posted on 2018-07-05, 00:00authored byMichael
K. Hausmann, Patrick A. Rühs, Gilberto Siqueira, Jörg Läuger, Rafael Libanori, Tanja Zimmermann, André R. Studart
The
alignment of anisotropic particles during ink deposition directly
affects the microstructure and properties of materials manufactured
by extrusion-based 3D printing. Although particle alignment in diluted
suspensions is well described by analytical and numerical models,
the dynamics of particle orientation in the highly concentrated inks
typically used for printing via direct ink writing
(DIW) remains poorly understood. Using cellulose nanocrystals (CNCs)
as model building blocks of increasing technological relevance, we
study the dynamics of particle alignment under the shear stresses
applied to concentrated inks during DIW. With the help of in situ polarization rheology, we find that the time period
needed for particle alignment scales inversely with the applied shear
rate and directly with the particle concentration. Such dependences
can be quantitatively described by a simple scaling relation and qualitatively
interpreted in terms of steric and hydrodynamic interactions between
particles at high shear rates and particle concentrations. Our understanding
of the alignment dynamics is then utilized to estimate the effect
of shear stresses on the orientation of particles during the printing
process. Finally, proof-of-concept experiments show that the combination
of shear and extensional flow in 3D printing nozzles of different
geometries provides an effective means to tune the orientation of
CNCs from fully aligned to core–shell architectures. These
findings offer powerful quantitative guidelines for the digital manufacturing
of composite materials with programmed particle orientations and properties.