Finite reservoir effect on capillary flow of microbead suspension in rectangular microchannels

The present study reports a theoretical investigation of capillary transport of microbead suspension in microfluidic channels with finite size reservoirs at the inlet. The reservoir-microchannel combination is often the case in Lab-on-a-Chip (LOC) where biomolecules are transported using capillary force. To demonstrate such finite reservoir effect, the reservoir is placed vertically above the microchannel. Under such condition, the pressure field expression at the rectangular microchannel inlet is deduced. Appropriate correlations for effective physical properties are used to account for the presence of microbeads in the working fluid, mimicking biomolecules in actual LOC. The non-dimensional governing equation for capillary flow with finite size reservoir is derived based on the balance among the surface, viscous and gravity forces acting on the fluid front. The numerical solution of governing equation is obtained to investigate the impact of several operating parameters on the flow front progression. It is observed that the aspect ratio of the microchannel and reservoir play vital roles in deciding the behavior and magnitude of flow front progression in the microfluidic channels. Capillarity and gravity force dominant regions during the progression is observed. The microchannel width and reservoir width decide the interplay between gravity and capillarity. Although higher fluid level in the reservoir has an added advantage for more gravitational head, the resistance from the reservoir makes the flow front progress slowly at the beginning of the capillary transport. It is also found that microbead volume fraction in the working fluid plays an important role in delaying the capillary transport under various operating conditions. Hence, it can be concluded that the use of reservoir at the inlet of microfluidic channels has an impact on the overall capillary transport of biomolecules in LOC devices.