Thermodynamic and mesoscopic modeling of tumbling nematics, of shear-thickening fluids and of stick-slip-like flow behavior

Shear thickening, i.e. the increase of the viscosity with increasing shear rate as it occurs in dense colloidal dispersions and polymeric fluids is an intriguing phenomenon with a considerable potential for technical applications. The theoretical description of this phenomenon is patterned after the thermodynamic and mesoscopic modeling of the orientational dynamics and the flow behavior of liquid crystals in the isotropic and nematic phases, where the theoretical basis is well-established. Even there the solutions of the relevant equations recently yielded surprises: not only stable flow alignment and a periodic behavior (tumbling) are found as response to an imposed stationary shear flow but also irregular and chaotic dynamics occurs for certain parameter ranges. To treat shear-thickening fluids, a non-linear Maxwell model equation for the symmetric traceless part of the stress tensor has been proposed in analogy to the equations obeyed by the alignment tensor of nematics. The fluid-solid transition is formally analogous to the isotropic-nematic transition. In addition to shear-thickening and shear-thinning fluids, substances with yield stress can be modeled. Furthermore, periodic stick-slip-like motions and also chaotic behavior are found. In the latter cases, the instantaneous entropy production is not always positive. Yet it is comforting that its long-time average is in accord with the second law

Flow Properties Inferred from Generalized Maxwell Models

The generalized Maxwell model is formulated as a nonlinear relaxation equation for the symmetric traceless stress tensor. The relaxation term of the equation involves the derivative of a potential function with respect to the stress tensor. Two special cases for this potential referred to as “isotropic” and “anisotropic” are considered. In the first case, the potential solely depends on the second scalar invariant, viz. the norm of the tensor. In the second case, also a dependence on the third scalar invariant, essentially the determinant, is taken into account in analogy to the Landau-de Gennes potential of nematic liquid crystals. Rheological consequences of the model are presented for a plane Couette flow with an imposed shear rate. The non-Newtonian viscosity and the normal stress differences are analyzed for stationary solutions. The dependence on the model parameters is discussed in detail. In particular, the occurrence of a shear-thickening behaviour is studied. The possibility to describe substances with yield stress and the existence of non-stationary, stick-slip-like solutions are pointed out. The extension of the model to magneto-rheological fluids is indicated.DFG, SPP 1104, Kolloidale magnetische Flüssigkeiten: Grundlagen, Entwicklung und Anwendung neuartiger FerrofluideDFG, SFB 448, Mesoskopisch strukturierte Verbundsystem

VR Hackfest

We built the future of the web — today! Our four-person eLibrary team designed an afternoon workshop and corresponding network-connected public exhibit centered around two cutting-edge internet technologies: IPFS and A-Frame

When grassroots innovation movements encounter mainstream institutions: implications for models of inclusive innovation

Grassroots innovation movements (GIMs) can be regarded as initiators or advocates of alternative pathways of innovation. Sometimes these movements engage with more established science, technology and innovation (STI) institutions and development agencies in pursuit of their goals. In this paper, we argue that an important aspect to encounters between GIMs and mainstream STI institutions is the negotiation of different framings of grassroots innovation and development of policy models for inclusive innovation. These encounters can result in two different modes of engagement by GIMs; what we call insertion and mobilization. We illustrate and discuss these interrelated notions of framings and modes of engagement by drawing on three case studies of GIMs: the Social Technologies Network in Brazil, and the Honey Bee Network and People's Science Movements in India. The cases highlight that inclusion in the context of GIMs is not an unproblematic, smooth endeavour, and involves diverse interpretations and framings, which shape what and who gets included or excluded. Within the context of increasing policy interest, the analysis of encounters between GIMs and STI institutions can offer important lessons for the design of models of inclusive innovation and development