Francisco Rodríguez-González

Rheo-PIV of a yield-stress fluid in a capillary with slip at the wall

An analysis of the yielding and flow behavior of a model yield-stress fluid, 0.2 wt% Carbopol gel, in a capillary with slip at the wall has been carried out in the present work. For this, a study of the flow kinematics in a capillary rheometer was performed with a two-dimensional particle image velocimetry (PIV) system. Besides, a stress-controlled rotational rheometer with a vane rotor was used as an independent way to measure the yield stress. The results in this work show that in the limit of resolution of the PIV technique, the flow behavior agrees with the existence of a yield stress, but there is a smooth solid–liquid transition in the capillary flow curve, which complicates the determination of the yield stress from rheometrical data. This complication, however, is overcome by using the solely velocity profiles and the measured wall shear stresses, from which the yield-stress value is reliably determined. The main details of the kinematics in the presence of slip were all captured during the experiments, namely, a purely plug flow before yielding, the solid–liquid transition, as well as the behavior under flow, respectively. Finally, it was found that the slip velocity increases in a power-law way with the shear stress.

Axial annular flow of a viscoplastic microgel with wall slip

The fully developed velocity distributions of a viscoplastic Carbopol microgel [0.12 wt. % poly(acrylic acid) in water] flowing axially in the annular gap between two long concentric cylinders were determined. The ratios of the length over the gap and the inner over the outer radii, κ, of the annulus were 442 and 0.78, respectively. The particle image velocimetry, PIV, measurements revealed that the viscoplasticity of the microgel in axial annular flow is manifested via the formation of plug flow and wall slip. The plug flow region spanned the entire or part of the annular gap depending on whether the absolute values of the shear stresses at the two walls were below or above the yield stress of the microgel, respectively. For all flow rates, the velocities at the two walls were similar indicating that the absolute values of the wall shear stress were also similar. Equality of the shear stresses at the two walls suggested that axial annular flow at a relatively high κ can be used as a viscometric flow for ...

Properties of Edible Films Based on Oxidized Starch and Zein

The objective of this work was to investigate the effect of zein and film formulation on mechanical and structural properties of native (FNS), and oxidized with 2.5% (FOSA) and 3.5% (FOSB) banana starch. The oxidized starch showed differences from native starch due to the oxidation process, showing a decrease in lipids, proteins, and amylose. The increase of the sodium hypochlorite increased the content of carbonyl and carboxyl groups in the ranges 0.015–0.028% and 0.022–0.031%, respectively. The film obtained from FOSB displayed the highest tensile strength (5.05 MPa) and satisfactory elongation value (27.1%). The zein addition caused a decrease in these mechanical properties, as well as a significant decrease in water vapour permeability (WVP). However, films from FOSA and FOSB showed higher permeability than that of the native starch. The addition of glycerol and the level of oxidation increased the films moisture. Micrographs showed that, during the oxidation process, impurities were largely eliminated from the starch granule, noting more homogeneous structures both in granules and films.

Origin of shear thickening in semidilute wormlike micellar solutions and evidence of elastic turbulencea)

The origin of shear thickening in an equimolar semidilute wormlike micellar solution of cetylpyridinium chloride and sodium salicylate was investigated in this work by using Couette rheometry, flow visualization, and capillary Rheo-particle image velocimetry. The use of the combined methods allowed the discovery of gradient shear banding flow occurring from a critical shear stress and consisting of two main bands, one isotropic (transparent) of high viscosity and one structured (turbid) of low viscosity. Mechanical rheometry indicated macroscopic shear thinning behavior in the shear banding regime. However, local velocimetry showed that the turbid band increased its viscosity along with the shear stress, even though barely reached the value of the viscosity of the isotropic phase. This shear band is the precursor of shear induced structures that subsequently give rise to the average increase in viscosity or apparent shear thickening of the solution. Further increase in the shear stress promoted the growing of the turbid band across the flow region and led to destabilization of the shear banding flow independently of the type of rheometer used, as well as to vorticity banding in Couette flow. At last, vorticity banding disappeared and the flow developed elastic turbulence with chaotic dynamics.

Slip and frictional heating of extruded polyethylene melts

Extrusion of polymer melts with slip at the die generates frictional heating. The relationship between slip flow and frictional heating during the continuous extrusion of a non-slipping linear low-density (LLDPE) and a slipping high-density polyethylene (HDPE), respectively, both pure as well as blended with a fluoropolymer processing aid (PA), was investigated in this work by Rheo-particle image velocimetry and thermal imaging. Significant rises in temperature were measured under slip and no slip conditions, being these much higher than the values predicted by the adiabatic flow assumption. Clear difference was made between viscous and frictional heating before the stick-slip regime for the LLDPE, even though they could not be distinguished from one another at higher stresses. Such a difference, however, could not be made for the slipping HDPE, since overall in the presence of slip, frictional and viscous heating act synergistically to increase the melt temperature.

Barrier properties improvement using additives

Abstract Food products must be of good quality, safe, good sensory attributes, inexpensive and with a good shelf life. The pure polymers are often insufficient to be used for food packaging, due to low-medium gas barrier attributes, thermal stability, and low solvent water resistance. A critical issue in food packaging is that of migration and permeability. In general, permeability of a polymer to oxygen or moisture is dependent on a large number of interrelated factors, including polarity and structural features of polymeric side chains, hydrogen bonding characteristics, molecular weight and polydispersity, degree of branching or crosslinking, processing methodology, method of synthesis, and degree of crystallinity. Nanotechnology can provide solutions for these, for example, modifying the permeation behavior of foils, increasing barrier properties, improving mechanical and heat-resistance properties, developing active antimicrobial and antifungal surfaces, and sensing, as well as signaling microbiological and biochemical changes and creating the nanobiodegradable packaging.

Iron nanoparticles on colloidal substrates

Iron nanoparticles are of interest for possible applications such as remediation of soil and water, and for their magnetic properties. In the present work, a simple method for producing iron nanoparticles on a colloidal substrate is presented. These composites have potential applications related to their surface and magnetic properties.

Visualization of Vorticity Banding in a Shear Thickening Micellar Solution in Couette Flow

The shear thickening behavior and flow instabilities of an equimolar semidilute aqueous solution of cetylpyridinium chloride and sodium salicylate were studied in this work by using rheometry coupled with flow visualization. The experiments were conducted at 25°C in a stress controlled Couette rheometer with a transparent flow cell. A detailed flow curve was obtained for this solution, which includes five different regimes. At very low shear rates a Newtonian behavior was found, followed by a shear thinning one in the second regime. In the third regime, a strong orientation of the micelles consistent with shear banding served as a precursor of the shear induced structures (SIS) and shear thickening. The fourth and fifth regimes in the flow curve were separated by a spurt-like behavior, and they clearly evidenced the existence of shear thickening accompanied by stick–slip oscillations at the rotating cylinder. The use of the combined method allowed the detection of SIS. The visualization of this solution in the shear thickening regime showed the build-up of SIS (turbid fluid) in the shear banding regime followed by large variations in shear rate and viscosity, corresponding to the oscillation of clear and turbid bands stacked in the vorticity direction. These alternating bands and shear rate variations are in agreement with the presence of the vorticity banding instability as well as with the creation and destruction of SIS.