Catarina Marques Mendes Almeida da Rosa Leal

Electro-rheology study of a series of liquid crystal cyanobiphenyls: experimental and theoretical treatment

In this work we study the electro-rheological behaviour of a series of four liquid crystal (LC) cyanobiphenyls with a number of carbon atoms in the alkyl group, ranging from five to eight (5CB–8CB). We present the flow curves for different temperatures and under the influence of an external electric field, ranging from 0 to 3 kV mm−1, and the viscosity as a function of the temperature, for the same values of electric field, obtained for different shear rates. Theoretical interpretation of the observed behaviours is proposed in the framework of the continuum theory of Leslie–Ericksen for low molecular weight nematic LCs. In our analysis, the director alignment angle is only a function of the ratio between the shear rate and the square of the electric field – boundary conditions are neglected. By fitting the theoretical model to the experimental data, we are able to determine some viscosity coefficients and the dielectric anisotropy as a function of temperature. To interpret the behaviour of the flow curves near the nematic–isotropic transitions, we apply the continuum theory of Olmsted–Goldbart, which extends the theory of Leslie–Ericksen to the case where the degree of alignment of the LC molecules can also vary.

Rheological Properties of Acetoxypropylcellulose in the Thermotropic Chiral Nematic Phase

Abstract 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 f...

Experimental Results on Electrorheology of Liquid Crystalline Polymer Solutions

The electrorheological (ER) effect is known as the enhancement of the apparent viscosity upon application of an external electric field. Suspensions of polarizable particles in non‐conducting solvents are the most studied electrorheological fluids, however, liquid crystalline materials may also present ER effect as long as their dielectric anisotropy is positive. In the liquid crystalline state of a positive dielectric anisotropy, the application of the electric field makes the director align perpendicular to the flow direction, thus increasing the apparent viscosity. In this work results of two liquid crystalline polymer solutions, acetoxypropylcellulose (APC) in dimethylacetamide (DMAc) and poly‐γ‐benzyl‐L‐glutamate (PBLG) in 1,4‐dioxane, presenting opposite behavior upon application of the electric field, will be presented. APC/DMAc (negative dielectric anisotropy) presents a decrease of the apparent viscosity upon application of the electric field, as expected, while PBLG/1,4‐dioxane (positive dielect...

NMR Study of Flow and Viscoelastic Properties of PBLG/m-Cresol Lyotropic Liquid Crystal

Abstract 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.

Rheology of the cytoskeleton as a fractal network

P. Patŕıcio, 2, ∗ C. R. Leal, 3 J. Duarte, 4 and C. Januário ISEL Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, 1959-007 Lisboa, Portugal. CEDOC, Faculdade de Ciências Médicas, Universidade Nova de Lisboa, 1169-056 Lisboa, Portugal. Centro de Investigação em Agronomia, Alimentos, Ambiente e Paisagem, LEAF, Instituto Superior de Agronomia, Universidade de Lisboa, 1349-017 Lisboa, Portugal. CAMGSD, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal (Dated: July 20, 2015)

Optimisation of Rodrun LC3000/PP Compatibilised Blends: Influence of the Compatibiliser and LCP Contents on the Rheological, Morphological and Mechanical Properties

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 different morphologies and may be correlated with the effectiveness of compatibilisers, which is not surprising since the flow behaviour of LCPs is highly complex and non-linear, with lower non-linear characters obtained for those blends that showed a higher fibrillar formation and better mechanical properties.

Electrorheological characterization of dispersions in silicone oil of encapsulated liquid crystal 4-n-penthyl-4 '-cyanobiphenyl in polyvinyl alcohol and in silica

The electrorheological (ER) effect is known as the change in the apparent viscosity upon the application of an external electric field perpendicular to the flow direction. In this work we present the electrorheological behaviour of suspensions in silicone oil of two different dispersed phases: foams of liquid crystal 4-n-penthyl-4′-cyanobiphenyl (5CB) encapsulated in polyvinyl alcohol (PVA) and nano/microspheres of 5CB encapsulated in silica. We will present the viscosity curves under the application of an electric field ranging between 0 and 3 kV mm−1. The ER effect was observed for the suspensions of 5CB/PVA but not in the case of 5CB/silica. For the case of the suspensions of 5CB/PVA, the effect of the viscosity of the continuum phase and the concentration of the dispersed phase was analysed, showing that the enhancement of the viscosity of the suspension increases with the concentration, as expected, however the continuum phase viscosity has no significant effect, at least in the investigated viscosity range.

A study of Rodrun LC3000/PP blends under different stationary and non-stationary shear conditions : the influence of LCP content and processing temperature

Blends containing liquid crystalline polymers and thermoplastics have been a topic of great interest for the scientific community due to their excellent performance and properties and thus, promising use in industrial applications. For that reason, from the eighties until today, these systems were widely studied and characterized in terms of their mechanical, morphological, and rheological properties under stationary conditions but not under non-stationary ones which are, in fact, those most relevant to processing sequences. Thus, despite all the published work on this subject, there is still a need to study the response of the materials under the latter conditions. The transient shear measurements performed on the blends of Rodrun LC3000 and PP showed an overshoot for the transient stress, the magnitude of which increases with increasing LCP content. This overshoot is attributed to the orientation and deformation of the LCP structures. The results obtained for the blends by Large Amplitude Oscillatory Shear, LAOS, were revealed to be highly sensitive (compared with those observed in steady shear measurements) not only to the LCP content, but also to the processing temperature. From the rheological point of view an unusual behavior was observed for these systems, which was characterized by an increase of the viscosity and storage modulus with the increase of the LCP content at low frequencies, but a decrease at high frequencies. The traditional and well-known decrease of steady shear viscosity and mechanical improvement, induced by the addition of liquid crystalline polymer to the thermoplastic, was also observed.

Shear Flows of LC Polymers: Reanalysis of Some Literature Data within a New Theoretical Framework

ABSTRACT A novel continuum theory of nematic liquid crystalline polymers has been recently proposed (Martins, 1993) and is applied here to interpret new and quoted data, for concentrated solutions of hydroxypropylcellulose (HPC) in acetic acid and poly(γ-benzil)L-glutamate (PBLG) in m-cresol, on shear rate ( γ ˙ ), molecular mass, and concentration dependence of the three flow functions η( γ ˙ ), N1( γ ˙ ) and N2( γ ˙ ) characterizing simple shear flows of these materials. The agreement between theory and experiment is in general very good. The five “Leslies viscosity coefficients” and two other structural parameters have been estimated. Some struture-properties relationships are derived from this analysis as well.

Rotational tumbling of escherichia coli aggregates under shear

Growing living cultures of Escherichia coli bacteria are investigated using real-time in situ rheology and rheoimaging measurements. In the early stages of growth (lag phase) and when subjected to a constant stationary shear, the viscosity slowly increases with the cells population. As the bacteria reach the exponential phase of growth, the viscosity increases rapidly, with sudden and temporary abrupt decreases and recoveries. At a certain stage, corresponding grossly to the late phase of growth, when the population stabilizes, the viscosity also keeps its maximum constant value, with drops and recoveries, for a long period of time. This complex rheological behavior, which is observed to be shear strain dependent, is a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties. Particular attention is given to the late phase of growth of E. coli populations under shear. Rheoimaging measurements reveal, near the static plate, a rotational motion of E. coli aggregates, collectively tumbling and flowing in the shear direction. This behavior is interpreted in the light of a simple theoretical approach based on simple rigid body mechanics.