(227m) The Effect of Variation in Channel Cross-Section on the Pressure Drop of Confined Microfluidic Droplets

Microfluidic devices have been increasingly popular in the fields of science and engineering due to their ease of fabrication. These devices have significant potential for a wide range of applications, including biomedical diagnostics and biosensing. Many of these applications involve the transportation, sorting and storage of droplets. However, moving drops in confined channels can cause localized pressure fluctuations as well as an overall increase in the pressure drop of the system. These pressure fluctuations can cause non-uniform drop size and spacing as well as affect the traffic of drops at bifurcations. There is a need to quantify this pressure drop in order to better design complex and robust microfluidic systems. Several studies have quantified the pressure drop of moving droplets in confined channels; however, it is unclear to what extent minor variations in channel cross-section affect the pressure drop due to flowing droplets. Factors that can cause channel geometry variation include: fabrication errors in soft lithography, flow- and swelling-induced deformation in elastomeric microchannels. In our experiments, we compared the pressure drop of a droplet in a square and a slightly distorted square cross-section channel. At higher capillary numbers (> 0.03), the pressure drop is greater in the distorted channel than in the square channel.  Furthermore, our analysis shows that at high capillary numbers (>0.07) the pressure drop data is contaminated by significant flow-induced channel deformation. We aim to resolve such issues that may have inadvertently affected previous studies by contrasting our current results with those obtained from pressure drop measurements in mechanically rigid channels.