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- Experimental results on electrorheology of liquid crystalline polymer solutionsPublication . Neves, S.; Leal, Catarina R.; Cidade, M. T.The electrorheological (ER) effect is known as the enhancement of the apparent viscosity upon apphcation 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 fiow direction, thus increasing the apparent viscosity. In this work results of two liquid crystalline polymer solutions, acetoxypropylcellulose (APC) in dimethylacetamide (DMAc) and poly-y-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/l,4-dioxane (positive dielectric anisotropy) presents the opposite behavior. For this last solution we will present the shear fiow curves for different electric fields in function of polymer molecular weight and solution concentration.
- Following micelles with rheo-NMRPublication . Almeida, Pedro L.; Feio, G.; Pereira, M.; Scheven, U.; R. Leal, CatarinaThe rheological behaviour of the micellar system Cetyltrimethylammonium Bromide (CTAB)/Sodium Salicylate (NaSal) in water is highly dependent on the components concentration and temperature. The surfactant CTAB forms spherical micelles in water above a certain concentration (CD), which upon addition of a salt (NaSal) elongate to form entangled wormlike structures, also known as “living polymers”. The viscosity of wormlike micelles solutions increases dramatically with the increase of salt concentration (CS), changing from quasi-newtonian to a viscoelastic behaviour in the gel-like, highly entangled state. Such rich rheological behaviour has already been characterized by some of the authors and in the literature, e.g., [1-3]. The rheological behaviour of CTAB/NaSal aqueous solutions in the regime CD/Cs>1 are presented in this work. Furthermore, we followed the rheological behaviour of these solutions by a rheo-NMR technique, allowing for an insight of “what happens” inside the sample during the application of a shear flow. To achieve this goal we performed quantitative flow rate measurements using velocity NMR mapping in the rheo-NMR experiments, developed by Callaghan [4]. Attempts to characterize the velocity profile in this system has already been described [5] using an optical-shear flow experimental setup, although for solutions with CD/Cs<1, where the surfactant [CTAB] has always a smaller concentration than the salt [NaSal].
- Bacterial growth screened by rheologyPublication . Portela, Raquel; Pereira, M.; Sobral, Rita; Almeida, Pedro L.; R. Leal, CatarinaThe study of bacterial growth is a challenging field since it aims to describe the behaviour of microorganisms under different physical or chemical conditions. Bacterial growth as a biofilm is of particular interest as these gel-like structures are detrimental for all applications where “clean” surfaces are most important, and are related to failure of infection treatment, food spoilage and oil pipeline contamination, amongst others. In the literature one can find several works concerning the characterization of the mechanical behaviour of bacterial biofilms, although mostly are implemented over solid biofilms, as they appear in real situations [1-3], to study the adhesion properties in surfaces. In this work we applied a different technique to monitor the growth rate of a coccoid shaped bacterial species, the human pathogen Staphylococcus aureus.
- Staphylococcus aureus strains rheology during growthPublication . Portela, Raquel; Almeida, Pedro L.; Patricio, Pedro; M.T., Cidade; Sobral, Rita; R. Leal, CatarinaThe study of the mechanical properties of living bacteria in a liquid rich medium, environment commonly used in laboratorial settings, opens a new perspective on the bacterial physiology and behaviour during population growth. In this work, the human pathogen Staphylococcus aureus was used as a study model due to its coccoid shape and regular morphology: MRSA strain COL [1] and its mutant strain RUSAL9 [2], which presents a deficient daughter-cell separation mechanism. Cultures were grown under a mechanical stress solicitation in parallel with optical density monitorization. Complex viscoelastic behaviour was revealed by these bacterial systems [3]. In particular the shear viscosity measurement during growth time, for a constant shear rate, showed an unexpected behaviour that cannot be observed by common optical experimental techniques.
- Cell necklaces behave as a soft glassy materialPublication . Franco, Jose M.; Patricio, Pedro; Almeida, Pedro L.; Portela, Raquel; Sobral, Rita; R. Leal, CatarinaPreviously we have reported a complex and rich viscoelastic behavior observed during the Planktonic growth of S. aureus strain COL1. In particular, in stationary shear flow, the viscosity Keeps increasing during the exponential phase and returns close to its initial value for the late phase of growth, where the bactéria population stabilizes. In oscillatory flow, the elastic and viscous moduli exhibit power--‐law behaviors whose exponentes are dependente on the bacteria growth stage. These power--‐law dependencies of G’ and G’’ match a Soft Glassy Material behavior. To describe this observed behavior, we have hypothesized a microscopic model considering the formation of a dynamic web--‐like structure, where percolation phenomena can occur, depending on growth stage and cell density. In this communication we describe the formation of these web--‐like structures, resembling cell necklaces at a specific time interval during bacterial growth. These findings were obtained by combining the previous data with new measurements performed in a rheometer with real--‐time image acquisition.
- 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.
- Rotational and translational motion observed in Escherichia coli aggregates during shearPublication . Portela, R.; Franco, J. M.; Patricio, Pedro; Almeida, Pedro L.; Sobral, R. G.; Leal, Catarina R.Recently, the growth of an Escherichia coli culture was studied using real-time and in situ rheology and rheoimaging measurements, allowing to characterize their rheological behavior during time [1]. In the lag phase, bacteria are adapting to the new environmental growth conditions, with a characteristic slow division rate. Accordingly, the viscosity shows a slow and constant increase with time. In the exponential phase the viscosity presents a dramatic increase, but exhibits several drops and recoveries. In the late phase of growth, the viscosity increase slows down, reaching na intermittent plateau of maximum viscosity, with several drops and recoveries. In this phase, the highest bactéria density is attained: bacteria still grow and divide, but at a lower rate; big and irregular bacteria aggregates are observed, which keep moving in suspension and no significant sedimentation is observed; the aggregates present translational motion in the shear flow direction and rotational motion in the vorticity direction; the aggregates become larger along time, due to the incorporation of smaller aggregates; due to the rotational motion, the aggregates become elongated along the rotational axis; apparently, the size of the aggregates does not influence the rotational motion, since almost all aggregates rotate with the same angular velocity, which is related to the applied shear rate. As a first approximation, and because an explicit individual motion of the cells within each aggregate is not observed, this behavior is interpreted in light of a simple rigid-body motion, in which shear rate and angular velocity are dependent, which will be presented as a microscopic model.
- S. aureus and E. coli Co-culture Growth Under ShearPublication . Portela, Raquel; Almeida, Pedro L.; Sobral, Rita; R. Leal, CatarinaGrowing monocultures of two different species of human commensal/pathogenic bacteria, Staphylococcus aureus – a non-motile grampositive coccus and Escherichia coli – a motile gram-negative rod, were characterized using a real-time in situ rheology and rheo-imaging strategy. Subjecting bacterial populations to a shear flow is a closer approximation to bacterial thriving in the host, where they experience mechanical forces such as arterial or venous pressure. For both cultures, as the cell density of the population increases, cells rearrange themselves in different aggregates, capable of strongly influencing their environment, and leading to very different physical rheological responses, where motility appears to be determinant. One of the most striking observations is the behavior of the viscosity growth curve, showing dramatic value variations, with no counterpart in traditional biological measurements, as well as the coupling between translational and rotational motion of the E. coli aggregates along the growth curve [1], while S. aureus cells tend to sediment [2], over long periods of time. In the present study, a similar approach was applied to a co-culture of these two bacteria, S. aureus and E. coli, to evaluate the effect of possible interspecies interactions on the viscosity curve of the culture, during growth, when subject to a shear flow. Surprisingly, the observed behavior of the viscosity growth curve was enhanced in comparison to each individual curve and reveals a combination of details specific of each monoculture, suggesting synergy between these two bacterial species. After the rheological analysis, the final co-culture was recovered and inoculated on different solid media that allow to distinguish the development of S. aureus or E. coli colonies. Unexpectedly, S. aureus showed the capacity to accelerate its growth rate relatively to E. coli, when the two-species community is subjected to a shear flow. This behavior may reflect the occurrence of specific growth adaptations during co-culture upon shear flow, getting one step closer to physiological conditions.
- Rheology of living cellsPublication . Portela, R.; Franco, Jose M.; Almeida, Pedro L.; Patricio, Pedro; Sobral, Rita; R. Leal, CatarinaThe mechanical behavior of living cells, during planktonic growth, has been thoroughly explored combining common biological techniques with rheology and rheo-imaging measurements. Under a shear flow, bacterial cultures of Staphylococcus aureus revealed a complex and rich rheological behavior not usually accessed in biological studies. In particular, in stationary shear flow, the viscosity increased during the exponential phase and returned close to its initial value at the late phase of growth, accompanied by the stabilization of the bacterial population. In oscillatory flow, the elastic and viscous moduli exhibited power-law behaviors whose exponents are dependent on the bacteria growth stage, and can be associated to a Soft Glassy Material behavior. These behaviors were framed in a microscopic model that suggests the formation of a dynamic web-like structure, where specific aggregation phenomena may occur, depending on growth stage and cell density. Furthermore, systematic measurements combining optical density and dry weight techniques presented new evidences, which confirmed that the observed cell aggregation patterns developed during growth, under shear, can not only be cell density dependent.
- Impedance spectroscopy and electro-optic switching times of a liquid crystal-hydroxypropylcellulose network compositePublication . MANAILA MAXIMEAN, DOINA; Barar, A.; Ganea, C. P.; Almeida, Pedro L.; Danila, OctavianThis paper presents electric and electro-optic studies conducted on a new composite, namely hydroxypropylcellulose network with liquid crystal fillings. The composite was manufactured using the electrospinning method, and deposited on indium tin oxide glass. In terms of electric characterization, the samples were subjected to impedance spectroscopy, their equivalent circuit model was deduced, and the corresponding electric parameters were computed based on the Cole-Cole diagrams and frequency plots of the two impedance components. In terms of electro-optic studies, the optical transmission of the sample under a varying steady-state alternative regime voltage was recorded, and the saturation threshold field was determined. Also, the switching times of the sample with respect to a step signal were determined.