Abstract
We present herein a novel method of bipolar field-effect control on DC electroosmosis (DCEO) from a physical point of view, in the context of an intelligent and robust operation tool for stratified laminar streams in microscale systems. In this unique design of the DC flow field-effect-transistor (DC-FFET), a pair of face-to-face external gate terminals are imposed with opposite gate-voltage polarities. Diffuse-charge dynamics induces heteropolar Debye screening charge within the diffuse double layer adjacent to the face-to-face oppositely-polarized gates, respectively. A background electric field is applied across the source-drain terminal and forces the face-to-face counterionic charge of reversed polarities into induced-charge electroosmotic (ICEO) vortex flow in the lateral direction. The chaotic turbulence of the transverse ICEO whirlpool interacts actively with the conventional plug flow of DCEO, giving rise to twisted streamlines for simultaneous DCEO pumping and ICEO mixing of fluid samples along the channel length direction. A mathematical model in thin-layer approximation and the low-voltage limit is subsequently established to test the feasibility of the bipolar DC-FFET configuration in electrokinetic manipulation of fluids at the micrometer dimension. According to our simulation analysis, an integrated device design with two sets of side-by-side, but upside-down gate electrode pair exhibits outstanding performance in electroconvective pumping and mixing even without any externally-applied pressure difference. Moreover, a paradigm of a microdevice for fully electrokinetics-driven analyte treatment is established with an array of reversed bipolar gate-terminal pairs arranged on top of the dielectric membrane along the channel length direction, from which we can obtain almost a perfect liquid mixture by using a smaller magnitude of gate voltages for causing less detrimental effects at a small Dukhin number. Sustained by theoretical analysis, our physical demonstration on bipolar field-effect flow control for the microfluidic device of dual functionalities in simultaneous electroconvective pumping and mixing holds great potential in the development of fully-automated liquid-phase actuators in modern microfluidic systems.
Highlights
The advent of micrometer-scale fluidic technology prompts the fundamental issue of how to deliver and mingle fluidic samples at the micrometer dimension, where Poiseuille flows and hydrodynamic instabilities from inertia effects are severely suppressed by liquid viscosity [1–8]
Due to normal field lines emitted from the fixed surface charge layer on channel sidewalls, a dynamic balance between electrostatic attraction and surface ion diffusion from diffuse charge dynamics induces a distribution of counterionic charge in a thin electrical double layer (EDL) at the solution/membrane interface to compensate for the fixed surface charge to maintain global electroneutrality [10–17]
Flow-FET has received considerable attention from the microfluidic community over the past two decades [37–45]. Most works on this interesting subject have focused on how to improve the pump flow rate from from the fixed surface charge layer on channel sidewalls, a dynamic balance between electrostatic attraction and surface ion diffusion from diffuse charge dynamics induces a distribution of counterionic charge in a thin electrical double layer (EDL) at the solution/membrane interface to compensate for the fixed surface charge to maintain global electroneutrality [10–17]
Summary
The advent of micrometer-scale fluidic technology prompts the fundamental issue of how to deliver and mingle fluidic samples at the micrometer dimension, where Poiseuille flows and hydrodynamic instabilities from inertia effects are severely suppressed by liquid viscosity [1–8]. Since it is originated by a fixed surface charge layer, this kind of diffuse screening cloud is named native EDL, which occurs spontaneously independent of any other applied voltages Under this situation, a tangential field component imposed across the source (S) and drain (D) terminals acts on the volumetric free charge within the thin EDL, and the resulting Coulomb force can effectively actuate the mobile counterions into unidirectional DC electroosmosis (DCEO) streaming flow along the channel length direction for active pumping of fluid samples (Figure 1b,c). This highly-integrated arrangement of gate terminal arrays of alternating voltage polarities makes it possible for us to fabricate complex microfluidic networks, from which we can obtain almost perfect liquid mixture at the downstream channel exit even in the absence of artificially-imposed pressure gradients, which supplies invaluable physical perspectives to the construction of smart on-chip liquid-phase platforms for fully-automated sample handling in contemporary microfluidic systems
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