The Modulation of Electrokinetic Streaming Potentials of Silicon-Based Surfaces through Plasma-Based Surface Processing

Abstract

A new plasma processing-based methodology for enhancing the streaming potential (V s ) that may be obtained in electrokinetic flows for a given pressure gradient over a silicon surface-based microchannel is indicated. The dependence of the V s on both the surface zeta potential and the electrolyte slip length was carefully determined through a series of experiments involving the variation of CF 4 -and Ar-based plasma parameters, incorporating pressure, exposure time, and power. It was determined through analytical estimates that, while the zeta potential is always increased, the slip length may be diminished under certain conditions. A record value of ∼0.1 mV/Pa was obtained using CF 4 plasma at 500 W, 10 mTorr, and 300 s of exposure. The implications of the work extend to the investigation of whether smooth surfaces may be effective for generating large V s 's for new modalities of electrical voltage sources in microfluidics-based applications.

Modulation of Electrokinetic Potentials Using Graphene-Based Surfaces and Variable Substrate Charge Density

“…The charges induced by the plasma are localized in the substrate. It was previously noted that plasma processing could additionally modify surface structure attributes . Our studies display the greater influence of charge density over dimensionality considerations with respect to the electrokinetic aspect.…”

“…It was previously noted that plasma processing could additionally modify surface structure attributes. 33 Our studies display the greater influence of charge density over dimensionality considerations with respect to the electrokinetic aspect. Extensive computational simulations involving charged liquid flows on the studied surfaces were carried out to validate our experimental findings and for gaining further understanding.…”

“…The results of additional investigations comparing the SLG and FLG transferred onto charged Si substrates are indicated in Figure 4b, where the measurements on SLG sample constituted channels yielded a larger FOM, i.e., the FOM decreased from ∼0.1 mV/Pa for SLG to ∼0.07 mV/ Pa for FLG on plasma-treated Si surfaces. It should be noted that while the charges induced by the plasma are localized in the substrate and their effect is significant over a testing period of a few weeks, 33 the long-term influence as well as the stability of the induced charges over time do need further investigation. As previously done, contact angle measurements of the SLG and FLG on the plasma-treated surfaces, as shown in Figures 4c−f, were used to correlate the measured θ CA to a slip length: b.…”

“…The relationships between the slip length b and the static contact angle (θ CA ), utilized in our paper, have been previously discussed. , It is generally not straightforward to determine the b, and the adopted methodology correlates the measured contact angle (θ CA ), related to hydrophobic interactions with the surface, to a slip length: b . Subsequently, a b (= b S–C ) in the range of 0.5 nm −1.3 nm was estimated from an adapted Shan-Chen model, i.e., with a b normalS − normalC = 0.3 0.25em normalnm × false( e θ CA / 56 − 1 false) , following previous work . Knowing the b and using the Helmholtz–Smoluchowski ( H–S ) relation, i.e., V s normalΔ P = ε η σ ζ eff , where the effective zeta potential: ζ eff   = ζ(1 + b λ D ), we compute the surface zeta potential: ζ.…”

“…To gain better control of the surface charge density, multiple plasma processing, e.g., involving argon 31 and nitrogen 32 plasmas, may be deployed to purposefully introduce electrical charges onto the 2D material and reference silicon substrate. 33 A significant aspect would be to probe the influence of charge inhomogeneities 34 as well as substrate interactions 35 in/of the 2D materials with respect to performance metrics, such as the FOM. Here, for instance, insights on the ion adsorption at the graphene/electrolyte interface 36 would help in such investigation.…”