analytica chimica acta 605 ( 2 0 0 7 ) 175–184 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/aca Electroosmotically driven capillary transport of typical non-Newtonian biofluids in rectangular microchannels Suman Chakraborty * Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721302, India article info Article history: Received 21 August 2007 Received in revised form 23 October 2007 Accepted 24 October 2007 Published on line 7 November 2007 Keywords: Capillary filling Non-Newtonian Electroosmotic abstract In this paper, a detailed theoretical model is developed for studying the capillary filling dynamics of a non-Newtonian power-law obeying fluid in a microchannel subject to elec- trokinetic effects. Special attention is devoted to model the effects of the electroosmotic influences in the capillary advancement process, variable resistive forces acting over dif- ferent flow regimes, and the dynamically evolving contact line forces, in mathematically closed forms. As an illustrative case study, in which the flow parameters are modeled as functions of the hematocrit fraction in the sample, the capillary dynamics of a blood sample are analyzed. Flow characteristics depicting advancement of the fluid within the microflu- idic channel turn out to be typically non-linear, as per the relative instantaneous strengths of the capillary forces, electroosmotic forces and viscous resistances. Non-trivial implica- tions of the blood hematocrit level and the imposed electric field on the progression of the capillary front are highlighted, which are expected to be of significant consequence towards the dynamics of electroosmotically aided capillary filling processes of biofluidic samples. © 2007 Elsevier B.V. All rights reserved. 1. Introduction Capillary-driven filling of microchannels finds important applications in the lab-on-a-chip-based microdevices and micro-total analysis systems. A comprehensive theoretical understanding of the capillary filling process can guide the designer to optimize the internal structure of the chip, such as the chambers, binding pillars, splits, valves, electrical and electronic circuitry etc., in order to avoid any potential filling problem (including air-bubble entrapment and clogging) and to achieve high throughputs. The moving contact line problem associated with capil- lary filling has eluded the researchers over a long period of time, primarily attributed to the difficulties in treating the contact line problem through conventional considerations of continuum fluid mechanics. Research on such contact line problems dates its origin back to more than one century ago, ∗ Fax: +91 3222 282278. E-mail address: suman@mech.iitkgp.ernet.in. with the introduction of the classical Washburn equation [1], dealing with the balance of viscous force, surface tension force, and the gravitational force for capillary transport. This analysis, however, suffered from an inherent constraint that it did not attempt to account for some additional inevitable forces acting on the system during the initial transients. Several modifications to the Washburn equation were subse- quently reported to include some of these effects [2–7]. Most of the above-mentioned studies, however, assumed constant contact angles between the advancing liquid front and the capillary wall, despite the fact that experimental observa- tions had revealed strong dependences of the contact angle on the rate of capillary meniscus advancement [8,9]. Considering the dynamic evolution of the contact angle, several empirical or semi-empirical correlations were subsequently introduced into the literature for prescribing the contact angle as func- tions of time [10–13]. However, most of these considerations 0003-2670/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2007.10.049