Patterned surface charges coupled with thermal gradients may create giant augmentations of solute dispersion in electro-osmosis of viscoelastic fluids

Abstract

Augmenting the dispersion of a solute species and fluidic mixing remains a challenging proposition in electrically actuated microfluidic devices, primarily due to an inherent plug-like nature of the velocity profile under uniform surface charge conditions. While a judicious patterning of surface charges may obviate some of the concerning challenges, the consequent improvement in solute dispersion may turn out to be marginal. Here, we show that by exploiting a unique coupling of patterned surface charges with intrinsically induced thermal gradients, it may be possible to realize giant augmentations in solute dispersion in electro-osmotic flows. This is effectively mediated by the phenomena of Joule heating and surface heat dissipation, so as to induce local variations in electrical properties. Combined with the rheological premises of a viscoelastic fluid that are typically reminiscent of common biofluids handled in lab-on-a-chip-based micro-devices, our results demonstrate that the consequent electro-hydrodynamic forcing may open up favourable windows for augmented hydrodynamic dispersion, which has not yet been unveiled.

Temperature effects on electrical double layer at solid‐aqueous solution interface

“…Here, we consider the system is under elevated isothermal situation only in this work. Otherwise, the thermodiffusion effect or the Soret effect may play an important role on ion transport [40][41][42][43][44]. Figure 2B demonstrates that the acquired surface charge (σ 0 ) enhances by increasing the solution temperature.…”

Temperature-gradient-induced massive augmentation of solute dispersion in viscoelastic micro-flows

“…Hydrodynamic dispersion is the band broadening of a solute which mainly arises from the non-uniformity in the flow field (Taylor 1953;Aris 1956Aris , 1959Chatwin 1970Chatwin , 1975Chatwin & Sullivan 1982;Smith 1982;Barton 1983;Watson 1983;Mazumder & Das 1992;Ng & Yip 2001;Zholkovskij & Masliyah 2004;Ng 2006;Ghosal 2006;Jansons 2006;Sounart & Baygents 2007;Dutta 2008;Datta & Ghosal 2008;Ghosal & Chen 2012;Arcos et al 2018;Chu et al 2019). Under ideal circumstances, the velocity profile of electro-osmotic flow does not contribute to shear-induced axial dispersion because of the flatness of the velocity profile as opposed to the case of Poiseuille flow (which is parabolic in nature) (Gaš, Štědrý & Kenndler 1997;Ghosal 2004;Mukherjee et al 2019). However, in practice, any inhomogeneity in the flow condition or flow domain can give rise to strong perturbation in the flow field, thereby inducing an axial pressure gradient, which is accompanied by the generation of secondary flow component in order to maintain the flow continuity.…”

“…Recently, incorporation of a thermal gradient has emerged as an alternative tool in augmenting dispersion where interplay between thermal and electrical effects over small length scales, almost exclusively, dictates the flow physics (Chen et al 2005;Sánchez et al 2018;Mukherjee et al 2019). In addition, it may be noted that with the emergence of new-generation medical devices, complex bio-fluids have more prominently come into the paradigm of microfluidics Berli & Olivares 2008;Olivares, Vera-Candioti & Berli 2009;Berli 2010;Zhao & Yang 2011.…”

“…Such fluids exhibit strikingly distinct behaviour compared to the fluids obeying Newton's law of viscosity (Owens 2006;Fam, Bryant & Kontopoulou 2007;Moyers-Gonzalez, Owens & Fang 2008;De Loubens et al 2011;Brust et al 2013;Silva, Alves & Oliveira 2017). Some recent studies have demonstrated that the constitutive behaviour of these biological fluids has close resemblance to the rheology of viscoelastic fluids, and therefore the inclusion of fluid rheology and viscoelasticity in dispersion characteristics has attracted significant attention lately (Brust et al 2013;Arcos et al 2018;Hoshyargar et al 2018;Mukherjee et al 2019). While considering the thermally induced electrokinetic flow of viscoelastic fluids, an additional source of nonlinearity crops up as mediated by the constitutive behaviour of the fluid (Afonso, Alves & Pinho 2009;Coelho, Alves & Pinho 2012;Afonso, Alves & Pinho 2013;Ferrás et al 2016;Ghosh, Chaudhury & Chakraborty 2016;Mukherjee et al 2017a,b).…”

“…While considering the thermally induced electrokinetic flow of viscoelastic fluids, an additional source of nonlinearity crops up as mediated by the constitutive behaviour of the fluid (Afonso, Alves & Pinho 2009;Coelho, Alves & Pinho 2012;Afonso, Alves & Pinho 2013;Ferrás et al 2016;Ghosh, Chaudhury & Chakraborty 2016;Mukherjee et al 2017a,b). Moreover, the degree of viscoelasticity, which is determined using physical properties like fluid viscosity and relaxation time, is a strong function of the prevalent thermal gradient, augmenting the complexity of the problem to a large extent (Bautista et al 2013;Mukherjee et al 2019).…”