Soft Matter, Rheology and Technology Section
The aim of the Soft Matter, Rheology and Technology (SMaRT) division is to foster and conduct research on soft matter and the rheology of complex fluids on a fundamental level as well as their application to technical processes. Such complex fluids span a wide range of materials, including polymer solutions, melts and blends, emulsions, colloidal dispersions, surfactant solutions, etc.
A characteristic feature of such complex fluids and soft matter is that they exhibit much more complex flow and deformation behaviours than do simple Newtonian liquids or Hookean solids normally associated with fluid or solid mechanics. The aim of the SMaRT section is thus to study the fundamental dependence of the molecular and microstructure of such systems to their complex and non-linear phenomena such as shear and extension thinning and thickening, linear and non-linear viscoelasticity, yielding, and thixotropy.
Such a link between the microscale and macroscopic material response typically starts with well-defined model materials and systems, which enables us to experimentally determine the fundamental principles and underlying mechanisms that determine the flow and deformation behaviour of complex fluids and to develop relevant rheological constitutive models. This allows us to subsequently understand and describe the behaviour of more complex systems in actual applications, but also to quantitatively design the microstructure and thus a flow and deformation behaviour of formulations to meet and even surpass the requirements of current soft matter product properties.
To achieves these goals, the division SMaRT has a state-of-the-art rheological infrastructure. These encompass traditional stress and strain controlled rheometry (both rotational and capillary) as well as novel techniques to measure elongational properties. Furthermore, a wide range of techniques to characterize the variable and flow-induced microstructure is available including (confocal) microscopic techniques, light scattering, rheo-optical and dielectric methods, as well as equipment for processing of polymers, powders and dispersions and instruments to measure interfacial rheology and dynamics of liquid structures at interfaces. This range is enhanced by (rheometrical) equipment developed for investigations with high-brilliance X-ray sources at the synchrotrons in Grenoble and Hamburg, and neutron scattering.
The general research topics that the division SMaRT is currently tackling are the following.
In the area of free surface flows, the focus is on the breaking dynamics of liquid jets structured with polymers, micro-and nanoparticles, covering a range of viscosities from water-based systems to honey. This is extended to the general extensional flow behaviour of polymer solutions and melts with the aim to quantitatively describe the general elongational behaviour in non-linear deformational flows. Resulting application oriented research includes the electrospinning of complex nanofibers and the electrospraying of core-shell nanoparticles for pharmaceutical and energy applications.
Confinement effects on the rheology are investigated for products with microstructures spanning the range of micro-to nano-meters originating from particle suspensions, microgels and emulsions. The investigations cover controlled confinement situations (thin-film rheometry) to controlled normal load tribo-rheometry up to tribological investigations of complex fluids, with applications in lubricants and oral food processing.
The structure and rheology of suspensions is also studied within the SMaRT research group. The stability, aging, and flow-induced changes to the structure are monitored using rheo-optical techniques. Devices such as a counter-rotating rheometer allow us to investigate, e.g., the link between floc size and thixotropy and a thermo-shear cell mounted to a confocal microscope can be used to investigate network structure and yield. Such investigations have the aim to directly link the micro- and network structure of particle networks with their response in order to predict and control the behaviour.
We are also using a novel, capillary-induced method to induce network formation in suspensions. The structure of these capillary suspensions is examined both as a function of the composition and in response to external shear. Their structure is linked back to the rheology to give information about the flexibility or rigidity of the network and highlight methods to tune the rheological response. Application oriented research for capillary suspensions such as in the rapid production of porous materials and crack-free films for printed electronics are also conducted.