Effectiveness of flow obstructions in enhancing electro-osmotic flow

Microfluidics and Nanofluidics, Mar 2017

In this paper the influence of obstructions on microchannel electro-osmotic flow is investigated for the first time. To carry out such a study, regular obstructions are introduced into microchannels and flow rates are numerically calculated. The effect of channel width on flow rates is analysed on both free and obstructed channels. The solid material considered for channel walls and obstructions is silicon, and the electrolyte is deionised water. The parameters studied include channel width, obstruction size and effective porosity of the channel. The effective porosity is varied between 0.4 and 0.8 depending on other chosen parameters. The results clearly demonstrate that, under the analysed conditions, introduction of obstructions into channels wider than \(100\,\upmu \hbox {m}\) enhances the flow rate induced by electro-osmosis.

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Effectiveness of flow obstructions in enhancing electro-osmotic flow

Microfluid Nanofluid Effectiveness of flow obstructions in enhancing electro‑osmotic flow S. Di Fraia 0 1 N. Massarotti 0 1 P. Nithiarasu 0 1 0 Department of Engineering, University of Naples 'Parthenope', Centro Direzionale Isola C4 , 80143 Naples , Italy 1 Biomedical Engineering and Rheology Group, Zienkiewicz Centre for Computational Engineering, Swansea University , Swansea SA2 8PP , UK In this paper the influence of obstructions on microchannel electro-osmotic flow is investigated for the first time. To carry out such a study, regular obstructions are introduced into microchannels and flow rates are numerically calculated. The effect of channel width on flow rates is analysed on both free and obstructed channels. The solid material considered for channel walls and obstructions is silicon, and the electrolyte is deionised water. The parameters studied include channel width, obstruction size and effective porosity of the channel. The effective porosity is varied between 0.4 and 0.8 depending on other chosen parameters. The results clearly demonstrate that, under the analysed conditions, introduction of obstructions into channels wider than 100 μm enhances the flow rate induced by electro-osmosis. Microchannels; Flow obstructions; Flow enhancement; Width effect; Numerical modelling 1 Introduction Electro-osmotic flow (EOF)-driven systems have been employed in various branches of engineering and technology, such as biomedical, geophysical, energy and chemical. Over the last century, electro-kinetic effects have been widely exploited in micro- and nanofluidic devices. The most common applications include pumping (Berrouche et al. 2009; Chen et al. 2008; Kang et al. 2007; Li et al. 2013b; Wang et al. 2006, 2009; Yao and Santiago 2003; Yao et al. 2006), capillary electrochromatography (Liapis and Grimes 2000; Rathore and Horváth 1997) and recently dehumidification and regeneration of desiccant structures (Li et al. 2013a, b). EOF in micro- and nanosystems with and without porous media has been investigated both experimentally and numerically by many, and recently, the behaviour of non-Newtonian fluids under EOF has also been examined (Chen et al. 2014). Due to the dimensions involved in microchannels, producing experimental data is difficult and therefore numerical modelling is very useful in predicting EOF (Li et al. 2013a, b; Wang et al. 2006). As introduced by Gouy (1910) and Chapman (1913), the internal potential for a planar surface can be described by the Poisson equation (see Sect. 2.1) that can be linearised for small values of electric potential by using the Debye–Hückel approximation (Patankar and Hu 1998). In numerical modelling of EOF in porous media, other simplifying hypotheses have been commonly assumed. Most authors have considered only the charge of channel walls neglecting that of solid particles (Scales 2004), both in the equation governing the internal potential and in that describing fluid flow. Recently, some researchers have attempted to estimate the contribution of solid particles to EOF. Several authors have analysed EOF at the pore level (Chen et al. 2014; Li et al. 2013a, b; Wang and Chen 2007; Wang et al. 2006), while others have used a generalised model for porous media flow and added a source term in the momentum equation, depending on the charge density of porous medium and the applied electrical field (Scales 2004; Tang et al. 2010). Although it has been found that the main driving force is due to the charged particles rather than the channel walls (Wang et al. 2006), it appears that the internal potential equation has not been appropriately modified to take into account the charge of solid particles, except for boundary conditions (Tang et al. 2010). To consider the charge of both solid particles and channel walls, Kang et al. (2005) split the velocity into two components and then coupled them to obtain the overall macroscopic EOF velocity. The first component was derived as per the fluid flow in standard channels, by assimilating the porous medium to an assembly of parallel tortuous cylinders. The second component was obtained by applying the Brinkman extension of the Darcy equation, in which the inertia terms were neglected because of the low Reynolds number. The dimensionless Darcy velocity was found to increase with the particles size, the applied electric field and the difference between zeta potential of particles and channel walls, and it was also found to decrease with increase in channel width. Many authors have focused on EO porous pumps and found that the thermodynamic efficiency significantly increases with the addition of a porous medium in a channel, as much higher pumping pressures can be generated (Wang et al. 2006). In general, the velocity has been found to increase with the increase in diameter of solid particles or pores (Berrouche et al. 2009; Chai et al. 2007; Chen et al. 2008; Kang et al. 2005; Tang et al. 2010; Wang and Chen 2007; Yao et al. (...truncated)


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S. Di Fraia, N. Massarotti, P. Nithiarasu. Effectiveness of flow obstructions in enhancing electro-osmotic flow, Microfluidics and Nanofluidics, 2017, pp. 46, Volume 21, Issue 3, DOI: 10.1007/s10404-017-1881-z