(539c) Relating Interfacial Vapor Void Phenomena And Fluid Actuation In Hydrophobic Microfluidic Devices Using Electrokinetic And Atomic Force Microscopy Characterization

Introduction

     We report the modeling and measurement of electrokinetic behavior in hydrophobic microfluidic channels with special attention to the effects of solvent changes and temperature cycling on interfacial electrokinetic phenomena.  Since electroosmosis dominates over pressure in micro-scale fluid transport, electrokinetic actuation is ubiquitous in microfluidic devices.  Hydrophobic substrates such as polytetrafluoroethylene (PTFE, Teflon) and cyclic olefin copolymers (COCs, e.g., Zeonor) are commonly used in microfluidics applications [1,2], making accurate and rigorous theoretical modeling of electrokinetic actuation in these substrates critical; however, the electrokinetic performance of these substrates has often been reported as variable and unpredictable.  Challenges to modeling and predicting these phenomena include pragmatic issues such as substrate preparation and surfactant effects, as well as modeling challenges which include proposed nano-scale interfacial phenomena such as slip, hydrophobic surface charge formation, and depletion region or vapor void formation.

     Many microbioanalytical techniques such as electrochromatography and PCR rely on temperature cycling and/or solvent changes, which exacerbate modeling challenges owing to the uncertainty of the role of temperature and solvent cycling on adsorption or void formation processes at the interface.  By using a model-based framework which carefully accounts for temperature-dependent chemical kinetics, slip, and the presence of interfacial vapor voids, we propose to determine the physical origins of these uncertainties.  Here we measure changes in electrokinetic behavior with controlled temperature and solvent cycling, and compare those results to changes in surface topography measured through atomic force microscopy.  We expect to identify correlations between nano-scale phenomena at the interface and macroscopic electrokinetic behavior.

Langmuir Adsorption Model

     The physical mechanisms for the origin of surface charge on PTFE substrates are not well understood, and generally disputed.  Some putative sources of charge include impurities [4], adsorption of buffer anions onto the surface [5], and adsorption of hydroxyl ions [6].  Experimental results for the pH dependence of the electrokinetic potential, zeta, in PTFE microchannels are shown in Figure 1.  Because the zeta potential of PTFE scales with the electrolyte ion concentration in a manner which is consistent with double layer shielding effects, it is unlikely that adsorption of salt ions is the dominant mechanism for surface charge formation.  On the other hand, the zeta potential of PTFE becomes increasingly negative with increasing pH in a manner reminiscent of the titration of binding sites.  These observations have led us to develop a model for the electrokinetic behavior of this system based on hydroxyl ion adsorption.  Our model assumes Langmuir kinetics in order to derive the fraction of hydroxyl ions bound to the surface as a function of temperature and pH (Figure 2).  In this framework, we have parameterized the equilibrium state with a free energy change due to adsorption.  The fractional surface binding can be related to the zeta potential through the ion charge, and the surface site density.

Electrokinetic Characterization

     The electrokinetic potential of PTFE and Zeonor substrates were measured using automated current monitoring and phase sensitive streaming potential experiments.  Current monitoring [7] is a technique used to measure the electroosmotic velocity, from which the electrokinetic potential is calculated.  In current monitoring (Figure 3), a microchannel composed of the material under study connects two reservoirs filled with salt solutions of slightly different (~5% different) conductivities.  Fluid is then driven by electroosmosis from one reservoir to the other, and the electrical current is simultaneously measured as a function of time.  As fluid from one reservoir displaces fluid from the other in the microchannel, the resistance in the system changes which leads to a measurable change in the current.  The electroosmotic velocity is obtained by measuring the time it takes for the current to change from a constant high to a constant low and vice versa.  We have automated current monitoring experiments through LabView scripting, and are exploring the dependence of observed phenomena on temperatrure.

Current monitoring results for the electrokinetic potential as a function of pH in Teflon microchannels are shown in Figure 4.  The pH and temperature dependence are qualitatively consistent with the Langmuir model, and a model fit at room temperature gives a free energy change of adsorption of -23 kJ/mol.

     In phase sensitive streaming potential experiments, a sinusoidal driving pressure is applied to a microfluidic channel composed of the material under study (Figure 5).  This results in an electrical potential across the channel, the streaming potential, which is also sinusoidal and is measured directly.  Fourier processing of the streaming potential allows us to select out the signal at the driving frequency, greatly reducing noise and ambiguities.  The streaming potential, like the electroosmotic velocity, is directly related to the electrokinetic potential.

     We have used phase-sensitive streaming potential experiments to observe transient electrokinetic effects due to methanol solvent rinses in zeonor microchannels (Figure 6).  The electrokinetic potential takes several hours (~7.3 hours) to stabilize after introduction of methanol, which indicates that changes in surface properties due to the solvent change have a slow equilibration time.  Since methanol affects the solubility of gas in the liquid, this could be indicative of an initial increase in the presence of interfacial vapor voids, which would lead to an initial increase in the apparent electrokinetic flow rate.  Experiments by Attard et. al [8] have shown that nanobubbles at the interface have a similarly slow equilibration time, suggesting a possible link between nanobubbles and the electrokinetic performance of hydrophobic microchannels.  This is important in PCR, for example, where thermal cycling leads to the generation of vapor voids at the interface, causing transient electrokinetic behavior that decays with slow time scales.

Atomic Force Microscopy Characterization

     AFM experiments are focused on direct observation of interfacial nanobubbles and their generation/dissolution over time with solvent rinses and temperature cycling.  The combination of these experiments with electrokinetic characterization will help define the putative link between nano-scale interfacial vapor voids and macroscopic electrokinetic behavior.  AFM data for a dry Zeonor surface is shown in Figure 7, which shows roughness features of height 5-15 nm.  AFM measurements [8] for hydrophobic surfaces in fluid, on the other hand, show features 200 nm in diameter and 30 nm in height, indicative of the presence of vapor at the interface.  We are currently developing AFM techniques for measuring the effects of temperature and solvent cycling on interfacial vapor void formation.

Figures

Figure 1. Comparison of data from several sources for the electrokinetic potential as a function of pH for PTFE at room temperature.  Data taken at different buffer ion concentrations (pC = -log C, where C is the counterion concentration in M) collapses onto one curve.  External data from [3].

Figure 2. Langmuir adsorption isotherms for hydroxyl ion adsorption calculated from the proposed model.  This model assumes that binding events can be described by a single free energy of binding, and that there are a finite number of binding sites.  Such modeling allows surface charge formation to be thermodynamically related to engineering parameters such as pH and temperature.

Figure 3. Schematic of a current monitoring experiment.  A microchannel made of the material under study connects two reservoirs filled with solutions of slightly different conductivity.  When driven by electroosmosis, fluid from one reservoir displaces the other leading to a change in resistance in the system.  The velocity may be obtained by measuring the time it takes for the current to change from a constant high to a constant low and vice versa.

Figure 4.  Observed electrokinetic potential, ζ, for PTFE as a function of pH and temperature measured with current monitoring.  Data points are shown as a mean ± standard deviation, and n>20 for all points.  All experiments were run with 0.1 mM potassium phosphate buffer (pC = -log C = 4, where C is the counterion concentration in M).

Figure 5. (a) Schematic of a streaming potential experiment.  The redistribution of ions in a pressure gradient results in an electrical potential.  (b) In phase-sensitive streaming potential experiments, a sinusoidal driving pressure is applied.  The resulting streaming potential signal is also sinusoidal.  (c) Noise in streaming potential vs. pressure data can lead to ambiguous linear fits.  (d) Fourier processing removes the ambiguity.

Figure 6.  Observed electrokinetic potential for Zeonor as a function of time after methanol-water solvent exchange, indicating a change in surface properties with a characteristic decay time of 7.3 hours.  The data is empirically well fit by an exponential function.

(a)

Figure 7. (a) AFM data shown as a height contour map of a 1μm x 1μm area of a Zeonor 1020 polymer surface.  (b)  3D view of the same area.

(b)

References

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