Colloids and Surfaces A: Physicochemical and Engineering Aspects
Experimental observation of induced-charge electro-osmosis around a metal wire in a microchannel
Introduction
Increased access to microfabrication technologies has spurred growing interest in the possibilities and the benefits of miniaturization in chemical analysis and biomedical screening. Microfluidics holds the promise of ultra-fast biological assays, analysis of ultra-dilute samples and characterization of a previously unrealized compounds [1], [2], [3]. In principle, these advances could be fully miniaturized, enabling new implantable medical devices and diagnostics, but there is still a need to develop robust techniques for pumping in portable microfluidic devices.
Conventional pressure-driven flows are commonly used in microfluidics [6], as long as the channel dimensions are not too small (10 m) and they have the advantage of being robust and easy to operate, regardless of the fluid composition. Nevertheless, pressure-driven flows have poor scaling with miniaturization and do not offer simple local control of flow direction and circulation. The pressure gradient needed to maintain a constant fluid velocity increases as the inverse square of the channel width and rather bulky (non-portable) pressure sources are often required.
Electrokinetics offers an alternative means to pressure gradients to drive microflows, which is gaining increased attention [4], [5]. Perhaps, the best known example of electrokinetics is the electrophoresis of charged colloidal particles [7], where an applied electric field acts on diffuse double-layer charge to produce fluid slip. By the same mechanism, an electric field applied down a glass or polymer microchannel can drive a plug-like flow in capillary electrophoresis.
Standard electrokinetic effects are linear in the applied field, which presents some disadvantages for microfluidics, especially in miniaturization. Aside from being fairly weak, linear electro-osmotic flows suffer from the requirement of direct current and thus electrochemical reactions at electrodes to maintain a steady flow. Alternating current suppresses reactions and thus can allow larger driving voltages without bubble formation or sample contamination by reaction products, but linear flows time-average to zero in alternating fields. Another problem for miniaturization is the large power requirement of linear electro-osmosis, since the voltage is applied globally down the channel, rather than locally across the channel. As a result, to achieve typical electric fields over 100 V/cm, one must apply over 100 V across a 1 cm long microfluidic chip. It would be preferable for portable or implantable microfluidics to make use of the much smaller voltages (1–10 V) supplied by microbatteries by applying fields locally at the scale of the channel width.
Non-linear electrokinetic phenomena provide a promising alternative mechanism for flow control in microfluidic devices. The first non-linear electrokinetic phenomenon described in the microfluidic literature was “ac electro-osmosis” at microelectrode arrays, independently discovered by Ramos et al. [8], [9] and Ajdari [10] and studied extensively ever since [11], [12], [13], [14], [15], [16]. Similar flows were also observed by Nadal et al. [17] around a dielectric stripe on an electrode by Thamida and Chang [18]. Recently, Bazant and Squires [19], [20] pointed out that the physical principle behind ac electro-osmosis – an electric field acting on its own induced double-layer charge at a polarizable surface – is more general, requiring neither electrodes nor ac fields and they suggested the more descriptive term, “induced-charge electro-osmosis” (ICEO) to describe the basic mechanism. They also noted that essentially the same effect had been described earlier in the Russian literature on metallic colloids by Murtsovkin and co-workers [21], [22], which speaks to the generality of the phenomenon and suggests new directions for research in microfluidics.
The fundamental effect of ICEO at an inert (non-electrode) metal surface has been predicted theoretically, but remains to be demonstrated experimentally in a microfluidic device. Here, we present experiments which reveal ICEO convection rolls around an electrically isolated platinum wire in a polymer channel driven by an ac voltage. The setup is similar to the canonical problem of a metal cylinder [19], [20] (or sphere [21]) in a suddenly applied uniform field, shown in Fig. 1, although the geometry is more complicated since the wire rests on one wall of the channel. We make detailed maps of the flow field inside the channel and compare with various theoretical models, solved numerically for the specific experimental geometry. Analogous experiments on metal colloids have also been reported by Gamayunov et al. [23], but the fixed and simple geometry of a microfluidic channel allows more direct, quantitative testing of the theory, as well as new technological applications.
Section snippets
Basic setup
As a simple first experiment to illustrate ICEO flow at a non-electrode metal surface in a microfluidic device, we considered the experimental setup shown in Fig. 2. A platinum wire of circular cross-section was attached to the wall of a polymer microchannel containing low-concentration KCl electrolyte. An ac voltage was applied from distant ends of the channel (without any electrical connections to the wire) to produce an oscillating background electric field transverse to the wire. The
The electrochemical problem
Given the experimental conditions, it is reasonable to test the simplest “equivalent circuit” model of ICEO, which assumes a homogeneous, neutral electrolyte with thin, charged double layers, represented by linear circuit elements. In this ubiquitous approximation [9], [10], [14], [19], [20], [21], [22], the electrochemical problem simplifies to that of a leaky dielectric (the bulk electrolyte) with a capacitor skin (the metal’s double layer). The electrostatic potential satisfies Laplace’s
Results
We begin by studying the spatial profile of the velocity from -PIV near the tip of the wire at a driving frequency of 300 Hz. The raw data is shown in Fig. 7 for applied voltages ranging from 35 to 100 V. At higher voltages bubbles form at the electrodes at low frequency and at smaller voltages velocities are too small to measure accurately with our -PIV setup.
The inverted optics microscope records images of fluorescent tracer particles in an optical slice roughly 20 m thick, set by the
Discussion
Overall, we find reasonable agreement between theory and experiment, sufficient to conclude that we have in fact observed ICEO. This is an interesting result on its own, since it demonstrates the same physical mechanism as ac electro-osmosis around an inert (non-electrode) metal surface with a very different frequency response, including steady electro-osmotic flow in the dc limit. The flow scales with the square of the applied voltage and the shape of the velocity profile and the frequency
Acknowledgments
This research was supported by the U.S. Army through the Institute for Soldier Nanotechnologies, under Contract DAAD-19-02-0002 with the U.S. Army Research Office (JAL, YB, TT, MZB). Some additional funding also came from the MIT–France Program (JAL, VS) as well as the NSF Mathematical Sciences Postdoctoral Fellowship and Lee A. Dubridge Prize Postdoctoral Fellowship (TMS).
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