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Marques Mendes Almeida da Rosa Leal, Catarina
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- Living S. aureus bacteria rheologyPublication . Portela, R.; Franco, J. M.; Patricio, Pedro; Almeida, Pedro L.; Sobral, R. G.; Leal, Catarina R.The rheological characterization of Staphylococcus aureus cultures has shown a complex and rich viscoelastic behavior, during the bacteria population growth, when subject to a shear flow [1,2]. In particular, in stationary shear flow, the viscosity keeps increasing during the exponential phase reaching a maximum value (∼30x the initial value) after which it drops and returns close to its initial value in the stationary phase of growth, where the cell number of the bacterial population stabilizes. These behaviors can be associated with cell density and aggregation patterns that are developed during culture growth, showing a collective behavior. This behavior has no counterpart in the bacterial growth curve obtained by optical density monitorization (OD620nm and cfus/ml measurements). In oscillatory flow, the elastic and viscous moduli exhibit power-law behaviors whose exponents are dependent on the bacteria growth stage. These power-law dependencies of G’ and G’’ are in accordance with the Soft Glassy Material model [3], given by: G’~ ωx and G’’~ ωx To describe the observed behavior, a microscopic model considering the formation of a dynamic web-like structure was hypothesized [1], where percolation phenomena can occur, depending on the growth stage and on cell density. Recently, using real-time image rheology was possible to visualize the aggregation process associated with these dramatic changes in the viscoelastic behavior. In particular, the formation of web-like structures, at a specific time interval during the exponential phase of the bacteria growth and the cell sedimentation and subsequent enlargement of bacterial aggregates in the passage to the stationary phase of growth. These findings were essential to corroborate the microscopic model previously proposed and the main results of this study are compiled and presented in this work, see Fig.1.
- Aging effects on the rheology of LC solutions of hydroxypropylcellulosePublication . Leal, Catarina R.; Godinho, M. H.; Martins, A. F.; Fried, F.We apply the recently developed continuum theory of liquid crystalline polymers1 to interpret the behavior of the shear viscosity η(γ.) and the first normal-stress difference N1(γ.) measured for liquid crystalline solutions (c = 37%, c > c*) of HPC (Aldrich, Mw = 100,000) in acetic acid (AA) with different ages: 1 day, 1 month and 2 years. η(γ.) and N1(γ.) were measured over four decades in γ. N1(γ.) is observed to change from positive to negative and again to positive, as the shear rate γ. increases. η(γ.) shows a small newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ. The rate of decrease of η(γ.) in this region is not monotonous, as usual, but shows an “hesitation” similar to one previously observed in a different system2. The aging effect promotes a “depolymerization”3 of HPC. This, in turn, should have a strong influence on the behavior of Nl(γ.) and η(γ.) which is indeed observed4. All these observations can be rationalized within the framework of the theory1.
- Rheo-NMR velocimetry characterisation of PBLG/m-cresolPublication . Leal, Catarina R.; Feio, Gabriel; Almeida, Pedro L.NMR spectroscopy was used to characterise the velocity profile in nematic solutions of poly(gamma-benzyl-L-glutamate) (PBLG) in m-cresol during the application of a simple shear-flow at constant shear-rate values. In this study, the rheo-NMR technique was used that allows an insight over the fluid dynamics during the application of a shear flow in the presence of an external magnetic field, in a perpendicular direction to the shear gradient. Using this Magnetic Resonance Imaging (MRI) based technique, the velocity profile developed inside the system is accessed.
- NMR study of flow and viscoelastic properties of PBLG/m-Cresol lyotropic liquid crystalPublication . Veron, Alain; Gomes, Alexandre E.; Leal, Catarina R.; Klink, Jacques Van Der; Martins, Assis F.Director distortion patterns and backflow effects in transient flows of nematic solutions of PBLG/m-cresol are studied by deuteron NMR. The simulation of the observed NMR spectra as a function of time allows to characterise the director field pattern and to evaluate most of the viscoelastic properties of the system. A good agreement with classical rheometry is observed.
- Temperature dependence of the rheological properties of acetoxypropylcellulose in the thermotropic chiral nematic phasePublication . Cidade, M. T.; Leal, Catarina R.; Godinho, M. H.; Navard, P.The rheological behaviour of two different molecular weights of a thermotropic liquid crystalline cellulose derivative, acetoxypropylcellulose (APC), at T=I2OoC, has already been reported [ l]. In this work we present the temperature dependence of the rheological properties of two molecular weights of APC. The shear viscosity, q, and first normal stress difference, N,, were measured at temperatures T=120, 130 and 140°C. for Mciw = 94000 g/mole, and T=120 and 140°C for Mw = 129000 g/mole, and for shear rates, +, between 0.01 and 10 s-’. The shear viscosity q(+) decreases with increasing T, for both samples, showing a strong shear thinning behaviour at all temperatures and over the whole range of + studied, except for shear rates between about 0.2 and 1 s-’ (depending on IGiw and T), where a quasi-Newtonian plateau is observed. The first normal stress difference N1 (+) also decreases with increasing T at a given +. It increases with shear rate over the whole + range studied, and shows an inflection at + values slightly above + = 1/r, where z is the relaxation time of the polymer memory function. The temperature dependence of the shear viscosity was fitted to an Arrhenius law, giving an apparent activation energy (E,) in the order of 8-15 kcaUmole, depending on molecular weight and shear rate. The activation energy was found to increase with molecular weight (at a given + ). The variation of E, with +, , for Mw = 94000 g/mole, shows a minimum at ;i x 2 s-l. Using a continuum theory for nematic polymers, proposed by Martins [2], some fundamental parameters were obtained from the fit of the theory to the experimental data. Using these parameters it was possible to construct a “master curve” for the viscosity, q(+), in good agreement with the experimental data.
- An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4'-cyanobiphenylPublication . Cidade, M. T.; Leal, Catarina R.; Patricio, PedroAn experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4'-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie-Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm-1, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log-log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie-Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.
- Rheological properties of acetoxypropylcellulose in the thermotropic chiral nematic phasePublication . Cidade, M. T.; Leal, Catarina R.; Godinho, M. H.; Martins, A. F.; Navard, P.Experimental data for the rheological behavior of two thermotropic liquid crystalline (LC) samples of acetoxypropylcellulose (APC) with different molecular weights, at 120°C, and in shear rates between 0.01 and 10s −1, are presented and analyzed in the framework of the continuum theory for LC polymers recently proposed by Martins1. The viscosity η(γ.)shows a strong shear thinning in the range of shear rates γ. studied, with an hesitation at shear rates of about 0.1-0.2 s−1, depending on the molecular weight, and the first normal stress difference Nl(γ.) shows only positive values, increasing with shear rate γ., with an hesitation at shear rates of an order of magnitude higher, i.e. about 1-2 s−l, also depending on the molecular weight. The hesitation points of the flow functions are displaced towards lower values of the shear rate, with increasing molecular weight. For small and intermediate γ. the shear viscosity of the higher molecular weight sample is greater than the corresponding viscosity for the lower molecular weight sample, but this pattern is reversed at higher γ., the crossover point being at γ. = 1.5 s−1. The molecular weight dependence of the first normal stress difference follows a similar pattern. All these observations can be interpreted by Martins' theory. The expressions for η(γ.) and N1(γ.) derived from this theory fit very well to the experimental data, therefore allowing for some fundamental viscoelastic parameters to be estimated.
- Influence of type of compatibilizer on the rheological and mechanical behavior of LCP/TP blends under different stationary and nonstationary shear conditionsPublication . Filipe, Susana; Maia, João M.; Duarte, Ana; Leal, Catarina R.; Cidade, Maria T.The addition of small amounts of liquid-crystalline polymers to thermoplastics leads to the formation of in situ–reinforced materials, with improved processability and mechanical properties. Nevertheless, the lack of adhesion between the thermoplastic and the liquid-crystalline polymer often occurs, thus requiring the use of compatibilizers. In this case, the results of several previous works show that there is an improvement of strength, usually accompanied by a decrease of toughness and, thus, the interest of LCP/TP blends for industrial applications will certainly increase if both strength and toughness are obtained. Additionally, the emphasis of previous studies has been on the evaluation of the properties of the blend under stationary conditions and not under non-stationary ones, which are, in fact, those most relevant to processing sequences. Thus, the present work focuses on the influence of type of compatibilizer on the mechanical and rheological properties of polypropylene/LCP blends under nonstationary conditions. In terms of mechanical properties, the traditional increase of tensile strength was obtained for all compatibilizers, which was essentially due to the formation, during processing, of thinner and longer fibrils of LCP dispersed in the matrix than those observed for the noncompatibilized blends. Additionally, an improvement of the impact strength and flexural modulus was also observed for the blend in which a compatibilizer with an elastomeric nature was used. Rheologically, the experiments most sensitive to the structure were those performed in transient shear, with an increase of the transient stress (in the form of an overshoot) of different magnitudes being observed for the different compatibilizers.
- Rheological properties of lyotropic solutions of acetoxypropylcellulose in dimethylacetamide. A comparision with the thermotropic casePublication . Cidade, M. T.; Leal, Catarina R.The rheological properties of a thermotropic acetoxypropylcellulose (APC) were already described in previous works [1,2]. In this work we present the rheo logical properties, viscosity, Z, and first normal stress difference, N1, of lyotropic solutions of APC in dimethylacetamide (DMAc). The flow curve of the lyotropic solution presents the typical behavior for this type of system (Onogi and Asada Curve) [3] however, contrary to what happens with that other lyotropic system [4–7], the APC/DMAc system presents a con tinuous increase of N1 in function of the shear rate, which we attribute to the fact that APC has higher flexibility than the other liquid crystalline polymers studied so far—namely hydroxypropylcellulose and poly-benzyl-L-glutamate— that present a negative N1 region, as well as a low molecular weight. We will also present some remarks concerning rheo-optical studies preformed with APC/DMAc, by comparison with previous studies on thermotropic APC [8,9].
- Optimisation of rodrun LC3000/PP compatibilised blends: influence of the compatibiliser and LCP contents on the rheological, morphological and mechanical propertiesPublication . Filipe, S.; Maia, J. M.; Leal, Catarina R.; Menon, A. R.; Duarte, A.; Cidade, M. T.The present study focuses on the optimisation of LCP/TP compatibilised systems. This study was essentially devoted to the influence of different LCP and compatibiliser contents on the final morphological, rheological and mechanical properties of the blends. Rheologically, we used both standard small amplitude oscillatory shear and large amplitude oscillatory shear (LAOS) experiments in order to improve the understanding of the mechanisms behind the mechanical reinforcement that might be helpful in the optimisation of liquid crystalline polymer and thermoplastic blends: The results revealed that LAOS, in particular, is highly sensitive to diferente morphologies and may be correlated with the effectiveness of compatibilisers, which is not surprising since the flow behaviour of LCP sis highly complex and non-linear, with lower non-linear characters obtained for those blends that showed a higher fibrilar formation and better mechanical properties.
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