Wetting, evaporation and deposition processes in interaction of complex liquid formulations with porous substrates

Abstract

Complex liquid formulations are typically mixtures containing a bulk liquid phase (water or other solvents) and a dispersed phase, which is usually the phase which typically carries the active ingredient. Common examples are particle suspensions, emulsions, and dispersions of surfactants & lipid assemblies such as micelles, vesicles etc. In everyday applications such as fabric and hair conditioning, and in industrial applications such as surface modification of paper and textile, the end goal is to deposit active ingredients onto substrate surfaces. The deposition of active ingredients produces the desired surface modification effect i.e. softening, conditioning, hydrophobicity etc. Therefore, a good understanding of the liquid formulation-substrate interaction is of great significance. Although the interaction of complex liquid formulations with well-defined flat solid surfaces has been investigated in earlier works, very few works have considered substrates which are closer to real-life substrates such as textiles, paper and hair. A common feature of these real-life substrates is that they are porous in nature, and the porous structure of these substrates significantly influences their interaction with the complex liquid formulation. This PhD thesis aims to investigate the processes involved in the interaction of complex liquids with porous substrates. Dispersions of cationic vesicles are used as model complex liquid for the experiments. In the first part of the thesis, we consider the formulation-substrate interaction under fully immersed conditions wherein attractive electrostatic forces drive the deposition of cationic actives on anionic substrate surfaces. Single cellulose fibers and porous cotton yarns are used as deposition substrates. Experiments with single cellulose fibers are aimed at understanding the fundamental mechanism of vesicle deposition on cellulose surfaces. In the case of porous cotton yarns, the investigation focuses on the distribution of deposited vesicles across the porous substrate. The investigations reveal that the cationic vesicles deposit on cellulose fiber surfaces as intact entities in contrast to the more usual case in which the deposited vesicles disintegrate forming supported lipid bilayers. The surface roughness of cellulose fibers and the lipid bilayer phase behavior was found to significantly affect the deposition process. In the case of porous cotton yarns, the lipid bilayer phase behavior was found to significantly influence the location of vesicle deposition. Additionally, the bulk electrical conductivity also influences the distribution of deposited vesicles. The later part of the thesis focuses on the spray-application of complex liquid formulations which is of great significance to industrial applications. Cellulose fiber filter papers are used as substrates for the experiments. We investigate the interaction of vesicle dispersion droplets with these porous substrates which involves, (i) wetting of the porous substrate by the droplets, and (ii) evaporation of the bulk liquid from the wetted area and subsequent stain formation. The wetting of porous substrates by vesicle dispersion droplets involves simultaneous spreading of the droplet over the porous substrate and imbibition of the liquid into substrate pores. In the final stains left on the porous substrate, deposition is observed to be localized close to the periphery of the wetted spot. It is similar to the typical “coffee ring effect” obtained when colloidal droplets evaporate on flat solid surfaces. Using the experimental results, the mechanism of formation of the coffee ring effect on porous substrates is explained. Furthermore, the factors which influence the droplet evaporation process and the resultant coffee ring stain are identified. The results of this PhD thesis are expected to be helpful in devising approaches for achieving improved performance in several applications involving complex liquid-porous substrate interactions such as fabric softening, hair conditioning, and industrial surface treatment of paper, textiles etc. The results are also significant for other applications such as transdermal drug delivery, dried spot sampling of blood and other biological samples.