Staff

Abstract:
Two decades of research on droplet formation in microchannels have led to the widely accepted view that droplets form through the squeezing mechanism when interfacial forces dominate over viscous forces. The initially surprising finding that the volume of the droplets is insensitive to the relative importance of these two forces is nowadays well understood from the constrained deformation of the droplet interface during formation. In this work, we show a lower limit of the squeezing mechanism for droplets produced in microfluidic cross-junctions. Below this limit, in the leaking regime, which was recently discovered for droplets produced in T-junctions, the volume of the produced droplets strongly depends on the relative importance of interfacial and viscous forces, as captured by the capillary number. We reveal a fundamental difference in the mechanisms at play in the leaking regime between T- and cross-junctions. In cross-junctions, the droplet neck elongates substantially, and unlike the case of the T-junction, the magnitude of this elongation depends strongly on the value of the capillary number. This elongation significantly affects the final droplet volume in a low capillary number regime. Generalizing the classical squeezing law by lifting the original assumptions and incorporating both identified mechanisms of leaking through gutters and neck elongation, we derive a model for droplet formation and show that it agrees with our experiments.

Keywords:
Microfluidics,Cross-junction,Flow-focusing device,Droplet formation,Two-phase flow,Scaling law,Squeezing regime

Abstract:
Reliable experimental data are essential for good research. Also, the interaction of drop and liquid surface is ubiquitous and of practical importance in our lives. However, beginning researchers often face an unavoidable long learning curve due to a lack of systematic collection of failure experiences. These include somewhat trivial but also influential factors, especially in cases of low impact velocities, e.g., the feeding process into a liquid tank, the dropping into a drip chamber, and/or neglecting the initial velocity associated with the dripping drop generation in the experimental preparation processes, etc. Inconsistent experimental comparisons may arise from different methods of thin-film preparation and measurement or from nonunified parameter definition, e.g., the penetration depth of vortex rings induced by low-velocity drop impact, which makes experimental data analysis and comparison even more challenging. Here, we have accumulated some of our experience in preparing drop and liquid surface interactions including a short review of preparing thin (or very thin) target liquid film of well-defined thickness; the method to minimize the commonly neglected current underneath the target liquid; the method of generating the satellite-free drop along with the correct prediction of drop oscillation phase in the free-falling period after pinch-off, and the role of drop initial velocity, etc. These archival data are expected to shorten the learning period and facilitate benchmarking of related experiments for future users in our community.

Abstract:
Due to various reasons, the concepts of thermodynamics are not easy to grasp for undergraduate students. One of the major reasons is that the students are mostly unfamiliar with the thermodynamics devices discussed in the courses. Offering courses with experiments is an effective approach to solve this issue. However, it is not practical or possible for universities to own devices that operate at high temperatures and with high pressure fluids. With the cooperation of a nearby electric company, undergraduate students of a thermodynamics course from the Department of Mechanical Engineering measured thermal performances of a commercial combined cycle and its sub-systems at the President University. After learning about the theory of thermal cycles, the students analyzed the thermal performances of actual thermodynamics cycles. Subsequently, they analyzed the thermal efficiency improvements when reheating or regeneration is applied to the simple Rankine cycle in the combined cycle. At the end of the course, the students gave presentations before the electric company’s management and engineering personnel, akin to professional engineers. This course is structured to familiarize undergraduate students with thermodynamics cycles and devices.

Keywords:
thermodynamics, combined cycle, Brayton cycle, gas-turbine engine, Rankine cycle