Investigation of the effect of complex flow fields on cellulose nanofibril suspensions by birefringence imaging
Typ
Examensarbete för masterexamen
Program
Applied physics (MPAPP), MSc
Publicerad
2020
Författare
Karlsson, Sara-Louise
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Cellulose is an interesting renewable material with many unique properties. A type
of nanocellulose is cellulose nanofibrils (CNF). It has potential to be used as reinforcement
in composites, to modify the rheology of complex systems in 3D printing
and in fiber production, among other areas. They are relatively long and flexible
particles with both crystalline and amorphous parts. This material can be used to
make stronger materials by increasing the alignment of the fibrils. In all of these
cases, it is crucial to understand the behavior of the material in flow, which is the
focus of this thesis.
The aim with this project is to study how the alignment in cellulose nanofibril
suspensions is affected in different flow types (extensional, shear and two mixed flow
types), how it changes with different flow rates (1 - 50 μL/s) and if it is affected
by the concentration of the suspensions (0.1, 0.25 and 0.5 wt%). To achieve the
different complex flow fields, a fluidic four-roll mill was used. The alignment of
the nanofibers were then determined by a birefringence imager which measured the
retardance, angle of the fast axis and the intensity.
The results showed that the used setup with the fluidic four-roll mill and birefringence
imager can successfully measure the alignment in CNF suspensions, and that
the experiments can be performed continuously with any one wavelength. Dependence
on the concentration of the suspensions, flow rate and flow type was found.
The alignment increased in extensional flow with increasing concentration, but not
in shear flow. The flow rate dependence showed that the alignment increased with
the flow rate for the highest concentration (0.5 wt%), but did not have much effect
on the lowest concentration (0.1 wt%). It was found that extensional flow achieved
more alignment than shear flow in the two higher concentrations of suspensions, but
that the flow type had less effect in the lowest concentration. An unexpected dependency
on the age of the sample and the assembly of the device was also observed.
Beskrivning
Ämne/nyckelord
cellulose nanofibrils , fluidic four-roll mill , birefringence imager , alignment