Understanding the relation between microscopic structure and macroscopic behavior is a question pertinent to almost all fields of science. This relation is particularly complex in the colloidal domain which is common to soft materials and biological systems. Here several length- and timescales coexist. Complex structures with unique properties often self-assemble from simple building blocks. On the one hand, the existence of natural structures whose constituents can assemble, disassemble, and reassemble either autonomously or on command has enabled the creation of materials which are known throughout the biological world that are capable of self-repair and multi-tasking properties . One of the major reasons for studying the self-assembly process is therefore to better understand the principles of biological organization. On the other hand, there is a technological demand for smart and functional synthetic materials with desired properties far more complex than those of the traditional metals, ceramics, and polymers, and exhibiting many levels of functionality and hierarchical organization, as is typical with biological structures. To support such levels of structural complexity and functional diversity, such materials must actively consume energy and “live” outside of equilibrium.
A fundamental question in non-equilibrium systems is how collective dynamics and ordering emerges from the dynamics of the discrete interacting components, their interactions and parameters of an external energy injection. Presently however, our knowledge of colloidal microscopic details by far supercedes our understanding of this emergent behaviour. This disparity presents a major hurdle on our way to understanding the principles of biological organization and to explore novel strategies in self-assembly and active materials design. Colloidal particles have been a subject of increasingly active research since they can be used as building blocks to fabricate functional micro and nano-structures via self-assembly processes . Due to the controllability of their size, shape and interaction profiles they serve as a promising model system and a starting point to enter the rich world of out-of-equilibrium dynamics and dynamic self-assembly.
A generic out-of-equilibrium dissipative system subjected to a time-dependent energy injection develops nontrivial collective dynamics and large-scale coherent structures, which are far more complex than their equilibrium counterparts [3-15]. The main source of such emergent behavior is the many body dissipative interactions among colloids (steric [16,17], electrostatic , magnetic [4,5]), external energy injection and coupling of particles dynamics to the fluid flow around them. Understanding dynamic interactions in such complex nonequilibrium dissipative system presents a significant theoretical and computational challenge.
To force colloidal systems out of equilibrium and thus to create new self-organized structures alternating magnetic, electric and hydrodynamic fields are successfully used. Dynamic structures, which are generally not available through equilibrium thermodynamics, have been reported [3-9]. Static electrical fields in combination with the electro-hydrodynamic convective flows create emergent structures like toroidal vortices and pulsating rings when applied to metallic microparticles immersed in a poorly conducting liquid . AC electric fields and patterned electrodes have been exploited to achieve dynamic templating of 3D colloidal patterns , and the polarization of electric double layers of colloidal rods due to an external alternating electric field is found to give rise to rich phase behavior . Biaxial rotating electric/magnetic fields are in general a versatile tool to direct colloidal assembly . Variety of low dimensional structures like chains, foams, membranes and clusters exhibiting viscoelastic properties have been observed [4,5,7,8]. Magnetic colloidal particles suspended at a liquid–air interface and energized by alternating magnetic fields self-assemble into complex structures and swimmers exhibiting remarkable dynamic behavior due to the additional coupling of particles to liquid through the interface excitations [3,6,13,14].
Theoretically, some features of the emergent dynamics in driven colloids can be understood in the framework of the amplitude equation (Ginzburg-Landau type equation) [6,9,15] coupled to the conservation law equation describing the evolution of the particle density and the Navier-Stokes equation for hydrodynamic flows. Nevertheless, the fundamental microscopic mechanisms leading to the dynamic self-assembly and their relations to the emergent behavior often remain unclear. Computer simulations are practically the only method to theoretically investigate such questions. However, modeling the nonequilibrium self-assembly presents a huge computational challenge due to the complex many-body interactions and collective dynamics on very different time scales. One of the main challenges is to properly account for the particle-fluid coupling. In very confined geometries the structure of the fluid (usually water) governs the dynamics: to properly describe this one needs to resort to atomistic simulations with proper description of hydrogen bonds etc. . Naturally, the length- and time scales accessible are then very limited. On a more coarse grained level, the fluid flow around colloids is modeled by molecular dynamics methods like Lattice-Boltzmann  and Multi Particle Collision Dynamics [21,22]. Flow around a single colloid is characterized by low Reynolds number, which simplifies the treatment of hydrodynamic interaction in these models, however, it becomes difficult to describe collective dynamics of self-assembled structures, which have a larger effective Reynolds number implying that inertia hydrodynamic terms become important. Alternative approach is to describe the colloidal dynamics by molecular dynamics simulations coupled to the Navier-Stokes equations describing large-scale time-averaged hydrodynamic flows generated by the colloids . In all of the coarse grained approaches, exact accounting of the particles-fluid coupling in response to external periodical excitations is computationally challenging and usually simplifying assumptions have to be made. Nevertheless, such models still provide vital information on the mechanisms governing out-of--equilibrium dynamics and self-assembly in colloidal systems. The planned workshop will be an ideal opportunity to create a constructive discussion between researches in the field of non-equilibrium self-assembly on the role of the fluid- particle coupling in the process of dynamic self-assembly and to define appropriate simulation methods to treat this coupling.