solution, Eur. Phys. J. Special Topics 225, 1595-1607, 2016.

[3] Zavadlav, J., Melo, M. N., Marrink, S. J., Praprotnik, M., Adaptive resolution simulation of polarizable supramolecular coarse-grained water models, J. Chem. Phys. 142, 244118, 2015.

[4] Zavadlav, J., Melo, M. N., Cunha, A. V., de Vries, A. H., Marrink, S. J., Praprotnik, M., Adaptive resolution simulation of MARTINI solvents, J. Chem. Theory Comput. 10, 2591-2598, 2014.

[5] Zavadlav, J., Marrink, S. J., Praprotnik, M., Adaptive resolution simulation of supramolecular water: The concurrent making, breaking, and remaking of water bundles, J. Chem. Theory Comput. 12, 4138-4145, 2016.

*Particle trajectory entanglement in confined flows*

*hydroclusters*[1] only bonded by hydrodynamic interactions. While classical studies on non-Brownian self-diffusivity [2] report average particle displacements of fractions of the particle diameter, the trajectories observed in our system involved displacements of several particle diameters. Trajectory statistics are obtained experimentally for different shear rates and particle relative sizes. The results are compared with

*particle dynamics simulations*and analyzed to investigate the nature of the hydrodynamic interactions entering into play [3]. The emerging complexity in such a simple system could be applied to promote advective mixing in micro-channels [4] or self-assembly of droplets and/or solid particles in motion.

References.

[1] Wagner, N. & Brady, J. F. Shear thickening in colloidal dispersions, Physics Today (October 2009).[2] Leighton, D. & Acrivos, A. Measurement of shear-induced self-diffusion in concentrated suspensions of spheres, J. Fluid Mech. 177, 109-131, 1987.

[3] Batchelor, G. K. & Green, J. T. The hydrodynamic interaction of two small freely-moving spheres

in a linear flow field. J. Fluid Mech. 56, 375-400, 1972.

[4] Souzy, M., Yin, X., Villermaux, E., Abid, C. & Metzger, B. Super-diffusion in sheared suspensions.

Phys. Fluids 27, 041705(7), 2015.

*Migration and swimming of soft-particles in (time-dependent) Stokes flows*

*Chair –*Andrea Scagliarini

*Brownian motion of colloidal particles with arbitrary shapes*

The characteristic time scales of the translational and rotational Brownian diffusion for nanoparticles are typically much smaller than time resolution of the experiments. In this case, nanoparticles can be treated as point-like, and described by the standard Brownian theory. However, formicroparticles, the characteristic Brownian time scalesare of the order of seconds, and therefore non-negligible in comparison to the typical time scales of the measured Brownian motion. For microparticles of complex shapes, a more general theoretical approach is needed.

The exact analytical expressions for the time-dependent cross-correlations of the translational and rotational Brownian displacements of a particle with arbitrary shape have been recently derived [3,4] , and it has been demonstated how to benefit from these results while analyzing experimental data [5].

References.

[1] Cichocki B., Ekiel-Jezewska M. L., Wajnryb E., Communication: Translational Brownian motion for particles of arbitrary shape, J. Chem. Phys., 136, 071102, 2012.[2] Adamczyk Z., Cichocki B., Ekiel-Jezewska M. L., Slowicka A. M., Wajnryb E., Wasilewska M., Fibrinogen conformations and charge in electrolyte solutions derived from DLS and dynamic viscosity measurements, J. Colloid Interface Sci., 385, 244, 2012.

[3] Cichocki B., Ekiel-Jezewska M. L., Wajnryb E., Brownian motion of a particle with arbitrary shape, J. Chem. Phys. 142, 214902, 2015.

[4] Cichocki B., Ekiel-Jezewska M. L., Wajnryb E., Note: Brownian motion of colloidal particles of arbitrary shape, J. Chem. Phys., 144, 076101, 2016.

[5] Cichocki B., Ekiel-Jezewska M. L., Wajnryb E., Translational and rotational Brownian displacements of colloidal particles of complex shapes, under review in Archives of Mechanics.

*Melting driven by turbulent thermal convection*

A key question related to this process is the prediction of the evolution of the melting-rate which is connected to the heat-flux dynamics at the liquid-solid interface. In order to shed light on this process and in particular on its scaling laws, we study here the dynamics of a model system.

Our setup is a fluid layer heated from below and in contact with a solid-to-liquid melting interface on the top-side. Similar to the Rayleigh-Bénard (RB) system, for sufficiently large vertical temperature gaps a convective instability develops. The resulting flow exhibits a rich dynamics as the Rayleigh number is increased. In the present case however, the top lead melts at the pace of the local heat flux across the fluid layer, the resulting shape of the lead may in turn modify the organization of flow structures with a feedback on the heat transport. One may wonder if this coupled process enhances or rather reduces the heat flux and the mixing in the system as compared to the RB case.

We investigate the described model system by means of numerical tools. In particular, we perform Direct Numerical Simulations built on a enthalpy based Lattice Boltzmann algorithm to address the high Rayleigh number regime both in two- and three-dimensional setups. We focus on the scaling of global quantities, Nusselt and Reynolds numbers, and on the characterization of the geometry of the melting interface.

*The many faces of the bead-spring microswimmer*

**Coffee Break**

*Chair –*Ana-Sunčana Smith

*Emergent Hydrodynamic Bound States Between Magnetically Powered Active Micropropellers*

References.

[1] F. Martinez-Pedrero, A. Cebers, P. Tierno, Soft Matter (2016), 12, 3657.[2] F. Martinez-Pedrero, A. Ortiz-Ambriz, I. Pagonabarraga, P. Tierno Phys. Rev. Lett. (2015) 115, 138301.

*Light-driven actuation of interfacial swimmers*

References.

[1] A. Girot, N. Danné, A. Würger, T. Bickel, F. Ren, J.-C. Loudet, and B. Pouligny, Langmuir 32, 2687 (2016)[2] N. Kavokine, M. Anyfantakis, M. Morel, S. Rudiuk, T. Bickel, and D. Baigl, Angew. Chem. Intl. Ed. 55, 11183 (2016)

*Cilia-based transport networks*

Cerebrospinal fluid conveys many physiologically important signaling factors through the ventricular cavities of the brain. We investigated the transport of cerebrospinal fluid in the third ventricle of the mouse brain and discovered a highly organized pattern of cilia modules, which collectively give rise to a network of fluid flows that allows for precise transport within this ventricle. Our work suggests that ciliated epithelia can generate and maintain complex, spatiotemporally regulated flow networks. I shall also show results on how to assemble artificial cilia that I call synthoneme.

**Lunch**

*Chair –*Álvaro Marín

*Spatio-temporal dynamics and control of confined colloidal suspensions under shear*

In the present talk I will focus on suspensions of interacting spherical colloids under the combined influence of shear flow and restricted geometry. We have analyzed such systems by many-particle computer simulations and effective single-particles models. By varying the externally applied shear rate (“open-loop” control) these colloidal films display a sequence of states characterized

by pinning, shear-induced melting, laning, and moving crystalline order with synchronized oscillations of the particles. We also discuss the appearance of moving local density heterogeneities (kinks and antikinks) and relations to the behavior observed in driven colloidal monolayers. Finally, by adding an additional feedback equation of motion we are able to stabilise specific properties such as the degree of hexagonal ordering or the shear stress. This opens the route for a deliberate control of friction properties.

*Capillary forces on colloids at fluid interfaces*

Colloidal particles, which are trapped at fluid interfaces, deform them. This gives rise to effective lateral interactions among such colloids. Under favorable conditions this capillary interaction resembles two-dimensional gravity or electrostatics. Accordingly, these systems can be described in terms of permanent or induced capillary multipoles. This provides a simple interpretation of numerous experimental observations.

In a further analogy, the presence of thermally excited capillary waves generates Casimir-like forces between the colloidal particles, but with a larger variety of possible boundary conditions as compared with the case of the standard Casimir force.

The capillary attraction among the colloids drives their collective dynamics. This dynamics exhibits an instability which is formally analogous to the gravitational instability on cosmological scales. Corresponding analytic and simulation studies are discussed.

*Non-Equilibrium Stratification of Colloids by Size during an Evaporation Process*

References.

[1] Fortini, A. et al. “Dynamic Stratification in Drying Films of Colloidal Mixtures,” Phys Rev Lett (2016) 116, 118301[2] Martin-Fabiani, I. et al. “pH-Switchable Stratification of Colloidal Coatings: Surfaces ‘On Demand’,”ACS Appl. Mater. Interfaces (2016) 8, 34755.

** Coffee Break**

*Chair –*Siegfried Dietrich

*Molecular Simulations of the Interfacial structure of complex fluid interfaces*

*Non-equilibrium dynamics of colloidal particles at interfaces*

Experiments with suspensions of ellipsoidal colloidal particles suggest a transition in the statistical properties of the stain left by an evaporating drop, depending on the eccentricity of the particles [1]. We proposed a stochastic model to show that the very-strong anisotropic capillary attraction between particles stemming from the deformation of the interface can be responsible for such transition [2,3]. We will discuss the main mechanisms involved and compare the quantitative results with experiments.

With the experimental groups of Erika Eiser (Univ. Cambridge) and Jasna Bruijc (New York University), we have shown that the long-range capillary attraction and consequent formation of kinetically trapped structures of colloidal particles at interfaces can be avoided using DNA-coated colloids on complementary functionalized interfaces (oil droplet) [4], where we keep the irreversible interfacial binding but suppress the strong attraction, resulting in a fully ergodic colloidal dynamics. We will discuss how the coverage of the oil droplet by colloidal particles and the self-assembled structures depend on different system parameters, such as temperature and bulk concentration of colloidal particles.

References.

[1] P. J. Yunker et al. Physical Review Letters 110, 035501 (2013).[2] C. S. Dias et al. in preparation.

[3] C. S. Dias, N. A. M. Araújo, M. M. Telo da Gama, EPL 107, 56002 (2014).

[4] D. Joshi et al. Science Advances 2, e1600881 (2016).

*Influence of polymer-plasticizers interactions on the flexibility of film coatings*

We propose and compare different multi-scale methods to predict the compatibility between plasticizers and film formers. The methods are based on, i) Solubility parameter calculation using molecular dynamics, ii) Mesoscale simulation using DPD, where we propose a coarse-grain model, and iii) Experimental DSC analysis for validation. The methods are tested for various polymer-plasticizer blends including HPMC-PEG, MCC-PEG and PVP-PEG.

The different methods showed similar results; PEG plasticizer diffuses inside HPMC and PVP polymer chains, thereby extending and softening the composite polymer. However, MCC surrounds PEG molecules without diffusing in its network, indicating low PEG-MCC compatibility. We also found that DPD simulations offer more details than the other methods on the miscibility between the compounds in aqueous solid dispersion, and can predict the amount of plasticizer that diffuses in the film forming polymer network.

*Chair –*Pietro Tierno

*Clustering of red blood cells: A model system for soft deformable colloids*

*Interface-resolved simulations of particle-laden channel flow*

*Dynamics of rigid and soft red blood cells in deterministic lateral displacement devices with non-circular pillars*

**Coffee Break**

*Chair –*Peter Schall

*Polymer flow and polymer topology: linear chains, rings and knots flow differently*

Modifications of the topological state of polymers are extremely interesting and relevant operations for a vast domain of scientific inquiry ranging from knot theory and polymer science all the way to materials science and biophysics, where cyclic and knotted DNA plays a key role in biological processes. Recent work has demonstrated that joining the two ends of a linear chain to form a cyclic (ring) polymer has a number of significant consequences in the structural [1,2], and rheological [3] properties of concentrated or semidilute solutions of the same. Accordingly, a number of questions arise regarding the behavior of linear, cyclic and knotted ring polymers under flow: how does the topology of the dissolved polymer affect its orientational resistance, as well as its rotation-, tumbling- or tank-treading motion under Couette flow? What consequences does shear flow have for knot localization along a sheared polymer? Can one make use of the different flow properties of various polymer topologies to build microfluidic devices that act as filters/separators of topologically different polymers? By applying hybrid (MPCD/MD) simulation techniques that take into account the hydrodynamics, we address the questions above for polymers of varying topologies, knotedness and stiffness and we analyze quantitatively the influence of polymer topology on single-polymer properties under flow[4]. Polymer properties under Poiseuille flow will also be analyzed and on this basis concrete suggestions for the construction of topology-separating microfluidic devices will be presented [5].

References.

[1] M. Z. Slimani, P. Bacova, M. Bernabei, A. Narros, C. N. Likos, and A. J. Moreno, ACS Macro Letters 3, 611 (2014).[2] P. Poier, S. A. Egorov, C. N. Likos and R. Blaak, Soft Matter 12, 7983 (2016).

[3] M. Kapnistos, M. Lang, D. Vlassopoulos, W. Pyckhout-Hintzen, D. Richter,

D. Cho, T. Chang, and M. Rubinstein, Nature Materials 7, 997 (2008).

[4] M. Liebetreu and C. N. Likos, in preparation (2017).

[5] L. Weiß, A. Nikoubashman, and C. N. Likos, in preparation (2017).

*Dynamics of semiflexible filaments in shear flow*

Propulsion of micro-organism is induced by the motion of cilia or flagella whose motion in the surrounding flow generates the propulsive forces. At these small scales, the fluid dynamics is dominated by viscous forces, a regime quantified by low Reynolds numbers. Fluid-structure interaction describes the non trivial coupling between deformation and/or transport of a deformable object and the flow. During the past years we studied different experimental model situations where the properties of both the objects and the flow are well-controlled. One example is the case of semi-flexible polymer in simple shear flow. A rigid fiber transported in a such a flow will perform Jeffery orbits in which the fiber is mainly transported aligned with the streamlines but will experience fast tumbling due to the rotational component of the flow [1]. Much less is known, however, for the case of flexibles fibers [2]. In our experimental study the fibers are actin filaments, a biopolymer which plays a fundamental role in cell mechanics. Actin filaments can be polymerized, fluorescently labelled and stabilized into semi-flexible micron-long filaments that can be followed when flowing into a micro-channel onto a microscope.

The filament experiences different modes of deformations depending on the length and the shear rate. The transition between no deformation (case A, in the figure) and buckling (case B) is an instability governs by the elasto-viscous number which compares elastic and viscous effects. This transition has been predicted to be responsible of the appearance of normal stresses in a suspension of flexible fibers [3] but no experimental study has confirmed this prediction. The present study can be considered as a first step in this direction.

References.

[1] Jeffery, G. B. The Motion of Ellipsoidal Particles Immersed in a Viscous Fluid. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 102(715), 161-179, 1922.[2] Harasim, M., Wunderlich, B., Peleg, O., Kro, M. Direct Observation of the Dynamics of Semiflexible Polymers in Shear Flow. Physical Review Letters, 110 10830, 2013.

[3] Becker, L. E., Shelley, M. J. Instability of Elastic Filaments in Shear Flow Yields First-Normal-Stress Differences. Physical Review Letters, 87, 198301, 2001.

*Fluid-Solid interactions at interfaces for atoms and particles*

1) the transition from a solid to a fluid (e.g. relevant for the release of avalanches and landslides, or the clogging of a nano- or micro-channel) as well as the transition from a fluid to a solid-like behavior, related to jamming and shear-thickening in various materials like suspensions or soft and granular matter.

2) the interaction of the fluid with the solid boundaries, which leads to short-ranged ordering, layering or cristallization, e.g. a wall-induced microstructure that renders the fluid behaving much differently from a bulk-fluid due to the existence of this fabric/structure on the atomistic or particle scale.

**Lunch**

*Chair –*Othmane Aouane

*Self-assembled surface swimmers and micromanipulators*

Soft magnetic particles floating on a liquid can spontaneously assemble into ordered structures [1]. This process is controlled through the amplitude and orientation of an external magnetic induction field. Complex behaviors can arise under a time-dependent magnetic field [2]. In particular, assemblies of three particles or more can undergo deformations in non-time-reversible sequences, a necessary condition for low Reynolds number locomotion [3]. Such microswimmers can follow precisely controlled trajectories [4], as depicted in the figure. As a consequence, these self-assembled structures can be used as microrobots to perform different tasks, such as the capture, transport and release of a microcargo, the mixing of fluids at low Reynolds number, and more. This latter point is the main focus of this communication.

References.

[1] Vandewalle, N. et al., Symmetry breaking in a few-body system with magnetocapillary interactions, Phys. Rev. E 85, 041402 (2012).[2] Lagubeau, G. et al., Statics and dynamics of magnetocapillary bonds, Phys. Rev. E 93, 053117 (2016).

[3] Grosjean, G. et al., Realization of the Najafi-Golestanian microswimmer, Phys. Rev. E 94, 021101(R) (2016).

[4] Grosjean, G. et al., Remote control of self-assembled microswimmers, Sci. Rep. 5, 16035 (2015).

*Self-assembled magnetocapillary swimmers*

Magnetic fields can be used to avoid the contact of micrometric magnetic particles at an air-water interface [1]. Indeed, the induced dipole-dipole interaction can balance the capillary attraction, the so-called “Cheerios effect”. Using additional oscillating magnetic fields, it has been shown that clusters of such particles can spontaneously swim along the interface via hydrodynamic coupling [2] while in the low Reynolds approximation.

In this presentation, we discuss the criterion required to observe swimming dynamics, the “Scallop theorem”, and its experimental fulfilment. This discussion will be centred on the recent realization of the paradigmatic “Najafi-Golestanian swimmer” (see Figure ) with magneto-capillary structures [3]. By tuning the natural frequency of the magneto-capillary bonds linking the particles, we show that non-reciprocal deformations occur, therefore leading to locomotion. The efficiency of this swimmer is studied as a function of the different experimental parameter as well as its minimization to smaller scales. Finally, thanks to the insight provided by this model, we consider bi-dimensional swimmers. In particular, we discuss the case of triangular assemblies of particles and their swimming dynamics along the interface.

References.

[1] Vandewalle N., Obara N., Lumay G., Mesoscale structures from magnetocapillary self-assembly, Eur. Phys. J. E 36, 127, 2013.[2] Grosjean G., Lagubeau G., Hubert M, Vandewalle N., Remote control of

self-assembled magnetocapillary microswimmers, Sci. Report 5, 16035, 2015.

[3] Grosjean G., Hubert M, Lagubeau G., Vandewalle N., Realization of the Najafi-

Golestanian microswimmer, Phys. Rev. E 94, 021101, 2016.

*Dynamics of magnetocapillary microswimmers: Lattice Boltzmann simulations*

Stable two-dimensional arrangements of three or more ferromagnetic particles placed at a fluid-gas interface – termed magnetocapillary microswimmers – are the focus of the intensive theoretical [1] and experimental research nowadays [2]. The arrangemets are achieved via an interplay of attractive capillary and repulsive magnetic dipole-dipole interactions. Application of a set of a static and a time-dependent magnetic field allows to manipulate the position and the velocity direction of the swimmer, which makes the swimmers potentially interesting for various applications ranging from a controlled cleaning of liquid interfaces to even a drug delivery or other medical applications in vivo.

By means of a hybrid lattice Boltzmann and discrete element method we demonstarte a dependence of the average speed of the swimmer on the frequency of the time-dependent magnetic field for various parameters like surface tension, number of particles and fields strengths. The control of the direction of the swimmer motion using B-fields is also illustrated. In addition we perform analysis the obtained results based on analytical models.

References.

[1] R. Chinomona, J. Lajeunesse, W.H. Mitchell, Y. Yao and S.E. Spagnolie, Soft Matter 11, 1828 (2015).[2] G. Lumay, N. Obara, F. Weyer and N. Vandewalle, Soft Matter 9, 2420-2425 (2013); G. Grosjean, G. Lagubeau, A. Darras, M. Hubert, G. Lumay and N. Vanderwalle, Sci. Rep. 5, 16035 (2015).

**Coffee Break**

*Chair –*Christos Likos

*Complex transport processes at fluidic interfaces - numerical simulations based on thermodynamically consistent models*

Onsager’s variational principle of minimum energy dissipation allows to derive thermodynamically consistent models describing a variety of phenomena.

In the first part of the talk, we focus on models for two-phase flow with electrolyte solutions which couple the Nernst–Planck equations to standard pde-systems for two-phase flow. Numerical simulations in two and three spatial dimensions will be presented which address field-induced droplet break-up, droplet coalescence and contact angle hysteresis.

In the second part of the talk, we present a micro-macro-model for two-phase flow of dilute polymeric solutions. Orientation and elongation of the polymer chains, which are described as dumbbells, are captured by a generalized Fokker–Planck equation which is coupled to the momentum equation via the Kramers stress tensor. Numerical simulations in two and three spatial dimensions illustrate the effect, polymer chains exert on the flow field. Moreover, they indicate that such models are capable of capturing the alignment of amphiphilic surfactants with the normal of the interface.

*Electrohydrodynamic assembly and manipulation of particles and drops*

Particle covered drops are important in emulsion technology, and can also serve as templates for producing particles and capsules [1]. Our own studies in this area show how electrohydrodynamic circulation flows in drops can assemble solid or fluid colloidal particle films on drop surfaces, and ho electric fields can be used to structure particles on drop surfaces [2,3,4],. Recently we have performed further investigations of both the rheology and dynamics of Pickering drops subjected to electric fields. This is considered in the context of previous studies which include the response of particle covered drops or bubbles exposed to shear flow [5] or other mechanical forces [6]. We find that there is critical electric field corresponding to a deformation yield point of the colloidal capsule above which a capsule deforms plastically. Below the yield point, we observe electro-orientation, weak capsule deformation and crumpled and folded states. By using electric fields to induce Quincke rotation [7] dynamics of colloidal shells, we also observe tumbling (“stiff”) rotation dynamics to tank-treading (“deformable”) dynamics, as well as propulsion of counter rotating Pickering drop pairs [9].

References.

[1] Zeng, C., Bissig, H. & Dinsmore, A. D. Particles on droplets: From fundamental physics to novel materials. Solid State Commun. 139, 547-556 (2006)[2] Rozynek, Z., Mikkelsen, A., Dommersnes, P. & Fossum, J. O. Electroformation of Janus and patchy capsules. Nature communications 5 (2014)

[3] Dommersnes, P. et al. Active structuring of colloidal armour on liquid drops. Nature Communications 4, 2066 (2013)

[4] Rozynek, Z., Dommersnes, P., Mikkelsen, A., Michels, L. & Fossum, J. O. Electrohydrodynamic controlled assembly and fracturing of thin colloidal particle films confined at drop interfaces. Eur. Phys. J. Special Topics 223, 1859-1867 (2014)

[5] Ha, J. W. & Yang, S. M. Electrohydrodynamic effects on the deformation and orientation of a liquid capsule in a linear flow. Phys Fluids 12, 1671-1684 (2000)

[6] Subramaniam, A. B., Abkarian, M., Mahadevan, L. & Stone, H. A. Colloid science: non-spherical bubbles. Nature 438, 930, (2005)

[7] T.B. Jones, Quincke Rotation of Spheres, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IA-20, 845-9, (1984)

[8] P. Dommersnes, A. Mikkelsen & J. O. Fossum, Electro-hydrodynamic propulsion of counter-rotating Pickering drop Eur. Phys. J. ST 225, 699–705 (2016)

*Mesoscopic simulations of electrokinetic phenomena: electrowetting dynamics*

Electrokinetic effects play a crucial role in many natural and technological systems, from the biological to nano-fluidics. Numerical simulations of conducting fluids presents a significant challenge mainly due to the variety of length-scales involved, and the presence of two long-range interactions: hydrodynamic and electrostatic. We show an electrohydrodynamic mesoscopic model, discuss its validity, and focus on the electrowetting effect, where a droplet changes its contact angle with a substrate when an electric field is applied. The hydrodynamics of two fluids is solved using the lattice-Boltzmann method. Ions present in the solvents are considered at the level of the Nernst-Planck equation, which is solved via a finite-volume, finite-difference discretization, following the link-flux method. We show that the simulation scheme is robust and remains valid in a wide range of parameters. Furthermore, the model is able to quantitatively capture the electrowetting effect, and allows to study in detail the mechanisms that control the deformation of oil-in-water drops.

*Chair –*Marisol Ripoll

*Inferring earthquake statistics from soft-glass dynamics below yield stress*

*Dynamics of chemically active colloids near or at interfaces*

Here we focus on the case of chemically active particle in the vicinity of, or trapped at, a liquid-fluid interface. The inhomogeneous distribution of reactant and product molecules, due to the chemical activity of the particle, at the interface can induce local variations of the surface tension. This leads to the onset of so-called Marangoni flows, which extend into the bulk phases. When such particle is in the vicinity of the interface, the Marangoni flow drives the particle close to (or far away from) the interface. This effective interaction is long ranged and may provide means to control particle accumulation at fluid-fluid interfaces [3]. We further consider the many-body dynamics of a monolayer of such spherically symmetric active particles trapped at a fluid–fluid interface. For typical experimental

conditions, we show that the induced Marangoni flow can prevent the clustering instability driven by capillary attraction if the produced chemical species tends to decrease the surface tension of the interface [4].

References.

[1] R. Golestanian et. al., New J. Phys. 9, 126 (2007)[2] W.E. Uspal et. al., Soft Matter 11, 434 (2015)

[3] A. Dominguez et. al., Phys. Rev. Lett. 116, 078301 (2016)

[4] A. Domínguez et. al., Soft Matter 12, 8398 (2016)

*Towards the simulation of dense particle systems interacting with complex media*

Particulate suspensions are ubiquitous in nature and industrial applications, and the understanding of their flow properties represents therefore a challenging technical problem. Although the dilute and semidilute rheological behaviors for suspension with a simple Newtonian matrix are well understood, when the solid concentration increases towards the maximum packing fraction several new issues arise. In dense systems (Fig.) particles under flow can get very close entering the lubrication regime [1].

From a computational perspective, reproducing correctly the lubrication interaction between two particles in a very thin separation gap is a very challenging task due to the singular character of the force. Issues of stability and accuracy in the lubrication problem has been recently solved by means of a novel and general semi-implicit splitting strategy which allows to integrate efficiently the particle equations of motion and speed-up the simulations significantly [2].

Another challenging fundamental aspect is represented by the choice of the lubrication model in the case the matrix is non-Newtonian. This is critical to extend the current simulations of dense systems towards complex suspending media. In this talk we will discuss novel models of lubrication forces in the case of dense particulate system suspended in shear-thinning media [3], pseudo-yield stress fluids [4] and possible extensions to discontinuous shear-thickening media. Some comparisons for the rheology of these systems with experimental data will be also provided.

References.

[1] Vazquez-Quesada, A., Ellero, M., Rheology and microstructure of non-colloidal suspensions under shear studied with Smoothed Particle Hydrodynamics, J. Non-Newt. Fluid Mech 233, 34-47, 2016.[2] Bian, X., Ellero, M., A splitting integration scheme for the SPH simulation of concentrated particle suspensions, Computer Physics Communications 185, 53-62, 2014.

[3] Vazquez-Quesada, A., Tanner, R.I., Ellero, M., Shear Thinning of Noncolloidal Suspensions,Phys. Rev. Lett. 117, 108001, 2016.

[4] Vazquez-Quesada, A., Ellero, M., Analytical solution for the lubrication force between two spheres in a bi-viscous fluid, Phys. Fluids 28, 073101, 2016.

**Coffee Break**

*Chair –*Stefan Luding

*Interfaces and fracture in cohesive colloidal systems*

We also assemble colloidal solids by increasing the critical Casimir force, and study fracture of these cohesive colloidal materials. The advantage is that in these weak colloidal solids, fracture speeds are many orders of magnitude slower than in regular solids, allowing us to follow the fracture process in great detail on the single particle level. We can follow single bond breaking processes as well as study the influence of the attractive interaction strength on plastic processes preceding fracture.

*Bubble-bubble interactions in 2D, just above the jamming transition*

Two-dimensional wet foams or weekly compressed emulsions

are often modelled as circular disks whose overlap is resisted by a force that varies linearly with distance. Despite the insight that this so-called

bubble model [1] has provided for foam rheology (when augmented by a viscous dissipation term)[2,3], it is unrealistic in various respects. In particular Morse and Witten [4] have shown that drops/bubbles just above the jamming transition do not obey simple pairwise force laws. On the other hand, numerical simulations based on balancing surface tension and pressure force (2D software PLAT) or minimising interface energy (Surface Evolver), which work well for dry foams, have limited success for wet foams.

Here we present the development of Morse and Witten’s approach to relative forces and positions for wet foams in a scheme analogous to that of Hohler and Cohen-Addad in 3D [5]. We hope to clarify the discrepancy that has recently emerged in the variation of average contact number of bubbles with liquid fraction, namely a square root variation in the bubble model, and a linear relation in PLAT simulations which consider adjustments of bubble shapes [6]. We also comment on the possibilities for developing a meaningful dynamical model based on the Morse-Witten formalism.

The Morse-Witten force law deserves to take its place as one of the canonical laws of soft matter, but has hitherto been poorly appreciated, largely on the account of the difficulty of interpreting the original paper. Our re-formulation of the theory, initially in 2d, should offer clarification.

References.

[1] Durian, D.J., Foam mechanics at the bubble scale, Phys. Rev.Lett. 75, 4780-4783, 1995. [2] Durian, D.J., Bubble-scale model of foam

mechanics: Melting, nonlinear behavior, and avalanches, Phys. Rev.

E 55, 1739-1751, 1997. [3] Langlois, V.J., Hutzler, S. and Weaire, D., Rheological properties of the

soft-disk model of two-dimensional foams, Phys. Rev. E 78

021401, 2008. [4] Morse, D.C. and T.A. Witten, Droplet elasticity

in weakly compressed emulsions, Europhys. Lett. 22, 549-555, 1993. [5] Hohler R. and Cohen-Addad S., Many-body

interactions in soft jammed materials, Soft Matter, (submitted, 2016). [6] Winkelmann, J., Dunne, FF., Langlois, V.J.,

Mobius,M.E., Weaire,D. and Hutzler,S.

2D foams above the jamming transition: Deformation matters,

Colloids Surf., A, (submitted, 2016).

*Flow and mass transfer around a core-shell reservoir*

**Coffee Break / Small Lunch on Site**

*Chair –*Federico Toschi

*Confinement affects colloid assembly in emulsion droplets*

Complex structures can be achieved by varying composition, interaction, or shape of the constituent colloidal particles.

Another method to modify the crystallization process is to utilize interfacial effects resulting from the introduction of confinement.

Recently it has been shown that entropy favors icosahedral symmetry for colloids assembling in spherical confinement.

In this joint experimental-theoretical work we use droplet-based microfluidics to create homogeneous emulsion droplets as sources for defined spherical confinement.

This allows to systematically explore the assembly behavior of clusters containing between 100 and 10000 near-monodisperse colloidal spheres.

We observe a discrete series of colloid clusters with icosahedral symmetry.

To understand and explain the formation of the clusters, we propose a geometric model and extract extremal principles.

*Amoeboid swimming in confined geometry*

*Dynamics of thin films and droplets of simple and polymeric liquids: lubrication theory and mesoscopic simulations*

character can have highly non-trivial implications [5].

When the film thickness reaches the nanometric scales, the validity of a fully hydrodynamic description breaks down: thermal fluctuations must be accounted for, whose interplay with surface tension and fluid-substrate interaction may lead to a significantly different behaviour [6]. Within the framework of fluctuating hydrodynamics [7], I will derive a stochastic lubrication equation for power-law fluids. By means of numerical solutions of such equation, I will study the stability of thin films, showing the effect of thermal noise and non-Newtonianity on the dewetting inception and long time coarsening as well as on the spectrum of capillary waves and on the growth of a nucleated hole. Finally, I will discuss a novel method based on a lattice Boltzmann model for viscocapillary shallow water equations; recent results on the dewetting of thin films and on the sliding of droplets of Newtonian and viscoelastic fluids obtained with numerical simulation of the method will be also presented.

References.

[1] P.-G. de Gennes et al., Capillarity and wetting phenomena, Springer (2004).[2] C. Redon et al, Phys. Rev. Lett. 66, 715 (1991).

[3] D. Bonn et al, Rev. Mod. Phys. 81, 739 (2009).

[4] J.H. Snoeijer and B. Andreotti, Annu. Rev. Fluid Mech. 45, 269 (2013).

[5] V. Bergeron et al, Nature 405, 772 (2000).

[6] K. Mecke and M. Rauscher, J. Phys.: Condens. Matter 17, S3515 (2005).

[7] L.D. Landau and E.M. Lifschitz, Fluid Mechanics, Pergamon Press (1987).

*Thermal lattice Boltzmann method for catalytic flows*

We present a thermal multicomponent model based on the entropic

lattice Boltzmann method (J. Kang et al., Phys. Rev. B 89, 063310 (2014)) to simulate catalytic reactions through porous media. This method reproduces the Navier-Stokes equations and allows the tracking of temperature dynamics. The viscosity, diffusivity, and heat diffusivity are calculated from the Lennard-Jones parameters of the gases, while the chemical surface reactions are incorporated in a very flexible fashion through the flux boundary conditions at the walls.

To show the strength and flexibility of this model and our implementation, we will report the catalytic turn-over for a wide range of porosities and reaction conditions.

*A direct numerical simultion approach for liquid-gas-solid flows*

Liquid-gas-solid systems are important in nature and industrial applications. Because analytic descriptions of complex flows are hardly possible, numerical simulation has become an important tool for investigations. However, only a small number of direct simulations of liquid-gas-solid flows have been reported in literature. Due to the multiple length scales typically involved in such systems, the computational complexity is considerably high.

In this contribution, a direct numerical simulation approach for liquid-gas-solid flows is presented [1]. The method allows fully resolved flow simulations involving gas bubbles and rigid particles in a containing liquid. Detailed studies of bubble-particle interaction or mixing of particle beds in the wake of rising bubbles become possible. An example is shown in figure , where particles follow the wake of a gas bubble.

References.

[1] Bogner, S., Direct Numerical Simulation of Liquid-Gas-Solid Flows Based on the Lattice Boltzmann Method, PhD Thesis, University of Erlangen-Nueremberg, 2016.*Direct Assembly of Magnetic Janus Particles at a Sessile Droplet Interface*

Self-assembly of nanoparticles at fluid interfaces is a promising route for bottom-up fabrication of novel functional materials. However, directing and controlling the particles to assemble into highly tunable and predictable structures is essential but challenging. The anisotropic particles with response to external fields are promising building blocks of reconfigurable and programmable self-assembled structures [1-3]. Here, we both numerically and theoretically investigate the behaviour of magnetic Janus particles at a spherical droplet interface interacting with an external magnetic field [4]. We use a multicomponent lattice Boltzmann method for the simulations of fluids. The particles are discretized on the lattice of the fluid solver and propagated using a molecular dynamics algorithm [5]. We show numerically that a single magnetic Janus particle moves to the location where the particle position vector (relative to the droplet center) is parallel to the direction of the applied magnetic field. When many magnetic Janus particles adsorb at the interface, the particles assemble into hexagonal arrangement. We develop a free energy model and find good qualitative agreement with simulation results. Finally, using an evaporation model recently developed in our group [6], we study the particle deposition after evaporating a Janus particle-laden droplet on a chemically patterned substrate and demonstrate that the direction of the magnetic field allows to tune the deposition. Therefore, our results provide a versatile platform for hierarchical materials assembly.

References.

[1] G. B. Davies, T. Kruger, P. V. Coveney, J. Harting and F. Bresme. Adv. Mater., 26:6715, 2014[2] Q. Xie, G. B. Davies, F. Gunther and J. Harting. Soft Matter, 11:3581, 2015

[3] Q. Xie, G. B. Davies, and J. Harting. Soft Matter, 12:6566, 2016

[4] Q. Xie, and J. Harting. In preparation, 2017

[5] F. Jansen and J. Harting. Phys. Rev. E, 83:046707, 2011

[6] D. Hessling, Q. Xie and J. Harting. J. Chem. Phys., accepted, 2017 ( co-first authorship)

*Transport of microcargos via hydrodynamic flow of magnetically driven colloidal ribbons*

*Phase diagram and critical phase transitions of driven granular matter in quasi 2d*

Driven granular matter exhibits a rich variety of nonequilibrium phases [1,2]. Recently, a critical transition to a state with quadratic order has been reported, with several critical exponents measurable [3]. We study this set-up by computer simulations, which consists of spherical particles between two horizontal plates. The particles are agitated by vibrating the plates in vertical direction. The energy injection is balanced by energy loss through inelastic collisions of the granular particles. Thus, the system reaches a steady state which exhibits phase behavior similar to equilibrium systems. The gap between the plates is about two particle diameters allowing the particles to form – besides fluid-like states – hexagonal and quadratic bilayers. We determine the relevant parameters for formation of ordered states, present a numerical phase diagram for this system, and study phase coexistence and criticality.

References.

[1] Melby, P. et al The dynamics of thin vibrated granular layers, J.Phys. Condens. Matter 17, S2689, 2005.[2] Vega Reyes, F., Urbach, J. Effect of inelasticity on the phase transitions of a thin vibrated granular layer, Phys. Rev. E 78, 051301, 2008.

[3] Castillo, G. et al Fluctuations and Criticality of a Granular Solid-Liquid-Like Phase Transition, Phys. Rev. Lett. 109, 095701, 2012.

*Multi-scale simulation of the drying process of polymeric films*

Polymeric films are found everywhere in modern life, for instance in paints and coatings.

Many still have volatile organic compounds (VOCs) inside which are required to plasticise the polymeric particles during drying to aid the formation of a homogeneous and strong film.

Governmental legislation however keeps demanding a decrease of VOCs due to the negative health aspects, while consumers are not willing to use paints with lower qualities and reduced properties that result from the complete removal of VOCs. To aid the targeted design of true VOC-free paints more fundamental understanding is needed on the different physical processes that happen in this multi-scale process.

Our work aims at obtaining more insight on the microscopic level for the interdiffusion between the polymeric particles, which plays a major role for properties like the mechanical strength of the dried coating. For this we use a molecular dynamics model where the characteristic monomers are coarse-grained into individual beads. This allows us to study the interdiffusion between two polymeric particles as a function of key variables like the glass-transition temperature.

The microscopic model is not able to describe the dynamics during the drying stages since it is limited to two or three particles. Many end characteristics of the coating are defined during the transition towards the close-packing state and the distribution of the particles. On this larger scale the polymeric nature however cannot be neglected, for example during skin-formation when the evaporation timescale dominates over the diffusive timescale. To tackle this we use a mesoscopic multi-component lattice Boltzmann method where the polymeric particles are coarse-grained as deformable capsules, as schematically visualised in figure 1. To capture the polymeric nature of the particles when they come close to one another, constitutive laws are extracted from the microscopic model and introduced between particles.

*Analytical and numerical study of microswimming using the bead-spring model*

An important class of designs for the construction of artificial microswimmers swimming either in monophase or multi-phase fluid systems is that of swimmers made of multiple beads moving in coordinated ways. If the individual bead motions can be controlled, then such a swimmer can be made to swim in desired directions.

Here we use a bead-spring microswimmer model, inspired by the Najafi-Golestanian three-sphere swimmer [1], to firstly discuss several principles of microswimming, and secondly to provide a theoretical framework for the description of many-bead swimmers swimming in muilti-phase fluids, as have recently been fabricated [2]. Our model consists of three beads of arbitrary shapes, and with possibly deformable surfaces, connected to each other with harmonic springs. A non-symmetric sinusoidal driving of the system causes the swimmer to propagate at negligible Reynolds numbers.

Solving the above system analytically by assuming small bead oscillations, we shed light on various principles of microswimming, such as why microswimmers sometimes swim faster when the fluid viscosity is increased, how swimmer elasticity can overturn the effect of the viscous drag force (for instance, making the same shapes either the most or the least optimal for motion) [3,4], under what conditions swimmer body deformability promotes or inhibits the motility, the effect of surrounding walls on swimmers, and the characteristics of large swimmer swarms [5].

Apart from their importance in understanding biologcal swimmer systems, our results serve to lay the ground for the theoretical description of complicated many-bead swimmers swimming in various fluid environments.

References.

[1] Najafi, A., Golestanian, R., Simple swimmer at low Reynolds number: Three linked spheres, Physical Review E 69, 062901, 2004.[2] Lumay, G., Obara, N., Weyer, F., Vandewalle, N., Self-assembled magnetocapillary swimmers, Soft Matter 9, 2420, 2013.

[3] Pickl, K., Gotz, J., Iglberger, K., Pande, J., Mecke, K., Smith, A.-S., Rude, U., All good things come in threes–Three beads learn to swim with lattice Boltzmann and a rigid body solver, Journal of Computational Science 3, 374, 2012.

[4] Pande, J., Smith, A.-S., Forces and shapes as determinants of micro-swimming: effect on synchronisation and the utilisation of drag, Soft Matter 11, 2364, 2015.

[5] Pickl, K., Pande, J., K ostler, H., R ude, U., Smith, A.-S., Lattice Boltzmann simulations of the bead-spring microswimmer with a responsive stroke–from an individual to swarms, Journal of Physics: Condensed Matter, https://doi.org/10.1088/1361-648X/aa5a40, 2017.

*Janus particles at fluid interfaces*

Janus particles are interesting as they combine the general colloidal properties (such as the role as emulsion stabilizer) with the properties of amphiphilic molecules. In the first part of the presentation, the influence of the interplay on the adsorption of a single particle at a flat fluid interface is studied and compared with redictions from free energy models. In the second part, the collective rotational behavior of an ensemble of Janus particles at a flat fluid interface is presented.

*Microswimming with inertia*

yet most theoretical approaches addressed the domain of negligible Reynolds number Re, ignoring inertia. In nature, however, in an intermediate range of Re, before turbulences arise, the inertial effects become important. In this work we conduct a theoretical study of how this regime emerges. For this we extend the swimmer model by Golestanian and Najafi [1], which has three beads attached in series in a fluid and moving along the axis of the swimmer, by inclusion of the beads’ masses. We do this by combining the Oseen-Stokes equations for the coupled motion of distant spheres in a fluid with Newton’s force-mass relations and obtain a coupled system of first-order differential equations. Solving these equations allows us to derive a closed-form expression for the velocity of the swimmer which explicitly takes inertia into account. This velocity expression compares considerably better to results obtained from lattice-Boltzmann simulations of the swimmer, for intermediately high bead masses or driving forces, than the inertia-free model of Golestanian and Najafi.

References.

[1] R. Golestanian and A. Ajdari, Analytic results for the three-sphere swimmer at low Reynoldsnumber, Physical Review E – Statistical, Nonlinear, and Soft Matter Physics , 77(3):1–6,2008. ISSN 15393755. doi: 10.1103/PhysRevE.77.036308*Thin-film simulation using Shallow water-Lattice Boltzmann Method*

We employ the solver to study for Rayleigh-Taylor Instability and for investigating the contact line motion during spreading and sliding of droplets.

*Jammed capsules as a model system for soft glassy mater*

References.

[1] T. Kruger et al. Particle stress in suspensionsof soft objects. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 369(1945) (2011): 2414-2421.

[2] J. Harting et al. Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 363.1833 (2005): 1895-1915.

*Induced long-lived anomalous thermal diffusion nearby elastic cell membranes*

*Ultrasound-triggered margination of microbubbles for targeted drug delivery*

located in the low-shear interior of the blood stream. Yet, when they are close to

the pathological region, they should attain a near-wall position for the most efficient

interaction with the endothelium. Using mesoscopic three-dimensional numerical simulations

we show that this apparent contradiction can be resolved by using phospholipid

coated microbubbles. Application of an ultrasound pulse triggers the rapid migration

of the microbubbles toward the endothelial walls due to the hydrodynamic interactions

with the red blood cells. The effect is caused by the oscillations of the bubbles, resulting

in alternations between a soft and a stiff state, as induced by the lipid shell. We find that

the effect is very robust, being triggered even if the time spent in the stiff state is five

times lower than the opposing time in the soft state.

*Cross-streamline migration of soft and asymmetric particles in oscillatory shearflow*

Using 1D dumbbells, 2D ring polymers, and 3D capsules as elementary representatives of such particles, we show that these may indeed migrate within the shear plane only if they have an intrinsic material asymmetry and if the shear gradient varies with time. The CSM exists over a wide range of parameters and, importantly, is a generic property, i.e., it does neither depend on the dimensionality of the particle nor on the details how the particle asymmetry is realized.

The migration velocity can be tuned by various parameters, including the frequency and amplitude of the time-dependent, linear shear flow as well as the elastic properties of the particle. Besides the fundamental importance of this phenomenon, the ability to tune the migration process externally has promising applications in microrheology such as particle separation.

*Migration-transition of sedimenting soft particles in vertical flows*

The dynamics of soft, heavy (light) particles is investigated in plane Poiseuille flows between two vertical walls and in the limit of a vanishing Reynolds number. We observe, that heavy soft particles migrate to the center of a parabolic Poiseuille flow profile with the flow direction parallel to the gravitational force, similar as for neutrally buoyant particles. For light particle we observe the opposite effect. If the flow direction is reversed and antiparallel to gravitation, we find a surprising reversal of the migration direction and heavier particles migrate away from the center of a parabolic flow profile. This transition of the migration direction is determined numerically by the Stokesian particle dynamics and the Lattice-Boltzmann-Method as well as analytically in case of small deformations of a ring polymer. The migration away from the center is slowed down due to hydrodynamic particle-wall interactions. The parameter dependence of the final off center particle position may be used for separating different particles.

*Formation of liquid bridges with angled collisions between wet particles: a micro-scale study*

This study investigates the effects of non-head-on collisions (i.e. collisions at various angles) and variation of velocity on the formation and rupture of liquid bridges. The dynamic process of formation and rupture of liquid bridges in 3-dimensions is simulated using the Incompressible Smooth Particle Hydrodynamics method. A pairwise force algorithm is used to model surface tension and wetting phenomena. Interesting deviations from theoretical estimations on the critical velocity of approach for wet particles to form agglomerates are presented.

*Stochastic rotation dynamics with isotropic interaction*

References

[1] Gompper, G., Ihle, T., Kroll, D. M., Winkler, R. G., Multi-Particle Collision Dynamics: A Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids, Adv. Polym. Sci.

221, 1-87, 2009. [2] Ihle, T., Kroll, D. M., Stochastic rotation dynamics. I. Formalism, Galilean invariance, and Green-

Kubo relations, Phys. Rev. E 67, 066705, 2003. [3] Mühlbauer, S., Strobl, S., Lee, K.-W., Pöschel, T., Stochastic rotation dynamics with isotropic

interaction, 2017. (manuscript in preparation)