Nanoparticle deposits formed at driven contact lines

Abstract

Drying of colloidal suspensions appears in many applications such as coatings (paints, ink printing, paving), colloidal assembly/templating , discrimination of particles with different size even medical diagnostics. Complex liquids, namely suspensions of solid particles, polymeric dispersions, emulsions.., and simple liquids behave in different way at interfacial regions. The formation of stains at the periphery of drying drops of any colloidal dispersion is known as the "coffee stain" effect or "coffee ring" effect.

Desiccation of colloidal suspensions is governed by the coupling between hydrodynamics, heat and mass transfer and wetting. It is not always clear what the mechanisms of formation for the different deposits are. Besides, although the time scale of the process is extremely long, it is not clear how to treat such a nonequilibrium situation. A better understanding of the driving mechanisms of the drying of colloidal suspension drops would improve the productivity and competitiveness of the concerning industries.

In this work, we propose standardize the contact line dynamics of sessile drops upon evaporation-like conditions but without any significant convective flows within the drop. This way, special attention was addressed to receding contact lines. As reported in literature, driven contact lines also enable the formation of particle deposits. Our methodology allows examining separately the impact of contact line dynamics and the properties of the nanoparticle suspensions (bulk concentration, surface electric charge, wettability). We probed the arrangement of nanoparticles at driven contact lines, with low capillary numbers and at time scales shorter than during evaporation. Unlike typical commercial curtain coating, we operated in the quasi-static regime of contact line motion where the viscous effects are excluded (Ca << 10-4). In this scenario, the observed contact angle was speed-independent.

The present dissertation is essentially arranged in two parts in order to explore separately the behaviour of receding contact lines with pure liquids (part I) and complex liquids (part II), namely nanoparticle aqueous suspensions. With a variable rate of withdrawal of liquid, we controlled the dynamics of receding contact lines of millimeter-sized drops (~ 100 ul). Following this approach, we were able to emulate the first stages of drop evaporation, but at shorter times. We monitored the contact line dynamics of shrinking drops containing nanoparticles to identify stick-slip events. We analyzed the effect of several parameters such as wettability contrast between particle and substrate, particle concentration, particle size and electrostatic interactions (particle-particle and substrate-particle) on the formation and morphology of the nanoparticle ring-like deposits. Special attention was addressed to the effect of pinning time on the formation of particle deposits. As far as we know, this is the first work devoted to study the role of the particle-particle and substrate-particle interactions on the deposits formation without macroscopic evaporation.