Drying of oil-in-water emulsions on hydrophobic and hydrophilic substrates

We have investigated the effect of the hydrophobic/hydrophilic character of the substrates on the drying behaviour of dilute silicone oil-in-water (o/w) emulsions by light microscopy and ellipsometry. The poly(dimethylsiloxane) (PDMS) emulsion droplets, which are stabilised by a triblock PEO/PPO/PEO copolymer, form a close-packed structure containing domains of hexagonally packed droplets on the hydrophilic substrate. We find that the hydrophilic substrate does not destabilise the emulsion droplets; the close-packed structures are very stable and coalesce very slowly only when most of the water has evaporated. This is supported by ellipsometry measurements, which showed that the emulsion droplets do not adsorb to the hydrophilic substrate. The hydrophobic substrate, on the other hand, destabilises the emulsion and we observed a significant increase in the coalescence rate. Ellipsometry measurements suggest that destabilisation is promoted by the strong interaction between the emulsion droplets and the hydrophobic substrate. We also find that the emulsion undergoes dewetting followed by a release of oil and rewetting of the substrate when the o/w emulsion film reaches a critical thickness on the hydrophobic substrate.