Lab

Rheology and Processing of Soft Matter


About the lab

Featured research (6)

Simultaneous rheological, polarized light imaging, and small‐angle X‐ray scattering experiments (Rheo‐PLI‐SAXS) are developed, thereby providing unprecedented level of insight into the multiscale orientation of hierarchical systems in simple shear. Notably, it is observed that mesoscale alignment in the flow direction does not develop simultaneously across nano‐micro lengthscales in sheared suspensions of rod‐like chiral‐nematic (meso) phase forming cellulose nanocrystals. Rather, with increasing shear rate, orientation is observed first at mesoscale and then extends to the nanoscale, with influencing factors being the aggregation state of the hierarchy and concentration. In biphasic systems, where an isotropic phase co‐exists with self‐assembled liquid crystalline mesophase domains, the onset of mesodomain alignment towards the flow direction can occur at shear rates nearing one decade before a progressive increase in preferential orientation at nanoscale is detected. If physical confinement prevents the full formation of a cholesteric phase, mesoscale orientation occurs in shear rate ranges that correspond to de‐structuring at nanoscale. Interestingly, nano‐ and mesoscale orientations appear to converge only for biphasic suspensions with primary nanoparticles predominantly made up of individual crystallites and in a high‐aspect ratio nematic‐forming thin‐wall nanotube system. The nano‐micro orientation propagation is attributed to differences in the elongation and breakage of mesophase domains.
A general procedure for combining material functions and numerical modeling to predict the orientation of highly filled wood polymer composites (WPCs) in a single screw extrusion and validation thereof is elaborated in this study. Capillary rheometry was used to determine the shear viscosity and wall slip functions as well as the melt density of the biocomposites. The numerical model consisted of a model film die where the melt flow was simulated using a finite element method in the generalized Newtonian constitute equation framework. Fiber orientation was modeled using the Folgar–Tucker approach and included fiber–fiber interaction during the process. Reference extrusion tests were performed on a single screw extruder on the biocomposites. The extrusion setup included two melt pressure transducers that were used to determine the die inlet initial conditions (end of the extruder/die inlet) and provide feedback on the wall slip boundary conditions (pressure discharge along the die). Overall, the pressure error between experiments and simulations was less than 6.5% for all screw speeds investigated in 20 wt. % WPCs. Extrudates were produced, and the wood fiber orientation was estimated based on scanning electron microscopy micrographs and image analysis and compared with the simulations of fiber orientation. We show that the general procedure outlined can be calibrated to predict the overall orientation distribution of wood fiber biocomposites during single screw extrusion.
For magnetic field orientation of nonstructures to become a viable method to create high performance multifunctional nanocomposites, it is of paramount importance to develop a method that is easy to implement and that can induce long‐range uniform nanostructural alignment. To overcome this challenge, inspired by low field nuclear magnetic resonance (NMR) technology, a highly uniform, high field strength, and compact magnetic‐field nanostructure orientation methodology is presented for polymeric nanocomposites using a Halbach array, for the first time. Potential new advances are showcased for applications of graphene polymer composites by considering their electro‐thermal and antibacterial properties in highly oriented orthogonal morphologies. The high level of anisotropy induced in the graphene nanocomposites studied stands out through: 1) up to four decades higher electrical conductivities recorded in comparison to their randomly oriented counterparts, at concentrations where the latter show minimal improvements compared to the unfilled polymer; 2) over 1200% improvement in thermal conductivity, 3) antibacterial surfaces at field benchmark levels with lower filler content and with the added versatility of arbitrary orientation of the nanofillers. Overall, the new method and variations thereof can open up new horizons for tailoring nanostructure and performance for virtually all major nanocomposite applications based on graphene and other types of fillers.
We study the flow stability and spatiotemporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor–Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations, for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elasticity and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration, a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. Based on rheo-SAXS measurements, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor–Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions.

Lab head

Roland Kádár
Department
  • Department of Industrial and Materials Science
About Roland Kádár
  • Research Group: www.rheo-chalmers.se

Members (10)

Marko Bek
  • Chalmers University of Technology
Viney Ghai
  • Chalmers University of Technology
Alexandra Aulova
  • Nouryon Performance Formulations
Reza Ghanbari
  • Independent Researcher
Arvindh Seshadri
  • Chalmers University of Technology
Sajjad Pashazadeh
  • Chalmers University of Technology
Sylwia Wojno
  • Chalmers University of Technology
Kesavan Sekar
  • Chalmers University of Technology

Alumni (5)

Pooria Khalili
  • University of Borås
Karolina Gaska
  • Hitachi Energy
Mina Fazilati
  • Chalmers University of Technology
Georgia C. Manika
  • Chalmers University of Technology