On-Chip Manipulation and Controlled Assembly of Colloidal Particles using Alternating Electric Fields

Abstract

Alternating (AC) electric fields have been investigated as a versatile tool for rapid particle and fluid manipulation in micro- Total Analysis Systems (μTAS). Different on-chip electrode geometries and different particle suspensions were explored in this study with an aim to acquire a fundamental understanding of particle behavior under applied fields. Aqueous suspensions of particles of sizes ranging from nanoparticles to microspheres and having varied electrical properties (dielectric or conductive) were studied. For each system, detailed electrostatic simulations were carried out to identify the forces acting on the particles and fluid. Control of the particle-field, fluid-field and particle-particle interactions, by fine tuning the applied field, lead to the desired assembly of particles. Dielectrophoresis (DEP) was used to assemble gold nanoparticles into microwires and for manipulating fluid droplets containing suspended particles. AC Electrohydrodynamics (EHD) driven liquid flows were used for the transportation, redistribution and collection of suspended particles inside experimental cells.

Suspensions of metallic nanoparticles in water were assembled via DEP into wires of micrometer thickness between planar electrodes. Control of the process parameters allowed making, for example, straight single connectors, or massively parallel arrays of microwires on the surface of the chip. The direction of microwire growth was guided by introducing conductive islands or particles in the suspension. DEP based manipulation of freely suspended droplets was used for designing a novel liquid-liquid microfluidic chip. Water droplets containing suspended particles were floated on dense fluorinated oil and manipulated by applying electric fields using electrodes present beneath the oil. The on-chip system was capable of transporting multiple droplets in parallel, for rapid mixing of the contents of droplets and for carrying out chemical reactions and precipitations.

AC fields were applied to dilute suspensions of latex microspheres enclosed between a patterned silicon wafer and an ITO-coated glass slide in a small chamber. The latex particles entrained by EHD flow became collected in the center of the conductive "corrals" on the silicon wafer. The particle collection efficiency and speed depended only on the frequency and strength of the field and were independent of the material properties of the particles or the electrodes.