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)