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The growth and shrinkage of water droplets at the oil-solid interface.

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Zhang, Ran 
Liao, Wei 
Wang, Yunpeng 
Wang, Yao 
Ian Wilson, D 


HYPOTHESIS: The mechanism for the spontaneous formation of water droplets at oil/solid interfaces immersed in water is currently unclear. We hypothesize that growth and shrinkage of droplets are kinetically controlled by diffusion of water through the oil, driven by differences in chemical potential between the solid substrate and the aqueous reservoir. EXPERIMENTS: The formation, growth and shrinkage of water droplets at an immersed oil/solid interface are investigated theoretically and experimentally with three silicone oils. The surface is hydrophobic and the droplets formed are truncated spheres with radius, a, less than 10 μm. The expansion and contraction of the droplets can be controlled by adjusting the difference in chemical potential. The growth kinetics are modelled in terms of water migration through the oil layer which predicts a2∝t. FINDINGS: This is the first study of possible mechanisms for the formation of such interfacial droplets. Several possible causes are shown to be unfavourable, negligible, or are eliminated by careful experiments controlling key parameters (such as oil viscosity, substrate chemistry). The rate constant for mass transport is proportional to difference in chemical potential and an estimate shows dissociation of surface groups on the substrate provides a driving chemical potential of the right magnitude.



Chemical potential, Diffusion, Droplet growth, Droplet shrinkage, Interfacial droplets, Oil-solid interface, Osmotic pressure, Surface speciation, Viscosity

Journal Title

J Colloid Interface Sci

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Elsevier BV
National Natural Science Foundation of China (21872078)