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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.
- 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.
- The first normal stress difference and viscosity in shear of liquid crystalline solutions of hydroxypropylcellulose: new experimental data and theoryPublication . Fried, F.; Leal, Catarina R.; Godinho, M. H.; Martins, A. F.The constitutive equations for liquid crystalline polymers recently proposed by one of us [1] are applied here to interpret the behaviour of the shear viscosity η equation image and the first normal stress difference N1($ \dot \gamma $equation image) measured for liquid crystalline (LC) solutions of hydroxypropylcellulose in acetic acid. N1(equation image) is observed to change from positive to negative and again to positive, as the shear rate $ \dot \gamma $equation image increases, at lower concentrations, in the LC phase. The $ \dot \gamma $equation image-values at which N1 changes sign depend on the molecular mass (degree of polymerization) and on the concentration. η $ \dot \gamma $equation image shows a small Newtonian plateau at low shear rates and a strong shear-thinning at higher values of $ \dot \gamma $equation image. The rate of decrease of η $ \dot \gamma $equation image in this region shows an “hesitation” similar to one previously observed in LC solutions of poly-γ-benzyl-L-glutamate PBLG. All these observations can be rationalized within the frame-work of Martins' theory. The expressions for N1($ \dot \gamma $equation image) and η $ \dot \gamma $equation image derived from this theory fit very well (quantitatively) to the experimental data and some fundamental viscoelastic parameters of the system under study are thereby obtained for the first time.