Hydrodynamic resistance and flow-induced squeezing of a trapped microfluidic drop
Swastika S. Bithi , Mehdi Nekouei and Siva A. Vanapalli*
Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409
*Corresponding Author’s E-mail: firstname.lastname@example.org
Understanding the flow resistance and the squeezing characteristics of a trapped viscous droplet are crucial for microfluidic applications where nanoliter-scale drops are used as microreactors as well as for enhanced oil recovery in porous media. The hydrodynamic resistance of a viscous drop immobilized against a microchannel constriction can depend on a wide range of parameters, such as capillary number, viscosity ratio, degree of drop confinement, geometry and wetting characteristics of the channel. In this work, we use a unique technique to immobilize drops against microchannel constrictions and measure the hydrodynamic resistance of the stationary droplets. We mainly focus on the effect of capillary number, viscosity ratio and drop confinement on the hydrodynamic resistance of the trapped droplet. We observe that the resistance of the trapped drop is constant at low capillary number (Ca¢ <0.7). It increases with capillary number due the increase in degree of blockage at the constriction for intermediate capillary number (Ca¢< 4). However, for high capillary number (Ca¢³ 4), the deformation of the occluded drop saturates causing no further increase in the hydrodynamic resistance. We also determine the fluid pressure needed to dislodge the trapped drops and investigate its dependence on system parameters. We complement our experimental observations with computational fluid dynamics simulations to understand the dependence of hydrodynamic resistance of a trapped drop on capillary number in channels with circular and square cross-section.