Wicking and chromatographic properties of highly porous functionalised calcium carbonate coatings custom-designed for microfluidic devices

Abstract

Devising controlled microfluidic liquid transfer in porous diagnostic devices presents challenges in respect to differentiation between surface/film flow and wicking within the bulk of the substrate. Rapid liquid translation is predominantly driven by film flow mechanisms in such devices. However, the more complex needs of in-pore reactivity between test fluids and reactive agents, followed typically by the need to perform chromatographic separation of eluents, demands the often contradictory facility of rapid spatial transport whilst allowing exposure to high internal surface area, i.e. by the mechanism of internal bulk wicking. The key parameters are permeability to saturated flow relatively far behind the wetting front, limiting the delivery rate of liquid to the wetting front, and capillarity within an ideally parallel high surface area pore network. We demonstrate a way to balance these properties using a coating with a discretely bimodal pore structure, whereby bulk flow is controlled by interparticle connectivity and interpore throat size, and capillarity and surface reactivity by ultrafine intraparticle pores. The distance travelled at time t by the wetting front is expressed as a power law function, tp . Evaluating p provides a measure as to the closeness of dependence on viscous-controlled bulk flow permeability as p tends to 0.5, i.e. √t. Deviation from bulk flow dependence indicates the presence of pore wall film flow or inertial plug flow, possibly combined with evaporation and/or material sorption onto the pore wall surface or into the binder matrix. In this context, a variety of coating formulations are evaluated, and wicking rate is shown to vary strongly as a function of the binder impact on interparticle pore connectivity and on the absorption behaviour of the binder itself. Microfibrillated cellulose is shown to be a highly suitable binder for microfluidic diagnostic coatings. Reactivity within the coating is illustrated using charge-driven chromatographic colorant separation.