Browsing by Author "Dias, Ricardo P."
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- Análise do escoamento sanguíneo em microcanais com bifurcaçõesPublication . Cidre, Diana; Oliveira, Brigitte; Lagoela, Marco; Fernandes, Carla S.; Lima, Rui A.; Dias, Ricardo P.; Balsa, Carlos; Ishikawa, Takuji; Yamaguchi, TakamiVários estudos revelaram que a informação obtida sobre as propriedades reológicas do sangue em capilares de vidro difere da situação in vivo (Pries et al. 1994). As principais causas potenciais para estas discrepâncias podem ser devidas à camada superficial formada por células endoteliais e às redes microvasculares compostas por segmentos de vasos irregulares e curtos que estão ligados por numerosas bifurcações e convergências (Maeda 1996).O principal objectivo deste trabalho é analisar as propriedades não-Newtonianas do sangue durante o escoamento em modelos microvasculares. Neste estudo foi utilizado o método de litografia suave para fabricar microcanais bifurcados com secção rectangular em polidimetisiloxano (PDMS). Usando um sistema “confocal micro-PTV” (Lima et al. 2008), mediu-se o efeito da bifurcação no escoamento de partículas fluorescentes diluídas em água pura e em suspensões concentradas de eritrócitos. Os resultados experimentais obtidos pelo sistema “confocal micro-PTV” foram complementados por modelos hemodinâmicos computacionais. Após a execução de várias simulações com o software comercial de elementos finitos POLYFLOW®, compararam-se alguns resultados experimentais com resultados numéricos. Os resultados preliminares sugerem que as trajectórias dos eritrócitos podem sofrer alterações na direcção transversal ao escoamento devido à colisão com os eritrócitos que se encontram na vizinhança do ponto de divergência da bifurcação (Brigitte et al. 2009).
- Análise do escoamento sanguíneo em microcanais com bifurcaçõesPublication . Cidre, Diana; Lagoela, Marco; Oliveira, Brigitte; Fernandes, Carla S.; Balsa, Carlos; Lima, Rui A.; Dias, Ricardo P.; Ishikawa, Takuji; Yamaguchi, TakamiA dinâmica do escoamento sanguíneo na microcirculação depende fortemente das redes microvasculares compostas por segmentos de vasos curtos irregulares que estão ligados por inúmeras bifurcações. Este trabalho apresenta a aplicação de um sistema micro-PTV confocal para medir o comportamento dos eritrócitos durante o escoamento em um microcanal polidimetisiloxano (PDMS) rectangular com uma bifurcação. O efeito da bifurcação no escoamento de partículas fluorescentes diluídas em água pura e em suspensões concentradas com eritrócitos foi medido através de um sistema micro-PTV confocal. Numericamente, estudou-se o escoamento do sangue recorrendo ao software comercial de elementos finitos POLYFLOW® utilizando diferentes modelos reológicos. Os resultados preliminares sugerem que as trajectórias dos eritrócitos podem sofrer alterações na direcção transversal devido à acumulação de eritrócitos na vizinhança do ponto de divergência da bifurcação.
- Analysis of the blood flow in a microchannel with a bifurcationPublication . Oliveira, Brigitte; Lagoela, Marco; Cidre, Diana; Fernandes, Carla S.; Lima, Rui A.; Dias, Ricardo P.; Balsa, Carlos; Ishikawa, Takuji; Yamaguchi, TakamiIn vitro experiments allow precise measurement and control over relevant physiological variables. Thus, in the present study we have used a lithography technique to fabricate a rectangular PDMS microchannel with a bifurcation. By using a confocal micro-PTV system, we have measured the effect of bifurcation on the flow behaviour of both fluorescent particles diluted in pure water and red blood cells (RBCs) in concentrated suspensions. After performing simulations with the commercial finite element software package POLYFLOW®, some experimental results were compared with the numerical results. Our preliminary results suggest that the RBC paths may suffer fluctuations on the transversal direction caused by RBCs obstruction around the neighbourhood of the diverging point of the bifurcation.
- Application of binary packing for starch separation by hydrodynamic chromatographyPublication . Mota, Manuel; Teixeira, José; Dias, Ricardo P.; Yelshin, AlexanderColumns packed with commercial glass beads of 5 and 19 microns average diameter and binary mixtures with finest fraction of 5 micron (30 % volume fraction of the mixture) were used to analyse starch by hydrodynamic chromatography (HDC). Experiments were carried out at 3 and 15 ºC. The observed resolution increased with the application of binary packing as compared with single-size packing. The best results were obtained at starch’s amylopectin and amylose separation with a glass beads mixture (5 + 19 micron) at 3ºC. In what concerns amylopectin and amylose separation, a lower pressure drop were obtained for the mixed binary packing when compared with the packing containing uniform 5 micron glass beads. For the Hylon VII starch RRT were 0.777 and 0.964 for amylopectin (AP) and amylose (AM), respectively, while for the Tapioca starch the obtained RRTs were 0.799 and 0.923. Application of unbound glass beads as column packing might reduce equipment and running costs in preparative scale separations.
- Asymmetry of red blood cell motions in a microchannel with a diverging and converging bifurcationPublication . Leble, Vladimir; Lima, Rui A.; Dias, Ricardo P.; Fernandes, Carla S.; Ishikawa, Takuji; Imai, Yohsuke; Yamaguchi, TakamiIn microcirculation, red blood cells (RBCs) flowing through bifurcations may deform considerably due to combination of different phenomena that happen at the micro-scale level, such as: attraction effect, high shear and extensional stress, all of which may influence the rheological properties and flow behavior of blood. Thus, it is important to investigate in detail the behavior of blood flow occurring at both bifurcations and confluences. In the present paper, by using a micro-PTV system, we investigated the variations of velocity profiles of two working fluids flowing through diverging and converging bifurcations - human red blood cells suspended in dextran 40 with about 14% of haematocrit level (14Hct) and pure water seeded with fluorescent trace particles. All the measurements were performed in the center plane of rectangular microchannels using a constant flow rate of about 3.0×10-12 m3/s. Moreover, the experimental data was compared with numerical results obtained for Newtonian incompressible fluid. The behavior of RBCs was asymmetric at the divergent and convergent side of the geometry, whereas the velocities of tracer particles suspended in pure water were symmetric and well described by numerical simulation. The formation of a red cell-depleted zone immediately downstream of the apex of the converging bifurcation was observed and its effect on velocity profiles of RBCs flow has been investigated. Conversely, a cell-depleted region was not formed around the apex of the diverging bifurcation and as a result the adhesion of RBCs to the wall surface was enhanced in this region.
- Blood flow in cylindrical stenosed channel – numerical approachPublication . Ferreira, Stéphanie; Rocha, Daniela; Dias, Ricardo P.; Fernandes, Carla S.Arthrosclerosis means literally “arteries hardening”. However, arthrosclerosis it is a generic term that is related with three patterns of vascular diseases, which have the hardening and loss of elasticity of the arteries walls as a common factor [1]. The dominant pattern is atherosclerosis, characterized by the formation of atheroma, which is comprised by fibrous plaques that generally exhibit a centre rich in lipids. In the present work, the flow of blood in a cylindrical channel, containing an atheroma at the walls, has been numerically studied using the finite-element software package POLYFLOW®. In the simulations, blood was considered an incompressible homogenous fluid and the flow regime was the laminar regime. The rheology of the mentioned fluid was described by distinct constitutive equations [2] - constant viscosity, power-law model and Carreau model. The local behaviour of properties such as pressure, interstitial velocities, shear rate and shear stress was explored in the present investigation. The local behaviour of these properties can help to understand the formation and detachment of thrombi.
- Blood flow in microchannels manufactured by a low cost technique: xurographyPublication . Pinto, Elmano; Taboada, Bruna Rafaela Pereira; Faustino, Vera; Cidre, Diana; Rodrigues, Raquel Oliveira; Miranda, João Mário; Garcia, Valdemar; Dias, Ricardo P.; Lima, Rui A.The xurography is a technique that has been used to make molds to produce microchannels. In contrast to soft lithography [1, 2], xurography uses equipments and materials commonly used in the printing industry, such as cutting plotters, vinyl and other materials. The main advantage of this technique is to fabricate microchannels at a reduced cost [3, 4]. The Fahraeus-Lindqvist effect is a well know phenomenon that happens in microcirculation, where red blood cells (RBCs) have tendency to migrate toward the centre of the microtube resulting in a marginal cell-free layer (CFL) at regions adjacent to the wall [5]. Recently several studies showed strong evidence that the formation of the CFL is affected by the geometry of the microchannel [1, 6, 7] and the physiological conditions of the working fluid, such as the hematocrit (Hct) [2, 8]. The main objective of the present work is to fabricate polydimethysiloxane (PDMS) microchannels by using a soft xurography technique in order perform blood flow studies. Additionally, a high-speed video microscopy system is used to measure the CFL thickness in two different geometries, i. e., bifurcations and confluences.
- Cell-free layer (CFL) measurements in complex geometries: contractions and bifurcationsPublication . Novais, Susana; Pinho, Diana; Bento, David; Pinto, Elmano; Yaginuma, Tomoko; Fernandes, Carla S.; Garcia, Valdemar; Pereira, Ana I.; Lima, José; Mujika, Maite; Oliveira, Mónica S.N.; Dias, Ricardo P.; Arana, Sergio; Lima, Rui A.In this chapter we discuss the cell-free layer (CFL) developed adjacent to the wall of microgeometries containing complex features representative of the microcirculation, such as contractions, expansions, bifurcations and confluences. The microchannels with the different geometries were made of polydimethylsiloxane (PDMS) and we use optical techniques to evaluate the cell-free layer for red blood cells (RBC) suspensions with different hematocrit (Hct). The images are captured using a high-speed video microscopy system and the thickness of the cell free layer was measured using both manual and automatic image analysis techniques. The results show that in in vitro microcirculation, the hematocrit and the geometrical configuration have a major impact on the CFL thickness. In particular, the thickness of the cell-free layer increases as the fluid flows through a contraction-expansion sequence and that this increase is enhanced for lower hematocrit. In contrast, the flow rates tested in this studies did not show a clear influence on the CFL thickness.
- Cell-free layer measurements in a bifurcation microchannel : comparison between a manual and automatic methodsPublication . Bento, David; Pinho, Diana; Pinto, Elmano; Yaginuma, Tomoko; Correia, Teresa Montenegro; Lima, José; Pereira, Ana I.; Fernandes, Carla S.; Dias, Ricardo P.; Lima, Rui A.In the present work, in vitro blood flowing through a bifurcation microchannel was studied. The aim was to measure the Trajectories of the cell-free layer (CFL) by using different methods, i. e., a manual and two automatic methods.
- CFD aplicada ao estudo de permutadores de calor de placas chevronPublication . Fernandes, Carla S.; Dias, Ricardo P.; Nóbrega, João M.; Maia, João M.Os fluidos alimentares com viscosidade elevada são frequentemente processados em permutadores de calor de placas (PCPs) em regime laminar. 0 desempenho termo-hidráulico dos PCPs do tipo chevron depende das características geométricas dos mesmos, nomeadamente do ângulo das corrugações sinusoidais das placas, B, do espaçamento entre placas e do factor de incremento de área, o, definido como a razão entre a área real das placas e a sua área projectada.