The Rheology of Complex and Polymeric Liquids in Confinements and Free Surface Flows

The investigation and interpretation of the flow behaviour of complex liquids is afascinating field of research. When designing materials with tailored properties, complexfluids and polymeric liquids can display unique combinations of properties that will becontrolled by the structure (the ‘morphology’ and internal conformation) of the material andthat would not be attainable with a simple single phase. Key to predicting and tailoring theproperties of these fluids is the understanding of the fundamental relations that govern theeffect of flow on the structural features over a wide range of length and time scales. Thisunderstanding provides the basis for the production and invention of new materials andapplications, ranging from simple day to day products as shampoo or spread cheese to highend pharmaceutical or medical products as stabilized colloidal antibiotic solutions. Theimportance of the understanding on the flow behaviour of structured fluids reflects also inrecent changes in chemical industries, whose focus has shifted over the last years from bulkchemical and commodity products towards speciality chemicals and higher value-addedmaterials. Following the definition of Cussler and Moggridge [1] these are in particularchemical products and formulations “whose microstructure, rather than molecular structure,creates value”. Flow related phenomena as droplet deformation, breakup, or coalescences,changes in crystal size or distribution as well as the structure alignment, association andconformation change of polymers will cause, as laid out by M. Hill, “changes inmicrostructure due to processing (that) are … unavoidable” [2], and require hence thefundamental understanding on how the flow of a complex or polymeric liquid is related to theits microstructural evolution and how this microstructure influences the flow and processingconditions.In the science and technology of complex 'structured fluids' one has to deal with theomnipresence of ‘confinements’ for the separate single phases, for example interfaces andsurfaces in emulsions, immiscible polymer blends, foams and suspensions of particles.However, in many cases it is the influence of external surfaces (walls or free surfaces) thatplays a dominant role on the evolution of the internal structure during flow. These effects arenot necessarily limited to multiphase materials, but are equally important for structure andorientation of dissolved macromolecules or colloidal dispersed aggregated structures. Someexamples include the recent trends towards miniaturized processing, the advent ofmicrofluidic techniques, the orientation of particles by flow near walls with unexpectedmacroscopic consequences, the importance of free surface flows in technological processessuch as ink-jet printing and even more “forward looking” techniques such as convectivenanoparticle self-assembly.When the length scale of the flow is reduced in at least one dimension, it can becomecomparable to the characteristic length scale of the internal structure of a fluid. The scalingrules and concepts that hold for bulk materials will then no longer be valid. There will be aninterplay between the internal structure of the material and the external surfaces, thuspotentially affecting the morphology development of multiphase materials or the orientationand conformation of polymers in solutions. But most important, the coupling between thepresence of walls and interfacial phenomena can make it possible to generate structures thatmay not be attainable with bulk processing. Recent investigations have indicated that, forexample, droplet break-up of polymer blends changes dramatically when the dimension of theflow geometry becomes comparable to the droplet size, or that tribological properties (friction6in confinement) of polymeric fluids can differs dramatically from the bulk properties, even forhydrodynamic lubrication, once the dimensions of the polymer coils are reached. However, acomprehensive investigation of the effects of confinements on the flow properties of complexliquids is still missing to date.On the other hand, free surfaces during the formation and break-up of liquid filaments, aspresent in atomization, extrusion or jetting operations, replace the ‘hard’ and inflexibleconfinement of processing equipment with the soft but flexible confinement of the stronginterfacial tension of a liquid-air surface. Also the free surface of a thinning filament canquickly reach the length scale of the enclosed high-interface fluid, however, much moreimportant is the possibility to induce strong elongational fields, enabling the creation ofhighly elongated and aligned internal structures that cannot be achieved with rigid surfaces.Understanding the behavior of free-surface flows is of enormous importance for a widevariety of applications in the chemical processing, food and consumer products industries.Operations such as ink jet printing, spraying of fertilizers, paint-leveling, misting, bottlefilling and roll coating are all controlled by interactions between the non-Newtonian stressesarising from the microstructure of the bulk, and capillary stresses at the deformable freesurface. The theory for the free surface flow and break-up of a Newtonian liquid is laid outand experimentally verified and a number of recent studies have therefore promulgated theidea of using the capillarity induced thinning of a liquid filament as a rheometric device forquantifying the properties of complex fluids in predominantly extensional flows. Entov andHinch provide a detailed discussion of the evolution of a perfectly cylindrical thread of aviscoelastic fluid undergoing capillary-driven thinning and breakup, and capillary-thinningdevices for the investigation of viscoelastic effects of polymeric solutions have beendeveloped recently by a number of laboratories. However, the boundaries of this technique forthe investigation of polymeric solution and melts, as well as the investigation of the responseof more complex fluid structures to the strong elongational flow field during a capillarybreakup are still unknown.

Jaar van publicatie:2008