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The architecture of microfluidic networks can significantly impact the flow distribution within its different branches and thereby influence tracer transport within the network. In this paper, we study the flow rate distribution within a network of parallel microfluidic channels with a single input and single output, using a combination of theoretical modeling and microfluidic experiments. Within the ladder network, the flow rate distribution follows a U-shaped profile, with the highest flow rate occurring in the initial and final branches. The contrast with the central branches is controlled by a single dimensionless parameter, namely, the ratio of hydrodynamic resistance between the distribution channel and the side branches. This contrast in flow rates decreases when the resistance of the side branches increases relative to the resistance of the distribution channel. When the inlet flow is composed of two parallel streams, one of which transporting a diffusing species, a concentration variation is produced within the side branches of the network. The shape of this concentration gradient is fully determined by two dimensionless parameters: the ratio of resistances, which determines the flow rate distribution, and the Péclet number, which characterizes the relative speed of diffusion and advection. Depending on the values of these two control parameters, different distribution profiles can be obtained ranging from a flat profile to a step distribution of solute, with well-distributed gradients between these two limits. Our experimental results are in agreement with our numerical model predictions, based on a simplified 2D advection-diffusion problem. Finally, two possible applications of this work are presented: the first one combines the present design with self-digitization principle to encapsulate the controlled concentration in nanoliter chambers, while the second one extends the present design to create a continuous concentration gradient within an open flow chamber. 相似文献
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Epimaque Niyibizi Emmanuel Sibomana Cyprien Niyomugabo Béatrice Yanzigiye Amini Ngabonziza Jean de Dieu Juliet Perumal 《Assessment & Evaluation in Higher Education》2018,43(7):1166-1182
Assessment in institutions of higher learning has been researched worldwide. However, there are gaps in research exploring the implications of assessment practices for learners’ access to the kind of knowledge enabling them to participate in their communities’ socio-economic transformation. This is the case in the African Great Lakes region, including Rwanda. This paper investigated the assessment practices at the University of Rwanda-College of Education to establish whether they enable students to access powerful knowledge for socio-economic transformation, which promotes critical thinking and creativity, or the knowledge of the powerful, which promotes memorization of knowledge produced by experts. The study adopted mixed methods, where 361 questions from 20 examinations papers for 2014/2015 and 2015/2016 academic years were analysed using Bloom’s revised taxonomy. Interviews with lecturers and group discussions with students were conducted. The findings revealed that assessment practices heavily encourage the knowledge of the powerful, with limited focus on the socio-economic transformative knowledge. Such practices are unlikely to lead to national socioeconomic transformation, which the government expects from education. The paper recommends revisiting assessment practices in Rwandan higher education to instil in the graduates the required knowledge for active contribution to socio-economic transformation. 相似文献
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