(184g) Strong Surfactant-Dependent Lift Forces On Drops and Bubbles in Microchannels

The transverse motion of drops and bubbles within liquids flowing in pipes and channels is determined by the combination of several types of hydrodynamic lift forces with mechanical confinement, buoyancy and external-field forces. In centimeter-sized and larger pipes, lift forces are usually weak compared to the buoyant forces corresponding to the buoyant difference between air and water, but they can be observed in neutrally buoyant systems.

Hydrodynamic lift forces are more prominent in microfluidic applications, where they have been used to position and sort particles with high efficiency and high accuracy. We used experimental, analytical, and computational methods to investigate lift forces on drops and bubbles immersed in a liquid, flowing in microfluidic channels under conditions characterized by low particle capillary numbers (0.0003 < Ca_P < 0.3) and low particle Reynolds numbers (0.0001 < Re_P < 0.1). We measured the lift forces directly and we developed a method to compare their strength across more than 20 different systems that were chosen for their relevance to microfluidic applications.

The  measured lift forces were often much larger (up to a factor of 1000) than the predictions of analytical models of inertial and deformation-induced lift. Numerical simulations that accounted for inertial and deformation-induced mechanisms revealed that the stronger forces can be explained in part by the confinement of the flow. The complete explanation, however, includes interfacial physicochemical effects: in one series of experiments with water drops in fluorocarbon carrier liquid, we observed a five-fold increase in the lift force as a fluorocarbon-soluble surfactant was gradually added to the system.

We will present measurements of lift forces and their dependence on hydrodynamic numbers. The dependencies separate the systems in (i) one group that exhibits a deformation-induced lift force enhanced by confinement effects, and (ii) another group that exhibits a lift force for which an analytical model does not exist yet, but to our best knowledge is based on physicochemical effects at the interfaces of drops and bubbles. The second group is most interesting from a practical point of view as well: these are the systems in which the lift forces were strongest—thus most useful to manipulate droplets in microchannels.