Frequency Dependent Mobility of Janus Spheres

Frequency dependent mobility of Janus spheres

Alicia Boymelgreen¹, Tov Balli¹, Gilad Yossifon¹,Touvia Miloh²

^,1Faculty of Mechanical Engineering, Micro- and Nanofluidics Laboratory, Technion – Israel Institute of Technology, Haifa 32000, Israel

²Faculty of Mechanical Engineering, Tel Aviv University, Tel Aviv 69978, Israel

E-mail: yossifon@tx.technion.ac.il

Non-linear electrohydrodynamic flows, including induced-charge and alternating-currentelectroosmosis have received much attention amongst the micro/nanofluidic community over the past decade for their potential to produce net flow even under ac electric forcing, with suggested applications ranging from electroosmotic pumps to externally controlled micromotors and mixers. The advantages of ac electric forcing over its more commonly used dc counterpart include lower applied voltages and the suppression of Faradaic reactions and bubble generation which can block and degrade microfluidic systems.

The underlying mechanism relies on the polarization of a solid suspended in an aqueous medium, under the application of an externally applied electric field in which case the solid acquires a non-zero surface charge. Accordingly, counterions from within the suspending medium are driven toward the polarized surface to shield it, creating a region of charge density or an induced electric double layer. The applied field may then act on this electric double layer, to ultimately drive electroosmotic flow. When the polarized medium is the actual electrodes to which the field is applied, the phenomena is termed “alternating current electroosmosis” (ACEO) while the more general term of “induced-charge electroosmosis” (ICEO) includes the presence of non-active polarizable structures or particles within the suspending medium. Nonlinearity arises from the fact that the field both induces the double layer and drives it to produce electroosmotic flow.

We examine the influence of frequency of a uniform applied ac electric field on the mobility of metallodielectric Janus particles, comprised of one metallic (gold) and one dielectric (polystyrene) hemisphere. It is well-established that at low frequencies (on the order of single kHz), the mobility is predominantly determined by the asymmetric induced-charge electrokinetic flow around the particle, which is strongest around the more polarizable hemisphere and acts to propel the particle perpendicular to the electric field in the direction of its dielectric end. However, it has previously been observed that the particle velocity decays well-before the charge relaxation time, , where  is the particle radius,  the thickness of the Debye layer and  the diffusivity of the suspending medium. Such a premature decay suggests the presence of competing forces, acting against pure ICEP, and result in a reduction of the Janus sphere mobility. By tracking both freely suspended Janus particles of varying size in electrolytes of differing conductivites, as well as using uPIV to examine streamlines around stationary Janus particles, we examine the interplay between induced-charge electrophoresis, dielectrophoresis and wall effects stemming from the proximity of the wall to the translating particle. This work is of both fundamental and practical importance and may be used to optimize the behavior of Janus micromotors or carriers in lab-on-a-chip analysis systems.