Analysis of Static and Dynamic Contact Angles of Ferrofluid Droplets for Magnetically Actuated Micropumps

Abstract

Ferrofluid plug driven micro-pumps are useful for manipulating micro-volume of liquids by providing remote actuation using a localized magnetic field gradient. Inside a microchannel, the ferrofluid experiences combined actions of different relevant body forces. While the pressure, viscous and magnetic forces can be estimated using established techniques, surface tension force cannot be readily. The presence of a second fluid (the fluid being pumped) and magnetic field (the driving dipole) alters the ferrofluid-wall contact angle (CA) in both static and dynamic fashions, which has not been reported in the literature. Therefore, realistic prediction of ferrofluid-plug driven micropump requires comprehensive data on variation of CA between the ferrofluid and glass capillary wall under different kinematic conditions. Here we perform an experimental characterization of static and dynamic contact angles of oil-based ferrofluid (EFH3) droplets on glass surface immersed in pure or surfacted distilled water. The relation between CA and the contact line velocity for ferrofluids is particularly important as the observed CA values fell beyond the classical Hoffman-Tanner equation. In the presence of an external magnetic field, a sessile ferrofluid droplet is seen to acquire interesting shapes. Growth of a droplet due to pumping of fluid from below the surface leads to a pinning and de-pinning effect. Our results also show a decrease in contact angle hysteresis (CAH) at higher contact line velocities as the droplets slide down an inclined plane.