We derive explicit analytical expressions for the recurrence relations using the analytical matrix method for frequency response and the Bautista-Manero-Puig model for complex fluids. The BMP model ...is derived from the Extended Irreversible Thermodynamics formalism and has been shown to be useful in predicting the complex rheological behavior of self-associative systems. All harmonics of the alternating normal and shear stresses in oscillatory shear with various amplitude oscillatory regimes (AOS) can be calculated analytically, i.e., small amplitude oscillatory shear (SAOS), medium amplitude oscillatory shear (MAOS), and large amplitude oscillatory shear (LAOS). We show that incorporating the effects of the first and second normal stress differences for all AOS regimes leads to the emergence of higher harmonics. We establish the limits between the different AOS regimes based on criteria suggested by the analytical method. For some typical systems, such as CTAB-NaSal, we found a satisfactory quantitative agreement with the measured behavior of AOS.
In this work, equilibrium molecular dynamics is used to predict the change in the Helmholtz free energy (
) for solutions of associative polymers of the telechelic type.
is calculated using the ...thermodynamic integration method, wherein molecular chains are simulated in a coarse-grain scale within an NVT ensemble. Simulations take place in the concentration interval
, such that aggregate formation is possible. The initial configuration is that of dispersed free chains; the increase of interactions among stickers signals the onset of the micelle formation process. The solution with high concentration experiences substantial micelle aggregation despite little conformation changes. Therein, transient networks among micellar flowers are formed. The decrease in free energy is related to the formation of aggregates to achieve stability. When solutions are thermodynamically stable, the free energy and configurational entropy decrease and increase non-linearly with concentration, respectively. Below critical micellar concentration, this behaviour is associated with a step-by-step process along which the sticker groups gather to form the first thermodynamically stable aggregate. Above critical micellar concentration, the most probable distribution increases with concentration, therefore, free energy decreases with increasing aggregates that form the network.
SEM image of microparticles obtained by Spray Drying of the encapsulated systems.
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•Aloe vera mucilage is an encapsulating agent of antioxidants using spray-drying.•The spray drying ...process produces stable microparticles for gallic acid delivery.•Microcapsules of Aloe vera loaded with gallic acid preserve its antioxidant capacity.•Morphology of the encapsulated systems evidenced particle agglomeration.•The rheological characterization evidenced strong particle-particle interactions.
Mucilage of Aloe vera (Aloe barbadensis Miller) is an alternative agent for microencapsulation by spray drying of labile bioactives, such as gallic acid. The microstructure of powders prepared with Aloe vera and maltodextrin (used for comparison purposes) with and without gallic acid revealed defined and undamaged particles. Aloe vera particles without gallic acid exhibited a quasi-modal distribution while the gallic acid loaded samples presented a bimodal distribution (i.e., particle size of 2–3 μm). In maltodextrin samples, the particle size presented a modal distribution (with and without gallic acid), but the mean particle size was almost ten times larger than that of Aloe vera (about 20 μm), as reflected in the low viscosity values of these samples. Under steady-state shear flow, the reconstituted samples depicted a shear-thinning behavior (n<1). In small-amplitude oscillatory flow the viscous component G" dominated over the storage modulus G’. The FT-IR analysis confirmed the effectiveness of the wall material encapsulating gallic acid. Likewise, antioxidant activity results revealed that the encapsulated phenolic compound retained its antioxidant capacity and stability due to the short contact time in the spray dryer (<5 s). Comparison of the release profiles (i.e., maximum release time and release percentage) revealed larger magnitudes in samples of Aloe vera containing gallic acid. Thus, the observed release profiles assess the suitability of encapsulation systems based on Aloe vera mucilage for potential applications in functional foods and pharmaceuticals, where maltodextrin is commonly used.
SEM image of microparticles obtained by Spray Drying of antioxidant laurel infusions at the optimum conditions. The results evidenced that the best conditions for laurel encapsulation by SD were ...160°C inlet temperature and 8mL/min feed rate. The encapsulated systems may be optimum delivery systems according to release profiles.
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•Laurel infusions with antioxidant capacity were spray dried.•Optimum conditions were found to produce stable and well-formed microcapsules.•Produced microcapsules preserve both phenolic content and antioxidant capacity.•The rheological characterization evidenced strong particle–particle interactions.•Encapsulated systems may be optimum delivery systems according to release profiles.
The effect of maltodextrin as an encapsulating agent on spray dried (SD) laurel infusions was studied (inlet temperatures: 140, 160 and 180°C, and feed rate: 8 and 10mL/min at fixed flow atomization). In the SD samples, the phenolic content (TPC), antioxidant capacity (DPPH*), morphology (SEM), chemical structure (FTIR), rheology properties and release profiles were studied. The results show that laurel infusion had 42.10 (±0.23) mg gallic acid equivalent/g of laurel and EC50 of 0.40 (±0.10) mg laurel/mL of DPPH*, the SD microparticles showed defined morphologies. Encapsulation of laurel infusion was achieved with an efficiency of ∼70%. The reconstituted SD powders solutions showed a shear-thinning rheological behavior (n<1). The results evidenced that the best conditions for laurel encapsulation by SD were 160°C inlet temperature and 8mL/min feed rate.
•Rheological and SALS data in CTAT + NaCl solutions are described by the BMP model.•Agreement with the Cox-Merz rule at high salt content agrees with fast breaking kinetics.•The shift of the ...scattering patterns with shear rate is predicted by the kinetic (BMP) model.•These predictions involve a variable relaxation time, which is a function of the kinetics.
In this work we present theoretical and experimental studies on the structure factor in micellar systems. Cetyl trimethyl ammonium tosilate (CTAT) solutions in absence of salts exhibit a flow behavior similar to polymer solutions, while upon an increase in the ionic strength with NaCl, their behavior changes to that of a kinetic-controlled breakage-reformation process. The relaxation mechanism thus changes from that of a spectrum of relaxation times into an averaged dominant relaxation mode, representative of reversible kinetics.
Micellar solutions exhibit mechanical instabilities under flow, which may be increased by concentration fluctuations. Elastic stresses increase concentration fluctuations, which are experimentally observed by rheo-optical techniques, such as Rheo-SALS, depicting characteristic “butterfly” patterns. Scattering patterns provide information on the structure factor, arising from concentration fluctuations of the flowing solution. Upon addition of salt to a CTAT solution, we observe agreement with the Cox-Merz rule, revealing that the solution is in the fast-breaking regime approaching a single relaxation time kinetics. This behavior cannot be predicted by polymer solution theories which do not incorporate the breakage-reformation kinetics. Here, we propose a kinetic model by which the structure factor can be calculated for these micellar solutions. A shift in the maximum of the scattering patterns as the shear rate is increased is predicted by the new theory, which can be explained by considering flow-induced changes in the structure of the solutions with the underlying kinetics of breakage-reformation. Agreement with experiments is exhibited.
•High-We solutions for wormlike micellar systems (Wecrit∼400).•Constitutive (ABS) and centreline (VGR) corrections proposed.•Theoretical-to-numerical stress-component-eigenvalues identities at ...centreline.•Second eigenvalue of conformation tensor as numerical stability marker.•General applicability to non-thixotropic PTT models (Wecrit∼5000+).
This study is concerned with the numerical modelling of thixotropic and non-thixotropic materials in contraction–expansion flows at high Weissenberg number (We). Thixotropy is represented via a new micellar time-dependent constitutive model for worm-like micellar systems and contrasted against network-based time-independent PTT forms. The work focuses on steady-state solutions in axisymmetric rounded-corner 4:1:4 contraction–expansion flows for the benchmark solvent-fraction of β=1/9 and moderate hardening characteristics (ε=0.25). In practice, this work has relevance to industrial and healthcare applications, such as enhanced oil-reservoir recovery and microfluidics. Simulations have been performed via a hybrid finite element/finite volume algorithm, based around an incremental pressure-correction time-stepping structure. To obtain high-We solutions, both micellar and PTT constitutive equation f-functionals have been amended by (i) adopting their absolute values appealing to physical arguments (ABS-correction); (ii) through a change of stress variable, Π=τp+(ηp0/λ1)I, that aims to prevent the loss of evolution in the underlying initial value problem; and finally, (iii) through an improved realisation of velocity gradient boundary conditions imposed at the centreline (VGR-correction). On the centreline, the eigenvalues of Π are identified with its Π-stress-components, and discontinuities in Π-components are located and associated with the f-functional-poles in simple uniaxial extension. Quality of solution is described through τrz, N1 and N2 (signature of vortex dynamics) stress fields, and Π-eigenvalues. With {micellar, EPTT} fluids, the critical Weissenberg number is shifted from critical states of Wecrit={4.9,220} without correction, to Wecrit={O(102),O(103)} with ABS–VGR-correction. Furthermore, such constitutive equation correction has been found to have general applicability.
A biocomposite based on montmorillonite clay mineral (MMT) and hybrid biopolymer microparticles (MP) carrying atorvastatin (AC), these were designed for oral drug carrier was characterized by ...scanning and transmission electron microscopy (SEM &TEM), differential scanning calorimetry (DSC) and in vitro biocompatibility by cell viability on fibroblast cells. The rheological synergism relation (by linear oscillatory tests at different pH and frequency) with the mucoadhesive properties revealed interactions between isolated mucin and the MP/MMT biocomposite and those between the mucin and the individual components. Results of these tests showed that the MP/MMT biocomposite provides an improvement in the thermal stability and hence good biocompatibility exhibiting anti-proliferation effects at high concentrations. To our knowledge, no previous reports exist on the determination of the rheological synergism in biocomposites with MMT clay either alone. Moreover, we found a dependence of the rheological synergism on pH and frequency provided by linear viscoelastic data at low frequencies. The MP/MMT biocomposite possesses the highest rheological synergism related to the strongest interaction with mucin at pH 1.6.
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•A raspberry-like structure with MMT adsorbed on the MP surface was observed•MP/MMT composite exhibited improved thermal stability•The composite showed good biocompatibility due to high cell bioavailability•Strong pH-dependence of the interaction between mucin and MP/MMT composite was observed by rheological measurements•Solid-like confirmation is proposed to avoid false-negative rheological mucoadhesive assessment
The main objective of this work is to demonstrate the agreement between the two-fluid linear Langevin formulation and that described by the extended irreversible thermodynamics (EIT). The two-fluid ...model, originally proposed by de Gennes, has been widely analyzed by many authors in various flow situations, especially to compare predictions with experimental data of the structure factor in many complex flows. The canonical Langevin equations together with the fluctuation-dissipation theorem ensure consistent thermodynamic behavior for constitutive equations. Therefore, agreement between the EIT formulation and the two-fluid Langevin equations demonstrates the thermodynamic consistency of the EIT formulation. Extension of this analysis to include normal stresses is also considered.
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