Hydrodynamic entrance length for laminar flow in microchannels with rectangular cross section

This work presents a detailed numerical investigation on the required development length (L=L/B) in laminar Newtonian fluid flow in microchannels with rectangular cross section and different aspect ratios (AR). The advent of new microfluidic technologies shifted the practical Reynolds numbers (Re) to the range of unitary (and even lower) orders of magnitude, i.e., creeping flow conditions. Therefore, accurate estimations of L at Re≤O(1) are important for microsystem design. At such low Reynolds numbers, in which inertial forces are less dominant than viscous forces, flow characteristics become necessarily different from those at the macroscale where Re is typically much larger. A judicious choice of mesh refinement and adequate numerical methods allowed obtaining accurate results and a general correlation for estimating L, valid in the ranges 0≤Re≤2000 and 0.1≤AR≤1, thus covering applications in both macro and microfluidics.The authors acknowledge the support by CEFT (Centro de Estudos de Fenómenos de Transporte) and Project PTDC/EMS-ENE/3362/2014—POCI-01-0145-FEDER-016665, funded by FEDER funds through COMPETE2020 “Programa Operacional Competitividade e Internacionaliza” (POCI) and by national funds through FCT “Fundac ao para a Ciência e a Tecnologia”, I.P. L.L. Ferrás would also like to thank FCT for financial support through scholarship SFRH/BPD/100353/2014 and projects UIDB/00013/2020 and UIDP/00013/2020. A. Sucena, A. M. Afonso, M. M. Alves and F. T. Pinho are also grateful to FCT for funding through projects UIDB/00532/2020 and UIDP/00532/2020. A. Sucena thanks FCT for the Ph.D. Grant SFRH/BD/115547/201

Análise computacional de defeitos intersticiais em nanocristais de Si

Mestrado em FísicaNeste trabalho são estudadas diferentes propriedades de defeitos intersticiais em silício cristalino e nanocristais de silílico. Os modelos são baseados na teoria do funcional da densidade. Numa fase inicial são feitos testes de convergência para escolher parâmetros de modelação, em particular as funções de base. Para clarificar a identidade de alguns centros de origem intersticial, são estudados vários modelos de defeitos tri- e tetra-intersticiais numa supercélula de silício. São estudadas a geometria dos defeitos representados por estes modelos, bem como a sua energia de formação e níveis elétricos. Posteriormente, são calculados a população de Mulliken e o acoplamento hiperfino. Os mesmos cálculos são feitos para o carbono interstícial como forma de estudar a validade do modelo. Com o objetivo de estudar a segregação e estabilidade de autointerstícios em nanocristais de silício, para diferentes posições de um defeito autointersticial em nanocristais de silício de diferentes tamanhos é calculada a energia de formação do defeito.In this work, di erent proprieties of interstitial defects are studied in crystaline silicon and silicon nanocrystals. The models are based on the Density Functional Theory. In an initial phase, several convergence tests are made in order to choose modelling parameters, in particular the basis functions. To clarify the identity of some centers with interstitial origin, several models of tri- and tetra-interstices are studied in a silicon supercell. The geometry of the defects represented by these models is studied, as well as the formation energy of the electronic levels. After that, the Mulliken population and the hyper ne coupling are calculated. The same calculations are made for the interstitial carbon as a way to study the model validity. Aiming to study the self-interstitial segregation and stability in silicon nanocrystals, for di erent positions of an self-interstitial defect in silicon nanocrystals of di erent sizes, the formation energy of the defect is calculated