Title: The Rheology of Complex and Polymeric Liquids in Confinements and Free Surface Flows
Authors: Clasen, Christian
Issue Date: 2008
Publisher: University of Hamburg
Series Title: Kumultive Habilitationsschrift
Abstract: The investigation and interpretation of the flow behaviour of complex liquids is a
fascinating field of research. When designing materials with tailored properties, complex
fluids and polymeric liquids can display unique combinations of properties that will be
controlled by the structure (the ‘morphology’ and internal conformation) of the material and
that would not be attainable with a simple single phase. Key to predicting and tailoring the
properties of these fluids is the understanding of the fundamental relations that govern the
effect of flow on the structural features over a wide range of length and time scales. This
understanding provides the basis for the production and invention of new materials and
applications, ranging from simple day to day products as shampoo or spread cheese to high
end pharmaceutical or medical products as stabilized colloidal antibiotic solutions. The
importance of the understanding on the flow behaviour of structured fluids reflects also in
recent changes in chemical industries, whose focus has shifted over the last years from bulk
chemical and commodity products towards speciality chemicals and higher value-added
materials. Following the definition of Cussler and Moggridge [1] these are in particular
chemical 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 and
conformation change of polymers will cause, as laid out by M. Hill, “changes in
microstructure due to processing (that) are … unavoidable” [2], and require hence the
fundamental understanding on how the flow of a complex or polymeric liquid is related to the
its microstructural evolution and how this microstructure influences the flow and processing
In the science and technology of complex 'structured fluids' one has to deal with the
omnipresence of ‘confinements’ for the separate single phases, for example interfaces and
surfaces 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) that
plays a dominant role on the evolution of the internal structure during flow. These effects are
not necessarily limited to multiphase materials, but are equally important for structure and
orientation of dissolved macromolecules or colloidal dispersed aggregated structures. Some
examples include the recent trends towards miniaturized processing, the advent of
microfluidic techniques, the orientation of particles by flow near walls with unexpected
macroscopic consequences, the importance of free surface flows in technological processes
such as ink-jet printing and even more “forward looking” techniques such as convective
nanoparticle self-assembly.
When the length scale of the flow is reduced in at least one dimension, it can become
comparable to the characteristic length scale of the internal structure of a fluid. The scaling
rules and concepts that hold for bulk materials will then no longer be valid. There will be an
interplay between the internal structure of the material and the external surfaces, thus
potentially affecting the morphology development of multiphase materials or the orientation
and conformation of polymers in solutions. But most important, the coupling between the
presence of walls and interfacial phenomena can make it possible to generate structures that
may not be attainable with bulk processing. Recent investigations have indicated that, for
example, droplet break-up of polymer blends changes dramatically when the dimension of the
flow geometry becomes comparable to the droplet size, or that tribological properties (friction
in confinement) of polymeric fluids can differs dramatically from the bulk properties, even for
hydrodynamic lubrication, once the dimensions of the polymer coils are reached. However, a
comprehensive investigation of the effects of confinements on the flow properties of complex
liquids is still missing to date.
On the other hand, free surfaces during the formation and break-up of liquid filaments, as
present in atomization, extrusion or jetting operations, replace the ‘hard’ and inflexible
confinement of processing equipment with the soft but flexible confinement of the strong
interfacial tension of a liquid-air surface. Also the free surface of a thinning filament can
quickly reach the length scale of the enclosed high-interface fluid, however, much more
important is the possibility to induce strong elongational fields, enabling the creation of
highly 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 wide
variety of applications in the chemical processing, food and consumer products industries.
Operations such as ink jet printing, spraying of fertilizers, paint-leveling, misting, bottle
filling and roll coating are all controlled by interactions between the non-Newtonian stresses
arising from the microstructure of the bulk, and capillary stresses at the deformable free
surface. The theory for the free surface flow and break-up of a Newtonian liquid is laid out
and experimentally verified and a number of recent studies have therefore promulgated the
idea of using the capillarity induced thinning of a liquid filament as a rheometric device for
quantifying the properties of complex fluids in predominantly extensional flows. Entov and
Hinch provide a detailed discussion of the evolution of a perfectly cylindrical thread of a
viscoelastic fluid undergoing capillary-driven thinning and breakup, and capillary-thinning
devices for the investigation of viscoelastic effects of polymeric solutions have been
developed recently by a number of laboratories. However, the boundaries of this technique for
the investigation of polymeric solution and melts, as well as the investigation of the response
of more complex fluid structures to the strong elongational flow field during a capillary
breakup are still unknown.
Publication status: published
KU Leuven publication type: ABa
Appears in Collections:Soft Matter, Rheology and Technology Section

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