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Cluster of Excellence
Engineering of Advanced Materials
MP1305 COST Action “Flowing Matter” WG2+WG3
28 Feb 2017 - 3 Mar 2017
Just after the “Fasching” festivities, the research unit “Dynamics of complex fluids and interfaces” of the Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy organized a four day workshop under the umbrella of the EU COST Action “Flowing Matter” and EAM.
Taking place in the beautiful recently opened location “Haus der Kirche” on Erlangen’s Bohlenplatz, the goal of the workshop was to bring together scientists working on theoretical modelling and simulation of fluid systems where a non-trivial interplay emerges between flow and interfacial dynamics. In the spirit of the COST initiative, the meeting was meant as an opportunity to stimulate discussions and promote collaborations on new topics at the crossroad between soft matter, active systems and turbulent flows.
The workshop enjoyed the participation of almost 70 attendees from 13 different European countries, including eminent scientists working in the fields of Soft and Active Matter and Turbulence, as well as early stage researchers, that provided more than 40 invited talks and a selection of about 20 posters. The Friedrich-Alexander University of Erlangen-Nürnberg and the Cluster of Excellence “Engineering of Advanced Materials” have been represented by 18 researchers, out of which 4 have given talks and 5 have presented posters. The high quality of the presentations was witnessed by the numerous discussions that each of them motivated. The not excessive size of the workshop, in fact, proved to be optimal to promote interactions and incipient collaborations among the participants.
A common and distinctive feature of soft materials like suspensions, emulsions, foams, gels, etc, is the co-existence of multiple phases and/or components. As a consequence, their structural complexity is characterised by interfaces, whose properties determine (often more relevantly than bulk properties) the macroscopic dynamics of these systems. The stability of foams and emulsions, for instance, relies crucially on surface active agents (surfactants) and their rheology results from the micro-mechanics of interfaces and films separating the elementary constituents (droplets and bubbles). The presence of interfaces, moreover, is known to affect profoundly the swimming/propulsion mechanism (and hence the collective behaviour) of microorganisms and artificial active particles.
Turbulent multi-phase and multi-component flows are of utmost importance for a plethora of natural and technological situations, like cavitation and emulsification, where the multi-scale energy transfer and dissipation processes typical of high Reynolds number flows are intimately connected with the continuous breakage and formation of interfaces controlled by surface tension.
Generally speaking, the lesson learned is twofold: on the one hand, the presence of surfaces separating phases in flowing matter phenomena calls for the need to deal with complex boundary conditions, that make modelling a challenge (F. Bresme, M. Praprotnik, S. Metzger, A. Scagliarini); on the other hand, controlled generation of interfaces and three-phase contact lines leads to the formation of small-scale structures. The evolving shape of the solid/liquid interface affects profoundly the heat flux and the melting rate in melting-driven turbulent thermal convection in magma chambers and ice lakes (E. Calzavarini); “complex” fluid flow, in turn, determines the mass transfer process from the “simple” surface of a core-shell reservoir (B. Kaoui). We have seen how resolving fluid-structure interactions at the particle scale proves to be fundamental to explain the flow characteristics and rheology of suspensions, both at high (in channel flows; L. Brandt) and low (S. Luding, M. Ellero) Reynolds numbers, and the dynamics of blood flow, in terms of red blood cells clustering (C. Wagner) and biomedical techniques exploiting microfluidic devices (T. Krüger) or in situ (S. Gekle). It was shown that a complex solid/fluid interfacial dynamics occurs inside ventricular cavities of the brain, where the action of cilia induces flow of the cerebrospinal fluid, with relevant physiological implications (E. Bodenschatz).
The study of the behaviour of suspensions under different flow conditions involved also: colloidal suspensions under shear (S. Klapp); the diffusion properties of rotating colloidal rods (M. Marechal) and anisotropic colloids (M. Ekiel-Jeżewska); the migration of soft particles under time-dependent Couette and Poiseuille flows (W. Zimmermann); the probability distribution of velocity fluctuations of particles in a suspension flowing in very narrow channels (A. Marín); the response of linear, ring and knotted polymers (C. Likos) and semiflexible filaments (O. du Roure) to shear flows. When the volume fraction of the dispersed phase (colloidal particles, droplets, bubbles, etc) is high enough, the system enters in a dynamically arrested state, or jammed (S. Hutzler), state, characterising foams and dense emulsions. Cohesive colloidal materials can be nucleated, nonetheless, by a proper tuning of the particle-particle interactions, e.g., by Casimir critical forces; such materials appear to be an optimal test case to study fracture dynamics in solids (P. Schall). In the same spirit, it was highlighted how, from the plastic activity emerging in numerical studies of a model system of dense emulsions, one may infer statistical laws of earthquakes (F. Toschi).
Many instances were presented, from natural and technological situations, where solid/colloidal particles and/or droplets are not suspended in the bulk of a continuous phase, but either trapped at a fluid-fluid interface, or deposited on solid substrates, or even on more complex materials, like soft solids (J. Snoeijer). Fluid interfaces deformed by particles give rise to effective interactions among them (S. Dietrich), that has been exploited to design novel ways to engineer nanostructures (N. Vogel, M. Engel) or to deposit desired structures from a drying suspension of colloids (J. Keddie, N. Araújo) or polymer-plasticisers (A. Jarray) on a substrate. Such capillary interactions play an essential role when combined with electric fields, to manipulate particle-stabilised droplets (P.G. Dommersnes, N. Rivas), or, under applied magnetic fields, to assemble artificial microscale self-propelled devices (G. Grosjean, M. Hubert, A. Sukhov). Other self-propulsion mechanisms for interfacial swimmers are based on chemical reactions catalysed at the particle surface (M. Popescu), or on the Marangoni effect for laser-heated colloids (T. Bickel); in the latter case, in particular, it was shown that very high propelling speeds can be achieved. The proximity to a solid bounding surface is determinant also for the hydrodynamic interactions that determine self-propulsion in a type of magnetically powered chain-like micro-propellers (P. Tierno). Fundamental kinematic aspects of microswimming, finally, were investigated addressing the dynamics of model swimmers (A.-S. Smith, A. Farutin), while collective behaviour (swarming) was addressed in simulations of thermophoretic dimers (M. Wagner).
Andrea Scagliarini and Jens Harting (HI ERN)