Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars

Abstract

Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with highspeed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the nonuniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 lm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.

A comprehensive theoretical model of capillary transport in rectangular microchannels

“…A microscopic energy balance is used by Newman (1968) and Szekely et al (1971), as opposed to the quasi-steady state approximation of Washburn (1921) and they have claimed that the Washburn approach (Washburn 1921) can not be used for short capillary transport. Further, Saha and Mitra (2009b) demonstrated numerically and experimentally (Saha et al 2009) that Washburn prediction deviates for micro-scale geometries. Moreover, Saha and Mitra (2009a) have performed the numerical simulation of capillary filling process in a pillared microfluidic geometry and reported that variation in the capillary filling length does not follow the Washburn equation.…”

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