Elasto-inertial focusing and particle migration in high aspect ratio microchannels for highthroughput separation

The combination of flow elasticity and inertia has emerged as a viable tool for focusing and manipulating particles using microfluidics.Although there is considerable interest in the field of elasto-inertial microfluidics owing to its potential applications,research on particle focusing has been mostly limited to low Reynolds numbers(Re<1),and particle migration toward equilibrium positions has not been extensively examined.In this work,we thoroughly studied particle focusing on the dynamic range of flow rates and particle migration using straight microchannels with a single inlet high aspect ratio.We initially explored several parameters that had an impact on particle focusing,such as the particle size,channel dimensions,concentration of viscoelastic fluid,and flow rate.Our experimental work covered a wide range of dimensionless numbers(0.05<Reynolds number<85,1.5<Weissenberg number<3800,5<Elasticity number<470)using 3,5,7,and 10μm particles.Our results showed that the particle size played a dominant role,and by tuning the parameters,particle focusing could be achieved at Reynolds numbers ranging from 0.2(1μL/min)to 85(250μL/min).Furthermore,we numerically and experimentally studied particle migration and reported differential particle migration for high-resolution separations of 5μm,7μm and 10μm particles in a sheathless flow at a throughput of 150μL/min.Our work elucidates the complex particle transport in elasto-inertial flows and has great potential for the development of high-throughput and high-resolution particle separation for biomedical and environmental applications.

Inkjet printing technology for increasing the I/O density of 3D TSV interposers

Interposers with through-silicon vias(TSVs)play a key role in the three-dimensional integration and packaging of integrated circuits and microelectromechanical systems.In the current practice of fabricating interposers,solder balls are placed next to the vias;however,this approach requires a large foot print for the input/output(I/O)connections.Therefore,in this study,we investigate the possibility of placing the solder balls directly on top of the vias,thereby enabling a smaller pitch between the solder balls and an increased density of the I/O connections.To reach this goal,inkjet printing(that is,piezo and super inkjet)was used to successfully fill and planarize hollow metal TSVs with a dielectric polymer.The under bump metallization(UBM)pads were also successfully printed with inkjet technology on top of the polymer-filled vias,using either Ag or Au inks.The reliability of the TSV interposers was investigated by a temperature cycling stress test(−40℃ to+125℃).The stress test showed no impact on DC resistance of the TSVs;however,shrinkage and delamination of the polymer was observed,along with some micro-cracks in the UBM pads.For proof of concept,SnAgCu-based solder balls were jetted on the UBM pads.