1,046 research outputs found
Influence of the Polarity of the Electric Field on Electrorheometry
Uniaxial extensional flow is a canonical flow typically used in rheological characterization to provide complementary information to that obtained by imposing simple shear flow. In spite of the importance of having a full rheological characterization of complex fluids, publications on the rheological characterization of mobile liquids under extensional flow have increased significantly only in the last 20 years. In the case of the rheological characterization of electrorheological fluids, the situation is even more dramatic, as the ERFs have been exclusively determined under simple shear flow, where an electrorheological cell is attached to the rotational rheometer generating an electric field perpendicular to the flow direction and that does not allow for inverting the polarity. The very recent work published by Sadek et al., who developed a new electrorheological cell to be used with the commercial Capillary Breakup Extensional Rheometer (CaBER), allows for the very first time performing electrorheometry under extensional flow. By means of the same experimental setup, this study investigates the influence of the polarity of the imposed electric field on the filament thinning process of a Newtonian and an electrorheological fluid. Results show that a polarity against the gravity results in filament thinning processes that live longer or reach a stable configuration at lower intensities of the applied electric field
Flow of low viscosity Boger fluids through a microfluidic hyperbolic contraction
In this work we focus on the development of low viscosity Boger fluids and assess their elasticity analyzing the flow through a microfluidic hyperbolic contraction. Rheological tests in shear and extensional flows were carried out in order to evaluate the effect of the addition of a salt (NaCl) to dilute aqueous solutions of polyacrylamide at 400, 250, 125 and 50 ppm (w/w). The rheological data showed that when 1% (w/w) of NaCl was added, a significant decrease of the shear viscosity curve was observed, and a nearly constant shear viscosity was found for a wide range of shear rates, indicating Boger fluid behavior. The relaxation times, measured using a capillary break-up extensional rheometer (CaBER), decreased for lower polymer concentrations, and with the addition of NaCl. Visualizations of these Boger fluids flowing through a planar microfluidic geometry containing a hyperbolic contraction, which promotes a nearly uniform extension rate at the centerline of the geometry, was important to corroborate their degree of elasticity. Additionally, the quantification of the vortex growth upstream of the hyperbolic contraction was used with good accuracy and reproducibility to assess the relaxation time for the less concentrated Boger fluids, for which CaBER measurements are difficult to perform
The building blocks behind the Electrohydrodynamics of non-polar 2Dinks
This work provides a complete rheological characterization of 2D inks in
electric fields with different intensities and orientations to the imposed flow
field. 2D nanomaterials used in this study are graphene nanoplatelets,
hexagonal boron-nitride, and molybdenum disulfide. These materials with
different electric properties are dispersed in a non-polar solvent (Toluene)
with different concentrations of Ethyl Cellulose (EC), providing Newtonian or
viscoelastic characteristics. Shear rheology tests show that the presence of
nanoparticles barely changes the fluid behaviour from the carrier fluid, and
the application of an electric field perpendicular to the flow does not result
in electrorheological behaviour. However, extensional experiments, which mimic
the actual EHD jet printing conditions, allowed the observation of the
influence of both the particles and the electric field aligned on the filament
thinning process. It was observed that the electric field generates vortices
due to an electrophoretic effect in the carrier fluid when EC is present in the
formulations, which has severe consequences on the stability of the liquid
bridges, whereas it scarcely affects the shear viscosity; additionally, the
kind of 2D nanoparticles modifies also the conductivity and permittivity of the
solution, inducing Maxwell stresses that also make the filament more stable
against surface tension
Nanogel formation of polymer solutions flowing through porous media
A gelation process was seen to occur when Boger fluids made from aqueous solutions of polyacrylamide (PAA) and NaCl flowed through porous media with certain characteristics. As these viscoelastic fluids flow through a porous medium, the pressure drop across the bed varies linearly with the flow rate, as also happens with Newtonian fluids. Above a critical flow rate, elastic effects set in and the pressure drop grows above the low-flow-rate linear regime. Increasing further the flow rate, a more dramatic increase in the slope of the pressure drop curve can be observed as a consequence of nanogel formation. In this work, we discuss the reasons for this gelation process based on our measurements using porous media of different sizes, porosity and chemical composition. Additionally, the rheological properties of the fluids were investigated for shear and extensional flows. The fluids were also tested as they flowed through different microfluidic analogues of the porous media. The results indicate that the nanogel inception occurs with the adsorption of PAA molecules on the surface of the porous media particles that contain silica on their surfaces. Subsequently, if the interparticle space is small enough a jamming process occurs leading to flow-induced gel formation
Optimised cross-slot microdevices for homogeneous extension
Microfluidic cross-slot devices can generate wide regions of vorticity-free strong extensional flow near the stagnation point, resulting in large extensional deformation and orientation of the microstructure of complex fluids, with possible applications in extensional rheometry and hydrodynamic stretching of single cells or molecules. Standard cross-slot devices, with sharp or rounded corners, generate a flow field with a non-homogeneous extension rate that peaks at the stagnation point, but decays significantly with distance from the stagnation point. To circumvent this limitation, an optimized shape cross-slot extensional rheometer (OSCER) was designed numerically and shown to generate constant extension rate over a wide region of the in- and out-flowing symmetry planes [Haward et al., Phys. Rev. Lett., 2012, 109, 128301]. Since the OSCER device was based on a 2D flow approximation, the practical implementation requires a large aspect ratio, which cannot be reproduced by standard soft-lithography techniques. Here, we propose a set of new designs for optimized cross-slot geometries, considering aspect ratios of order 1 and different lengths of the homogeneous inlet/outlet-flow regions. Micro-particle image velocimetry experiments were carried out in order to validate the flow kinematics, and the velocity profiles were found to be linear along the in- and outflow centrelines in good quantitative agreement with the numerical predictions
Changing the Lives of People With Primary Immunodeficiencies (PI) With Early Testing and Diagnosis
Correlation between the rheology of 2D-inks precursors and the droplet size generated from a capillary nozzle in dripping regime
This study provides a complete rheological characterization of 2D
nanomaterial dispersions, employed as 2D-inks precursors in printed
electronics. Three different 2D nanomaterials (molybdenum disulfide (MoS2),
graphene, and hexagonal boron nitride(hBN)) were dispersed in a Newtonian fluid
(toluene) and a viscoelastic fluid (toluene + ethyl cellulose) with different
polymer concentrations. The presence of nanoparticles does not change the shear
rheology of the carrier fluid. Regarding the extensional rheology, the results
showed that the pinch-off phenomenon is present in all Toluene suspensions;
however, the presence of the ethyl cellulose introduces elasticity in the
system, even leading to the formation of beads-on-a-string, and the relaxation
times of the suspensions depends on the kind of nanoparticles present in the
fluid. As controlling the droplet size when dispensing 2D-inks is of paramount
importance for printed electronics, as well as for many other applications,
here it is presented a correlation between the rheological properties of these
2D-inks precursors and their droplet size when generated from a capillary
nozzle in dripping regime
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