A microfluidic viscometer: Translation of oscillatory motion of a water microdroplet in oil under electric field

Abstract

The electric field induced motion of a charged water droplet suspended in a low‐dielectric oil medium is exploited to evaluate the rheological properties of the suspending medium. The time‐periodic electrophoretic motion of the droplet between the electrodes decorated in a polymeric micro‐well is translated into a proof‐of‐concept microfluidic prototype, which can measure viscosities of the unknown fluid samples. The variations in the instantaneous velocities of the migrating droplet have been measured inside silicone oil of known physical properties at different electric field intensities. Subsequently, a balance between the electric field to the viscous force has been employed to evaluate the experimental charge density on the droplet surface. Thereafter, a comprehensive scaling law has been devised to find a correlation between the charge on the droplet to the dielectric permittivity of the surrounding medium, size of the water droplet, and the applied electric field intensity. Following this, the scaling law and force balance have been employed together to evaluate the unknown viscosity of an array of suspending mediums by simply analyzing the electrophoretic motion of water droplet. The model proposed is also found to be consistent when a solid amberlite microparticle has been employed as a probe instead of the water droplet. In such a scenario, minor changes in the exponents of the scaling law are found to be necessary to reproduce the results obtained using the water droplet. The method paves the way for the making of an economical and portable microfluidic rheometer with further finetuning and translational developments.

Electrophoresis of a dielectric droplet with constant surface charge density

“…Among them, Equation ( 18) is the generalized Rybczynski-Hadamard condition, the continuity of total stress upon the droplet surface, including the electric Maxwell stress [25]. Continuity of fluid velocity on the droplet surface, Equation (17), is also applied. Equations ( 15) and ( 16) indicate that the droplet surface is ions impenetrable.…”

“…To manipulate liquid droplets as desired, an electric field is often applied as the driving force since colloidal droplets are essentially all charged in practice. As a result, it is of vital importance to investigate the electrophoresis of droplets, especially the dielectric droplets, which constitute the majority of droplets in use [16][17][18][19][20].…”