4 research outputs found
Mechanics of Platelet-Reinforced Composites Assembled Using Mechanical and Magnetic Stimuli
Current fabrication technologies
of structural composites based on the infiltration of fiber weaves
with a polymeric resin offer good control over the orientation of
long reinforcing fibers but remain too cumbersome and slow to enable
cost-effective manufacturing. The development of processing routes
that allow for fine control of the reinforcement orientation and that
are also compatible with fast polymer processing technologies remains
a major challenge. In this paper, we show that bulk platelet-reinforced
composites with tailored reinforcement architectures and mechanical
properties can be fabricated through the directed-assembly of inorganic
platelets using combined magnetic and mechanical stimuli. The mechanical
performance and fracture behavior of the resulting composites under
compression and bending can be deliberately tuned by assembling the
platelets into designed microstructures. By combining high alignment
degree and volume fractions of reinforcement up to 27 vol %, we fabricated
platelet-reinforced composites that can potentially be made with cost-effective
polymer processing routes while still exhibiting properties that are
comparable to those of state-of-the-art glass-fiber composites
Ultrahigh Magnetically Responsive Microplatelets with Tunable Fluorescence Emission
Tuning the optical properties of
suspensions by controlling the
orientation and spatial distribution of suspended particles with magnetic
fields is an interesting approach to creating magnetically controlled
displays, microrheology sensors, and materials with tunable light
emission. However, the relatively high concentration of magnetic material
required to manipulate these particles very often reduces the optical
transmittance of the system. In this study, we describe a simple method
of generating particles with magnetically tunable optical properties
via solāgel deposition and functionalization of a continuous
layer of silica on ultrahigh magnetically responsive (UHMR) alumina
microplatelets. UHMR microplatelets with tunable magnetic response
in the range of 15ā36 G are obtained by the electrostatic adsorption
of 2 to 13% of superparamagnetic iron oxide nanoparticles (SPIONs)
on the alumina surface. The magnetized platelets are coated with a
20ā50 nm layer of SiO<sub>2</sub> through the controlled hydrolysis
and condensation reactions of tetraethylorthosilicate (TEOS) in an
NH<sub>3</sub>/ethanol mixture. Finally, the silica surface is covalently
modified with an organic fluorescent dye by conventional silane chemistry.
Because of the anisotropic shape of the particles, control of their
orientation and distribution using magnetic fields and field gradients
enables easy tuning of the optical properties of the suspension. This
strategy allows us to gain both spatial and temporal control over
the fluorescence emission from the particle surface, making the multifunctional
platelets interesting building blocks for the manipulation of light
in colloid-based smart optical devices and sensors
Dynamics of Cellulose Nanocrystal Alignment during 3D Printing
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 <i>via</i> 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 <i>in situ</i> 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 coreāshell architectures. These
findings offer powerful quantitative guidelines for the digital manufacturing
of composite materials with programmed particle orientations and properties
Locally Reinforced Polymer-Based Composites for Elastic Electronics
A promising approach to fabricating elastic electronic
systems
involves processing thin film circuits directly on the elastic substrate
by standard photolithography. Thin film devices are generally placed
onto stiffer islands on the substrate surface to protect devices from
excessive strain while still achieving a globally highly deformable
system. Here we report a new method to achieve island architectures
by locally reinforcing polymeric substrates at the macro- and microscale
using magnetically responsive anisotropic microparticles. We demonstrate
that the resulting particle-reinforced elastic substrates can be made
smooth enough for the patterning and successful operation of thin
film transistors with transfer characteristics comparable to state-of-the-art
device